Jerry wrote:
"ve2pid" wrote in message
...
Seems to me, that relay should always be closed during operation
from the base feedpoint so there will essentially be zero volts
across the contacts. Since the relay should not be switched
with the key down, your question seems irrelevant.
To be more precise, the loop would remain intact, but I want to
disconnect the coax from the inactive source, and I am looking for a
way to do this using a relay near the source (or at the shack). The
power would the full legal power.

Any idea ?

Hi Pierre

I submit that you will have no luck with your plan to run the two
transmission lines from the antenna to the shack where the switch chooses
which one to use. The unused line will present an impedance across the
"gap" which is a function of the length of the line.
Correct me if I am wrong.

Jerry KD6JDJ

There are two ways to get the gap voltage. One is to insert a
zero-amplitude current source in a continuous wire at the gap position.
The source will act like an open circuit, but the voltage across it can
be seen via the Src Dat (source data) button. The other is to insert a
high impedance load at the gap position and use Load Dat. In either case
it's necessary to set the power level in the Options Menu.

Regarding using dual feed lines, I've done this successfully many times.
The trick is to use low loss lines of the correct length. For example,
my "Field Day Special" antenna
(http://eznec.com/Amateur/Articles/Field_Day_Special.pdf) requires the
unused line to look like an open circuit, so its length is an integral
number of half wavelengths and it's left open at the
transmitter/receiver end. Low loss is important, since even a
surprisingly small amount of loss will lower the impedance of a line
intended to present an open circuit and raise the impedance of a line
intended to look like a short.

EZNEC v. 5.0 has the ability to include lossy transmission lines in the
model, so you can directly model the effect of transmission line loss
and imperfect length.

Roy Lewallen, W7EL