Topband: The Forbidden Frequency

[Tom Rauch]

> Maybe this has been covered here before, but what's up
with the QRM (steady
> carrier) on or about 1830khz?

It's a combination of harmonics from 610 and mixing products
of AM BC stations on other frequencies.

The same thing occurs on 1810, 1820, 1840, and so on.

> Almost everyone across the country regards 1830 as a "bad"
frequency and
> avoids it, but where does the signal come from?  I've read
many DX spot
> posts that call it BCI, but is it really? 1830khz would be
the second
> harmonic of 915khz, but MF AM stations in this country are
spaced at 10khz
> intervals. 1830khz is also the third harmonic of 610khz.
There's not an AM
> station operating on 610khz anywhere near me, so for so
many op's in all
> parts of the US to hear the same signal, there would have
to be mis-tuned
> 610khz transmitters everywhere. And, if it's BCI the
source would have to
> be located close by beacuse I hear it 24/7, not just
during the hours of
> darkness.

You might have something local going on in additional to
problems at a distance. At my QTH the dominant 1830 product
is from the NE, although there is crud from multiple other
directions. None of mine is local, some has been identified
as third-order mixing from the Meadowlands of NJ.

> Actually the closest I can zero-beat the signal puts it at
1830.24 plus or
> minus a few hertz.

I don't know how you can zero beat a signal without an
external oscillator or a stable internal additional tone
oscillator. The internal SSB oscillator normally can't be
used to zero beat because of filter shape and audio
response. On CW, the oscillator is normally offset by
several hundred Hz so you'd have to do a "tone match" to the
oscillator offset.

> only 1000W in power. Could it be a harmonic or
sub-harmonic of some
> modulation process generated by and common to all TV
and/or FM
> transmitters? (Like the TV color burst carrier on
3579.545khz)

Virtually impossible to be that. There is no such thing as a
"sub-harmonic" occurring in a transmitter amplifier stage.
There is such a thing as an oscillator leaking out. Say a
3.5MHz transmitter with a 1.75 MHz VFO driving a multiplier
stage to 3.5, where some of the 1.75 MHz energy remains
after passing through low pass filters. It could be a
harmonic, but the transmitters are so well filtered and the
attenuation in the signal path so extreme it is unlikely to
have stage leakage or blow-by except in an amateur type
transmitter that employs only a few stages and low-pass
filters in the tank system.

Or could it
> be some oscillator signal or artifact coming from within
our own transceivers?

Some of it could be, but significant signal energy on 1800
and multiples of ten kHz above that are very real. They are
absolutely due to transmitter problems at AM BC stations. At
my QTH, NONE of those products are generated in my own
system. I clearly hear mixing products from transmitters in
the NE USA (some 700-800 miles or more), Florida, the west,
and so on.

I know first hand the FCC has busted stations with problems,
including a Korean language station in the Meadowlands of
NNJ  that mixes with Bloomberg radio in the Korean language
station's transmitter. I've been directly involved in about
ten or fifteen cases where BC stations have been cited,
including more than one where a station received a
substantial fine!

In addition, there are harmonics of low power NDB stations
(airport LF beacons) populating 160. I hear about 5 or so
here regularly. Some from as far as the northern edge of the
USA and the west coast. This is because the FAA and the
manufacturers of some NDB transmitters do not have good test
procedures and the transmitter design so far as harmonic
suppression is terrible. Of course the NDB's do not fall on
even tens, except by dumb luck.

There are also a few intentional radiators on 160 meters
used to mark ocean fishing equipment that is allowed to
drift around and catch fish.

The advice for DX stations to avoid transmitting on
multiples of ten when working USA is very good advice. This
has all been covered before.

73 Tom