| Resolving 6 Meter Amateur Interference
Transmitting operations in the 50-54 MHz range offer some unique problems that
over the past 50 years have stymied station owners, forcing them to tolerate quiet
hours and hostility from family and neighbors attempting to enjoy other electronic
services. The 6 Meter band has some very special qualities, and its cases of interference
are resolved in a somewhat different manner than radio transmissions at other frequencies.
Here are some easy steps to "silent running" and the pursuit of enjoyment
of this fine communication band.
First, the station. In all cases try to keep radio equipment on or below the ground
where earth ground connections can be made and kept short. This always reduces case
and near-field equipment radiation. Elevated stations always suffer more from interference
and when a ground lead is a wavelength long (about 18 feet on 6 Meters), it's almost
useless. In fact, grounding of an elevated 6 Meter station may make interference
worse.
Second, always try to place
6 Meter transmitting antennas as high as possible and away from all other
services to increase isolation. Use good quality coaxial line with
at least 95% shield. Always crank RF connections down tight
with pliers and use anti-oxidants to avoid arcing. Install
a lowpass filter with a cutoff frequency right above
the 6 Meter band as a matter of good station practice. Although
an LPF won't help a great deal with interference to low TV channels
it's still a good hedge against higher
frequency generations and transmitter stability.
It also prevents strong local broadcast signals from entering and interfering
with 6 Meter signal reception. And always make sure that coaxial
cable shield lines for both 6 Meter antennas and cable television lines
are grounded directly to a ground rod using a grounding block, lightning
arrestor, or homebrew means. For best results keep the distance from
the coaxial connection point to the earth entry point
at near zero lead length. It's important for interference,
receiver noise, and lightning protection too. And be sure to inspect
the transmitter's shield case to insure that the cabinet paint is not preventing
good electrical shield contact with the main chassis.
If your interference case
is the common disruption of channel 2, 3, 4, etc. of the low VHF channels,
then recognize that 50-54 MHz transmissions are not in any way related
to the low VHF channels except by proximity. The reason that
such interference occurs with regularity is because TV tuners are not filtered
in such a way that 6 Meter transmissions are rejected. Thus, near-field
transmitting causes huge potential RF voltage to enter the tuner, and destructive
interference is easy after that. Run this test. Disconnect
the TV antenna or cable lead from the set and see if interference persists
during 6 Meter transmission. If it does it indicates that the TV
set may be suffering interference from pickup via the
AC line or chassis directly. Install an AC line filter or EMI filter to help decouple
the set from the line and run the test again. In all cases try disconnection tests
to see what the specific entry point of the interference is. In cases of low VHF
channel interference the only practical solution is to stop the introduction of
6 Meter transmission signals to the TV set's tuner, and that can be done with a
good highpass filter that has a steep slope of rejection beginning at 55 MHz One
or several filters of this type can often introduce enough front-end attenuation
to 6 Meter signals that the threat of interference is eliminated. keep repeating
the disconnection tests and makes notes as you go. A resolution is possible in nearly
all cases if you're persistent and follow good basic troubleshooting practice.
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