Over that range, the equivalent series capacitance changes from 59pF
at the low end to 138pF at the high end, and at least by NEC2's
prediction, the impedance changes especially quickly around 51MHz--
both reactive and resistive parts. 50.75MHz: 10.3-j31.76; 51MHz: 3.91-
j22.56, quite a large percentage change in 250kHz. Having the
effective series capacitance change that quickly will cause the
matching network to behave very differently than it would with a
capacitance element that is fixed.

That is exactly the point! It would not be correct to calculate bandwidth on the
basis of the Q factor at resonance and assuming that the capacitive antenna
reactance is equivalent to that of a fixed capacitor.

Today I have discovered another shortcoming of that antenna. After raining cats
and dogs, the antenna resonant frequency gets lowered by about 130 kHz due to
the influence of the wet terrain. That is really a lot if you consider that,
after making very accurate measurements with a Bird wattmeter, the antenna
bandwidth is only 100 kHz at 1.4 SWR!

I am considering to re-build the driven element for 50-ohm match, by using a
longer driven element and a 1:1 balun. However it will not be easy to find the
optimum situation because there are two variables to be adjusted, that is the
driven element length and the hairpin length.

Also, I am not too sure on to which extent using a longer driven element would
influence the antenna radiation pattern.

Any comment?

73

Tony I0JX