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


Hi Tom,

the results you got on EZNEC are encouraging. Nevertheless I would not like to
try using a lengthened element in conjunction with a capacitor, as the
difference between that configuration and the original configuration would be
the maximum (although it would be much easier to adjust a capacitor than the
inductance of an hairpin).

What puzzles me is that the antenna manufacturer reported me having sold several
hundreds of those antennas, and no one has reported him the bandwidth being too
narrow or the exagerated wet terrain influence.

I am not sure on what I am going to do, also because I am not 100% sure on
whether the bandwidth problem is only due to the matching system, or it is also
due to the particular antenna design.

My original intention was to compare this 50-MHz long Yagi antenna (32-foot
boom) against a smaller antenna (11-foot boom) I have on another tower, so as to
determine how much a bigger antenna really helps during multiple-hop sporadic
openings to US and Japan.

Probably for the forecoming sporadic-E season (May-August) I will leave things
as they are, and just try to assess the practical advantages of the bigger
antenna. After that I will see what I shall do.

Thanks very much for the useful discussion.

73

Tony I0JX