Monday, October 5, 2015

A Hardwired Loop For DSP Radios

The introduction of DSP ultralight radios using the Silicon Labs Si473x chip about four years ago proved to be an experimenter's playground. The new design allows for a simply-wound ferrite loopstick antenna with a single coil winding, connecting to the radio's circuit board on just two soldered pads. No more complicated coil with a secondary local oscillator winding or multiple coil taps. The radio's chip tunes the single coil inductance for longwave through the top of the mediumwave band. Replacing the ferrite antenna with something else is now easily possible.

The common mod to these radios has been to remove the internal ferrite loopstick and replace it with a longer one, often mounted outside the radio on top. This new ferrite loop is then tuned by the circuitry of the radio just as the old loop was. The longer (or beefier) ferrite gives the radio much greater sensitivity. What about an air core loop?

Early on in reviewing the Si473x chip's documentation, I noticed the manufacturer showed an option of using an external, air core loop antenna as a substitution for the internal ferrite loop. Their suggestion was for a loop of minimal turns connected to the circuit board's antenna terminals through a 1:5 winding, 25x step-up ferrite core transformer, thus providing the correct coil inductance (180-450 micro-Henries). It was apparent that using a full inductance loop was also possible. This would also result in greater signal gathering ability.

So, let's get started.

Currently I own the Tecsun PL-380 and Eton Traveler III DSP radios. After removing the cases of the two and examining their internal ferrite loopsticks, it was evident that the Eton's loopstick would be the easiest to remove. One end is held down with some flexible sticky glue and is easily removable with a small jeweler's screwdriver. Pull the ferrite bar with coil out of the case and unsolder the two pads on the circuit board.

Ferrite loop removed.

I decided to make this a totally breadboard-type operation, so I drilled two tiny holes through the top of the radio's case and soldered two short pigtail leads through them to the board. I then connected two micro alligator clips to the ends of the pigtails. These will be how we attach the radio to the loop we will build. Using clips, it will also enable easier experimentation with other loop devices.

Holes drilled and pig-tails and clips attached.

Initial experiments with a 12 inch PVC loop of 16 turns (about 215 micro-Henries) showed excellent signal strengths. In researching optimum coil inductances for the Si473x chip, the figure of 400 micro-Henries seemed to be mentioned in the forums, so it was decided to build an 18 inch loop of 17 turns***, which calculated out to about 410 micro-Henries. The additional loop area (2.25x greater area) would also provide better signal gathering ability.

***17 turns proved to be way to many turns, as I will explain below.

Radio complete.

About the simplest box loop frame to build is one from 1x2 inch lumber. Cut four pieces to make a square 18 inch frame and nail (I also glue) the corners. Using a fine saw or hacksaw and file, notch each corner about 3/4 inch as shown. This will contain the coil windings. You will need about 65-70 feet of insulated wire, about 22-24 gauge should do it.

Now, let's talk about wire. Unless someone gives you a quantity of insulated wire for free, the least expensive wire source I know of is to buy 4 conductor telephone cable. Remove the outer sheath and you have four solid, insulated 22 or 24 gauge copper conductors. Using a straight edge razor held between your fingers just right against a hard table surface, draw the sheathed wire under it, scoring it lengthwise, and you will find the sheath peels away quite easily. A 50 ft. roll will cost you about $8 at Home Depot. That's 200 ft. of wire for about 4 cents a foot. I have used this wire in loops for years, and in the loops described here. Regular hookup wire at Radio Shack is about double that cost.

18 inch box frame showing notched corners.

Before starting the loop winding and to secure the coil ends, I drilled two small holes through the bottom of the frame. Near these holes on the inside, drive two small headed-nails which will secure the wire ends, as shown in the picture just below.

I decided to close-wind the coil, that is, to place each turn right next to each other. I haven't found this to be a huge detraction in passive loops. The close turns tend to increase the inductance a bit, and add a small amount to the inherent internal capacitance of the loop, but not as much as you might think.

Bottom detail showing drilled holes and nails.

Secure one end of wire to one nail through the hole, and start winding. I brought the last turn back through the other hole and secured it to the other nail. Be sure you've left two pigtails off the nails and strip the wire ends. Make a small loop at each wire end, 1/8 inch diameter or so. We will attach the alligator clips to them.

After winding the frame, I used electrical tape to wrap the sides in strategic places to keep the loop turns together. Over time, the loop turns will tend to relax some and get loose, and the tape will help to keep the loop turns corralled.

Loop winding completed.

Last, cut two 18 inch side slats from 1x3 lumber for the bottom of the frame, as shown on the completed loop photo. They will provide a slot to hold your radio and also broaden the base of the frame so it will sit upright on a flat or nearly-flat surface.

You are done! Time to try it out. Place the radio in the slot and connect the micro clips to the loop ends. Turn the radio on (use headphones), tune to a known station and rotate the loop for maximum signal. This will be off the ends of the loop.

Disappointment at first. 17 turns wouldn't tune to 1700 KHz, or anything above about 1580 KHz. Signal strengths were also not much better than the 12 inch loop. Too much inductance, apparently. About all I could get at the upper end of the band was a weak KMIK-1580 (50 KW), in Tempe, AZ, at 139 miles. This station came in with tremendous signal on the 12 inch loop. Mexican station (ESPN Radio) XEKTT-1700 (10 KW), between Tijuana and Tecate along the US border (169 miles) was weak but copyable on the 12 inch loop. It was non-existant on the 18 inch with 17 turns. In fact frequencies above 1600 KHz were almost full quieting, indicating that the radio was not tuning this high. Recalculating, I found that 12 turns was about the equivalent inductance as the 12 inch loop, so I removed 5 turns. The loop now tunes perfectly between 530 and 1700 KHZ.

The final product, the 18 inch loop with 11 turns.

What a difference. Huge daytime signal increases over the radio's stock internal ferrite antenna are apparent using this loop, and a noticeable increase over the 12 inch loop. KNX-1070 (50 KW) at Los Angeles, at 237 miles distant now shows an RSSI strength reading of 44/15 and is beautiful copy. It was not receivable before with the stock ferrite antenna, at 15/02. Calculated field strength for KNX-1010 at this location is on the order of 0.03 mV/m, an extremely small signal, not receivable at all on a stock ultralight. The Panasonic RF-2200 and the Eton E1 will receive it fairly well. The Eton Traveler 3 now beats them both.

Nulls are well defined with this loop, and fairly narrow in beamwidth. I am able to receive KTIE-590 (2.5 KW), San Bernardino, CA at 178 miles and KSUB-590 (5 KW), Cedar City, UT at 284 miles by rotating the loop and nulling out each station. The stations are approximately 90 degrees to each other. Calculated field strengths here are extremely weak, at 0.02 mV/m for KTIE and 0.03 mV/m for KSUB.

KTNN-660 (50 KW), Window Rock, AZ at 327 miles produces a decent daytime signal as well. I have heard KKOB-770 (50 KW) at Albuquerque, NM (447 miles) on certain days. It's calculated signal strength is extremely low, at 0.01 mV/m. KALL-700 (50 KW), N. Salt Lake City, UT (515 miles) is an easy catch. Most of the big-gun San Francisco, CA stations, though usually weak, (about 525 miles) are receivable as well: KNBR-680, KGO-810. KCBS-740 is somewhat masked by semi-local KDIR-740 (1 KW) in Phoenix.

I had a small, plastic entertainment center receiver loop frame I've been saving. It measures 5.5 x 6 inches. You've seen them - they are everywhere until you want one. I found one at a Goodwill store some time ago for about 50 cents and stockpiled it. Calculating the inductance required, I wound 31 turns of wire on it and connected it to the Eton via the clips. Signal strengths were definitely down from the 12 and 18 inch loops. They were slightly above the stock ferrite loopstick, however. This little loop might make a usable antenna for playing the radio in the house.

Mini-receiver loop. A fairly poor performer.

Here's a curious tuning tip that may not be apparent at first. It occurred to me that when the radio is attached to the loop and powered on and tuned to a frequency, say 590 KHz, the loop should act the same way as a passive loop does which has an external variable capacitor across it tuned to 590 KHz. I placed a second radio in the slot within the loop and tuned it to 590 KHz. The pronounced signal at 590 KHz was also induced into the second radio. Other frequencies were not. If I tuned the Eton off 590 KHz, the second radio, still tuned to 590 KHz went quiet. Interesting, but it makes sense. Using this technique, we have a way to use this loop arrangement also as a passive loop for another radio, with the benefit that we know precisely the tuned frequency of the loop.

The 12 inch loop is made from 1/2 inch PVC. Purchase four 3-way corners. The pieces fit together tightly without glue and allow it to be disassembled. To each corner I glued a grey plastic, 3/4 inch x 1-1/2 inch sprinkler system nipple. We will use the thread indentations on these nipples to create an perfectly and evenly-spaced loop of 16 turns. Drill tiny holes through the nipples at each end to secure the wires. The wires are further secured with a zip-tie.

The 12 inch loop, with neatly-spaced turns, performs remarkably for its size.

The 12 inch loop.

If you are an experimenter and don't mind cutting into your DSP ultralight, try building a hardwired loop for it. It will open up a new world of listening. 12 or 18 inch loops will result in signal sensitivities exceeding those of the stock Panasonic RF-2200 and Eton E1 radios. Coupled with the excellent stock sensitivity and selectivity of the Eton Traveller III, you will have a great performer.

Tuesday, September 22, 2015

Impressions Of The Eton Traveller III

Passing through Phoenix, Arizona last April, I stopped into one of those Radio Shacks that were closing and picked up the new Eton Traveler III DSP radio for about $45 out-the-door. Not a bad price, near 40% off suggested retail of $69.99. I've spent the summer evaluating it, and had a chance to use it while crossing the country.

The Eton Traveler III

The last couple of DSP radios I bought were disasters so I quit buying them. The Tecsun R2010D and the Kaito KA321 both had odd tuning quirks, seemingly a product of their engineering. Quality is generally down these days. Sensitivities are generally poor. I gave both radios away at a yard sale two summers ago, then decided to take a hiatus from buying Asian consumer DSP radios until something changed, or someone reported an improvement worth looking at.

In a radio, I only care about a few things. Mainly, but not limited to: does it have reasonable sensitivity and selectivity, does it tune acceptably well, and does it have a readable display? I don't care about jillions of memories, ETM, alarms, sleep timers, or temperature display. Just give me a radio that receives stations. My old 1962 Sears Silvertone seven-transistor radio, "ice blue", received stations, and well too. Elementary silicon technology, it was. If they could do it some 53 years ago, they can do it now.

And so we have the Eton Traveler III. Not perfect, but promising. I bought it for two reasons. People had reported that the annoying soft-mute (see other DSP radio reviews on this blog) had been disabled or at least minimized, and that the radio's sensitivity was pretty good. The 'Traveler III is basically the next iteration of the original Tecsun PL-300WT, and Eton's Grundig G8/Traveler II offering of several years ago. It uses the new version of the Silicon Labs' Si473x chip.

The Eton Traveler 3 (I'll use the number 3 from here on out), is small, basically pocket-sized, but has some heft to it. It fits well in your hand. It is a beautiful little thing to the eye, and it's got a nice build quality. The sound out of its ample, front-facing speaker is pleasant.

The Eton engineers have nailed three important things on this unit: smoothness in tuning for a DSP radio (though points off for poor tuning encoder), defeat of soft-mute, and sensitivity.

They have also failed in three categories: the tuning encoder, the AGC (automatic gain control) setup, and the display.

The good news first.

Ahhh, smoothness in tuning. Fond memories. As an old radio "Ham" and general DXer, I've been tuning receivers for more than 50 years. Memory drifting.... Slowly we sweep across the band. Vibrant, reverberant (somewhat microphonic) signals rise up out of the ether, then smoothly fade away under the noise as we pass by. Pure magic. Think of zeroing in on Radio Moscow on a cold winter's morning in about 1960, announcer Joe Adamov booming in on a late 1930s vacuum tube Zenith stand-up console radio.

Analog tuning, the old superheterodyne, the smoothest of the smooth, seems to only exist in some cheap consumer pocket varieties now, many of them hold-overs of best-selling consumer items of 15-20 years ago like the Sony ICF-S10MK2, the Sony SRF-59, still on sale at your local drug store. And there are others of course - newer, cheaper, Asian analogs. Digital PLL (Phase-Locked-Loop) radios are still with us, introduced on the public many years ago. They tune adequately, although exhibit a sort of lack of "presense", a hard sensation to describe.

Back to our story.

Enter the consumer digital DSP chip world, circa 2009. It was (and still is) promising. But we seem to have gotten away from smooth tuning. Consumer design teams have struggled getting it right with these new Silicon Labs DSP chips for some reason. I was starting to believe it wasn't possible.

After several years of using these radios, it started to become evident that soft-mute was the main culprit here. Combine soft-mute with oddly-set AGC characteristics, then couple that with the DSP chip's odd infatuation with peak AM signal-lock and you have a recipe for tuning weirdness.

Soft mute, which is programmable, has finally been turned off in the Eton Traveler 3. No more muting of noise next to a strong (or weak) station. No more dead air where there is no station. Only the beautiful, low-level hiss of the noise floor and the resounding crashing of the ionosphere. Wait - I hear a carrier and faint audio under that hiss. I am back in 1962 once again! Sweet!

The soft mute change - its elimination - promotes smoothness of tuning, at last. Signals don't just "pop in", ala TV remote control. They rise and fade, nearly like the old days, as you sweep across the band***. Thank you Eton.

*** See the caveat, below.

The Eton Traveler 3's sensitivity is to be commended. Of all the ultralight DSP radios I own or have tried, it is the most sensitive one of all. In western Arizona, Los Angeles' KFI-640 (50 KW), at 217 miles distant, is armchair copy. The only other radio which offers a presentable signal, though weaker, is the Tecsun PL-380, another Si473x DSP radio. On the others, audio is strained or non-existent. No signal is even present on the Sangean DT-400W. The Sony SRF-59 struggles to even make it out.

Selectivity is good. It appears to be about 3 KHz wide, or possibly even 2 KHz, about equivalent to the Tecsun PL-380's 2 or 3 KHz setting. And of course the skirts are steep, using DSP technology. The only thing lacking is a way to set different bandwidths like the PL-380 can. That's not objectionable to me, though.

A nice and welcome software correction, the radio's display now updates signal strength (RSSI) and signal-to-noise values approximately three times per second. The Tecsun PL-380 only updated its display about once every two seconds. It's much easier to rotate the radio now and read the signal strength changes in somewhat realtime.

Now for the bad news.

Eton, you skimped on the tuning encoder. It's cheap, and mechanical, though a mechanical encoder would be what is expected in a radio of this price range. Tune upwards very slowly, like DXers do, and it skips two channels over, or tunes backwards a channel. Tune downwards slowly, same thing. Unacceptable. Mine isn't the only one, either. Reports are that many, if not most, act this way. One solution is to change the tuning step to 1 KHz which makes the channel skipping effect less noticeable. But it takes forever to get anywhere at 1 KHz without a keypad. Fast tuning, activated by spinning the tuning knob many, many times, doesn't kick in quite soon enough.

A little history. Automatic Gain Control, or AGC, was first implemented in radios many years ago for the reason of fading propagation, which required continuing manual adjustments of the receiver’s gain. The idea is that the AGC circuit will automatically maintain a constant signal level at the output, regardless of the signal’s variations at the input of the system. Besides the depth of its interaction, AGC has two other characteristics, attack rate and decay rate. The attack rate is the speed in which the AGC is applied, the decay rate is the speed in which it is relaxed. All three are important design considerations.

***I find the Eton's AGC to be a bit heavy handed. It's decay rate is extremely slow in reaction, especially for the DXer. Tune off a moderate or strong signal to a nearby weak one, and it seems seconds before the AGC relaxes and brings up the volume of the low level background signal and attended atmospheric noise. This is simply a programmatic adjustment on the DSP chip. It probably works for the intended consumer crowd, but not for the DXer. I prefer a relatively fast-reacting AGC.

Now, for the LCD display. Use this radio indoors, if possible. The orange digits on a deep black background are beautiful to behold in a dark or dimly-lit room. Take the radio outside, particularly on a sunny day and you think you have gone blind because the display disappears. You can't see the digits. The display also has an odd "best" viewing angle. It appears to be around 45 degrees, looking up at it from the bottom of the radio, not dead on. And that angle is not forgiving in the sunlight, either. A few degrees off and the display disappears again. You are constantly adjusting the viewing angle straining to read the information.

In summation, I found the Eton Traveler 3 to be a good buy at the price I paid. It has marvelous sensitivity for such a small unit. Except for the deficient tuning encoder, it tunes smoothly, nearly as good as the old analogs did. Soft mute has finally been eliminated. I could never understand why you would combine AGC with soft mute. One intends to increase the receiver gain so you can hear weak signals and the other masks the receiver output for weak signals. They are mutually-exclusive.

Maybe the Eton Traveler 4 will have tweaked AGC, a better tuning encoder, and a brighter display?

One last thing, the Eton Traveler 3 looks highly modifiable. It's ferrite loopstick is easily removed, allowing for experimentation. More to come on that!

Thursday, September 17, 2015

A Cross-Country DXing Story, Fall 2015

Driving from coast to coast is a great opportunity to watch the world go by, to think, and to DX the mediumwaves on your car radio. I admit I am a casual DXer. I rarely do it from inside the home (too much electrical buzz), preferring to take a portable or small ultralight out to a quiet location, or to DX right from the car using its own radio. Twice a year in recent years, I have driven between western New York and southwestern Arizona, staying for six months in each location.

The Ford Ranger pickup I had for eight years had a superb radio. It was extremely sensitive, and its noise floor was very low. A couple of extra feet added to the whip made it one screaming DX machine. It suffered a little from ignition noise under acceleration, but be gentle on the gas and she would quiet.

Click map for larger and more detailed image

For example, cruising east along I-40 out of Flagstaff, AZ last April, I chased Los Angeles' KFI-640 (50 KW) to within an hour of Albuquerque, NM. I started out in Flagstaff at 6 AM and drove for six hours until I lost KFI-640 in the noise just about noon. Of course at the outset their signal was full skywave strength as the sun was just rising. As the sun rose, it faded through steadily decreasing peaks and nulls till about 10 AM when it entered this sort of echo-ey nether-world, the signal just barely above the noise. Then it would fade for longer and longer periods nearing 30 minutes between peaks, in and out of the background noise. Audio would be readable for a few minutes, then enter a long fade again. And then it vanished totally. In the end, it was a daytime reception of 600+ miles. That's a taste of what you're in for.

The newish 2011 Honda CR-V I got this summer doesn't fare as well in the radio department. The noise floor is high, masking ultra-weak DX, and it desenses more readily near big signals. No ignition noise, though. None. Sensitivity is only fair. Overall the radio is a disappointment for mediumwave. I'll carry the recently-acquired RF-2200 along instead.

Now, most of what I report is directed at daytime reception, though much of DXing theory applies to nighttime skywave as well. The key to successful DX, I've found, at least in crossing the wide open western states, is to park yourself on a frequency and just listen. Listen for hundreds of miles if necessary. You will see the progression of fade in to fade out, and the interaction of co-channel stations come and go. Pick a few favorite frequencies, or try some promising new ones, and see what happens. It's kind of like fishing in a stream - don't be too anxious to pull your line out too soon.

One of the main routes I follow when crossing the country is Interstate 70. I-70 runs from near Baltimore, MD through the central US states of Pennsylvania, Ohio, Indiana, Illinois, Missouri, Kansas and Colorado to the I-15 junction in western Utah, about 50 miles past Richfield. From Rochester, NY, our starting point, access to I-70 means a trip southwest to Columbus, OH via the I-90 thruway. I-90 leads you west across the bottom of Lake Erie to Cleveland, OH, then southwest via I-271 and I-71 to Columbus to join I-70.

Notable at the outset right out of Rochester is the massive 50 KW signal of WWKB-1520 at Buffalo, NY, pushing a nicely-formed cardiod pattern slightly north of east from its three-towered array. Sitting perfectly in the cardiod's notch is tiny, adjacent-channel WMCE-1530 (1 KW) at Mercyhurst College in North East, PA, broadcasting AM-stereo, and only 65 miles southwest of the 50 KW monster. Now this is a marvel of pattern engineering, sandwiching a 1 KW adjacent-channel signal right next to a 50 KW blockbuster, both of them sitting right next to huge fresh water lakes! And it works, too. 1530 KHz is filled with horrendous WWKB-1520 splatter as you pass Buffalo. Within 20 miles, WWKB-1520's hash and desense have evaporated to nothing, revealing the oldies-radio format of WMCE on 1530. Somebody should get an award for this engineering. WMCE-1530 is a great station, by the way.

An interesting listen on the Rochester, NY to Cleveland, OH segment is Canadian CHTO-1690, Toronto, Ontario (6 KW), with a variety of international programming and music. The signal is good and propagation is enhanced as it beams across both Lake Ontario and Lake Erie, about 188 miles as we near Cleveland. Here, nearly all of the propagation path is across fresh water, some 140 miles of it. The signal soon dissipates after you make the left turn south, leaving Lake Erie and Cleveland behind as you head toward Columbus.

I overnight at Mansfield, OH, off I-71 and about 60 miles south of Cleveland. In the dark just before sunrise, Denver's KOA-850 is weak, in and out, more out than in, but it does make an appearance. KOA-850 is difficult though not impossible to hear in Rochester, at 1430 miles distant. Here, near Mansfield, at 1175 miles, it makes the trip a little more easily. What's a few hundred miles you ask? Ohio's WKNR-850 in Cleveland (4.7 KW at night) beams north and we are south of them about 60 miles in somewhat of a dead skip zone. But the big difference, and something I stand by after years of DXing, is the single hop skip distance factor. Extreme single hop skip distance at mediumwave is on the order of 1300-1400 miles. We are within KOA's single hop skip distance at Mansfield. Rochester, NY is just barely outside that distance. Beyond that skip distance, signal strength takes a distinct downward turn, making reception that much harder.

Onward to Columbus, still dark, the famous Grand Ole Opry station WSM-650, Nashville, TN (50 KW) is in solid due to skywave. The sun will rise and it will be with us all day across I-70 almost to the Mississippi River, through the states of Ohio, Indiana, and most of Illinois. 650 KHz is an interesting and somewhat vacant frequency. Only a handful of stations in the lower 48 (seven to be exact) broadcast on 650 KHz, three of them broadcasting under 10 KW power output. From the Mississippi River west, more than 400 miles of dead air reigns until nearly central Kansas, where little KGAB-650 at Cheyenne, Wyoming appears out of the northwest. At Orchard Valley near Cheyenne, still some 400+ miles distant at this point, KGAB-650 commands amazing coverage for an 8.5 KW monopole. The secret here is the excellent ground conductivity of the mid-west, that is, the land west of the Mississippi River. A pipeline of mediumwave signals barrels east out of the front range of the Rocky Mountains across Kansas where ground conductivities hit 30 mS/m and the land is flat and treeless.

WJR-760, Detroit, Michigan (50 KW) hangs with me most of the day too. In fact, it was with me all day yesterday too. It's coverage is incredible. I can hear it weakly in Rochester during daytime hours. Today it hangs in there through Indianapolis nearly to the Illinois state line. Early afternoon, the corn-belt starts to appear, namely, WHO-1040, Des Moines, Iowa (50 KW) at 300 miles.

Overnighting in East St. Louis, this year I veer off I-70 and head south to Springfield via I-44, diverting into the beautiful undulating, treed-hills of central and southwestern Missouri. In this part of the country, roughly the center of the US land-mass, with a quick spin of the dial just before sunrise you can log all four sides of the continent within one minute - WSB-750, Atlanta, GA, WWL-870, New Orleans, LA, WCCO-830, Minneapolis, MN, and KFI-640, Los Angeles, CA. I tried it again. It never fails to impress me. Cuban stations are often in as well. Sunrise in Cuba.

Daylight breaks, skywave is dissipating, and out of Springfield, Missouri this morning the plan is to cross southern Kansas via US 400 to Greensburg, the town 95% destroyed by the EF5 tornado in May, 2007, then northward to Fort Hays. I park my radio on 630 KHz, 650 KHz, and 850 KHz, waiting for something to show up out of Denver or Cheyenne. I didn't have long to wait. In southeastern Kansas, 50 miles east of Wichita near the Butler County line, I hear evidence of carriers on the Honda radio. I pull over and get out the RF-2200. All three stations have readable audio - KHOW-630, Denver (5 KW at 487 miles), KOA-850, Denver (50 KW at 472 miles), and KGAB-650, Cheyenne (8.5 KW at 510 miles). It is late morning, just past 11 AM. KHOW-630 is perhaps the surprising catch. Its two-towered array pushes signal to the southwest into the Rockies and only about 3.8 KW is directed at southeastern Kansas on a beam of 106 degrees.

Nebraska's big-gun farm station KRVN-880 at Lexington (50 KW) is in there all the way across southern Kansas, broadcasting livestock reports. Greensburg is interesting, and gives off an eerie feeling as I pass through. Many new steel buildings, but also many vacant lots, some with concrete steps to nowhere. At Fort Hays, back up along I-70 again, the two Denver stations and Cheyenne are now armchair copy at only at 310+ miles. It is 3 PM in the afternoon. Onward to Denver tomorrow.

Just minutes west of Denver's mile high location lies the continental divide and the massive Rocky Mountains. Relatively poor ground conductivity along the 245 mile path to Utah (2 and 8 mS/m) and towering 14,000 ft. mountains take their toll on westward-propagating signals from the Denver area and east. By Grand Juction, a mere 215 miles, KOA-850 has disappeared, leaving only a weak KLTT-670 (50 KW) and weaker KKZN-760 (50 KW), both Denver area multi-towered arrays that push the brunt of their signal west. 40 miles further along I-70 at the little ghost town of Cisco, Utah, only KLTT-670 remains, extremely weak but readable.

Camp near Cisco, Utah. Quiet!

I leave I-70 and camp in the barren desert hills just beyond Cisco at mid-afternoon. The RF-2200 reveals Albuquerque's KKOB-770 (50 KW at 310 miles) with readable signal. One time here several years ago at mid-afternoon I clipped 60 feet of wire to the truck's whip and strained for Los Angeles' KFI-640. It was weak, yet readable, at 600 miles. But talk about quiet, even my cell service has disappeared. Moab is 40 miles to the south. Now camped, both at nightfall and daybreak I listen for WWL-870, New Orleans (50 KW). Nothing. WWL-870 is a medium-tough catch in western Arizona, but absent at this time.

Recent talk has been of defunct Spanish language KXOL-1660 (10 KW daytime, 1 KW nighttime), out of Brigham City, Utah, up by Salt Lake City. The FCC has canceled its license due to expiration of its silent status filing. It has reportedly been heard on the air. I listened on 1660. Nothing on daytime groundwave (a little too far for Brigham City), but skywave at night is strong, particularly right after sunset. They identify as "La Raza", which is what KXOL identified as. I can only conclude that this is indeed KXOL as reported. Further south, near Blanding, UT and beyond, this station and another Spanish station mix. My guess is the other is KTIQ-1660 out of Merced, CA. Not a positive ID, but they mentioned area locations.

Near Moab, Utah

One of the most beautiful drives in the country is the 30 mile drive through the Colorado River canyon on Utah 128. Much of the canyon is narrow, between towering, sheer rock walls, with the road clinging to one edge and the river many feet below. Few signals penetrate. Grand Junction's KNZZ-1100 signal (50 KW and only 65 miles distant) is in and out during the drive. Out of the canyon and into Moab proper, it is strong. One station is in the Moab area, KCPX-1490, a 1 KW graveyarder at Spanish Valley, just south of Moab. Two Grand Junction stations are the next closest, KNZZ-1100 being one of them. We are getting into lonely mediumwave territory here..

While in Moab, the side-trip south along the muddy-brown Colorado River to Intrepid's Potash Mine is always a great experience. At the mine complex the road ends in a dead-ended, washboard mess of rocky jeep trails. I sit down for lunch and listen for Los Angeles' KFI-640 again. Nothing. Two years ago KFI-640 on the truck radio was very readable at the noontime lunch hour. Distance, 570 miles. It was later in the year, though, further into fall. That may have made the difference. A funny place for any mediumwave station to show up, really.  We are still deep within canyon walls. Isn't propagation interesting?

Headed south the next day at the noon hour, we enter Indian territory. Navajo to be exact, the largest Indian reservation in the US. At about 130 miles distant, KNDN-960 (K-Indian), out of Farmington, NM (5 KW) pops up. They broadcast almost entirely in the Navajo language. If you've never heard the Navajo language, listen for it sometime, it is a treat. English words are interspersed where no Navajo word exists to describe the modern noun or action. With us all the way from Cisco is the other Navajo station, blockbuster KTNN-660, Window Rock, AZ (50 KW). Much of their broadcast is in the Navajo language as well. Both stations intersperse Indian pow-wow dance music between their usual country-western format, another listening treat.

I overnight at Flagstaff, AZ. Headed out early the next morning, KFI-640, Los Angeles is prominent on skywave. It is with me the remainder of the six hour, 300 mile drive to southwestern Arizona where Los Angeles is a mere 217 miles distant and Mexico is but 85 miles south. Familiar KBLU-560 out of Yuma, AZ (1 KW) reappears as an old friend south of Kingman, AZ, nearly 275 miles distant.

Settled in my western home, fall and winter DX is at hand! I'll try out the new Panasonic RF-2200 and see what she can do.

Tuesday, August 18, 2015

An RF-2200 Find

Remember what I told you? Summertime is the time to check yard sales, flea markets, church rummage sales. Check Goodwill and Salvation Army stores at any time of the year. For radios, of course!

A visit to the Asbury United Methodist Church's basement antique sale in Rochester, NY a couple of weekends ago resulted in the find of the year, a Panasonic RF-2200. Price? $50 - marked down on the last day from $200 because they couldn't sell it. The power cord was missing ($6 on EBay), but all the controls seemed to work freely and it was in clean shape, so I went for it. I could hardly contain my glee and keep from doing back flips as I passed the $50 to the cashier. Fingers crossed as I drove home, as there was no way to test it.

After a thorough cleaning with Q-tips, soft cloth and mild detergent, fixing a slightly bent switch lever, and removing a small amount of corrosion from the battery compartment, I installed batteries. Not only does the unit look new again, it operates wonderfully. The carrying strap is still in place, and the whip antenna is intact and undamaged. There are a two very tiny nicks in the cabinet at the top edge, but it's otherwise pristine. What remains is to pull the front and rear covers off and hit the controls and switches with electronic contact cleaner to remove some scratchiness. The radio is nearly flawless. What a find!

Mediumwave sensitivity is great, and mostly lives up to the review hype. The rotatable gyro ferrite antenna is in perfect shape and is totally functional. An RF gain control is included, something rarely found on a portable. Selectivity is good, and certainly much better than a non-DSP ultralight. Two bandwidths are available, wide and narrow. I do live in a high signal area, so there was a little overload at the high end of the mediumwave band near WXXI-1370, 5 KW at 1.2 miles distant. Also contributing to overload is WROC-950, 1 KW and WHTK-1280, 5 KW, each at 2 miles distant. The radio is big, however, and heavy. It takes four D-sized batteries. I found four D alkalines on sale at CVS for $5, another bargain. The earphone jack appears to be mono, so requires an adapter for stereo headphones. Check, I have that.

The next day I spent an hour or so at a card table outside in the bright daylight (can't see) and pulled the front and back covers off. I got the controls and switches clean using CRC QD Electronic Cleaner spray. The tuning scale reads about 25 KHz high. I may look into that this winter and do a bit of tweaking. Otherwise the complicated dial mechanism is in great shape and is tight. Dial slop is extremely small. The radio case was a bugger to get back together. Be wary and be careful! Old plastic is brittle.

The Panasonic RF-2200 receives the AM, FM and shortwave bands. Ranges: 525-1605 KHz, 3.9-8 MHz, 8-12 MHz, 12-16 MHz, 16-20 MHz, 20-24 Mhz and 24-28 MHz plus FM. This radio features a very accurate analog dial down to 10 KHz, and two tuning rates. It has an S-Meter (tuning strength 0-10), BFO, FM AFC, Bass, Treble, Record Jack, Dial Lamp, Dial Lamp Switch, RF Gain, Crystal Calibrator 125/500 KHz and External Antenna Terminals. The dual conversion circuit (1.985 MHz/455 KHz) features wide and narrow selectivity (reportedly 5/3.4 KHz at -6dB). The 8 inch rotatable ferrite rod antenna makes the RF-2200 a strong medium wave performer. Remarkable sensitivities are FM: 2 µV, MW: 14 µV/m, SW: 0.5 µV. Early production models where gray, later models where black.

On a side note, and just to further prove the point that bargains can also be had at thrift stores, last winter I found an old HP 32SII Programmable Calculator at a Salvation Army thrift shop in Arizona. It was pristine and had its manual and slip cover. I saw it sitting in the glass case at the counter. When I asked the woman attendant if I could see it and how much it was, she picked it up, looked at it with this questioning look on here face (I could almost see her mind spinning - just another dumb old calculator), and said "2 dollars". Check them out on eBay - they are worth anywhere from $70 to $200 or more depending on condition and whether they come with manual and case! Lesson - don't ignore thrift stores!

This new Panasonic RF-2200 radio will be a joy to DX with in remote Arizona this winter. I am headed cross-country again in about a week so we shall see!

The beautiful Panasonic RF-2200. Click to see a larger image.

Tuesday, August 4, 2015

KEZW-1430 Denver, Colorado

Late last April I passed through Denver, Colorado and got a nice shot of KEZW-1430. The transmitter site sits south of Denver and west of Interstate 25 between the I-470 loop and E. County Line Rd., and just inside Douglas County. The beautiful, in-line symmetrical 5 tower array stands tall against the front range of the Rocky Mountains.

KEZW is the largest radio station in the United States to be powered by the sun. The 100 KW solar array is 300 x 80 ft. and normally dispenses 80 percent or more of the station's daily power requirement. The ground-mounted solar array is 12 tables with 36 panels on each table, surrounded by fencing. Tower lighting has been converted from power-hungary incandescents to LEDs. 85 KW are available from the inverters.

During the design phase, it was decided to forego battery power and simply feed the inverters directly into the power grid. This meant the station would be on the grid during nighttime hours. It also simplified the installation. Concern was also expressed over what effect the solar array might have on the signal pattern. It turned out to be small, and fine tuning of the array solved the differences.

KEZW operates on 1430 KHz with 10 KW daytime and 5 KW nighttime using a Nautel MD-10 transmitter. In the photo below, tower #1 is to the right and tower #5 to the left. During daytime hours, tower#1 is used as a monopole, producing an omni-directional pattern. At night, all 5 towers are fed to push a 9.2 dB gain signal lobe northward at 10 degrees (42 KW equivalent), in-line with the towers and out the right side of the picture.

Curious is the center tower, which appears to be about twice the height as the others. FCC engineering records show all towers, including tower #3 to be 136 electrical degrees tall (the fed portion), which equates to 259.8 ft, or 0.378 wavelength. So, taking this record as true, it means that only the bottom half of the center tower is being fed. It leaves me wondering what the top half is used for, as I see no other hardware on it, though it does appear it could be insulated in the middle.

See the interesting article on KEZW published in Radio World:

Radio Mag Online also has an interesting artilce on the construction phase:

If you ever get to the Denver area, be sure to take a drive on I-470 west from I-25 and see this impressive array.

KEZW-1430 Denver, Colorado

Thursday, April 9, 2015

US Ground Conductivity Map

Have been doing a lot of work this winter on the various mediumwave FCC databases. I've also torn apart the published Canadian database offered by Industry Canada, and added all licensed entries to the combined database which I keep. Primarily the work this winter has been constructing groundwave and skywave pattern plots for all stations.

In the process of doing this, I also tidied up the FCC's M3 sequence file, which defines the distinct areas of ground conductivity across mainland U.S., Hawaii, Canada, and Mexico. Missing Great Lakes summer conductivities were also ferreted out from other sources. With that in hand, I created a nice Google-based HTML map of U.S. ground conductivities and thought I would share it with you. It's fully zoomable like all Google maps.

It can be downloaded from the link at the upper right of this blog. It's self-contained like my pattern maps. Unzip the file and click on the HTML file to view.

Hope you enjoy it.

Static rendering of the FCC's M3 ground conductivity map.

Tuesday, February 24, 2015

The dBµ vs. dBu Mystery: Signal Strength vs. Field Strength?

We've just talked about Tecsun's use of the term dBµ (Greek letter µ 'mu') in a previous article. They use it as a measure of received signal strength on their DSP radios. But there is another dBu (lowercase 'u' this time), also a measurement of strength, and more commonly used. What's the difference?

We first need to identify dBu's cousin, millivolts per meter.

You may have seen the term mV/m, or millivolts per meter, used as a measurement of field strength. The common unit used in measuring E-field "strength" is volts per meter, or V/m. Volts per meter is a lot when we are dealing with small received signal levels, so millivolts per meter 'mV/m' (one-thousandth of a volt per meter) is usually used. We have all seen mV/m used in a receiver's sensitivity specs, or to represent a station's received field strength at a given distance. Defined, an electric field of 1 mV/m is an electrical potential difference of 1 millivolt existing between two points that are 1 meter apart, perhaps along a one meter length of wire or between two parallel planes placed in the path of a signal. Technically, a millivolt per meter (mV/m) is achieved if a voltage of 1 millivolt is applied between two infinite parallel planes spaced 1 meter apart.

dBu (yes, lowercase 'u'), in reality is another improper contraction - a shortened version of dBµV/m (there's that Greek letter µ 'mu' again). dBµV/m is commonly and usually written nowadays as dBu, using the lowercase letter 'u'. It is the term used worldwide by engineers and the FCC for measuring electric field strength of AM, FM, and TV broadcast stations at prescribed distances. dBu is directly related to mV/m (mV/m = 1000 times µV/m), and is the logarithmic representation of mV/m.

Graph depicting measured dBu levels for KOA-830 (1934)

Have a look at the 1934 graph above depicting various dBu levels for station KOA-830, Denver. Confusing things even more, in the old days dBµ was indeed used as the shortened version of dBµV/m, and not dBu. (note: KOA currently is allocated to 850 KHz).

It is interesting to see what the different dBu values represent in terms of field strength. Several levels are represented. I have converted the dBu values to millivolts per meter:

88 dBu: 25 mV/m (urban)
74 dBu: 5 mV/m (residential)
54 dBu: 0.5 mV/m (rural)
36 dBu: 0.06 mV/m (atmospheric noise level)

For propagation aficionados, some other interesting things are of note here, in terms of propagation of this 50 KW signal over the excellent ground conductivity of the mid-west:

1. The ionospheric signal (the skywave) is strongest in the 300-500 kilometer range.
2. At about 200 km distant, the skywave strength essentially matches the groundwave strength, at about 63 dBu (1.4 mV/m).
3. The skywave signal level rises above the atmospheric noise level at just 30 km distant.
4. The groundwave signal level doesn't drop below the atmospheric noise level (36 dBu) until about 550 km distant., a site most are familiar with, uses the following mV/m values to represent different reception zones:

2.5 mV/m (68 dBu, local, red line)
0.5 mV/m (54 dBu, distant, purple line)
0.15 mV/m (43.5 dBu, fringe, blue line)

They are essentially in agreement.

Confusion continues to exist between Tecsun's dBµ (their version of dBµV), and dBu. They are constantly confused as the same thing, though they are very different. Note, however, that dBµV is indeed indirectly related to mV/m, and dBu.

So let's define them again, concisely:

dBu (letter 'u') from (dBµV/m): the decibel (logarithmic) representation of electric field voltage above or below one microvolt per meter.

dBµ (mu 'µ') from (dBµV): the decibel (logarithmic) representation of voltage above or below one microvolt across a load.

All you have to do is remember two things:

1. dBu (letter 'u') is actually another name for dBµV/m, related to mV/m. It came into common use many years ago.

2. dBµ (mu 'µ') is somebody's shortening of dBµV. Tecsun re-coined this one.

Important! You cannot convert dBµV as shown on the DSP radios to mV/m or dBu! The values are not interchangeable. The difference is found in what is called Antenna Factor, or the ability (actually efficiency) of the antenna to convert the passing field to an electrical voltage which can then be received by the detection process. As each antenna is different, each will transfer a different signal voltage to a radio's input. Each antenna (a ferrite loopstick is an antenna) will have a different Antenna Factor.

It matters little whether (at reception time) the received signal is ultimately impressed on a ferrite bar or rod, or a long wire, or a bed spring, in that the receiver will take whatever tiny voltage induced and convert it into intelligible audio if it is strong enough. Remember, as stated before, the iron core ferrite rod is basically a signal concentrator. The longer the rod and thus the more iron ferrite, the more the concentration, and the greater signal voltage, at least to a point.

The FCC offers a conversion calculator to convert from mV/m to dBu and back.

If you'd like to figure it yourself, you can by using the following formula:

dBu = 20 * Log(mV/m * 1000)

To reverse the computation, converting dBu back to mV/m:

mV/m = 10 ^ (dBu / 20) / 1000

(Log is the common logarithm, or base 10).

How, then, do we go about measuring millivolts per meter, mV/m?

Millivolts per meter (mV/m) is a way of defining a station's expected (or measured) field strength at a receiving location. Field strength can be measured by a device specifically designed to measure the strength of the passing wave. Potomac Industries makes the model 4100, a device which measures field strength. It was the subject of a previous blog post. Formulas to calculate approximate field strength can also be used.

Potomac Industries 4100 measurement.

Please note that a mediumwave station's expected field strength at a receiving location depends on many factors. One is transmitter power. Two, the distance from the transmitter. Three, the ground conductivity variations along the path between the transmitter and receiver. Four, the frequency of the wave. There are other factors too.

I did some articles on signal measurements and ferrite antennas on my blog a couple of years ago. Maybe they will help with introducing some of this field strength material.

Field Strength Calculations (3 parts)

An Unassuming Antenna - The Ferrite Loopstick

Field Strength Calculations: A History

I hope this helps in identifyng the difference between dBµ and dBu.

Saturday, January 31, 2015

The Ultralight dBµ Mystery, S-Meters, And Field Strength

On our little ultralight DSP receivers like the Tecsun PL-380 and PL-310, etc. you may have noticed the dial face has some numbers which vary with the received signal strength. Next to them are marked the letters "dBµ". It seems like a field strength meter of some kind. Just what is that? Well, it is indeed an indicator of signal strength. Let's dig into what it means and see how it is directly related to the S-meter of old, the field strength meter.

National Radio Company

Tecsun's use of dBµ (a funny-looking, backwards 'u', which is the Greek letter µ 'mu', meaning micro, or one-millionth) is really an improperly-used, shortened version of the term dBµV. Warning! You may have also seen the term "dBu" (lowercase "u") written in various publications, associated with field strength also. It refers to something different (actually the E-field of the passing wave). We can handle that one in a separate article, so be sure not to confuse Tecsun's dBµ with dBu!

Back to Tecsun's dBµ. Let's break it down:

dB = decibels, or simply a way of expressing magnitudes of a value, like voltage, logarithmically
µV = microvolts, or millionths of a volt

Consequently, dBµV is a voltage expressed in dB above (or below) one microvolt. This is measured across a specific load impedance, commonly 50 ohms. Important! Here we have a real received voltage measured across a specific load impedance like a tuned circuit!

The 'dB' or decibel measurement is a logarithmic ratio as you may know. In terms of voltage, an increase of 6 dB is a doubling of voltage. So, if our little Tecsun receives a signal at 28 dBµ and it increases to 34 dBµ, the received voltage has doubled. Coincidentally, this is also an increase of one S-unit! Now we are getting somewhere.

Let's translate our received dBµV into actual received voltage:

dBµV   µV(millionths of a volt)
 94    50000.0
 84    15810.0
 74     5000.0
 64     1581.0
 54      500.0
 44      158.1
 34       50.0 (the S-9 of old!)
 28       25.0
 22       12.5
 16        6.3
 10        3.2
  4        1.6
 -2        0.8 (less than 1 µV sends the dB ratio to a negative value!)
 -8        0.4
-14        0.2

The following formula is used to convert dBµV to millionths of a volt:

µV = (10 ^ (dBµV/ 20))

To convert millionths of a volt back to its decibel representation:

dBµV = 20 * Log(µV)

(Log is the common logarithm, or base 10).

The modern DSP receivers like the Tecsun PL-380, 310, etc. which employ the Silicon Labs chips, measure and display dBµV as received at the tuned front end across a load. They call it the RSSI indicator. Our radio's antenna, the iron core ferrite rod, is basically a signal concentrator. The longer the rod and thus the more iron ferrite, the more the concentration, and the greater the signal voltage transferred to the radio's tuned input.

So what exactly is this so-called dBµ indicator on our DSP radios telling us?

Some time ago, more than a year ago, I posed this question to Scott Willingham, who was on the design team for the SiLabs DSP receiver chips used in these radios.

He stated:

"The RSSI (dBµ) readings are referred to the pins of the chip, which are the inputs to the LNA. In the Tecsun radios operating in the MW band, this is also the voltage across the loopstick. In SW bands, the Tecsun ULRs use a preamp/LNA on the circuit board between the whip antenna and the Si4734. In that case, the RSSI readings reflect the signal at the output of Tecsun's external LNA."

Essentially for mediumwave, the received signal is measured in microvolts right off the loopstick and then converted to dBµ, which is decibels above a base of one microvolt. Remember again, dB is just a logarithmic ratio. Of course a PL-380 is not going to read the same dBµ as a PL-310 or a PL-398, etc., because the antenna setups (loopstick lengths, whip extension, tuned circuit efficiency) are different and each will induce different received voltage levels to the radio.

A curious measurement, yes, but there is also some meaningful information here in comparing signal strengths within the same radio just like an S-meter did, and in fact there is a direct correlation to the S-meter.

The analog S-meter us old guys remember in now ancient receivers was based on S-9 indicating a 50 µV (microvolt) input signal to the antenna circuitry, at a load impedance of 50 ohms. That is, the S-meter read S-9 if the receiver S-meter was calibrated right, as the meter was further down the IF chain, and usually responded to the AGC (automatic gain control) level. Each S-unit is 6 dB apart, meaning a signal reading S-9 is 6 dB stronger than a signal reading S-8. S-9 +10dB is 10 dB greater than S-9, or one S-unit plus 4 more dB.

What does this mean? An S-9 signal is twice as strong as an S-8 signal. The received voltage is double. An S-9 signal is four times as strong as an S-7 signal. The received voltage is doubled twice.

Some direct correlation can be attempted with the SiLabs DSP chip dBµ readings used in the Tecsun radios.

S-unit        µV  dBµV  dBm
S9+60dB  50000.0   94   -13
S9+50dB  15810.0   84   -23
S9+40dB   5000.0   74   -33
S9+30dB   1581.0   64   -43
S9+20dB    500.0   54   -53
S9+10dB    158.1   44   -63
S9          50.0   34   -73
S8          25.0   28   -79
S7          12.5   22   -85
S6           6.3   16   -91
S5+4.9dB     5.6   15   -92
S5           3.2   10   -97
S4           1.6    4  -103
S3+1.9dB     1.0    0  -107
S3           0.8   -2  -109
S2           0.4   -8  -115
S1           0.2  -14  -121
Look at the S-unit and dBµV columns. As can be seen, a 34 dBµV signal (again, the Tecsun DSP radios label it dBµ) is essentially equivalent to an S-9 signal on the old S-meter setup. The 25 dBµ signal shown in the picture below represents a signal halfway between S-7 and S-8.

On the Tecsun PL-380 (at least the version I own, which registers from 15 dBµ - 63 dBµ), somewhere around 15 dBµ seems to be the signal detection threshold which translates to just below the old S-6, at 6.3 microvolts of signal. As noted elsewhere, these modern drug store consumer radios are not as sensitive as the old communications receivers we remember. S-6 on an old vacuum tube receiver was virtually "arm chair" copy. This is where an FSL or passive loop brings up the weak received signal to similar levels in the DSP radios.

So there you have it. Keep this chart handy and you can convert between Tecsun's dBµ and S-units.

Stay tuned for the second article in this series: The dBµ Versus dBu Mystery: Signal Strength vs. Field Strength?

25 dBµ, or between S-7 and S-8

Saturday, January 24, 2015

2015 US And Canadian Pattern Reference

A new US Mediumwave Pattern Reference, produced by Radio Data MW, has been uploaded. You will find it at the top of the right sidebar under LATEST US MEDIUMWAVE FILES. Radio Data MW, a program I have been working on for the last few years, accomplishes this mapping process.

Included is a complete set of GoogleMap-based, HTML-driven maps which show the most current pattern plots of all licensed US mediumwave broadcast stations from 540 - 1700 KHz. The set includes all frequencies for the indicated services: Unlimited, Daytime, Nighttime, and Critical Hours. Individual maps are grouped by channel frequency: 540, 550, 560 KHz, etc.

I will attempt to make this a regular feature on RADIO-TIMETRAVELLER, with regular yearly updates. The sidebar at the top right will have the most current links. The link will change for each new posting, so I would avoid copying and pasting it into a forum or other web page. Come to the main page of this blog instead.


The maps are HTML-based, so no regular install is necessary. Simply unzip the downloaded file and click on the individual map file to run. The map will open up in your web browser. They are self-contained, with image icons embedded right into the code. You must have an internet connection to view the maps.


Using the actual FCC database files Radio Data MW will auto-generate an interactive HTML pattern map, showing the pattern plots for all stations included at the discretion of the user. A complete set of mediumwave pattern maps can be generated in a matter of minutes. Radio Data MW generates a real pattern plot based on ground conductivity, ground dielectric constant, and can display actual (but approximate of course) signal level boundaries for Local, Distant, Fringe, Extreme mV/m levels, or any custom mV/m level chosen by the user.

The online Google Maps API is used to generate and plot each station on a map of the US. An accurate flag pin is placed at each transmitter location, and in satellite view may be zoomed in to see the actual transmitter site. Map flags are color-coded to indicate Unlimited, Daytime, Nighttime, and Critical Hours services. Each flag has a tooltip-type note, and when hovered over with the mouse will display a note on the station.

A pattern plot for each station is generated and displayed. Each pattern is calculated using standard formulas used by the FCC to compute the base values at one kilometer, and field strength formulas at distance based on the works of many people over the years. See Field Strength Calculations: A History and Field Strength Calculator One, previously posted on RADIO-TIMETRAVELLER.

Finally, an accurate ray path can be drawn from all transmitters to a user-specified receiving location by inputting latitude-longitude coordinates. Super-imposed on the pattern plots, the ray paths show the listener where he or she falls on each station's pattern, a handy guide to knowing where you stand.

Note that these maps are web-based. As stated, they use Google Maps and thus require access to Google. In order to view them you need a connection to the internet. In desktop or laptop use, they have been tested in the Internet Explorer, Firefox, Chrome, Opera, and Safari browsers. If using Internet Explorer, best results are had with the latest version. Chrome works best.

These maps will work on some tablet or phone browsers. I have tested them on an Android device and it's handy to be able to display them while DXing outside or on the road. Some browsers will not allow pinch-to-zoom, where others will. Some browsers don't render the map controls correctly. Response is fair to poor on the tablet or phone due to the sheer number of HTML lines and processing required to render the maps. Such is the current state of tablet and phone browser rendering.

Hope you enjoy these pattern maps and find them useful.