[SOC] Matching G5RV

[Paul Bartlett]

Hi again,

Thanks for the responses so far. I'll describe the problem again. Apologies
for the repeats.

Here's my (poor and qualitative analysis) of what I have to date - without
ATU.

The centre of the 104 foot G5RV goes via ladder line (300R?) down to a co-ax
connector. A 50R feed (approx 30') runs from this connector back to the
shack and thence to K2. I've coiled about 10 turns of the co-ax close to the
connector to form a choke balun to provide some kind of match  between the
balanced ladder line and the unbalanced co-ax feed.

This arrangement seems to load up fine on 40m with barely a flicker on the
meter measuring reverse power.

Things get worse as I move up the bands and from the literature I gather
that with a G5RV an ATU is necessary for a proper match as frequency
increases.

I have available an ex-mil ATU which has a variable shunt cap to ground on
the input followed by a series variable inductor (up to 50 turns on a
cearmic former spaced over six inches or so with a diameter of about one and
a half inches). I believe that the ATU was designed to match a 50R feed to a
whip antenna and a substantial ground stake.

There are three screw terminals on the back end of the ATU: ground; the
other end of the inductor and a third that's connected via an as yet
unmeasured fixed cap to the output end of the inductor.

Hmmmm. As Bob points out, it's essentially an L network if you ignore the
third terminal but turns into a PI network if you tie said terminal to
ground. Albeit with a fixed cap on the output side and a variable one on the
input.

I think I'll do the experiment....

Paul M3CRQ :-)