Cecil Moore wrote:
Here's a question overflowing from eHam.net and it's
not a trick question.

Assume that the radiating portion of a 40m vertical
is made out of 33 feet of RG-213 and the braid is
the radiator. The center conductor of the coax is
left floating at both ends. How much RF voltage
and/or current will be induced in that center wire
when using the outside braid as the radiator for
100 watt operation? How much of an EM field can
exist inside the coax braid?

I know this will shock regular group users, but he has taken things WAY
out of context in the question above!

The actual problem is this:

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.

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.

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.

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.

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

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.

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.

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. I went up 1/4 wl above the base,
and bonded the cables to the tower. I spaced the cables a foot or so
off the tower face on large insulators, so it formed an open 1/4 wl
very low loss stub. This made the differential mode impedance of the
open stub end at the ground very high, and allowed the cables to be
brought away from the hot tower base without interacting a large amount
with the tower base impedance.

This is a very simple common system that is often used in antennas
(often in BC systems) , and once in a while used incorrectly by Hams
and Ham antenna manufacturers (like Gap and MFJ and a few manufacturers
of Ham log periodics).

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

73 Tom