|Fighting Back Against BCB Interference
Broadcast band interference is a common nuisance to radio reception efforts
and is caused by powerful nearby AM broadcasting station transmitting in
the 550kHz to 1,700kc frequency band. A station in this area may be very
close to where you live and employ a receiving station. If you're receiving
interference of this kind you will probably know it right away, especially
if you can hear music or commercial announcements on dozens of frequencies
well separated from the AM broadcast band. Sometimes the station can be
heard every few kilohertz across a broad spectrum of the HF frequency range.
Can you fight back against
this legal onslaught? You bet, but there are a few important steps to take
first. To start, try to identify what type of interference exists.
If the offending station can be heard all over the place in the HF frequency
range or commonly mixes with other station's signals you likely have a
case of fundamental RF voltage overload caused by the offending station's
primary transmission. Identify which station is the offender by flipping
back and forth between the interference and the AM broadcast band, trying
to match the voice or music with the various loud stations heard. If the
problem appears to be of the overload type a BCB filter is the best answer.
Choose one that puts the offending station the furthest possible into the
filter's stopband, or frequency reject area. BCB filters can often
insert 70db or more of attenuation, making the offending station manageable.
The other type of BEB interference is caused by harmonic generation from the AM broadcast
station's transmitter site. Harmonics are the tendency of a radio transmitter to
exhibit a small amount of its content output on multiples of the original transmission
frequency. These spurious radiations are filtered by the transmitter's final circuitry,
but not fully. The result is a 5KW station with several measureable watts output
on frequency multiples. Example: a station on 1,200kc will have outputs on 2,400
kc, 3,600kc, 4,800kc, etc.
Problems of this kind can
only be dealt with by the transmitter's owners. Best bet - if you're suffering
from this kind of interference try to identify the offending station and
tell them about it. Call or write the station and address your correspondence
to the chief engineer, requesting that he take a look at the station's
content output and see if some additional filtration is warranted or possible.
Sometimes the station may claim to meet FCC harmonic specifications and
they are required to do nothing else. They are right, so be diplomatic
and polite. Ordinarily the FCC will not intervene if the station meets
the Commission's purity standards. Unfortunately, that still
does not solve your problem. Coop diplomatic work and possibly a visit
to the station personally may be helpful.
Another type of interference
is caused by re-radiation of AM broadcast signals and effects from the
powerful local signal mixing with electrical power distribution systems
or overhead telephone lines. These cases are pretty rare and somewhat difficult
to find. Special equipment and technical expertise available from a local
radio club may be helpful in these instances.
In your efforts to live
with AM broadcasters keep in mind that some local efforts at your own station
can also help, such as good short distance equipment grounding, shielding,
operating with horizontally polarized antennas,
and sometimes even the use of bandpass filters for HF application or attenuators.