Tracking Down RF Interference
Page content last updated Nov. 11, 2011
Copyright © 2011 Larry Benko, W0QE
RF Interference is generally where an inadvertant radiator (computer, power line, appliance, etc.) produces RF energy which interferes with a device (receiver) intended to receive RF energy. These inadvertant radiators produce low power levels but the received signals may be large due to the close distance from the radiator to the receiving antenna. However these signals are generally not large enough to cause receiver blocking or intermodulation distortion. Another form of RF interference is true overload, blocking, etc. which is produced when multiple deliberate RF sources are located in close proximity such as a contest station or a neighbor who also an amateur radio operator.
Interference from a deliberate radiator (licensed transmitter) to a device (TV, stereo, computer, etc.) is generally defined as RF susceptability which is a weakness of the device in the presence of the RF signal.
Click on picture for a larger image.
Power lines radiate by an arc occurring somewhere and then radiating on the connected wires. Appliances and computers radiate either from within the device or due to common mode currents flowing on the wiring connected to the device. For most devices with the exception of large TVs and large appliances the devices themselves are not large enough to efficiently radiate at HF without common mode current flowing on some/all the connected wiring. The antenna factor due to the connected wiring varies greatly with frequency and the frequency where interference is heard may vary due to the installation. For example, an untuned 20m dipole is a very inefficient antenna on 40m. Likewise the connected wiring to an inadvertantly radiating device will radiate better on some frequencies than others.
Due to these variables it is very hard to ask questions like, "What could the S8 40 meter interference I hear be due to?" and get an answer that is helpful in any way.
Interference is almost always tracked with the tracking receiver in the AM or SSB mode since virtually all interference contains amplitude variations which are suppressed if the receiver was in the FM mode. Power line noise which is very broadband is usually tracked at higher frequencies such as VHF since antennas have sharp patterns and it is much easier to pinpoint the source. Sometimes parabolic dish acoustic detectors are used.
However most other interference needs to be tracked at the frequency where it is an interference issue. This frequency is usually too low to have a omni-directional antenna with a narrow pattern. Small loop antennas have been used for this purpose for years with good results in the hands of nearly anyone who is willing to spend some time learning how to track down interference.
The picture at the right shows a loop built on a piece of PVC tubing by Tom, W0IVJ. He regularly uses this loop which can be tuned from 3.5 MHz thru 15 MHz. The loop is peaked via a compression trimmer capacitor and a small switch which adds additional fixed capacitance to achieve resonance. Tom and I have practiced tracking down numerous deliberately hidden RF noise souces in order to understand the antenna use. Additionally a lot was learned by coupling an RF signal source to the house wiring and tracking it down. Tracking down interference takes less than an hour and is easier to track down than it is to complain about it. It does take some up front practice but not much more.
How the Antenna Works
Small loop antennas are bi-directional in nature unless a sense antenna is coupled into the loop. The use of a sense antenna complicates things since the amplitude and phasing of the sense antenna relative to the loop varies with frequency reducing the ability of the loop to be used on more than one small frequency range. Therefore most small loop antennas used to track interference do not use a sense antenna which in practice is only a minor inconvenience.
A small loop antenna, high above the ground, oriented vertically and fed on the bottom will have a figure 8 pattern to both horizontally and vertically polarized signals. However the loop has the null to the horizontally polarized signals in line with the loop and for vertically polarized signals the null is broadside to the loop. This would appear to make the loop worthless for tracking interference. However we want to use the loop by carrying it (about 4 ft. above ground) and we are concerned with signals at an elevation angle of less than 5-10 degrees above the horizon. Taking this into account the loop pattern is still at right angles for vertical and horizontal polarization BUT the sensitivity of the loop to horizonatal polarization is now more than 20dB weaker than to vertical polarization. Thus the loop appears to ignore horizontal polarization for distances to the interference of more than about 25 ft. The signal peaks inline with the loop and is minimum broadside to the loop.
If you wish the loop can be turned so that it is still oriented vertically but the feed point is on the side. Now the loop still basically only sees vertical polarization and still has nulls broadside to the loop and peaks inline with the loop.
Tracking Down the Interference
There are two ways to track the interference:
First method is to track signal maximums. Start by taking a bearing and guess which direction to go. Walk in the chosen direction and if the signal increases you probably guessed correctly. Otherwise if the signal decreases then reverse your direction. This method works better if the receiving radio has a signal strength meter.
Second method is to use the signal minimums. This method works better if the signal is stronger. Start by finding the direction of the nulls. Then walk in a direction at right angles to the previous direction of the nulls and make another measurement. If you walked far enough relative to the distance to the source of the interference the two null lines should converge in one direction. That will be the direction to the source of the interference.
It is almost easier to put out a signal from an RF signal generator and practice tracking it down than it is to explain!