[Home] [Radio Page] [Web links] [Interests] [Location] [Personal] [Aerials] [1.933 PAGE]

Page 2


Check out your Transmissions!

First step is to check your transmitter. Connect to a dummy load and transmit using the same bands etc. If interference is still present it may be due to RF propagation down the mains, or radiation from the chassis of the transmitter (if in close proximity). Check grounding and shielding of the transmitter. Ensure the affected equipment does not use the same earth. Do not use a ground connection more than a couple of meters long (it may not be possible to earth your station if your are on the first floor or higher) and try to keep the wire close to ground and in the horizontal position. Be sure never to attach a ground to the mains earth or central heating pipes (which are normally earthed) as they can act as a long wire antenna.

Ensure that your antenna  system is not at fault, i.e. always feed a balanced system with open wire line or coax plus a balun. Try to ensure that long wires are fed at a point outside the shack and away from buildings so that high levels of RF are kept away from house wiring and domestic antennas. As a general rule transmitting antennas should be placed as far away from domestic receiving antennas as possible, particularly high gain VHF/UHF beam antennas. Check your antenna system for poor or corroded connections to prevent non-linear rectification. If coax is in use avoid higher loss (e.g. RG58) or poor quality types, as they can radiate signals particularly over long stretches. Maintain low VSWR on coaxial fed antennas.

Examine the susceptible equipment for correct installation, i.e. quality of coax on TVs and VHF broadcast receivers, coax plugs connected correctly etc. Often perished coax or poor connections are the culprit. However, NEVER start taking your neighbours electronic equipment to bits!

 Install a low-pass filter (for HF) or band pass filter (VHF or UHF) as required on your transmitter. Try to avoid leaving SWR/power meters in line as they can generate spurious emissions. If you need to leave a meter in line, place it ahead of the low pass filter. If a problem still exists it is unlikely to be harmonic radiation after a low pass filter has been installed.

Continuing to troubleshoot

The majority of interference  problems are not caused by transmitter harmonics. Most of the time fundamental overload is the culprit. If interference persists after ensuring the station is clean it is time to try remedies for the affected device.

If the affected equipment is a TV, radio, video etc. disconnect the antenna. If interference is still present it may be mains borne or entering the equipment directly. For combinations of TVs and videos etc. try with the TV on it’s own to begin with as the interference may be superimposed on the required signal through the video. Of course it is possible (and quite likely!) that the interference could be affecting the mains leads of both TV and video. If mains borne interference is suspected, install a choke filter on the mains lead (or a few turns of mains lead on a ferrite ring). If a TV preamp is installed, fit a high pass filter on both the input and output together with a ferrite choke on the power lead.


Further reading

ARRL RFI Handbook, if you can’t cure it by reading this book you never will!

Some types of filter

How to form a common mode choke

with a ferrite ring on a mains lead


Page 8 of 10


[Home] [Radio Page] [Web links] [Interests] [Location] [Personal] [Aerials] [1933 PAGE]