Contour Maps

Converting the contours stated by Radio-Locator in field intensity units of millivolts/meter (mV/m), for AM broadcast stations:

Local (2.5 mV/m) = 68 dBµV/m

Distant (0.5 mV/m) = 54 dBµV/m

Fringe (0.15 mV/m) = 44 dBµV/m, approx

Of interest here is that the Radio-Locator map for KMOX, 1120 kHz, shows their "Local" (2.5 mV/m or 68 dBµV/m) contour to be about 5 times further away from their transmit antenna on a radial crossing the center of St Louis, MO than the distance where a PL-310 located ~near the geographic center of the St Louis metro reportedly showed 68 dBµ on its front panel display.

This tends to support the point that the "field strength" displays on such DSP-based receivers are quite INaccurate.

Inaccurate as it is, does the every-day Part 15er have anything else to use for field observations, aside from portable and auto radios along with guess-work?

If my memory is accurate, a very large "if", you have also explained (I think) that a Part 15 AM setup that is compliant with respect to input to the RF final (100mW) and length of the antenna and ground leads (within 3-meters), can STILL be found non-compliant by an inspector. This makes no argument in favor of dBu scales on small radios, but would suggest we are sitting ducks in a shooting gallery.

Hope I'm mistaken about that last impression, but I think you made that point.

This is kind of an off-thread topic, but if those parameters are strictly accurate in terms of physical principles, then an FCC field inspector would have no legitimate/legal basis for issuing a citation (NOUO) to such an operator.

Proving such in a legal action might be possible, though expensive.

Carl, unfortunately, since the Part 15 rules are words (and not equations), I believe that you are absolutely correct when you say that you can still be complying with Part 15 (at least, in your own mind) and still be found non compliant.

Examples of this abound.  Prior to KENC, FCC inspectors were allowing (i.e., finding Part 15 compliant) elevated installations grounded to metal objects such as masts.

Post KENC, that is not the case.  The Part 15 rules have not changed, but the interpretation of the words has.

But Rich is correct - this is off topic.

Well, your results seem to show a lot of discrepancies in relation to the Radio-Locator maps.

You talked about discrepancies in directional orientation of the radio when it was indoors. Then when you went outdoors, you said you took all the readings with the radio on a stool at a fixed location. Just for clarification, you didn't say how you oriented the radio on the stool for each reading. Did you rotate it all around 360 degrees to find the absolute maximum reading for each station regardless of which geographical direction the station was located? Did you find orientation discrepancies on the stool similar to what you found indoors? Did you get different readings with your hand on the radio vs. moving away from it?

From your readings, it seems there are problems somewhere. Are the R-L maps wrong? Are there significant variations in the actual reception vs. the lines on the maps? Is the Tecsun meter indicating very non-linearly? Was there a problem with how you oriented it for peak readings? ... too many questions.

So far your experiment seems to have raised doubt about the usefulness of the Tecsun for anything other than maybe tuning a transmitter to peak. It's not even clear that it is stable. Could it be relied on to monitor a part 15 transmitter day-to-day to check for transmitter problems?

I'm really not trying to sound negative. The variations in your measurements surprised me. I guess I was naively expecting better correlation to the R-L maps.

Maybe you could go out and re-measure some of the stations that gave unexpected results, checking to see if the second measurements agree with the first set.

YES I will remeasure the signals for a second sampling.

But I will say that the readings I got the first time correspond with general experience for the stations involved.

I think some of these stations have engineering problems.

For example there are two stations with almost identical circumstances, namely 690kHz at 1kW omni-directional and 770kHz at 1kW omni-directional, located very near each other.

But 770 has not ever been equal, dating into the 1960s.

If stations are operating below their authorized capacity does the FCC take notice?

...there are two stations with almost identical circumstances, namely 690kHz at 1kW omni-directional and 770kHz at 1kW omni-directional, located very near each other.  But 770 has not ever been equal, dating into the 1960s.

Carl, from Post #11 of this thread it appears that you are located about 1 mile from KFUO's tower in Clayton, MO.  Looking up the FCC data for KSTL (690 kHz) shows their antenna is located just across the Mississippi River from the St Louis Arch.  The path length from the KSTL tower to the KFUO tower is 8 miles.

Doing the same for WEW (770 kHz) shows their tower to be located on roughly the same bearing to KFUO, but that their path length to KFUO is 15.5 miles.

Both KSTL and WEW use ~90-deg towers, and earth conductivity for both paths to KFUO is 15 mS/m.

The FCC's groundwave propagation curves for these two 1 kW, omni stations shows that KSTL has a field intensity of about 21 mV/m in the vicinity of KFUO's tower, where WEW has a field of about 9.4 mV/m.

Both of these signals are substantial, but KSTL likely would have a better signal to noise ratio than WEW in AM receivers near the KFUO tower.

So your experience with these two stations makes sense.

Another consideration here is that the very high fields from KFUO within 1 mile of their tower might be affecting receivers tuned to 770 kHz differently than when tuned to 690 kHz.  KFUO has a field of 400 mV/m at 1 mile, and 500 mV/m at 0.82 miles.  A field intensity of 500 mV/m equates to 114 dBµV/m.

I buy that description of what's taking place with regard to 850kHz's influence on signals nearby on the dial.

Because of it's proximity to the KDX location, some radios have all kinds of symptoms in the 850 dial area...

850 also puts out slush on the side bands with I-BAH, which spills up to 870 and down to 830 on some receivers.

There is a big fat phantom of 850 on top of 800kHz on some radios.

As mentioned, there was an ambient noise level across the entire medium wave band, perhaps also a side-effect of the 850 field energy.

Another station I didn't mention, although the second nearest geographically, is 1320kHz with 5kHz daytime. Although their signal is hot on receivers, the spectrum analyzer doesn't show much.

IBOC is neither in band or on channel.

WDCX Posted: IBOC is neither in band or on channel.

I have the dial tuned to 860kHz in the medium wave band, hearing a hash-blast that obliterates any radio signals on that frequency.

For a signal that is not there, IBOC generates the illusion of being there.

The digital sidebands of AM HD stations lie within the AM broadcast band except for stations with analog carrier frequencies of 530, 540, 550, 1680, 1690 and 1700 kHz.

Using Contour Maps from radio-locator.com, following a suggestion by PhilB, we have been comparing the dBu readings from a TECSON PL-310 radio to the predicted dBu within three points in the circle of coverage, i.e., 60dBu= (L) Local, 50dBu= (D) Distant and 40dBu = (F) Fringe. The radio displays dBu/SN (signal-to-noise).

An important disclosure appears on the radio-locator maps: "This image is intended solely for entertainment purposes. Radio-Locator makes no claims as to the accuracy of this information, nor towards its suitability for any intended purpose."

During this inquiry Rich has informed us that dBu readings from the TECSON are not commensurate with professionally derived field readings using a different measurement scale, dBu/m, and by his calculations the amount of error is apt to be >20dB.

Our first set of readings were made yesterday around 1 PM CDT, and today's second set of readings take place also in the 1 PM hour with daytime conditions fairly equal, temperares in the 80s with sunlight.

I am seated on plastic stepping stool holding the radio by hand, after determining that readings do not vary when the hand is removed.

kHz    Map   Sep. 2  Sep. 3

550    L       68/25   67/25

590    L       54/25   53/25

630    L       68/25   67/25

730    D       30/02   32/06

770    L        51/22   51/22

800    D       xx/00    xx/00

850    L       86/16    86/16

880    L/D    41/15    42/15

920    L       53/25    53/24

940    F       xx/00    xx/00

980    D/F    xx/00    xx/00

1010  L       73/25    72/25

1080  L       42/16    42/12

1120  L       66/25    66/17

1220  D      xx/00     xx/00

1230  D      35/02    35/01

1260  L      51/16    42/14

1320  L      -----     80/25

1460  L      42/21    44/23

1510  D      35/03    35/02

1540  F      xx/00     xx/00

1570  L/D   34/07    34/08

Comments welcome.

...we have been comparing the dBu readings from a TECSON PL-310 radio to the predicted dBu within three points in the circle of coverage, i.e., 60dBu= (L) Local, 50dBu= (D) Distant and 40dBu = (F) Fringe.

Carl - as I stated in Post #16 of this thread, the Radio-Locator L, D & F coverage contours for AM broadcast stations do NOT have the field intensity values in dBµV/m shown in your clip above.  Apparently you are applying the "dBu" values for the R-L FM coverage maps to the R-L AM coverage maps.

Quoting from the R-L website:

" 3. What criteria do you use to define the "local", "distant" and "fringe" coverage areas?

• The "local", "distant" and "fringe" lines on the FM maps correspond to the predicted 60, 50, and 40 dBu field strength contours respectively.
• The "local", "distant" and "fringe" lines on the AM maps corresponds to the predicted 2.5, 0.5, and 0.15 mV/m contours respectively (of the horizontal groundwave propogation only). "

The correct conversion of the 2.5, 0.5 and 0.15 mVm field intensities is as follows:

Local (2.5 mV/m) = 68 dBµV/m

Distant (0.5 mV/m) = 54 dBµV/m

Fringe (0.15 mV/m) = 44 dBµV/m, approx

So if the dBu display on your Tecsun PL-310 (and on similar radios) was accurate, it would read very close to to the values in the three lines of text next above, at those physical locations.

But you are reading 66 dBµ on your Tecsun at only about 20% of the distance to the 68 dBµV/m Local contour shown for KMOX on the Radio-Locator coverage map.  This is a gross error.

So, while your readings would be useful in optimizing the tuning/field from a Part 15 AM setup, and in showing change from one day/month/season to another, they have no real value as an accurate measure of the true field intensity at the location of the Tecsun receiver.

Sorry.

Rich described a mistake I definitely made:

"Apparently you are applying the "dBu" values for the R-L FM coverage maps to the R-L AM coverage maps."

I thought all contour maps followed the same scheme and failed to note the difference between "AM" and "FM" measurement notation.

Maybe I should retrospectively cancel the fun it was.

"Maybe I should retrospectively cancel the fun it was."

whaaaaaaaat?? NEVAH!

🙂