[Antennas] Antenna length measurements

[Robert Lay]

Dear Buck, et al,

This is a copy of what I erroneously sent only to Jack.

I would like to add a few words about one point that Jack has already
discussed at length - and that is the issue of "resonance" of the dipole
itself.

It should be understood that operating a Herzian (center fed, balanced,
horizontal) antenna  at ANY frequency produces different radiation
patterns in both horizontal and vertical planes which are essentially
dependent on height above ground and the length of the antenna - both in
relation to a wavelength.

Now what all that translates to in terms of resonance is no more that to
say that a particular radiation pattern is associated with the length =
1/2 wave (the resonance frequency).

The only other consideration is the reactive term of its input
impedance, which happens to vanish at many different frequencies, all of
which are harmonically related. That, however, is only of importance
regarding the match between the antenna and the feedline and the
resulting SWR, which can certainly affect efficiency and the actual
amount of power reaching the antenna in relation to the maximum amount
of power that can reach the antenna.

So, in essence, what I am saying is that provided you get a sufficiently
good match to be able to realize a high percentage of the power getting
TO the antenna, the issue of the antenna then being at resonance is only
an issue in regards to the radiation pattern. Even tho an antenna has a
slightly different radiation efficiency at different operating
frequencies, it is essentially not a factor worthy of concern in
comparison with the greater concern over SWR and mismatch losses.

So, in even more succinct terms, it should not matter whether your
dipole is as much as 10 or even 20 % off resonance, except for the
rather subtle pattern changes and the less subtle effects on SWR and
feedline efficiency.


Bob Lay (W9DMK), Dahlgren, VA
w9dmk at crosslink.net
http://www.qsl.net/w9dmk