Owen Duffy wrote:
I played around a bit using the feedpoint impedances that I modelled
for my "Feeding the G5RV" article. With 31' of 554, I needed about
2000pF to "tune" it for low 50 ohm VSWR at 3.6MHz.

I'm just reporting what it took for my actual antenna under the
existing conditions at my QTH. The cap is actually 950 pf for
a minimum SWR of 1.3:1 on 3.9 MHz. The optimum value of the cap
would no doubt change at lower frequencies. With 22.5' of
Wireman #554 and a 950 pf cap, the 3:1 bandwidth is 145 kHz.
Adding sections of ladder-line lowers the resonant frequency.

Incidentally, this is a method for modifying the G5RV to work,
not only without a tuner, but with built-in tuners. When using
a built-in tuner, the antenna configuration doesn't have to
be changed as often. My IC756PRO will tune my present configuration
from 3.72-4.0 MHz. or 280 kHz.

I plotted the impedance presented to the coax for a range of
frequencies from 3.5 to 3.8MHz, they are at
http://www.vk1od.net/temp/G5RV-W5DXP.GIF . The Smith chart is
normalised to 50 ohms. The solution seems fairly narrow band, the VSWR
at 3.55 was 6, at 3.6 it was 1.3, and at 3.65 it was 5.

Changing the length of the series section will shift the resonant
frequency. I can vary mine from 22.5 ft. to 38.5 ft for a near-
perfect SWR on all HF ham frequencies.

Of course, implementations will have slight differences in actual
feedpoint impedances, and the outcome is very sensitive to slight
differences in feedpoint Z. This "no-tuner" matching scheme will
probably need significant customisation for each implementation.

IMO, that is what ham radio is all about - warm up the old MFJ-259B
and get with the program. :-)
--
73, Cecil http://www.qsl.net/w5dxp