Cecil Moore wrote in message ...
Brian Kelly wrote:
I'll have some fairly long runs of coax to HF antennas (21Mhz max) to
Field Day antennas. One of the antennas will be an 80M G5RV with their
usual nasty SWRs so a foam-type cable is indicated.

Hi Brian, what bands will the G5RV be used on? Only 80m?
The SWR is not all that nasty on 80m being about 20:1
on the ladder-line and 2:1 on the coax.

Greetings Cecil: I've been on the road for the past three days and am
just now catching up with my e-mail and such.

The answer to your question is that I intend to use a Van Gordon 80M
G5RV at about 40 ft. on 80/75/40 as a minimum. I'm "ambidextrous", I
motor-mouth and twitch paddles on FD so I need "bandwidth" on 80/75.
I'm building a close-spaced wires duo-band dipole for 20 & 15 which
will be hung at around 25-30 ft. If that antenna doesn't work right
I'll also have to use the G5RV on 20/15 too. In the ideal scenario
both antennas would work and I'll be able to swap antennas around for
20 & 15 depending on the pattern & propagation condx of the moment.
I'll use 8X to feed the G5RV balun which will be 8-10 foot above
ground level to keep the weight on the G5RV down

From there I'll run a length of RG-8 to the operating position. Looks
like a 75 or so foot run. I'll use my LDG-11MP ATU to tame the G5RV.
With all this coax involved the ATU won't have much "work" to do . . .

Here's one of my rules-of-thumb for which I am infamous. :-)

Heh.

Given a 102 ft. Dipole at 40 ft. fed with 450 ohm ladder-line
and used on 3.8 MHz, for a ladder-line length between about
25 ft. and 100 ft., the resistance looking into the ladder-line
is about equal to the length of the ladder-line. Thus for 25 ft.
of ladder-line, the impedance looking into the ladder-line is
about 25+jXL1. For a ladder-line length of 50 ft., the impedance
looking into the ladder-line is about 50+jXL2. For a ladder-line
length of 100 ft., the impedance looking into the ladder-line
is about 100+jXL3. Does that give you any hints on how to match it?

According to EZNEC, the impedance looking into 50 ft. of ladder-line
is about 50+j466. Use two series 180pf caps to neutralize the inductive
reactance and you have a feedpoint impedance of 50 ohms. A dual ganged
300pf variable would be ideal.

A 102 ft dipole 40 feet in the air fed with 50 ft of ladder-line
tuned with a dual-gang variable cap on the antenna side of a balun/
choke and no tuner box. Doesn't that sound like a perfect 75m field
day antenna?

I've read the whole thread thru Tuesday AM, I get your point and it
should work well on 80. I suspect the real advantage of your scheme is
that it might be possible to cover both 80 and 75 with low overall
losses which is difficult to accomplish with a fullsize dipole. Since
you already have the EZNec model on hand how 'bout running a 3.50-3.85
Mhz. sweep to see if it's possible to cover that range with some
single specific length of ladderline and the two-section BC variable
cap? If yes I think you have a real keeper idea.

Davis RF has "RG-213 Economy" cable, whatever that is, for $0.23/foot.
Good enough for Field Day.

Brian w3rv