[RFI] Link-coupled loop - more

[Roger Parsons]

I used to take part in 160m DF contests in a previous existence. Lots of tricks were used to make the competitions more difficult, and one of the favourite ones was to place the hidden stations very close to power lines. These can confuse bearings horribly - including making them 90 degrees from true. Your noise is almost certainly being re-radiated from a power line or other overhead conductor even if it originates within a building.

When you are DFing your noise you should make sure that you are as far away from any overhead conductors as possible. If you get ambiguous bearings go further out from your suspected site to a clear area - this is really important and does help a lot.

It is often helpful to have a sense antenna as well as the loop. When an omnidirectional antenna, such as a whip, is combined with the loop and with the correct relative amplitudes then a cardioid pattern results. 

I think that the major benefit of a shielded loop (and a balun) is that the two nulls will be closer to 180 degrees apart. With an unshielded loop it is harder to maintain balance and so the nulls may have several degrees of difference between the two directions.

I use a 'Suunto M2' compass which is about 2" diameter and can be directly mounted on a loop if necessary. It has an adjustment to correct for magnetic deviation and can easily be read to 2 degrees. Got it quite cheaply from a sporting goods store.

It has often been pointed out on this forum that it is highly desirable to use the highest frequency possible to track down noise sources - VHF is best if the signal can still be detected there. Relatively low frequencies such as 80m are horrible in built up areas just because of the number of conductors around.

Sorry if this has all been mentioned before, and good luck in your hunting.

73 Roger
VE3ZI