Jerry,
Very interesting stuff! I had not looked at your "parallel-tuned-circuit
in parallel" option before; I just modelled it in EZNEC to learn more
and saw some interesting results.
I took a dipole constructed of 128ft of #14 copper wire. It resonated in
Free Space at 3.745MHz, where it exhibited a resistance of 74 Ohms. It
had a 2:1 VSWR bandwidth of 150KHz.
Then I looked to see how the Susceptance changed for a small frequency
shift, and calculated the component values you would need in the
parallel tuned circuit to compensate. I was surprised how big a C was
needed (5,730pF) and how small the L (0.315uH). I tried that combination
in EZNEC and the VSWR bandwidth improved to 185KHz. What was interesting
was that, although the reactive element was being compensated quite
accurately, the presence of the tuned circuit caused the
series-equivalent Resistance to vary more quickly with frequency than in
the un-compensated case. So the bandwidth improvement was not as great
as I'd expected.
Next I tried your 2000pF figure and the 3m (lossless) stub. It didn't do
quite as well as my values, so I doubled the capacitance and shortened
the stub to compensate. The bandwidth was then back up to 185KHz.
Here's the interesting bit ..... only when I started to introduce some
losses into the stub did the bandwidth increase really significantly.
Once I put "real world" RG58 loss numbers in, the bandwidth shot up to
400KHz. But at that point the radiated power was down by 2dB!
Using RG213 loss figures produced a bandwidth of 305KHz and a drop in
signal of 0.9dB
I guess only individual ops can say whether these losses are prices
worth paying for the convenience of "tuner-free" operation.
73,
Steve G3TXQ
|