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:

http://www.radioworld.com/article/sun-rays-power--kw-am-station/217859

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

http://www.radiomagonline.com/deep-dig/0005/kezw-goes-green-with-solar/35230

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.

radio-locator.com, 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.

radio-locator.com


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.