+ RADIO FREQUENCY INTERFERENCE - by Ed Ewell, K7DXV
Associate Editor for Training - ewell2@cvc.net
RFI feedback can be a real problem, especially with our new
transceivers and multi band antennas.
You will normally get RFI when your antenna is very close to your shack
or you run a linear amplifier. Another cause for RFI is long cables. Try making
your cables and coax leads as short as possible.
RFI can be eliminated rather easily in most cases. Ferrite rods and
torroids work wonders in keeping RF out of cables and coax leads going to your
radio. Check all your antenna cable connectors to make sure you don’t
have a faulty connection. Check your antenna cables to make sure they are
in good condition and not leaking RF.
Also use shielded cable on all your audio cables. Make sure all
equipment is properly grounded. I use ferrite rods and torroids on almost all of
my shack’s cabling. I do this as a precautionary measure more than
anything.
You can have RFI on the transmit audio cable and not know it. If you have
a second radio, it’s always a good idea to listen to what your signal sounds
like. Or have someone on the air listen to your signal. Be careful
that the distortion of your audio signal is not FM modulation of your signal.
Some stations operate from batteries and when the voltage gets low the signal
will FM, and SSB signals that FM sound similar to RFI. Work at the problem
until you get it solved, we are trying to trap, or filter the RF signal that
rides on the outside of the coax and audio wiring from getting back into the
radio. “Clip on” RF chokes (available at most Radio Shack stores and
many other outlets) work great. You may have to place them on the speaker
leads, and power supply leads if you operate from a separate power supply.
I use a balanced antenna (Zepp) dipole with a ladder line feed. If
you use the common coax fed dipole antenna or a G5RV antenna, you will probably
have RFI on at least one band. You can choke off the RF imbalance at the radio
end of the coax with a homemade balun of 10 to 12 turns of 50 coax wound on a 4
to 6 inch diameter PVC pipe wrapped close together. RG8U or RG58U 50 ohm coax
will work well. Place the balun as close to the radio as you can, between
the radio and tuner if you use one. I have one of these on my radio between the
linear amplifier and the transceiver. Feed line lengths are important and
with some antennas you will want to experiment
with different lengths of coax.
If you happen to have a length that presents as a virtual
short or an open wire to your tuner then you will probably get RF in the shack.
Try adding about eight feet to the line and see if the problem clears up.
I had an RFI problem that would not go away until I found that one side of
the line broke loose at the antenna.
Be sure and check all of your connections, and make sure your mike
connector is on tight. When you install torroids, or RF chokes place them
at both the input and output points at the radio. In real tough cases you
might have to add .01 uf bypass capacitors at the audio input points of your
radio since some manufacturers do not have these points bypassed well.
The first step in locating an RFI problem is to disconnect all cables
except the antenna and microphone. If the RF problem is gone then, add each
cable you use until RFI shows up, then filter that cable. If you have RFI
on the first step, you will have to deal with your antenna and ground system.
The coax choke is real easy to make and it works well. And ALWAYS be sure
your radio, tuner, and all other components are well grounded.