Dealing with Interference

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 SWR 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.



If the affected equipment is a HI-FI check to see if the interference is still present while using tape or CD etc. For Hi-Fi separates try the amplifier on it’s own, and then with CD, Tape etc. to determine if the interference is getting in through the amp or associated devices. The interference may be entering the equipment via the mains, speaker leads or directly. A choke filter can be installed on speaker leads and mains cable.

If the signals are entering any equipment directly you may have to advise your neighbour to consult a qualified service technician. Alternatively you might try to relocate your antenna as far away from the affected equipment as possible or reduce your power levels.

For TVs and videos (or radio tuner on a Hi-Fi), if disconnecting antenna leads to the affected equipment cures the problem then you should install a filter to prevent your transmissions from entering the equipment via this route. For TVs a high pass filter should be installed. For VHF broadcast radio a band pass filter will be needed.

Direct Pickup

If you have ascertained that interference is not entering the affected equipment by the mains or input/output ports (antenna socket, speaker leads etc.), it may be possible that signals are being picked up directly by the internal circuitry or wiring of the device. In such cases it will be necessary to add more screening inside the device. Modifications should be carried out by a fully trained service technician, who is thoroughly familiar with the device in question.

Weird effects

You may often find that interference is an unfortunate combination of variables. Your transmissions may be entering the equipment by more than one route. You will need patience and a logical approach to the problem. Also be prepared for the unexpected! I heard of one guy who found that moving a lamp off the top of his neighbour’s TV cured the problem without filters or anything! It would appear that the lamp might have been acting as a non-linear rectifier, I even heard of one guy who heard voices coming from his electric oven when it was switched on, obviously another case of non-linear rectification!

Further reading

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

choke.JPG (7414 bytes)

How to form a common mode choke

with a ferrite ring on a mains lead


home page