Topband: Magnetic Loops

[Tom W8JI]

If you test some popular loop antennas for pattern, which I have done, you 
will find many have significant skewing in the nulls. The nulls often are 
not 180 degrees apart. This proves the loop is in multiple modes, one of 
which (electric field probe) would involve mast and feedline radiation. This 
is true even with an exceptionally clean mounting and feed arrangement.

This occurs because the feedpoint design causes the loop to be sensitive to 
common mode on the element, the mast, and the feeder.

The null skewing (and differing depth in each direction) certainly doesn't 
mean the loops can never make some people happy with a performance change. 
It simply shows lack of proper feed design, and sensitivity to common mode 
signals on the element, feedline, and supporting mast.

As I recall, the skew in one loop on 160 was about 20-30 degrees, and a few 
dozen dB null depth difference. This means CM noise on the feeder can be 
abnormally problematic, because more than the antenna is the antenna!

This throws another wild card in the deck.

73 Tom