Cec,
Behaviour of your feedline lengths for the various bands is interesting. But
it's likely optimum lengths for individual bands for anyone else will belong
to a different set. Your list is a guide just to the sort of thing to
expect.
Zo of the feedline has just as much influence on its impedance
transformation properties as does its length. Your measured Zo of 380 ohms
will have an effect considerably different from the nominal ladderline value
of 450 ohms or other impedances which other people may have.
There are, of course, other reasons for variability in optimum feedline
lengths. I don't think G5RV ever specified the impedance of his balanced
line - only that it should be 1/2-wavelength long around 14.15 MHz. Even
50-ohm coax would be OK on the 20m band. It's a fair match at either end.
----
Reg, G4FGQ
"Cecil Moore" wrote in message
...
These are actual measured values for the resonant lengths
of the series section transformer for all HF bands for
my optimized G5RV.
102' dipole, insulated wire. Ladder-line is approximately
Z0=380 ohms, VF = 0.9. Coax is RG-213
The insulation on the wire makes it electrically about 3%
longer, i.e. approximately the same as a 105' uninsulated
dipole. The ladder-line is Wireman #554, #14ga, "440 ohms",
but measures closer to 380 ohms.
Freq-MHz ladder-line length
3.8 23 ft.
7.2 35 ft.
10.125 20 ft.
14.2 29 ft.
18.14 36 ft.
21.3 27 ft.
24.95 29 ft.
28.4 38 ft.
Since my favorite bands are 40m and 17m, I keep the length
of the ladder-line at 36 feet most of the time. I rarely
work 30m so my minimum length is 23 feet. I have lengths
of ladder-line with banana plugs on one end and banana
sockets on the other end. I have one 8 ft. length, one
4 ft. length, one 2 ft. length, and one 1 ft. length so
I can achieve any length between 23 feet and 38 feet to
the nearest one foot. Of course, switches or relays could
be used instead of banana plugs.
--
73, Cecil, http://www.qsl.net/w5dxp
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