Does broadcasting in amateur radio interfer with AM/FM or other
boardcasting wavelength signals? Neighbours are concerned that if I
put up a 45 foot radio antenna in may backyard that this could
interfer with everyone's elses TV/radio receptions. Any information
out there that can be offered would be much appreciated.

In theory, if everything's working right, the answer is "No". You'd
be transmitting a signal falling entirely inside the amateur band,
there'd be no significant out-of-band radiation (harmonics or
intermodulation) falling into the TV or AM/FM bands, and everybody's
TV sets and radios would "ignore" the signals outside of their
operating bands.

Unfortunately, things often don't work right, and interference can
happen. There are a number of possible causes:

- A lot of consumer-electronic equipment is very poorly designed, with
regard to its ability to reject/ignore out-of-band signals. A
strong (but perfectly legal) amateur radio signal can result in
"fundamental overload" of a TV set or radio's RF front end due to
the high RF voltages present, and this can result in visible or
audible interference. Devices which aren't supposed to act as
radio receivers (e.g. intercoms, telephones) often do, and can pick
up your transmissions. Signals can get into such devices throught
their antenna/cable inputs, through the power line, through
speaker wiring, etc.

According to law, you (the amateur) are _not_ responsible for this
sort of problem, or for correcting it... it's a design problem in
the susceptible device. It can, however, be very helpful to
understand the cause of the problem, and some of the means for
curing it (e.g. adding filters) so that you can advise your
neighbors on how to fix their appliances... and if you happen to
have a boxful of filters and ferrite cores and etc. that you're
willing to hand out on request, that'll often soothe peoples'
feelings. [It's generally recommended that you _not_ actually
work on other peoples' TVs etc. to try to fix the interference, as
that might make you liable for any problems which occur with
them... give people advice and perhaps some filters, and let them
do the installations themselves.]

- Direct interference can occur if your station radiates out-of-band
signals - either harmonics of the frequency you're transmitting, or
intermodulation products. Corroded metal-to-metal junctions (e.g.
fence wires, rusty gutters, etc.) can act as rectifiers, picking up
your transmitted signal and then radiating harmonics or
intermodulation products.

You _are_ responsible for making sure that your rig's output
doesn't contain more out-of-band energy than the FCC rules allow...
if it does, fix it, or add an additional low-pass filter.

- Direct interference can also occur if your transmitted signal
bleeds into a cable-TV system. Cable channel 18 falls right into
the amateur 2-meter band, channel 16 is in the 70 cm amateur band.
If somebody's got a "leaky" cable-TV setup (inadequately shielded
cable or amplifier, or unterminated taps), outbound leakage can
cause QRM on the amateur bands, and inbound leakage of amateur
transmissions can hash up the picture.

You are not responsible for fixing this sort of problem.

As far as the tower goes - you can legitimately point out to your
neighbors that a tall tower is likely to *reduce* any interference
problems, compared with a lower-height antenna (assuming equal
transmission power). The higher up the antenna is, the further it'll
be from your neighbor's TVs, wires, etc., and the lower the induced RF
voltage and current is likely to be. Also, a higher antenna may give
you a better takeoff angle, allowing you to make your QSOs at a lower
power setting.

The ARRL sells a good book on the causes and cures of RFI problems...
it's worth having a copy around.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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