"Reg Edwards" wrote in message
...
Antenna bandwidth is usually expressed in terms of the limits of SWR such
as
1.5 to 1, or 2.0 to 1.

It can be calculated, or otherwise derived, on the assumption that at some
frequency in ONE band the SWR is EXACTLY 1 to 1.

But it never is exactly 1 to 1. The transmission line impedance Zo is
never
exactly equal to the resistive component of antenna input resistance at
resonance. It is usually well away from it. So in practice the actual SWR
bandwidth is always broader than the predicted, or stated, or claimed as
the
best possible value.

Purchasers should be wary of curves provided by antenna manufacturers (or
in
published articles) purporting to show SWR versus frequency which have a
perfect match to the line at some central point along a curve.

Although it should be stated there is much more importance attached to SWR
at HF than is warranted by the practical effects of SWR on system
peration - particularly in these days of almost universal use of antenna
tuners.

Perhaps undue importance arises because the SWR meter is the very last
indicating instrument to be found associated with transmitters. It is
quite
reassuring to see a needle jumping about as one gabbles into the mike.

But will the ubiquitous SWR meter eventually disappear as the automatic
tuner becomes the vogue? That will be a sad day.

Incidentally, sorry to be so disappointing, the SWR meter does NOT
indicate
SWR on the transmission line anyway.

I'm on Spanish red tonight.
----
Reg, G4FGQ

Hi Reg,

That's good to remember, thanks. When setting up a specific frequency dipole
(intending no tuner required for two frequencies, the main dipole and a
shorter dipole "fan" under it), the only logical place to test with the
bird-meter seemed to be at the transmitter output. Certainly that swr
changes as it gets to the radiator, as you say. But once the antenna was
trimmed to as near perfect swr as possible, what else would you do? Where
else would it be meaningful to measure the swr and best power output, and
then make wire length changes accordingly? It seems to work wonderfully as
trimmed to 1:1 (best you can read on the tuner anyway) from the tx output.
Both the bird and the MFJ-962D and tx meters all seemed to agree they liked
that setting.

73's

Jack
Virginia Beach Va