[RFI] Splatter from AM Radio Station WNTS Indianapolis

[Gary Peterson]

Paul,

I measure all harmonics from the 2nd to the 6th.  All harmonics, up to 5 MHz, are measured with a Potomac Instruments FIM 41 field intensity meter.  Any harmonics above 5 MHz are measured with a communications receiver.  It is rare to hear anything beyond barely perceptible, above the third harmonic.  It is not unusual for the the third harmonic to be somewhat stronger than the second harmonic.  I have experienced harmonics coming from a direction, other than that of the radio station’s antenna system.  Nonlinear junctions, combined with re-radiation, in utility lines and even barbed wire fences may be the culprit.  The use of loop receiving antennas results in my conclusion.  If a harmonic is not coming from the radio station, there is usually little that they can do about it.

The formula for determining the maximum permissible harmonic radiation is 43 plus 10 times the log of the carrier power in watts.  This provides the required, minimum dB of suppression from the carrier power.  A 1 kW AM must have all harmonics suppressed at least 73 dB below carrier power.  For stations licensed at 5 kW and above, all harmonics must be suppressed at least 80 dB below carrier power.  If a harmonic has the required degree of suppression, it still is not supposed to cause harmful interference.

A recently measured 5 kW AM station, on 1380 kHz, measured 102.4 dB suppression of the second harmonic.  The same station measured 87.1 dB of suppression of its third harmonic.  The fourth harmonic was barely perceptible on my receiver.  The fifth and sixth harmonics were not audible on a communications receiver.  All measurements were made very close to 1 km from the station’s antenna tower.

I hope I have addressed your questions.

Gary
KzeroCX  

Gary,
In your measure measurements during field testing do you test for 2nd and 3rd harmonic?
I think for lower power stations the FCC says -80 dBc  and -83 dBc for high power..  I don't know exactly.

What I was wondering if you have measure some actual values of 2nd and 2rd energy in the field.
Can you straighten me out on this.

Paul Cianciolo, W1VLF
ARRL Product Review Test Engineer