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
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