On Tue, 13 Jun 2006 15:58:00 GMT, "Jerry Martes"
wrote:

I have been thinking of the "string of ferrites" as a method of presenting
a high impedance to Conducted currents along the outside of the coax from
the antenna down to the radio.

Hi Jerry,

This may be what I've already described at the end of my response to
Owen.

73's
Richard Clark, KB7QHC

On Tue, 13 Jun 2006 07:05:51 GMT, Owen Duffy wrote:
Just think of the antenna as the "source" and consider its
characteristic Z. Now consider the common mode circuit of the
transmission line and its Z. So often this is an unknown, and if we

Richard, does Z mean the characteristic impedance of the line in the
"differential" mode or "common" mode?

Hi Owen,

It occurred to me that there are two Zs mentioned and you did not
really differentiate as I had.

The antenna Z will be something we all have expectations of attaining,
and we quite often measure it. For the dipole we merely assign a
value of 70 Ohms for this discussion.

Now, we have the Z of the "third wire" which is almost never measured,
especially for every application. If it presented an equal 70 Ohms,
it would then hog half the power - however, this is unlikely in the
scheme of things. Let us just say it will exhibit some value that may
tend to absorb and radiate power. This capacity both upsets antenna
patterns and contributes to RFI and feed line radiation.

If the source is 70 Ohms and we add 200 to 700 Ohms of resistivity in
line (through the application of ferrites as common mode series
resistances), then we have diminished its capacity to hog power. As
1000 Ohms of ferrite resistance is easily applied, this has generally
satisfied many conditions.

73's
Richard Clark, KB7QHC

On Tue, 13 Jun 2006 09:51:43 -0700, Richard Clark
wrote:


As this discussion has been largely coaxial based, the outer
conductive surface of the coax is the primary imbalance to a dipole
through its "third wire" connection at the dipole feed point. It
appears as one of three wires to the abstract source established at
that feed point and it presents an ad-hoc Z load. The value of this
load is rarely determined, except when one deliberately attempts to
make their feed line 1/2 wave long (or some multiple). Of course,
that means 1/2 wave for the velocity factor of the outside conductive
path of the coax. This is often accomplished through cut and try
rather than modeling or measuring currents, but these too would be
good first pass approximations.

Richard,

There is a focus on evaluating baluns as a passive component in a
bench test jig designed to characterise them by a simple equivalent
circuit to show their imperfections in certain scenarios (eg impedance
range, frequency range, common mode impedance with different balance
points on the load and floating loads). Collectively, we seem to have
done that to death. To date, I don't think anyone has discovered a
practical balun that is close to ideal in all applications.

Baluns (being any device that is designed to facilitate the transition
from un-unbalanced load (might be perfectly balanced, might not be) to
an unbalanced transceiver (one terminal approximately grounded) seem
to be usually employed to:
- reduce common mode RF currents entering the shack where they may
disrupt operation of equipment, and more recently may be a health
concern;
- reduce the feedline's participation in radiation or reception
(pattern distortion, unwanted noise pickup, EMC / proximity to other
equipment).

It seems to me that NEC models are a worthwhile tool in developing
insight into the effects that occur. I think NEC modelling of a coax
fed, centre fed dipole at various heights as a centre fed conductor
with generator at the centre of two wires, and the "third wire" to
ground through some loss resistance is quite revealing. (The coax
example is not to suggest that this phenomena is peculiar to coax feed
alone.)

You mention measuring currents. It seems that discussion on common
mode currents has spawned a new market for clip on RF current probes.
I even see suggestion that common mode current be continuously
monitored in much the same way as VSWR is monitored at a spot adjacent
to the tx.

NEC modelling reveals that the currents on the "third wire" (common
mode current) varies with position (no surprise there), and that
depending on the topology, can be insignificantly low at some points
while it is significantly high at other points. Measurement of common
mode current at just a single point does not necessarily provide
enough information to detect or assess a common mode current problem.
Spot measurement is a superficial approach.

So if there are maxima and minima in the common mode standing wave
current on the feedline, the influence of chokes on current in all
parts of the antenna system (hence pattern) and loss of chokes will
depend on where they are located.

That is not to suggest that it is all too complicated. I think there
are good reasons to routinely deploy baluns of appropriate type an
location, but they aren't a cut and dried idealised solution and
further work may be required to identify and rectify residual
problems.

Owen
--

If the Swr is better below 145 than above the radiator must be over
length. You need to trim it or match it. If you are measuring through
lot of coax the match at the antenna is a lot worse than 3:1.

Any kind of choke arrangement amounts to working on the wrong problem
to me.

On Mon, 12 Jun 2006 19:17:44 GMT, "K. Hastings"
wrote:


John Ferrell W8CCW

Thanks John - I may have thought the balun assembly was somehow involved.
I'm new to the group and a fairly new amateur, and your respone was the only
one that dealt with my question.

What I thought was a relevant description was mis-read as "excess verbiage"
I guess. I'll try trimming the loop, and hope also to cure the electronic
thermostat problem which is fairly significant.

73

Kevin

e"John Ferrell" wrote in message
...
If the Swr is better below 145 than above the radiator must be over
length. You need to trim it or match it. If you are measuring through
lot of coax the match at the antenna is a lot worse than 3:1.

Any kind of choke arrangement amounts to working on the wrong problem
to me.

On Mon, 12 Jun 2006 19:17:44 GMT, "K. Hastings"
wrote:


John Ferrell W8CCW