Notes On Mediumwave Loops And Wire Antennas

The following comments pertain to mediumwave reception.

I've constructed many loops here, both passive-tuned and hard wired for the DSP radios.

A question arose recently asking what is the equivalent air loop size to a 200mm (8 inch) length ferrite? Speaking from experience, I'll submit an educated guess and say the rough passive-tuned loop equivalent is an approximate square loop of 8-9 inches.

I've built smaller loops, on the order of 6 inches. They produce a signal strength about equivalent to the existing ferrite in a Tecsun PL-380, which is just shy of 4 inches long. The problem I've had with the really small loops of that size is that the nulling is pretty poor. The figure-8 pattern is not well-defined.

HARD-WIRED LOOPS

I have a hacked Tecsun PL-380 with the ferrite removed, and also an Eton Traveler 3 with the ferrite removed. I soldered wire leads in place and brought them out the tops of the radios for testing hard wired loops. I use micro-clips for all connections here for testing.

Two loops are currently in use here: an 8 inch and an 18 inch. Both are square loops, close-flat-wound with insulated telephone wire, about 24 gauge solid. The 8 inch has 26 turns and the 18 inch has 12 turns. If you wind a 12 inch loop, use 16 turns.

Both the 8 inch and 18 inch give good hard-wired results. Of course the 18 inch blows the doors off the 8 inch, and is the better nuller. You don't need a 5:1 step up transformer for impedance matching. Use the fully-wound loop. Inductance for both these loops is ballpark around 240 µH. I've found that is a good value to shoot for.

I will say, be careful if hard-wiring directly to the input of the DSP radios. They are very sensitive to static. I haven't blown one out yet, but I've come close. Any static or spike on the line when clipping to the chip will send it into desense for a few minutes, even if off. It eventually recovers, at least mine have so far.

At night, the PL-380 handles an 18 inch hard-wired loop pretty well without overloading. Not so much the Eton Traveler 3. Its sensitivity is a touch better than the PL-380, and otherwise will occasionally overload. A 12 inch loop might be a better bet for the Traveler 3. You shouldn't have any overload trouble with the 8 inch loop.

PASSIVE-TUNED LOOPS

Both 8 inch and 18 inch loops can be made passive and tuned with a 365 pF variable capacitor very nicely. The 18 inch ranges out nicely from 530-1700 KHz without further attention. The 8 inch only tunes to about 1430 KHz, so I jumper a clip wire across a few turns to get to 1700 KHz.

The advantage of the passive-tuned loop is that you can reduce the coupling by moving the loop a bit farther away from the radio, thus reducing the signal overload problem.

As others have stated, coupling a passive-tuned loop to a modern DSP chipped radio can be finicky because the radio gets de-tuned by the loop, then it re-tunes, then the loop is upset again. I have best results by coupling the passive loop off the end of the radio's ferrite. The tuning interaction is less. As so:

|
|
L
O
O  --RADIO FERRITE--
O
P
|
|

These DSP radios will start to overload much above 80 or 90 dBµ on the RSSI meter. Incidentally, the Eton Traveler 3 measures to 99 dBµ but the PL-380 only measures to 63 dBµ.

Remember, an RSSI reading of 34 dBµ is the equivalent of the old S-9 on vintage receivers us hams remember. 50 microvolts to the input = S-9. 34 dBµ also = 50 microvolts on these radios. A respectable signal. 80 dBµ is huge and it's not surprising this can cause overload.

Also, consider that the induced voltage in a loop increases linearly with the number of turns, the area of the loop, and the frequency. An 8 inch loop has nearly twice the voltage output as a 6 inch loop. Area 64/36 = 1.77 times the output. An 18 inch loop has more than twice the voltage output of a 12 inch loop. Area 324/144 = 2.25 times the output.

Here's a curious tuning tip that may not be apparent at first. It requires two DSP radios. Hard-wire a square loop to the first DSP radio (ferrite removed). This DSP radio tunes the loop in place of a 365 pF variable capacitor if you don't have a spare, sort of a "digitally tuned passive loop". Couple this loop inductively to the second, unmodified DSP radio like you normally would. Now you can accurately tune the passive loop by reading the frequency on the first DSP radio. Overkill, yes. But an interesting hack.

Bottom line on building a very small loop - 8 inches would be a minimal size for any appreciable gain.

FURTHER THOUGHTS ON HARD-WIRED vs. PASSIVE-TUNED LOOPS

I was all-in for awhile with the hard-wired loop, connected directly to the DSP's front end. The DSP chip tunes the coil just like the ferrite. What more could you want?

The problem here, again, is potential overload. These little DSP radios are not bullet-proof like the table-tops of the old days. An 18 inch or larger hard-wired loop produces massive un-tuned signal to the front end of the radio across the entire mediumwave band. The key word here is "un-tuned". You are relying solely on the DSP's existing circuitry to differentiate massive signals across a wide range of spectrum. Hard-wired loops above a certain size invite overload. We seem to crossover into this territory above the 12 inch range.

Hard-wired loops are great in quiet locations, especially for long distance daytime DX. Nighttime is a different story, as signal strengths can equal urban levels even without the loop-assist. You must find a way to incorporate selectivity to reject or lessen that which you are not interested in.

To summarize, using a passive tuned-loop and coupling inductively gives you two additional important advantages over the hard-wired loop:

1. A double-tuned front end. A tuned-loop has selectivity itself and its passband will essentially only present that large signal on the channel to which it is tuned.

2. The ability to decouple the loop from the radio by moving it farther away is sort of a poor man's RF gain control. If the radio is still overloading, move the loop away a bit.

An additional option to the passive-tuned loop is the successful FSL or Ferrite Sleeve Loop antenna. It is compact. If tuned with a variable capacitor, it will reject off-channel signals, lessening front-end overload as it serves as a double-tuned front end again. And it is directional of course. The C.Crane Twin Coil 200mm ferrite has similar qualities, a double-wound ferrite loop with a signal peaking ability.

ABOUT LOOPS AND NULLING

This applies not only to open air loops but to ferrite loopsticks as well. Rotating the loop off the station "peak" varies the induced signal voltage according to the trigonometric sine of the angle away from the peak. At 30 degrees off peak you have reduced the signal pickup by half the voltage (50%). At 45 degrees, 70% reduction. At 60 degrees, 86% reduction. At 90 degrees, theoretically 100% reduction, or zero signal. Now, we all know that 100% reduction is generally not theoretically obtainable, but sharp loops do come close.

The image above depicts the signal pickup for various angles of the signal arrival in relation to the side of the ferrite loopstick. Note that at only 30 degrees off the zero null the signal has increased to 50% of its full value. Passive box-type loops react exactly the opposite. Full signal occurs end-on to the loop instead of broadside.

WINDING FERRITE RODS AND BARS

Paul S., in Connecticut, who participates in the UltralighDX group, has some great tips on winding the ferrite coil:

The older and later 'pocket radio' designs were both made with a mix of price and quality. Older designs had weaker transistors/tubes and needed a high quality ferrite coil winding. A pocket model of the 60's had better transistors and could be made physically smaller, the circuit would compensate for the smaller ferrite rod.

Nonetheless, both designs are not optimum, as the most recent studies have shown. I refer to Ben Tounge's (of Blonder-Tounge Radio/TV) Article #29 at his namesakes website. I also note that there were mathematical errors in popularly published catalogs of ferrite material.

So what is a near optimum design one might ask?

1.) Keep the ferrite bar at least 10 times its diameter for nulling properties.

2.) Try to keep the coil winding between 1/2 and 1/3 of the ferrite bar length.

3.) Space the coil winding 3x the wire diameter (in typical designs its usually 2x the diameter). For example #30 gauge wire of 0.01" diameter is wound at 0.03" instead of the typical 0.02". This removes some "proximity effect" especially when many turns are involved.

4.) The wire diameter itself has to be minimized to reduce the "Skin Effect". This seems counter-intuitive because thin wire has more resistance per foot (meter). But, that's DC resistance... we are looking to reduce AC (AM radio signal) resistance. Such AC resistance gets larger when very little current flows in the center of the wire: the smaller diameter wire "saturates" the wire better, with more AM radio signal current flowing in the center of the wire.

5.) Because of #3 and #4 above, a single wire of small gauge (say #30 or better yet #32) can be wound upon the ferrite bar. However, the best choice is still Litz wire based upon #46 gauge strands. Its more expensive, and you do get what you pay for. I will also say that one would be surprised at how good a single #32 wire (0.008") spaced at 0.024" is.

6.) Using a single wire, one can wind directly on the ferrite bar (most of this 'magnet wire' is insulated), but most prefer using a layer of heat-shrink tubing over the ferrite bar.

So, in recap, the older radios got the wide spacing right, and the newer ones got the small gauge wire right, but neither got both right. One thing not mentioned in most commercial radio designs is the actual inductance of this ferrite bar antenna. I do know that some designs are intentionally high up to 700µH for Sony's in the 60's-70's. Modern DSP designs need 350-500uH, and can use short bar lengths (re: small wire diameter).

Regards,

Paul S. in CT

Thanks for the tips, Paul.

WIRE ANTENNAS

Loops in general are directional and have many advantages over simple wire antennas, should you have a radio which you can attach a wire antenna to. The non-DSP Sangean ATS-909X, the C.Crane EP Radio Pro, and Tecsun PL-880 are three which allow direct connection of wire antennas.

I find directly-connected (or inductively-coupled) horizontal short wire antennas for either DSP or non-DSP radios to be somewhat useless on the mediumwave band. Note, I am talking about SHORT wire antennas. Longer wires, and the longer the better, are preferred to the short variety. By longer, we are talking 200 feet or better. By short, I generally mean about 50 feet or less.

The Beverage antenna or "wave antenna" is a very long wire receiving antenna mainly used in the low frequency and medium frequency radio bands, invented by Harold H. Beverage in 1921. It's routinely run about six feet off the ground and may or may not be terminated at the far end with a resistor directly to ground. It has massive signal gathering ability and is also somewhat directional. A variety of Beverage, the BOG antenna (Beverage On Ground) has the same attributes going for it, laid directly on the ground. Neither are generally tuned, but can be. Beverage antennas work best if their length approaches a full wave or greater.

Another option is the flag or otherwise large-sized, single wire rectangular loop antenna. It may be 20 ft. tall by 50 feet long or larger. It is directional, and characteristically very quiet as loops are. It can be tuned as well. It has excellent performance.

THE VERTICAL ANTENNA

Lastly, a different option than the rest is the vertical antenna, an antenna poo-pooed by many but one which I've had many successes throughout a 55 year radio hobby. As a ham, over my active years I used verticals on the HF bands to work stations world-wide using only flea power at times. Using a simple vertical, I have worked long-path propagation from Denver to Sweden over the Pacific and Indian Oceans using CW (Morse) for example, a distance of some 19,000 miles. Their low angle take off is exceptional. But what about mediumwave?

Believe it or not, they are also useful for mediumwave reception. After all, the vertical is used by mediumwave stations to transmit!

The key to the vertical in all situations is matching it properly through a balun transformer of some kind. This will enable maximum signal (voltage) transfer to the receiver. It doesn't have to be tuned, just matched. I use a 25 ft. vertical here, matched through an RF Systems Magnetic balun, essentially a 9:1 transformer. 50 feet of coax is run from the RF Systems balun to the inside of the house. Coupling to the receiver is done inductively. Wind 15-20 turns of insulated wire around a 4 inch ferrite bar and connect to the coax, one side to center and one side to ground. Position the 4 inch ferrite near the radio's internal ferrite. The resultant increase in signal pickup is astounding.

A 25 foot vertical seems short for the mediumwaves, but its signal output is as good or better than a 48 inch passive-tuned loop. If there is a drawback it's that it is omni-directional versus the passive loop. However this can be advantageous for certain types of bandscanning. Park on a frequency for several hours while you are doing something else and you will hear the fade up and down of several stations. You will get to know the relative signal strengths of each over the long term listening period.

Also going for it is its same-plane alignment to the transmitted signal (vertical). The short horizontal wire antenna is 90 degrees to the mediumwave signal's polarization plane and perhaps even 180 degrees end-on or 90 degrees broadside to the station as well. A 20 dB or greater difference in signal strength can be realized in the vertical over the horizontal wire antenna. 20 dB is an increase of more than three times the signal pickup voltage.

Note that groundwave reception, even over extreme distances during daytime hours, is virtually in the vertical plane at all times. Nighttime skywave varies, but hardly less than 30 degrees off of vertical even at extreme distances. You can prove this yourself using your portable radio with internal ferrite loop antenna. At night, tune to a medium to strong distant station. Tilt the radio from horizontal towards vertical and the signal will be reduced dramatically as you approach the 45 degree point. The effect will be less-pronounced for stations closer than about 300 miles which have a higher angle arrival.

Much mediumwave fun can be had through experimenting with different antennas. Give it a try.

2019 US-Canadian Mediumwave Pattern Reference Is Here

Editor's note: The newest version of the pattern set (2024) is available. See the link at the upper right of this page.

The 2019 US-Canadian Mediumwave Pattern Reference for all stations is now available. Find the download link at upper right. Remember, the links change each time a new set is uploaded. Always look to this RADIO-TIMETRAVELLER site for the current link. Download is 52 MB.

DOWNLOAD LINKS

Media Fire link here.

If you download from the Media Fire link, be sure to click the DOWNLOAD button.

The Media Fire site is ad-supported and has several ad links on the page and will also issue an ad pop-under.

The other link at the upper right of this page is to Tomas Hood's NW7US site. It is ad-free. Thank you Tomas for mirroring this download. Both downloads are the same.

INSTALLING

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.

IMPROVEMENTS FOR 2019

1. The previous 2016 map version became crippled due to Google's new precondition for all Google Map usage now requiring a user key. I have applied for and received the requested key. It allows a fixed amount of map-usage bandwidth. All maps should now display fully.

Note: Google may throttle map-usage bandwidth based on excessive use. Whether this will happen is unknown at this time.

2. Skywave signal maps have been totally recoded for 2019. Many months of coding and testing were involved. Predicted signal levels are calculated in accordance with current ITU methods of recent years (1999 onward). The following new parametrics are now additionally considered in the skywave calculation:

   * Hourly transitional loss variance from sunset to sunrise.
   * Seasonal gain or loss, January - December.
   * Diurnal enhancement at the sunrise and sunset period.
   * Winter daytime skywave enhancement (only on maps created for times during the day).
   * Daily seasonal nighttime skywave enhancement.
   * Take off angle variances for stations at relatively close distances (experimental).

3. Colored plot (yellow) introduced for groundwave 0.1 mV/m level.

4. Small changes made to the map's title bar heading.

5. Unlimited, Daytime, and Critical Hours plots are at the 1.0 and 0.1 mV/m levels. Skywave this year is set at the 0.15 mV/m level. In all cases except for the nighttime graveyard stations, levels have been chosen to minimize pattern overlap and to more closely follow those which might be helpful to the mediumwave DXer.

ABOUT THE MAPS

Included is a complete set of GoogleMap-based, HTML-driven maps which show the most current pattern plots of all licensed US and Canadian mediumwave broadcast stations from 530 - 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, .. 1700 KHz, etc. Data for the plots in this offering is based on the current FCC and Industry Canada databases available at the time of its creation (January 16, 2019).

The daytime map series, in two parts, shows expected groundwave coverage patterns for Unlimited and Daytime (part 1), and Critical Hours (part 2) operations. Daytime signal patterns represent groundwave coverage at two levels, out to the 1.0 and 0.1 millivolts per meter contours. The choice of these levels is made in order to more closely match those which might be helpful to the mediumwave DXer. Note that daytime reception of signals out beyond the depicted 0.1 mV/m pattern is very possible, and in fact likely for the DXer. The contour line represents a signal strength at the station's extreme fringe distance, a level usually received on a sensitive portable radio with a low ambient local-noise level. I have chosen this signal level to give a good representation of what can be received by most DXers during sunlight hours.

The nighttime map series shows expected skywave coverage patterns for Unlimited and Nighttime operations. Nighttime signal patterns represent the standard SS+6 (sunset plus 6 hours, or approximately midnight Central Standard Time), 50% signal probability at 0.15 millivolts per meter. Note also that nighttime reception of signals out beyond the depicted pattern is very possible, and in fact quite likely for a skywave signal. The maps represent a signal strength between distant and fringe, a level generally easily received at night on most portable radios. I have chosen this signal level to give a good representation of what should be fairly easily received by most DXers on an average evening. The nighttime signal probability of 50% means that the signal will be received at this level approximately 50% of the time at Central Standard Time.

HOW THEY ARE PRODUCED

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 about five hours of processing time. Processing time has increased by nearly two hours in 2019 due to enhanced skywave calculations and other upgrades.

For daytime signal maps, Radio Data MW generates a real pattern plot based on transmitter power, antenna array efficiency and directivity, ground conductivity and ground dielectric constant of the path to the receiver. Increased conductivity of water paths over the Great Lakes are also accounted for. It 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.

For skywave signal maps, predicted signal levels are calculated in accordance with current ITU methods of recent years (1999 onward). A number of parametrics are now analyzed and accounted for in the calculation, namely diurnal and seasonal changes, and daily sunrise and sunset enhancements to the signal. The process is rather complicated.

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 (light red), Daytime (yellow), Nighttime (black), and Critical Hours (grey) 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 can be calculated using standard formulas used by the FCC or ITU 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. See the RADIO-TIMETRAVELLER blog at: http://radio-timetraveller.blogspot.com/

An accurate ray path can be drawn from all transmitters to a user-specified receiving location by inputting latitude-longitude coordinates on the heading bar at the top of the map. 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.

Individual station plots can be turned on or off by a checkbox. Click the station flag and you will see the option in a pop-up balloon. Check or uncheck the box, then click the ReDraw button. The entire plot set can also be turned on or off by buttons at the top of the map.

Included in each station's flag tooltip are FCC facility ID, engineering (application) ID, and distance of the station from the home latitude-longitude. Of interest to the DXer, by setting the home location latitude-longitude to your location and redrawing the map, each flag tooltip will have the distance from your location to the station.

SPECIAL NOTE ON SKYWAVE PLOTS

The skywave calculation has been totally overhauled and enhanced to more reflect actual signal expectations across the U.S. at night. The fact of life is that pattern overlay occurs on many frequencies. Simply select the plots you want to analyze. Check the No Plots checkbox then ReDraw to turn off all plots. Click any station flag and check the box to plot that station then ReDraw.

You will occasionally see a skywave plot which looks much smaller than surrounding plots. This is a case where the station's skywave signal did not meet the mV/m threshold (0.15 mV/m). The groundwave plot level is substituted in this case. The station does in fact have a skywave component, however small, it will be measurably less than the 0.15 mV/m level (very weak). It may be receivable!

The darker line defining the outer edge of the skywave plot shows the location of the 0.15 mV/m signal point at all compass points. Note that skywave signal strength does not decrease linearly with distance from the station. From the station outward, the signal strength will generally increase to a point usually 200-400 kilometers distant where it will peak, then decrease somewhat linearly from there.

Also note that the atmospheric background noise level on the mediumwave band is generally considered to be approximately 36 dBu (dBu in this case = dBµV/m), equivalent to 0.063 mV/m. Signals below that level will not be heard unless they fade up above the noise. A gain or directional antenna can be used to increase signal strength while limiting or even reducing the overall atmospheric background noise level.

Image below is an example of the 1370 KHz daytime ground contour map.

Hope you enjoy.

Daytime Ground Contour Map 1370 KHz

The KKOB-770 Synchronous Operation In Santa Fe

Returning back to Arizona from Denver after Thanksgiving, I decided to stay in Santa Fe, New Mexico. This was a perfect opportunity to check out the status of the KKOB-770 synchronous transmitter site in Santa Fe.

KKOB-770, as most of you know, is Albuquerque's 50 KW blowtorch outlet, "The Talk Monster". A single tower does duty during the daylight hours. At night, a second tower is switched on, the signal covering most of the western U.S. with a westerly-facing cardioid pattern. The nighttime notch at the base of the cardioid falls over Santa Fe.

KKOB maintains a 230 watt synchronized transmitter just one mile west-northwest of downtown Santa Fe, adjacent to the John Griego Vietnam Veterans Memorial Park. A single tower is visible just a couple of hundred yards or so from the parking lot of the park. The tower is a 231 ft. (70 meter) high affair, with an effective height of 65 electrical degrees.

The purpose of this operation is to fill in KKOB's nighttime pattern in the Santa Fe area. Santa Fe is 56 airline miles from Albuquerque.

Driving past the location at mid-afternoon about 2 PM, it was evident that the tower was transmitting during the daytime hours. Driving along the road past the park's parking lot, and closest to the tower at about 200 yards, I had the car radio tuned to 770 KHz. Weird signal fluttering occured for several hundred yards along the road as the Santa Fe and Albuquerque signals combined. For several seconds, the phasing of the two signals completely nulled, resulting in no signal at all.

It was a strange listening experience, as Albuquerque's KKOB-770, at 50KW, normally puts out a respectable signal to this area in the daytime.

The following synchronous operations are authorized through FCC license in the U.S. The FCC terms them as "Experimental":

Call          Frequency     Power     Location
---------     ---------     -----     -----------------
(D)WI2XAC      740.000       500      Ponce, PR
(N)WI2XAC      740.000       100      Ponce, PR
(U)KKOB        770.000       230      Santa Fe, NM
(D)KM2XVL     1220.000       170      Huntsville, TX
(N)KM2XVL     1220.000        11      Huntsville, TX
(U)WBZT       1230.000       800      Pompano Beach, FL
(U)KCOH       1230.000       410      Houston, TX
(U)KDTD       1340.000       200      Kansas City, KS

D = DAYTINE, N = NIGHTTIME, U = UNLIMITED

I've been wanting to check out this operation for awhile.

Click image for a larger version.

KKOB-770 Santa Fe. 230 watts synchronous operation.

KKOB-770 Santa Fe. 230 watts synchronous operation.

KKOB-770 Albuquerque. 50,000 watts. Single tower daytime, both towers at night.

Late 2018 IBOC Report, KNX-1070, KSL-1160

We talked at some length a few years ago about IBOC, or so-called HD radio. At the time, iBiquity Digital Corporation was the sole licensed vendor for the U.S. That has changed.

A NEW OWNER

Old news by now, in October, 2015, Dedicated To Sound, Inc. (a.k.a. DTS) purchased the rights to this HD Radio technology from iBiquity and is now the new sole owner.

DTS announced thusly:

"DTS is excited to announce it has entered into a definitive agreement to acquire iBiquity Digital Corporation, the developer of digital HD Radio technology for AM/FM audio and data broadcasting for approximately $172 million. DTS expects to finance the transaction through a combination of cash on hand and debt."

iBiquity was the exclusive developer and licensor of HD Radio technology, the sole FCC-approved method for upgrading AM/FM broadcasting from analog to digital. iBiquity’s partners included leading automakers, consumer electronics and broadcast equipment manufacturers, radio broadcasters, semiconductor and electronic component manufacturers and retailers.

The market that has shown some success is the automobile HD Radio market. The home market has not. Said DTS, "This transaction extends our strategy of delivering a personalized, immersive and compelling experience across the network-connected entertainment value chain, and complements our existing suite of technology and content delivery solutions while enabling us to strengthen our position in the large automotive OEM market. Consumers have come to expect a higher quality sound experience in their car, and we believe there is a tremendous opportunity for DTS to capitalize on the upgrade to HD Radio technology as cars are increasingly equipped with screens and advanced entertainment systems.”

iBiquity had successfully driven penetration of HD Radio technology in the North American automotive OEM market. HD Radio technology was built into approximately 35% of cars sold in the U.S. in 2014, and DTS expects the majority of North American vehicles to come equipped with HD Radio technology over time.

ANALYZING IBOC

The IBOC acronym stands for "In Band On Channel". Currently in the U.S., a hybridized version of the digital signal is imposed on the analog signal and the result transmitted. This is supposed to be an interim or transitional method. Stations that are licensed through the FCC for the digital add-on and have entered into contract with iBiquity and now DTS may transmit the hybrid signal. Current FCC records show a total of 237 stations are authorized to transmit IBOC hybridized digital. Many of those who made the initial licensing effort to get a digital signal on the air have terminated transmission due to resultant nighttime skywave interference problems on the broadcast band.

The misnomer here, at least for the hybridized version, is the term "On Channel". The 15 KHz digital sidebands either side of the main carrier bleed fully beyond the adjacent AM channel center and into the adjacent station's far sideband. Let me state this is another way: They have positioned the digital sideband information on the adjacent channel! No filter, on any receiver, regardless of bandwidth, can reject it. However, testing has shown some receiver's passbands are better than others. In order to provide room for these expanded sidebands superimposing digital information on top of analog audio, stations also have had to narrow their audio response to an absolute 5 KHz maximum, further reducing standard analog audio quality. Adherence to strict technical standards is now an absolute imperative, both in the transmitter, modulation technique, and antenna or you have an even worse interference problem. And we all know radio stations often fail in this area of strict adherence. This, currently, is what the FCC calls "Hybrid" digital operation, the precursor to going fully digital at some future date.

OBSERVED SKYWAVE RECEPTION PROBLEMS IN THE WESTERN U.S.

On the AM broadcast band, where this will all end up is anybody's guess. Will it ever go entirely digital? I suspect not, at least for the foreseeable future. The number of facilities registered to transmit hybridized IBOC digital has remained fairly stable over the last couple of years. In 2009 as IBOC was ramping up, 289 facilities were registered with the FCC. That number reached 293 a year later in 2010. Since the peaks of the initial few years, however, the overall decline has been sure but steady.

TopazDesigns runs a nice U.S. AM and Canadian AM station search site. Within it is also a page kept current with the latest IBOC stations on the air. Their current count as of this date shows only 117 transmitting, and only 35 at night. As is evident, interest has waned for digital on the mediumwave band.

Here in the far American West, there are some big boys still transmitting IBOC. Notably, the strongest received in southwestern Arizona at night is Salt Lake City's 50 KW KSL-1160, followed by Los Angeles's 50 KW KNX-1070.

Adjacent to KNX-1070 on its upper side is 50 KW KRLD-1080 in Dallas, Texas. A two tower array with 4.8 dB gain in my direction pushes a respectable 150 KW effective radiated power (ERP) at me. KRLD-1080 is essentially impossible to hear when KNX-1070 is faded up to full strength as digital hash blankets 1080 KHz. Careful nulling of the radio's loopstick does help, but the two stations are nearly 180 degrees opposed to each other.

I mentioned that some radios are better than others in reducing adjacent-channel hash, not withstanding the nulling technique. In my stable of super portables, the Sangean ATS-909X does the best job, followed by the Tecsun PL-880 and the C.Crane EP Radio Pro. The difference in the Sangean is fairly remarkable, I believe owing to the Silicon Labs Si4734/35 DSP chip being used down-chain in the I.F., and the outstanding fidelity of the audio section.

The worst IBOC offender here is Utah's KSL-1160 at 506 miles distant. I am in its sweet spot, propagation-wise. Its 455 ft., half wavelength (+) single tower with gain of 2.31 dB pumps about 85 KW ERP at my direction. KNX-1070 has an almost identical tower, however at only 237 miles distant I am in its skip zone and thankfully much of its signal is wasted overhead in the E-layer on its way to New Mexico and beyond.

My best chance at reception on 1170 KHz, adjacent to KSL-1160 is Tulsa, Oklahoma's 50 KW KFAQ-1170. But it's a long way off at 1057 miles and it takes some tremendous power to overcome KSL's signal. As luck would have it, the three tower array's main lobe is pointed directly at me in western Arizona. With respectable 5.56 dB gain, it pushes an astounding 179 KW ERP at me. KFAQ-1170 should boom in here at night, but caught in KSL-1160's digital hash on 1170 KHz, it barely makes a dent unless KSL is in deep fade. If KFAQ fades up as KSL fades down it makes an appearance. WBAP-820, a 50 KW station also in Dallas and clear of an IBOC hash problem, is in regularly. So is 50 KW WOAI-1200 in San Antonio.

Shown in the graphic is KSL's received signal in my SDRPlay RSP1a SDR receiver's waterfall. As you can see, the sidebands creep well into the adjacent channels 1150 and 1170 KHz. My guess is they are pushing the 5 KHz audio bandwidth, causing needless and excessive digital hash, that, or over-compressing the audio, or both.

We will see in the future where digital will go in the U.S. mediumwave band.

Click image for larger version.

KSL-1160 Salt Lake City, Utah