Mike,
Radials function to intercept the fields from the tower's entire length and
return that current efficiently to the feedpoint at the base of the tower,
rather than have it return via RF lossy earth. To get omni response from a
tower, the radial system must be made up of equal numbers of radials in all
directions such that radial currents cancel out and the radials do not
radiate.
Thus, there is not good reason for radials to be higher than the tower they
serve. Now, if they were elevated above earth, and tended to function as a
radiator, because they did not completely balance each other out, then you
might be more concerned about their length.
Many handbooks show how the field lines come down in an arc from the top of
the vertical and intercept the radials projecting out from the base of the
vertical. From this you can see that the length of the vertical, (maximum
length for a multi band one), and the radial length are linked.
73,
Stuart
K5KVH
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