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Cecil Moore · July 11th 06, 03:06 PM posted to rec.radio.amateur.antenna

wrote:
The actual problem is this:

Seems logical to discuss the simpler coax example before introducing
an example that is more complicated and harder to understand.

A fellow placed a relay at the top of a half square antenna to change
directions by switching from one flat top and drop wire to another.
This is a VOLTAGE fed antenna at the ground. The vertical wires at the
antenna ends have to be an electrical 1/4 wl long on the OUTSIDE for
the system to work properly.

Cecil suggested he simply run the relay wires up inside a "shield" to
the relay, and the shield would prevent the relay control wires from
affecting the very high feed impedance at the base. The shield could be
used as the actual vertical antenna lead.

I did not use the word "shield" or "sleeve". I said he could run
the control wires up inside a hollow 1/4WL radiator feeding the
rest of the half-square. Note that the two original antenna leads
were completely removed. The tubing becomes the radiator. Here's
the diagram of what I suggested where "RFC" is an RF choke with
RF bypass caps at A-B and C-D. FP is the half-square feedpoint.

FP 1/4 WL radiator 3/4WL
======================tubing====================== ===+---wire
A--RFC----------------------wire-----------------RFC--C--relay
B--RFC----------------------wire-----------------RFC--D--coil
======================tubing====================== ===

It is my contention that the RFCs located just inside the tubing at
both ends will prevent this configuration from acting like a stub
and that there will be little RF EM energy inside the tubing. How
many functional stubs has anyone seen with two RF chokes in the
conducting path?

Now I know to many people the problem is obvious. The problem is the
IMPEDANCE of the open stub formed at the bottom of the vertical sleeve
by the inner wire that has to go to a control system of some type and
the outer sleeve.

All DC circuits isolated by RF chokes inside the tubing and bypass
caps across points A-B and C-D.

That impedance has to be many ten's of kilo ohms so the shunting
impedance is high compared to the impedance of the sleeve.

Full RF voltage of the feedpoint is also across the gap where the
center wires leave the shield.

That's where the RF chokes are located inside the tubing. what
happens when RF voltage encounters an RF choke?

In order for the shield to have some meaningful effect on the system
other than simply running the wires down in parallel with the fed wire,
the impedance between the inner wire and shield must be VERY high at
the bottom. It can of course be a SHORT at the top, since the relay
just sits up there in the air with only the contacts making a
connection, so the top is easy to handle with some bypass caps.

But that wasn't the configuration I suggested.

What Cecil totally misses is he formed what is in effect the electrical
equivalent of a sleeve balun.

Please explain how a sleeve balun functions with two RF chokes
installed in the conducting path.

The velocity factor of the transmission
line forming this stub has to be the SAME as the outside of the sleeve
so the INSIDE is 1/4 wl long electrical, and the the loss has to be
very low. Otherwise the common mode impedance of the relay wires
exiting the shield will not be several times higher than the antenna
feed impedance, which is several k-ohms.

The impedance of the RF chokes is also pretty high.

I've seen antenna manufacturers make the same mistake Cecil just made,
and assume that running a cable down the center of a "hot" mast that is
part of an antenna means the wires have zero current and zero effect
since they are inside the shield, but anyone with any understanding of
how the system works would catch the flaws in this idea right away.

Don't forget the RF chokes inside the tubing and bypass caps where
the wires enter and exit the tubing.

The flaw is the differential IMPEDANCE between the shield and the shell
forming an antenna has to be several times the common mode impedance of
the shell or the system won't be worth a flip. Without that high
impedance, the inner wire might as well just run down the outside of
the sleeve and a couple good HV high impedance RF chokes be used to
supply relay control voltage.

RF chokes at each end present a pretty high series impedance. The
RF current at each end is virtually zero and the wires are a non-
resonant length.

As a matter of fact at AM BC stations, when using two way or RPU
antennas on the hot base insulated towers, I never bothered with
running the cables INSIDE the tower.

We weren't talking about a tower. We were talking about solid tubing
made from copper or aluminum.

Cecil will catch on with help I'm sure, I just don't have time to walk
him through the problem step by step.

I'm willing to learn but you cannot simply assert something that
seems to violate the laws of physics and then say you don't have
time to explain it or furnish a reference. Please explain how a
stub can be functional with two RF chokes in the conductive path.

I know you have a bunch of followers who consider your word to be
gospel and depend upon nothing except faith for their belief in you,
but I am not one of them.
--
73, Cecil http://www.qsl.net/w5dxp