Dealing with Interference

Aaagh! Interference- the scourge of amateur radio! Irate neighbours banging on your door! It is said there are two types of hams; those who have no interference problems and those who get on the air! The following is a guide to dealing with interference problems.

How interference is propagated

Interfering signals are propagated in one of three ways:

1/ Direct radiation – the signal passes through space as an EM wave, directly into the affected equipment. This can be via an input port on the affected device (e.g. antenna socket) or directly into some circuitry not normally designed to pick up such signals (this can occur due to poor shielding within a device).

2/ Conduction –the signal is picked up by cables and conducted into the affected equipment. Examples are mains cables, telephone wires, cable TV coaxial systems.

3/ Induction – a nearby source induces a current into a susceptible device by magnetic induction.

You can see this effect by placing a live transformer (e.g. in a power supply) near a radio receiver. Not normally a problem for amateurs as the interfering source normally has to be in fairly close proximity to the affected device. However, compact magnetic loop antennas often used on HF are a potential source of interference by induction. Telephones are often quite prone to this route of interference

Causes of Interference

There are four basic causes of interference, two of which are always related to the transmitter:

1/ Fundamental overload – the desired signal radiated from the transmitter is responsible for overloading the affected device. In most cases the device has not been designed to reject a strong nearby transmission or is not functioning correctly. The vast majority of TVs for instance, do not have adequate filtering in their front end to reject strong unwanted signals.

2/ spurious emissions- undesired signals radiated by the transmitter such as harmonics or parasitic oscillations (both of these are often seen in poorly constructed or badly tuned transmitters). Spurious emissions may also be generated externally to the transmitter by non-linear rectification. Non-linear rectification is a curious effect that results in new spurious signals. For instance a corroded connection on a TV antenna can act as a semiconductor junction producing unwanted mixing products. This effect is also known as the ‘rusty bolt effect’, with rusty garden fences etc. sometimes generating TVI in the presence of a perfectly clean amateur radio signal!

3/ Noise – both electrical and from RF sources, have the ability to cause interference. Not normally associated with a transmitter. Noise can also be generated within the affected equipment.

4/ Internally generated signals – sometimes a malfunctioning device may generate unwanted signals internally that may on their own cause interference, or after mixing with the transmitter signal.

Are you the cause?

Before you begin to trace the source of the problem you need to be sure that your transmissions are responsible. This may sound obvious, but there may be many cases where a neighbour may attribute some kind of interference to your transmissions i.e. electrical noise, unusual atmospheric conditions.

In the first instance the person(s) experiencing interference should make a note of the time and date(s) that interference occurs. From your logbook you will be able to tell if the times the interference occurred coincide with you transmitting. This is of course the main reason why hams are legally required to keep logbooks. and it is in your own interest to ensure you fill in your station log every time you transmit.

If you have established that your station is the source of the interference the next thing to do is to make a note on which bands, modes and power levels interference is experienced. It is often useful to have a friend assist you (preferably a fellow ham) who can monitor the interference or operate your station. Of course such an approach will require your neighbour’s consent. You should also establish what equipment is being interfered with, or if more than one device is susceptible (i.e. TV, video, HI-FI etc.)

In the first instance you should examine the installation of the affected equipment. For instance if it is a TV or radio tuner, examine antenna leads and plugs to ensure they are in good condition and that there is no corrosion present. If possible, also check out any antennas attached to the equipment, looking for corrosion and bad connections. Check out mains leads (when not plugged in!), speaker leads etc. for bad connections, improperly installed plugs.

If the interference is to a TV or radio, ensure that the device is receiving a strong enough signal; is the picture (or sound for a radio) of good quality (devices receiving weak signals are much more prone to overload from a nearby amateur transmitter)? Check to see if a TV preamp is installed, as these devices are notorious for poor immunity o breakthrough. For a Hi-Fi , check for hum on the speakers (which may indicate a ground loop due to improper installation)?

If your neighbour’s equipment does not appear to be installed correctly (e.g. he is using a TV antenna, which has been up for tens of years with coax that has perished) then it is possible that this is responsible for the interference. However, although incorrect installation may be the problem you should ensure that your own station is ‘clean’.

Does your neighbour get TV a picture like this?