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

[Steve Hunt]

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 
cancelling 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.

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 :)

73,
Steve G3TXQ