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

"Antonio Vernucci" wrote in
:

....
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).

Tony,

Some thoughts.

You are suggesting that it is easier to make a low loss capacitor that is
located at the feedpoint in a hostile environment, than it is to make a
low loss inductor (the hairpin).

Just as the hairpin is a s/c stub for inductive reactance, you could use
an o/c stub... but remember that transmission line elements are a path to
low Q reactors, use thick conductors for the transmision line (which for
an o/c stub will need to be much longer than for the s/c stub).


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.

Only hundreds?

Hy-Gain have used this feed system on 2m antennas for a very long time.
Yes, their gain figures seem a bit generous, but the hairpin is a viable
commercial option.

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 gut feeling is that optimised long Yagis have narrow bandwidth because
of the large number of elements with role that is frequency critical. A
short Yagi has wider bandwidth with the same feed system.

It is a long time since I read your first post, but narrow bandwidth can
be an advantage. It reduces out of band signal reach your first amplifier
where it will mix and produce IMD products that may be in-band. Narrow
band antennas help to provide the selectivity that is lacking in many /
most modern radios.

Owen

You are suggesting that it is easier to make a low loss capacitor that is
located at the feedpoint in a hostile environment, than it is to make a
low loss inductor (the hairpin).

What I was maning to say that is easier to determine the correct capacitance
(just using a variable capacitor) rather than the correct inductance (using a
sliding short on the hairpin). Once one determined the correct value, the
variable element should anyway be replaced with an equiavalent fixed element

Only hundreds?

Hy-Gain have used this feed system on 2m antennas for a very long time.
Yes, their gain figures seem a bit generous, but the hairpin is a viable
commercial option.

Well, this is an antenna just for 6-meter enthusiasts fabricated in Italy, and
there are not too many of them around here.

It is a long time since I read your first post, but narrow bandwidth can
be an advantage. It reduces out of band signal reach your first amplifier
where it will mix and produce IMD products that may be in-band. Narrow
band antennas help to provide the selectivity that is lacking in many /
most modern radios.

yes, but the bandwidth is so tight that it barely fits my needs. Also the SWR
response shift when it rains is a problem to which I have no solution

73

Tony I0JX