Hi to all,

I am modeling a square loop. The feed point is at the center of the
base segment. But I want to have a gap at the middle of a vertical
wire, in order to place a relay there to switch to this feed point
instead of the base point, so it would change polarization.

My question: does EZNEC provides a way to get the voltage at the
vertical (open point) mesured at the gap when the base source is
'active'? Just to be sure that arc won't be created there.

73 de Pierre VE2PID

ve2pid wrote:
I am modeling a square loop. The feed point is at the center of the
base segment. But I want to have a gap at the middle of a vertical
wire, in order to place a relay there to switch to this feed point
instead of the base point, so it would change polarization.

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.

My question: does EZNEC provides a way to get the voltage at the
vertical (open point) mesured at the gap when the base source is
'active'? Just to be sure that arc won't be created there.

You don't want the gap to even exist when the base source is
active. The relay contact should always be closed when the
base source is active. The base source relay should always
be closed when the side source is active.
--
73, Cecil http://www.w5dxp.com

"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

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

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

One can hook a DPDT relay up. Armature terminals go to the loop. One set
of the contacts are shorted, the other set connect to the feedline.

Yes, you'll need to know the voltage (relative to ground/feedline/relay
coil) at that point to choose a relay. Hook a large value resistor in
your NEC model from the point to "ground" (you should be modeling that
piece of coax as a single wire anyway.. so just put it in series). not
much current will flow through the resistor, so the pattern won't
change, but you'll be able to measure the voltage across it (or, more
properly, you can look at the current in that resistive segment and use
Ohm's law)