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

[Dr. Gerald N. Johnson]

On Sat, 2008-06-07 at 06:37 -0700, Jim WA9YSD wrote:
> Common Mode chokes built-in on every orifice  on the radio would work all so.
> 
> Maybe tomorrow I will visit that site.
> 
> Still 3 weeks and just hearing South America and a little of Europe with that big of improvement eliminates the short and long path theory.  No really good band openings.  Then along with the band conditions the way they were and the way they are I think thats a very very remote chance.
> 
All the nuances of a dipole and its feed won't do nearly as much about
opening up a band as a serious antenna array. Like one of the several
20m arrays at N0NI not far from here. A 200' rotating tower with 4
pieces of 4 element yagis stacked vertically. Its been reported that
such an antenna opens the long paths hours before the single yagi and
dipole antenna stations are heard overseas. That big array gets lots of
ERP, (maybe 14 dBd gain) at a low radiation angle, neither can be
accomplished with a dipole or two.

> Strange couple between the two antennas?  I moved the ends up and down spread one end  from parallel to 30 degrees and no change.  Not enough to disprove that theory?  Maybe?
> 
> Doc, your capacitor at the feed point with low antennas did not 
> work that well. 400 KC band width?  We tried 75 ohm cable, we 
> tried some high voltage 75 ohm stuff that I never seen before 
> that was used for a W2DU balun that had those small beads on it. 
>  We got a hold of some of that cable.  We also just tried a cap 
> across the feed point.  Thought we had all the right measurements. 
>  If what you say is true,  we missed a trick along the way some 
> where.  We had a loop of wire about 6 inches, I think we had a 
> 1000pf at 1KV cap.  Or was it a TV HV cap off the fly-back?   
> May be not enough?  One of the guys tested the combo at work. 
>  Been too many years to remember it.

That capacitor can't be alone. It has to be parallel resonated with a
coil or coax stub. I used a coax stub for better shielding. In my
antenna the wire was antenna was resonant at 3750 as I recall, and the
parallel tuned circuit with 2000 pf was resonant at 3850. That gave an
impedance that was a part of a circle centered away from 50 ohms and so
I added 5/16" wave of 75 ohm coax as a transformer (longer than a
quarter wave because the circle wasn't centered on a resistive value but
was partly reactive on the Smith Chart). Then it was found by users that
it worked better with a 50' length of 52 ohm coax feeding the 75 ohm
transformer. Then it gives a SWR of about 1.3:1 from 3.6 to 4.1 or a
little higher, never better than 1.3:1, not much worse until going
beyond that band, then the SWR rises very rapidly. I ascribed the need
for added coax to coupling from the antenna to the feed line, but it may
also come from the basic dipole being draped over house and garage, not
in free space or not even 1/8th wave above ground. One of those is near
and I might be able to run a SWR curve at the feed points, its been in
use at least 25 years.

Its probable that there is a better choice of antenna and compensating
circuit resonant frequencies that would dispense with the need for the
75 ohm feed line, and likely a better choice of capacitor to give a
smaller circle and so a lower SWR across the band. When I created and
computed the antenna, I had only slide rule and Smith chart, no NEC
computer program, and not much of a good network analysis program if
any. I had worked with network analysis programs but I think I had
graduated by the time I did that antenna and had lost access to that
mainframe and programs that worked with it. With vacuum tube
transmitters, I felt a 1.3:1 SWR was plenty fine and I quit the
development when I achieved that performance from 3.6 (80 m RTTY
frequency and past 4025 (Iowa MARS frequencies).
> 
>  Keep The Faith, Jim K9TF/WA9YSD
> 
73, Jerry, K0CQ