Contour Maps

What I had in mind when I suggested taking some readings of local satiations that happen to have one of the three R-L map contour lines passing over your location was a possible way to roughly calibrate the 310 readings by means of a conversion factor. The challenge would be to discover a number that when multiplied by the Tecsun dBu reading would result in the dBµV/m of the contour line. I didn't expect the Tecsun reading to be an accurate indication of actual field intensity.

But it seems that there are too many inconsistencies in your readings for it to be feasible to use the R-L maps for calibration. The inconsistencies are most likely due to inaccuracies in the maps and very local perturbations that change the readings. I have never seen any information about how accurate these maps may be.

It would definitely be possible to develop a conversion chart for the Tecsun that would show the actual field intensity in dBµV/m next to each of the 99 values displayed on the Tecsun. That would require a controlled environment, a calibrated FIM and an RF source. I thought maybe such a calibration chart would be what you would get from I.S.S. along with the higher than normal priced Grundig/Eton "Compliance Appliance" radio from I.S.S.

There is no guarantee from Tecsun/Grundig/Eton that the readings will be the same on each model within some tolerance, so creating one calibration chart for all radios of the same model would be useless. Each individual "Compliance Appliance" radio would need to get its own calibration chart. That would skyrocket the price, so it doesn't seem feasible.

Thank you Carl for your efforts. It was a grand experiment. Every experiment, whether it proves or disproves a hypothesis, contributes to the total body of evidence.

I have never seen any information about how accurate these maps may be.

The FAQ page on the R-L website ( http://www.radio-locator.com/cgi-bin/page?p=maps) addresses this quite well.  It is a good read.

Here are some clips from it:

" The coverage pattern for each AM station is based on the standard groundwave field strength pattern, the frequency of the station, and the ground conductivity of the local area.

These maps are generated using the same data and most of the same algorithms that the FCC uses when trying to predict coverage of radio stations and interference with other nearby radio stations.

Ground Conductivity To a large degree, AM radio signals are affected by the electrical conductivity of the ground over which they travel. Higher ground conductivity will enable AM radio signals to travel farther. The geology of the United States varies greatly from one area to another, and this affects the conductivity of the earth. Sea water has a much higher conductivity than the ground, so AM radio signals will always travel much farther over sea water than over land. WCBS-AM in New York City, for example, has one tower and a circular antenna pattern, but as you can see, its signal travels much farther over the water. You can learn more about ground conductivity at the FCC's website. "

Carl...

From Post #22 in this thread, the calculated field of KSTL close to the KFUO tower is about 21 mV/m (86.4 dBµV/m), and 9.4 mV/m (79.5 dBµV/m) for WEW.

So if those two stations are checked on your PL-310 at several open, uncluttered locations (if there are any) along a north-south line from the KFUO tower, and at least one mile away from the KFUO tower, you could see how your readings compare to those calculated fields.

The difference between the two readings also will be of interest.  If the PL-310 is equally sensitive at 690 kHz and 770 kHz, and the DSP chip/display is linear then there should be a difference of 7 dB in the average of the readings you get for each station.

The table shown in the graphic of Post #6 in this thread shows that for a difference in calculated fields of 5.2 dB, my Tecsun PL-310 measured a difference of 2 dB.  It also shows that the signal strength display on the PL-310 was lower than the calculated fields in dBµV/m by about 25 dB.  So this is one set of data, and yours would be a valuable addition.

You included readings for WEW in your Post # 26, which were 51 dBµ both days you measured it.  So that is 28.5 dB less than the calculated field at the distance of the KFUO tower.  You have stated that you are about a mile from the KFUO tower, but not whether your location is closer to WEW than the KFUO tower, or further away -- which could make a fairly small difference in what you measure for KSTL and WEW.

I second Phil's comments about your measurements.  They have been very instructive.

Staying on a north-south line from KFUO for your measurements not only will put your Tecsun at the distance where the fields for KSTL and WEW were calculated, but also will put the null of the loopstick in the radio more or less toward KFUO, which will help reduce any de-sense/overload issues from them.

You might even try to find the angle for the radio that produces a minimum reading when tuned to KFUO, and then without moving the radio check the readings for KSTL and WEW.

Thank you gentlemen for reviewing the TECSON's field readings.

Located directly south of the 850kHz (1 mi.) this location is the same distance from 690kHz and 770kHz as is the physical location of 650kHz.

Local perturbations include the topographic terrain which, while on the one hand being the highest location in the area, nick-named the "Hi Pointe" area, my exact spot is in a "bowl," surrounded 360-degrees by uphill mounding. In correspondence with a neighbor up on the top of the eastern ridge three blocks east, she had far more cluttered FM reception and a more difficult time finding an open frequency for her C. Crane FM transmitter. I am not sure how her AM reception would compare. She has since moved.

The "Afterthought About Simple Test" can be tried from the grocery parking lot 1-added-mile south, which stands outside of mound surround.

To be continued.

The "Afterthought About Simple Test" can be tried from the grocery parking lot 1-added-mile south, which stands outside of mound surround.

Very good, Carl.  Hope you can find a place with good clearance from any lighting poles in that parking lot, which can re-radiate the signals you are trying to measure and reject.

If the radio has a null of at least 25 dB for 850 kHz in the direction to KFUO, then probably the readings won't be affected much.

When I did compliance testing, I used a calibrated FIM with a loop antenna at frequencies below 30 MHZ. So the observed signal included cable loss and Antenna Correction Factors. The FIM was the classic Singer/Stoddart.

Here it is early evening and I want to go over to the store, but it's the wrong time of day to take dBu measurements in the parking lot, so I have to postpone the trip.

Then there's the terror that security teams will surround me with tazers drawn on suspicion of making radio contact with attack planes coming in low right down the Mississippi River Valley.

I'll have to send the dBu results from jail.

Part 15 takes risks.

The plan is good for noon.

Keep it hush hush.

11:45 AM CDT in parking lot of Save-So-Much near the bank, away from lamp poles, ready to sample RF field numbers with TECSON PL-310 radio. Few cars were parked nearby, and the weather was about the same as the past two tests.

For comparison, all three readings are posted here:

Frequency  9/2/13   9/3/13   9/5/13

850kHz      86/16     86/16    85/16

850kHz      -----      -----    56/14  The Eastern Null of 850kHz

770kHz     51/22      51/22    58/25

690kHz     63/25      none      65/25

Ready for more action.

Thanks for your efforts, Carl.  Hopefully you need no bail at this point.

Just wondering what people's conclusions are now, with your parking lot measurements versus the earlier comments in this thread...?

This is still an active thread, not only as we wait for comments on the latest parking lot measurements, but we still plan to try PhilB's suggestions for comparing daytime & nighttime dBu readings in a fixed location of weak reception, and a test to compare signal to noise in a location where noise interferes with the signal.

Having just completed a detailed review of this thread, I notice mistakes on my part, including the confusion already admitted between AM and FM measurement figures on radio-locator's contour maps, but at least one more instance...

Rich pointed out that the 770kHz transmitter was about 18 miles east, whereas the 690kHz tower was 8 miles east. I spanned right over that detail and continued claiming that the two stations were an equal distance from here.

My closing note at this time is to suggest that the TECSON and such dBu field measurement radios have another value to the Part 15er in addition to "peaking the transmit signal," and that is plotting a coverage pattern, i.e., I determined that my 1680 signal has the shape of a drip of water.

The results posted by Carl from his recent three measurements show that though the absolute numbers may not be accurate they are reproducible which is one of the prime aspects of any instrumentation. This means that if the TECSUN is used for day by day monitoring and if its position and orientation are unchanged then this is a good way to tell if something changes regarding field strength.

One of the problems encountered using the Yaesu FRG-100 here with a fixed dipole outdoor antenna was that the slope of the S-meter curve of voltage vs. antenna signal voltage was very low meaning that a large change in input voltage produced a small change in the S-meter voltage. This makes reading the S-meter voltage to assess field strength not a very good method since small changes in the S-meter voltage appear as a large field strength changes. This is typical of the AVC voltage of ICs used in some AM receivers such as the GE portable I have. I am not implying the TECSUN does this since I have no experience with it but mention this to illustrate some of the difficulties adapting AM receivers to measure field strength.

Neil

The AMT5000 Part 15 transmitter from sstran.com has a power adjustment control for the operator to fine-tune his 100mW output. It came to mind that watching the dBu scale on the TECSON PL-310 over the range of the AMT5000 power control might be an interesting thing to do. As a bonus, I also watched the peak on the screen of the spectrum analyzer (SA).

Audio is off with unmodulated signal turned on.

TECSON radio and spectrum analyzer are located 35' away at the far end of the building.

Minimum Power: 12mW : 61dBu/25-sn : SA= 33dBm

Mid Power:  101mW : 71dBu/25-sn : SA= 44dBm

Maximum Power:  261mW : 76dBu/25-sn : SA= 49dBm

What I want to do with future field measurements is to explore the very low end, where the signal-to-noise is just barely able to be detected by the dBu circuit in the radio. If we are not mistaken, I think FCC requires a sensitivity down to 1mW for testing equipment. It's somewhere in the rules. Maybe the real low end for sensitivity is 1 uW. Now I'm talking but don't don't know what about.