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.

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

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?
--
73, Cecil, W5DXP