The FCC and FAA informed me that (due to pressures from our community)
Gainesville airport AKA. FKV has ordered a new transmitter! Hopefully we
will no longer hear the fifth harmonic of the carrier (with a "negative CW
signal" on 1825) and the shift oscillator on 1826,27 etc.
Now FIO (Paduacha KY) is next, and BEQ third. The Feds are more receptive
to looking at these things now that they understand the problems they
cause.
> Dave, this is a problem everyone in the USA has to one degree
> or another. It is caused by the mixing products of our AM broadcast
> stations, which are spaced every 10 kHz, giving images on every even
> 10 kHz (1820.0, 1830.0, 1840.0).
One example of mixing is the garbage on 1855 kHz (the IMD does not have to
be on the tens, with modern switching PA's). If anyone can get an ID out of
that mix of two stations let me know, it's time to get them to fix their TX
system. When it is mixing, you will always hear both station's audio if the
signal is strong enough to decipher.
The other common problem is harmonics.
I reported an AM radio station on 1250 for QRM'ing WWV on 2.5 MHz. The FCC
visited the station within one week of the complaint. Seems the "engineer"
tuned the PA for the second harmonic, and then went outside and matched the
antenna on 2.5 MHz!!
BTW, the mixing problem is caused by a non-linear device generating
harmonics...which then mix with each other. For example a 1400 kHZ signal,
when passed through a non-linear device, will generate all hamonics of 1400
kHz. The second harmonic generated, 2800 kHz, might mix with a 980 kHz
transmitter to produce 2800-980=1820 kHz.
We also can get direct sum signals, for example 710 kHz plus 1140 kHz would
cause a problem on 1850. It would also cause odd order problems (3x one
plus or minus two times the other) and also on 2000 kHz and other
frequencies (3 X 1140 minus 2 X 710).
Like Bill pointed out, nearly EVERY even ten kHz is subject to QRM from
odd-order IMD, and as others pointed out harmonics are also a problem.
and of course we get to toss in the occasional parasitic...
WINV in Florida was forced to eliminate a wide raspy buzzing QRM to 1910
kHz. That only took two weeks for FCC action.
FCC field agents are now operating under "management suggestions" to
actively enforce rules, a reversal of policies of the recent past where
they were basically told to "be forgiving". Let's hope Congress let's them
do their jobs. It would not hurt to call your local politician and remind
them we want the FCC to enforce rules.
73 Tom
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