Part 15 AM Coverage - High vs. Low Carrier Frequency

Rich's picture

A NEC4.2 analysis shows this, for the system described:

  • 2.8-meter, Base-fed, Base-loaded, Vertical Monopole
  • Monopole Base Elevation = 0.2 meters above the Earth
  • R-F Ground = 16 Buried Radials, 3 meters in length (each)
  • Radial Common Point Concentric with Vertical Axis of Monopole
  • Earth Conductivity = 5 mS/m, d.c. 13 (about average)
  • Applied R-F Power at Loading Coil Input = 80 mW, unmodulated

At 1650 kHz...

  • Loading Coil R-F Resistance at System Resonance = 20 Ω (assumed)
  • Groundwave Field Intensity at 1 mile = 84 µV/m

At 570 kHz...

  • Loading Coil R-F Resistance at System Resonance = 30 Ω (assumed)
  • Groundwave Field Intensity at 1 mile = 34 µV/m


(First posted to the forums in a thread topic that would make it hard to find again.)



Is the difference due mostly to the increased assumed loading coil resistance or are other factors in play?



About the Differences

For the 1650/570 kHz systems:

  • Radiation resistance (Rr) =  0.1/0.01 Ω
  • Ground Connection R-F Resistance (Rg) = 7/7.2 Ω
  • Loading Coil R-F Resistance at System Resonance (Rc) = 20/30 Ω, assumed

Antenna system radiation efficiency = Rr / (Rr + Rg + Rc).

The reduced, groundwave propagation losses at 570 kHz are not able to compensate for the lower ERP that system produces.



I find this data to be encouraging, especially that the radials were only 3 meters. If my upcoming AM build experiment were to give me an average of only one quarter of a mile of usable range, I'd be quite content with it.

Hobbyist Microbroadcaster in Telford, Pa. Royalty-Free Music from WWW.MUSOPEN.ORG

Field Intensity vs Distance

Below is a plot of the groundwave fields produced from 0.1 to about 0.9 miles by the Part 15 AM setup defined in my opening post.

The field at 1/4 mile for this set of conditions is 0.43 mV/m.

A field of 0.15 mV/m might be useful to a good receiver setup in a low-noise area.  In this case that field occurs about 0.62 miles from the transmit antenna.

I am asking this question

I am asking this question because if I am having to move to Gloucester, VA because of my wife's medical condition I won't be able to operate FM due to the fact that there is an FM station right across the street from my step moms house WXGM 99.1. This leaves me to only be able to use AM. Now I have heard stories that people that use a Procaster AM transmitter has enjoyed 1 mile to 1 1/4 miles from a second story window.  So if this is true then why Worry about ground systems which I'm sure that my father wouldn't want me tarning up the yard to install in the first place. Besides he is a part of it HOA in this area. Remember this is if I have to move so I already want to know what I can do to get some decent range on AM in case this happens.Would Dad since he's sighted be able to easily wire up a carrier current coupler for me on a regular AC outlet?  Or am I better off using a simple transmitter like a Procaster or digital AM Transmitter where I'll still cover the neighbors in the Subdivision and still be able to enjoy radio. Again I have an agreement with the broadcaster across the street that I wouldn't try broadcasting on FM if I move here because I don't want to hurt him as he has been really nice to me. and has put up with some of the stuff I do lol.

Progressive Rock (Album Rock, Deep Tracks), Classic Rock

More Power for Hobby Broadcasters


I find it admirable that TheLegacy has entered into an agreement with the FM station that will be across the street from the new apartment.

Is the WXGM transmitter at that location? Or is it just the studio?

Of course, and I'm not suggesting that TheLegacy should rethink his agreement, but in the world of hard reality the part 15 rules give TheLegacy the right to operate on FM under 15.239 so long as it does not interfere with another station. But tuck that thought away for review next year.

All the best to Mrs. TheLegacy.

Carl Blare

Part 15 FM doesn't really

Part 15 FM doesn't really give you much range other than maybe one or two homes on either side of you. This is if I do it legally.


 when I was doing some testing with the Whole House FM 3.0 at that apartment or should I say house and I had the lightning bolt on I was able to get out. A Mile and I did walk around the street right up to where his transmitter was and my signal was coming in quite clear. He was concerned about what I was doing and I told him and his concern was that my FM signal could piggyback on his signal and cause intermodulation issues. Yes his transmitter is right across the street I could take a picture of it and show it to you. You can see the big antennas and this guy runs 6000 Watts.


 The gentleman knows how I operate in Deltaville and doesn't have a problem with it because it's a rural area. But now we're not talking real so much anymore so I have to be careful and not only that but I'm not out there to harm other people I'm out there to have fun and that the same time not cause harmful interference to anybody else. That to me is not having fun that to me is just being stupid and dangerous.


 if it does if it does come to that I will have to relinquish my FM signals and if I do use FM it would be at the part 15 level and mostly being used as an STL. But I think that using 900 megahertz or 2.4 gigahertz for an STL might be the better way if I have to do that to get to my AM transmitter.


 again I won't get rid of my FM transmitters till it does come to that point but it is something that I have talked over with my step mom has a possibility that we may have to move if my wife's appointments are going to be so closely scheduled because she lives about 30 miles from me so keeping that in mind I'm still keeping AM transmitters in mind.


  Already I will be saving money from donations that I get so that I can afford a decent and easy-to-use AM transmitter that doesn't have the hum that the Talking House has nor the issues the Talking House and yet will sound decent even if it goes out a quarter of a mile to a mile. It should still cover the subdivision and at the same time it won't cause any interference with this gentleman who has been very nice to me and at the same time I must do the same thing to him and follow the agreement.

Progressive Rock (Album Rock, Deep Tracks), Classic Rock

More Power for Hobby Broadcasters

Assuming the 1650 kHz field

Assuming the 1650 kHz field strength is correct let's see what the 570 kHz field strength would be.

The 570 kHz loading coil reactance will be 3X and assuming the Q is the same (about 200) the coil resistance goes from 20 ohms to 60 ohms. The radiation resistance goes from 0.12 ohms to 0.014 ohms. Let's assume the ground loss is the same and I'll use 30 ohms. 

The radiation efficiency ratio is 0.12/50 : 0.014/90 = 15.4:1 

The 570 kHz field strength at 1 mile will be 84uV/m x 1/15.4 = 5.5uV/m


My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

Changing the Model

My posts above showed:

  • Ground Connection R-F Resistance (Rg) = 7 Ω at 1650 kHz and 7.2 Ω at 570 kHz.  These are the values for Rg calculated by NEC4.2 for the radial wires I defined, when buried in the earth conductivity I defined (5 mS/m d.c. 13).  They are not assumptions on my part.  Rg in Reply 8 was taken to be 30 Ω, without stating any reason/basis for choosing that value.
  • Loading Coil R-F Resistance at System Resonance (Rc) = 30 Ω at 570 kHz, I assumed.  Reply 8 assumes 60 Ω coil R for 570 kHz.

Of course the changed values for coil R and ground R in Reply 8 necessarily will change the results I posted.

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