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

[Dr. Gerald N. Johnson]

On Sun, 2008-06-08 at 09:16 +0100, Steve Hunt wrote:
> 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 
> modeled 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.

Its great that the computer agrees with my design. If I recall
correctly, the 75 ohm line theoretically needed to be 56 feet long in
solid polyethylene dielectric, but users built it (with my suggestion)
using 50' because that was a standard length of a roll on the
distributor's shelf.
> 
> 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.

For a fixed length radiator, the resistive component of the conductance
increases as you go lower in frequency (e.g. parallel resistance
decreases), so with a shorter radiator the uncompensated impedance might
be lower and the transformation of the parallel tuned circuit might just
move it up to the 50 to 60 ohm range. However the antenna radiation
depends on the wire current times wire length and I doubt the shorter
radiator will have as much of either so it won't be as effective an
antenna.

The book I found later to cover the topic fully, "Very High Frequency
Techniques" used a series resonant circuit in the feed line probably to
do some impedance transformation and to gain further reactance
compensation for an even greater SWR bandwidth over the parallel tuned
circuit's bandwidth improvement.

Finding that work showed me a again that its really hard to invent a NEW
wheel.
> 
> 73,
> Steve G3TXQ

73, Jerry, K0CQ