[TenTec] Bazooka antenna.. More than you wanted to know!

[Dr. Gerald N. Johnson]

On Thu, 2008-06-05 at 03:41 -0700, Jim WA9YSD wrote:
> It is a wounder that these antenna companies has not made an HF yagi 
> with a folded dipole or a DB antennas as the driven element.  They all
> ways come up with some thing strange.  Must be too expensive.  The DB
> and FD has the best coupling factors that equals 1 or 0.9.  A dipole is 0.1 . 
> Now one would conclude that if you had wide spaced elements on a yagi
> that the DB and FD would be used.  You think?

Some VHF and UHF beams have used a folded dipole driven element. Many
use a T match or some other scheme. The choice is generally based on the
loading of the driven element by the directors and reflector. The best
condition is when the folded dipole has a feed Z of 200 ohms so a 4:1
coaxial balun can be used. That only works when the director/reflector
coupling gives a dipole center Z of 50 ohms. In most computer optimized
yagis, the dipole Z is lower or much lower than 50 ohms and the 4:1
transformation doesn't result in a convenient Z at the folded dipole,
like 50 ohms or 200 ohms. There the T, Gamma, Betta, or some form of
stub match is often used to step up the impedance to something
conveniently matched to a 50 ohm feed line.

Then for a multiband beam, I don't know of a folded multiband driven
element while traps work on a dipole.

There is no such difference in coupling between a folded or straight
dipole. And no good mechanical way to make a double bazooka to be
supported only in the middle. Besides the double bazooka is a sham
antenna anyway. It doesn't do anything for radiation or reception with a
decent receiver.
> 
> Strip the braid off of some RG8X and slide the center piece down the inside 
> of a driven element of you favorite yagi and tune the driven element and see what happens.
> 
That's a feed that works, but isn't necessarily weatherproof or broad
banded.

> So many people have computers these days, and antenna programs, that 
> they just do not go out side and experiment.  The Wheel has already 
> been invented folks.  It just needs a new groove to make it work better.

Sometimes the new wheel has been found to be faulty. I know I've used
EZNEC to invent a new antenna, but found the computation to be
inconsistent and the freshly built antenna to work far poorer on the
antenna range than the most optimistic computation. These programs
handle bends poorly, and bends that aren't at right angles worse than
that. The computation results depend the number of segments per piece of
a bent or straight element.

On segmentation, the originators of the NEC and MININEC programs
recommend starting with a low number of segments and note the results,
then increase the number of segments keeping track of the results, and
hope for the results to plateau. The expectation is that the results are
best along that plateau (gain or f/b vs number of segments). But I have
found that the plateau isn't always asymptotic to a great result as the
number of segments are increased further that the result wanders off
from that plateau. Yes, EZNEC and ELNEC compute minimum and maximum
segment numbers, but they don't take into account complications of the
antenna structure for all possible cases. Using NEC and MININEC is more
off an art form than an absolute computation. The results computed
aren't for sure good until the antenna has been built and properly
measured and even then there can be a dB or two of uncertainty.

Programs like YAGIOPT are based on simpler computations than NEC and
work only fair for single band yagis. Their computation is based on a
formula in Kraus (W8JK) original antenna text book that is only a good
approximation for PARALLEL elements very close to HALF WAVE RESONANT.
The program and the formula will give an answer for an interlaced two
band beam, but it won't work that way on the antenna range because that
answer is WRONG because the second band elements aren't near to half
wave resonant on the band being computed. But the simpler computations
are fast because they are simple.

There are other fundamental zits in the NEC family, but I'll leave those
to other times because I appear to be alone in the world calling them
errors.
> 
>  Keep The Faith, Jim K9TF/WA9YSD
> 
73, Jerry, K0CQ