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

[Dr. Gerald N. Johnson]

On Tue, 2008-06-03 at 11:27 +0100, Steve Hunt wrote:
> Jerry,
> 
> Spent a few more hours this morning investigating the 
> "parallel-tuned-circuit in parallel" option using a spreadsheet.
> 
> I learned a couple more things:
> 
> 1) Even when the tuned circuit does a reasonable job of tracking and 
> canceling the reactive component of the dipole's impedance, because it 
> is a PARALLEL compensation mechanism the resistive component gets 
> changed significantly - just like in the Beta Match. For example, 120KHz 
> below resonance the dipole's impedance was 67-j56Ohms, an admittance of 
> 8.7+j7.3mS (VSWR=2.61:1). The tuned circuit combination of 5730pF and 
> 0.315uH introduced a parallel component of -j8.9mS, resulting in an 
> aggregate 8.7-j1.6mS. At first sight this looks a pretty useful 
> cancellation of the reactive component, but this admittance translates 
> to an impedance of 111+j20, so although we've reduced the reactive 
> component we've also increased significantly the resistive component. 
> The resulting VSWR is improved ( to 2.32:1), but perhaps not by as much 
> as might first seem likely.

Look at my other post. Plot those resulting values on a Smith chart and
then transform them through a 5/16th wave length of 72 ohm coax and look
at the result then.
> 
> 2) As I mentioned in my previous posting, it seems to be the losses in 
> the short-circuit coax stub that are responsible for a majority of the 
> bandwidth improvement. At 3.5MHz, the losses in 3m of RG58 represent a 
> resistance in parallel with the feedpoint of about 145Ohms. No surprise 
> that a 145 Ohm resistor across the feedpoint improves the VSWR bandwidth 
> and takes the VSWRmin down to an impressive 1.03:1 :)

I assumed no losses when I computed the design, long before any NEC
programs with just a table of dipole impedance, a calculator or slide
rule, and a Smith chart.
> 
> 73,
> Steve G3TXQ
> 
73, Jerry, K0CQ