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
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