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.
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Reg, G4FGQ