On Apr 8, 12:31 pm, "Antonio Vernucci" wrote:
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

Though the Q calculation doesn't give the right SWR bandwidth for the
antenna/matching system, it does tell you that (with such a low loaded
Q), it should not be difficult to make a hairpin or even standard
helical coil inductor that has low enough loss that you can ignore the
effect.

I believe that the physical length of the driven element in a Yagi is
much less important than the tuning and spacing of the parasitic
elements. The question becomes something like this: what is the
relative amplitude and phase of the current in each parasitic element,
for some excitation of the driven element? A Yagi is a system of
coupled resonators, like a system of coupled pendulums. If one of the
pendulums is driven at a particular amplitude and frequency, even if
it's not that pendulum's natural frequency, the rest of the pendulums
will follow along pretty much the same as if the driven pendulum was
tuned to have that natural frequency. In the antenna, the difference
will only be in the coupling from the driven element to the others,
and I believe that changes only slightly as the length of the driven
element changes.

But I may be wrong about that, and await my re-education. ;-) But I
just ran EZNec on the example "NBS" 3-element 50.1MHz Yagi, varying
the nominal 110 inch long D.E. by +/- 10 inches, and saw the expected
fairly large variation in impedance, but only 0.02dB change in gain
over that whole range, with similarly small variation in F/B ratio and
beam width. The longest D.E. I ran was also the highest gain (by that
tiny amount), and provided enough inductive reactance that the
feedpoint could be tuned to resonance and present 200 ohms by shunting
with about 55pF capacitance. Next to try: compare the SWR bandwidths
of the hairpin (inductive) shunt of a shortened D.E. and the
capacitive shunt of a lengthened D.E.. Unless someone offers a better
test case, I'll use the NBS 3 element Yagi...

Cheers,
Tom