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

[Steve Hunt]

Jerry

> 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. 
>
I reckon you did a pretty good job without access to NEC. I just 
modelled the arrangement using EZNEC and tried a lot of different 
combinations of  L, C and the length of the 75 Ohm section. At best I 
beat your 2:1 VSWR bandwidth by only 40 KHz. My final values were: 
C=1575pF, L=1.14uH, length of 75 Ohm section: 920" (electrical). All of 
that was without any loss built into the model - if you include losses 
the bandwidth improves again of course.

I can find no way to dispense with the 75 Ohm section. The parallel 
compensation always has the effect of increasing the Resistive component 
of the impedance, and then you have to have some transformer section 
with an intermediate-value Zo to get back close to 50 Ohms.

73,
Steve G3TXQ