Induced signal?
 

wrote:
The shield is eight wires arranged in
a regular octogon with the tops and bottoms tied together.

That doesn't seem to me to simulate a coax shield and
is more akin to parallel conductors. Doesn't skin effect
enter into the coax braid equation? It seems to me that
by floating the inside conductor and using the coax
braid as the radiator, we are forcing most of the current
into common mode on the outside of the shield. With zero
current at each end of the floating center conductor, why
would current flow on the inside of the braid?
--
73, Cecil http://www.qsl.net/w5dxp

Induced signal?
 

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

Induced signal?
 

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

Induced signal?
 

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

Induced signal?
 

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

Induced signal?
 

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

Induced signal?
 

wrote:
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.

How about when there's two RF chokes in series?
--
73, Cecil http://www.qsl.net/w5dxp

Induced signal?
 

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

Induced signal?
 

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

Induced signal?
 

I think it's just a matter of degree. The more RF chokes there are,
the less current will flow.

It would be a straightforward matter to add more wire to the model and
include loads for bypass caps and chokes.

In either case (wire inside or outside of the shield) good decoupling
where the wire lead exits is going to be important. The model so far
may suggest that being inside the shield is better than being
outside... but without that lead trailing away from the antenna some
distance, it's not time to conclude much about the relay+half square
problem.

The 40m coax monopole answer would seem to be "yes, there's current on
the center conductor, but it's small and coupled in through the ends"

If you're using the standard EZNEC, you're going to have to knock
another section off the top of the cage... if you've got EZNEC+ then
just add away...

I may try it when I get home... knock another section off and try a
control wire...

Might have to send off my money to Roy and go for EZNEC+... I know I
can get around segment limitations with other programs but I do like
EZNEC. This is probably the ninth time since I got the program a few
months ago that I've hit the segment limit ... i like meshing things...


Gives me a question about the (EZ)NEC limitations... should I be
watching out for fine 2D meshes? It seems to work OK in this case...
the base impedance of the meshed monopole and the current distribution
viewed on the segments all makes sense, and it seems to me that there's
not much reason to doubt that the currents are calculated correctly in
the mesh as long as it's not coarse with respect to a wavelength... any
caveats in this regard?



73,
Dan