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
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