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Radio Direction Finding and Sense Antennas... Oh and the Equivalence Principle

For what's going on with research, scroll down.  But first, another study topic for the amateur radio extra class exam.  One of the questions asks what the purpose of a sense antenna is.  The correct answer is that "It modifies the the pattern of a DF antenna array to provide a null in one direction."  Whatever that means.  The DF referred to in the answer is direction finding.  Small loop antennas, like the one shown in the picture to the left (picture 1), can be used to find the direction that radio signals are transmitted from.  By rotating the loop until the received signal becomes the weakest, you can locate the direction.  There's only one problem, you can't tell whether the signal is coming from in front of, or behind the loop.  This is where the small vertical antenna in the picture comes in.  That's the 'sense antenna' or 'sense aerial'.  Its signal is added to the signal of the loop antenna and the net result is that it provides a uni-directional antenna pattern, (see the second picture).  I hope to be back soon with lots of cool information on exactly how it does it.

A little History
The first radio direction finder was invented by John Stone Stone an American electrical engineer and physicist.

Research Blog
I'm reading through several papers on the equivalence principal, which I used to think said that you can't tell the difference between a gravitational field and say a centrifugal force[2].  It looks like, at least at relativistic speeds, I may have been wrong.  There's been quite a bit written on this[4][5], and it's taking me a while, but I'll keep you up to date.

1.  Awesome listing of direction finding radios

2.  Equivalence principle

3.  Ham radio study exam

Muñoz G. & Jones P. (2010). The equivalence principle, uniformly accelerated reference frames, and the uniform gravitational field, American Journal of Physics, 78 (4) 377. DOI:
open access version:

Rohrlich F. (1963). The principle of equivalence, Annals of Physics, 22 (2) 169-191. DOI:


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