[TowerTalk] Perfect Field Day beam?
Joe Subich, W8IK/4
Tue, 4 May 1999 20:05:44 -0400
> We seem to be going around in circles. If different components of the
> radiated signal arrive at a distant point with equal and opposite phases,
> the net signal at that point is zero, right?
This is the very essence of beam formation and the mechanism responsible
for antenna gain/pattern. If an antenna has a pattern, ie. it radiates
other than equally in all directions, it has gain.
> I don't think it violates the conservation of energy to argue that some
> antennas may be more prone to this sort of cancellation than others.
Certainly, different antenna designs have different patterns and thus
more or less gain. If as previously stated however, an antenna shows
a F/B ratio of about 13 dB over a 90 degree sector it must have a forward
gain of 3 to 4 dB over a similarly situated dipole. The only possible
reason for a lower total radiated field from one antenna than another
given identical levels of applied power at the feedpoint is a lower
current in one antenna than the other. The lower current can be due to
a higher radiation impedence (not the same as feedpoint impedence) or
higher loss resistance.
> Moreover, why isn't it plausible that such cancellation effects could be
> more prevalent in the forward field than to the rear?
Such cancellation effects would effect only the shape of the pattern
(F/B, F/S, etc.) and not the total radiated field. Antenna "gain" is
like squeezing a baloon ... what one moves from one direction pushes out
in some other direction. It is the antenna designer's task to see than
the "air" is is pushed in a useful direction.
... Joe Subich, W8IK/4 ex-AD8I
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Administrative requests: towertalk-REQUEST@contesting.com