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

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

I reran my EZNEC coax model with a cage with an octagon loop every foot
in height... This is a pretty good shield at 40m, I think. The holes
are .002x.007 wavelength.

The difference is apparent.

Not much confidence that this is modeling the problem... but maybe it's
better:

Ran it with wire inside/outside the shield. Did #2 wire this time to
simulate thin wire down the center of a pipe. Before, the
inside/outside comparison yielded almost identical results. Now it
doesn't...

Wire 6 inches outside shield:

Wire No. 26:
Segment Conn Magnitude (A.) Phase (Deg.)
1 Open .00356 86.70
2 .00776 95.54
3 .01039 103.14
4 .01208 109.03
5 .01323 117.85
6 .01373 126.69
7 .01297 133.85
8 .01159 144.14
9 Open .00684 154.69

Wire centered in shield:

Wire No. 26:
Segment Conn Magnitude (A.) Phase (Deg.)
1 Open .00201 53.19
2 .00298 65.98
3 .00324 71.00
4 .00509 45.58
5 .00387 78.85
6 .00401 118.35
7 .00308 63.54
8 .00347 137.70
9 Open .00503 164.46


Anyway, the point that the original cage was a bad model is taken. An
additional approximation toward a full shield changes things a great
deal.

Should anyone want to take a look: http://www.n3ox.net/cage_coax.ez

Dan

wrote:
Anyway, the point that the original cage was a bad model is taken. An
additional approximation toward a full shield changes things a great
deal.

Thanks, Dan, for your considerable effort. You saved
us from a new old wives' tale. :-)
--
73, Cecil http://www.qsl.net/w5dxp

On 11 Jul 2006 07:46:49 -0700, "
wrote:

Should anyone want to take a look: http://www.n3ox.net/cage_coax.ez

Hi Dan,

Thanx for the work.

73's
Richard Clark, KB7QHC

Oh, the laws of physics don't preclude RF from getting in the ends of a
piece of coax, by the way.

There is no minimum cutoff frequency for the TEM mode in coaxial
waveguide.

There is in hollow waveguide with no center conductor.

You still need to be able to couple to the ends, and a floating center
conductor is not the best way to couple energy in. However, there's no
fundamental physical reason why currents *won't* flow on the center
conductor in an open-ended piece of coax.

Dan

I should add that sticking the wire even a little bit (six inches) out
the ends of the skeleton shield increases the current on the center
conductor...

I expect that the situation with a long wire exiting the bottom will
couple MUCH more energy into the center conductor.

So, in the context of control wires up an antenna element, the wires
coming away from the antenna and a load to represent a choke should be
included.

I'd also like to refine the shield mesh but I ran out of segments !

Dan

wrote:
I expect that the situation with a long wire exiting the bottom will
couple MUCH more energy into the center conductor.

My suggested solution over on eHam.net included RF chokes
and RF bypass caps at each end of the tubing.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:
wrote:
I expect that the situation with a long wire exiting the bottom will
couple MUCH more energy into the center conductor.

My suggested solution over on eHam.net included RF chokes
and RF bypass caps at each end of the tubing.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil,

You put great faith in passing you Mensa exam.

With that in mind, what possible difference does the shield make once
the inner conductor is bypassed and choked at each end?

In case you can't understand, the answer is NO difference.

Without the chokes and bypasses, your idea won't work. With the chokes
and bypasses, the idea isn't needed. The relay wire can run right down
the mast with no ill effects.

It's so basic and simple, even a Mensa member can follow it!

73 Tom

wrote:
You put great faith in passing you Mensa exam.

This member of MENSA is having difficulty in parsing
your statement. What does, "you Mensa exam", actually
mean? Does the pronoun, "you", address the Mensa exam?
Wouldn't it be better to address the Mensa exam as an
"it" rather than as a person? Your numerous deviations from
the accepted laws of physics and accepted English language
construction have me confused.

With that in mind, what possible difference does the shield make once
the inner conductor is bypassed and choked at each end?

What I said is that, contrary to your strange assertions, is that
the RF chokes and RF bypass capacitors block the stub function
upon which your entire argument rests. The result of my suggestion
is *NOT* a stub function as you have so stubbornly insisted. So I
ask you once again: Please prove that a stub with two RF chokes in
the conductive path is actually functional as a stub. If you cannot,
your entire argument falls apart.
--
73, Cecil http://www.qsl.net/w5dxp