On Minimum Frequency Spacing for AM Amateur Radio Stations

by Steve Ickes, WB3HUZ



The question of minimum spacing for AM amateur radio stations arises often and is usually the subject of vigorous debate. However, I've seen little in the way of either empirical or theoretical data to bolster these arguments. I happened across the "Handbook of Radio-Frequency Interference," volumes 1 to 4, published by Fredrick Research Corporation, Wheaton, MD in 1962, a few years ago (here after referred to as the Handbook). Therein I found a section on interference to radio communications systems. Various systems or modes of communication were covered, including CW Morse, radioteletype and AM.

The following is my take on the data presented in the Handbook on tolerable signal to interference ratios for AM (double-sideband telephony) and how it relates to AM operation in the Amateur Radio bands.

Data from the Handbook

Here is the information I gleaned from the Handbook. I photocopied a few pages, which I later typed into a computer and printed out for future use. Unfortunately, I did not get to read the handbook further to learn the details on how the data was derived.



Besides noise, the efficiency of radio communication systems can be limited by the interference produced by other radio communication systems. The amount of tolerable signal/interference ratio, and the determination of conditions for entirely satisfactory service, are necessary for the specification of the amount of harmonic and spurious frequencies that can be allowed in transmitter equipments, as well as for the correct spacing of adjacent channels.

Double-Sideband Telephony

The multiplying factor for frequency separation between carriers as required for various ratios of signal/interference is given in the following table. This factor should be multiplied by the highest modulation frequency.

The acceptance band of the receiving filters in Hertz is assumed to be 2x (highest modulation frequency frequency) and the cutoff characteristic is assumed to have a slope of 30 dB/octave. [Comment: Roughly equates to a  -6/-60 dB shape factor of 4:1. HUZ]


 Ratio of Desired
to Interfering
Carriers (dB)
 Multiplying Factor for Various Ratios of Signal/Interference
 20 dB  30 dB  40 dB  50 dB
 60 0  0  0  0
 50  0  0  0
 40 0  0  0.60 1.55 
 30  0.60 1.55  1.85
 20  0.60 1.55   1.85 1.96
 10  1.55  1.85  1.96  2.00
 0  1.85  1.96 2.0   2.55
 -10  1.96  2.00  2.55  2.85 
 -20 2.00  2.55   2.85 3.2
 -30  2.55  2.85  3.2 3.6 
 -40  2.85  3.2 3.6  4.0 
 -50  3.2  3.6  4.0 4.5 
 -60  3.6  4.0  4.5  5.1 
 -70  4.0  4.5   5.1 5.7 
 -80  4.5   5.1  5.7 6.4 
 -90  5.1  5.7  6.4 7.2 
 -100  5.7  6.4  7.2 8.0

Table 1. Multiplying Factors for Various Ratios of Signal/Interference



Comments and Interpretation

If we calculate the case where two signals (desired and adjacent channel interferer) are about the same signal strength (by using the 0 dB Ratio of Desired to Interfering Carriers row from Table 1) , and assume the highest modulation frequency to be 4 kHz, we get the following minimum spacing:

7.4 kHz spacing if the interference is to be at least 20 dB down from the desired signal, signal to interference ratio, (20 dB S/I);

7.84 kHz for a 30 dB S/I;

8.00 kHz for a 40 dB S/I;

10.2 kHz for a 50 dB S/I.

The case above would be typical when the two stations are in the same geographic region with typical late afternoon propagation on 75 meters, or daytime conditions on 40 meters. In a roundtable QSO it would not be out of the ordinary to have a situation where the adjacent channel interferer is stronger. If we use the -10 dB Desired to Interfering carrier ratio row from Table 1 we get the following:

7.84 kHz for a 20 dB S/I;

8.00 kHz for a 30 dB S/I;

10.2 kHz for a 40 dB S/I;

11.4 kHz for a 50 dB S/I.

For the cases where there is an even greater disparity in strengths of the desired and adjacent channel interferer, the spacing is larger.

The examples above assume an 8 kHz receive bandwidth. Many people use a 6 kHz bandwidth. Although a 6 kHz bandwidth can yield usable audio fidelity, I find it less than enjoyable. Since the main reason, I use AM is for the audio fidelity, reducing the fidelity needlessly (in many cases) due to interference from another AM station spaced too closely is unacceptable. But for fun, let's run the numbers for the two examples above. But this time we'll assume the highest modulation frequency is 3 kHz. Thus, the receiver bandwidth is now 6 kHz. For the zero dB Desired to Interfering carrier ratio case we get the following:

5.55 kHz for a 20 dB S/I;

5.88 kHz for a 30 dB S/I;

6.00 kHz for a 40 dB S/I;

7.65 kHz for a 50 dB S/I.

And for the -10 dB desired to interfering carrier ratio we get the following:

5.88 kHz for a 20 dB S/I;

6.00 kHz for a 30 dB S/I;

7.65 kHz for a 40 dB S/I;

8.55 kHz for a 50 dB S/I.

Many AMers think that a 5 kHz spacing is acceptable and many use such a spacing on the air. But notice that not even in these two more conservative cases is a 5 kHz spacing acceptable. This corresponds closely with my on air experience.

Future Work

It would be interesting to obtain the information on the derivation of the data presented in the Handbook. With this, one could look at some other cases such as where the receiving bandwidth is something other than twice the highest modulation frequency or where the receiver passband shape characteristic is something sharper than 4 to 1 (crystal and mechanical filters are usually sharper). Could these reveal a case where a 5 kHz spacing is acceptable? Probably. But I think in the real world these would occur far too infrequently to support a more general and widely applicable minimum acceptable spacing standard.


I firmly believe that in the majority of the on air cases where the signal strength of stations are within a 10 dB range, a 5 kHz frequency spacing between two stations or QSOs is not enough. A more acceptable minimum spacing appears to be around 7 kHz. Of course, we will only know for certain if this is tested in the real world with proper measurements and on the air comparisons.

I hope the data and analysis presented here will serve as a starting point in the discussion on minimum frequency spacing for AM amateur radio stations. I also hope it drives the discussion towards the use of theoretical and empirically derived data. Your thoughts and comments are welcome.


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4 August 2000