Topband: RX Loops - Some relevant facts

K8LV1 at K8LV1 at
Wed May 12 16:03:42 EDT 2004

Note: you can skip the following if you have had one good course in EMF 
theory. Otherwise, read on:

Consider a small loop (e.g. length=.1lambda) oriented with its plane parallel 
to he incoming wave direction, which would normally be, at least, 
approximately true. When illuminated by an incoming plane wave (i,e, the far-field of 
someone's TX antenna) the voltage induced in the loop by the 'electric component' 
of that plane wave simply equals the line integral of the E field (which is 
constant over the loop surface) over that surface. Since it forms a closed 
loop, and there is no phase shift along its path, that line integral is zero and 
there is NO induced voltage.

Now consider the voltage induced by the 'magnetic component', which produces 
a uniform flux of B (equal to mu-zero time H) across the area of the loop, 
when the loop is exactly parallel to k (multiply by cos-theta if not). This will 
induce a voltage equal to 2PI x F X B X loop area, and that is the received 
signal - plain and simple (to the first approximation level).

This is the basic physics of the small loop as taught to every EE sophomore. 
It clearly shows that the small loop "responds' primarily to the magnetic 
field, and for this reason, most field theory people refer to small loops as 
'magnetic dipoles.'  A short length of single straight wire, a/k/a an electric 
dipole, responds primarily to the electric field (the integral of E is NOT 0 along 
the wire). The distinction is very clear.The comments made by W8JI to the 
contrary are seriously in error. Unless he has some magic concept hidden in the 
term "field dominant", which to me is  vague and not a standard term within EMF 

Pick up a copy of "LINEAR ANTENNAS", by RWP King, which is THE authoritative 
reference on wire antennas, and you will find the term "magnetic dipole" 
appearing hundreds of times - always as a synonymn for the "small loop".

Eric von Valtier K8LV

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