Roy Lewallen wrote in
:

....
A choke at a high-impedance point doesn't do much, since there's no
substantial current at that location to block. You should put the
choke about a quarter wavelength from there where the current is
nominally high. The presence of the choke can move the locations of
high and low voltage and current points, so you'll need a choke at
least about every quarter wavelength to make sure there's no point
where substantial current can occur.

Roy, NEC models suggest that lossy chokes (eg suppression sleeves or cores
where Q is very small) don't modify the current distribution much unless
they are of sufficiently large impedance, and that introduction of low Z
chokes just introduces another loss without much impact on the current
distribution or resultant antenna pattern.

The magnitude of Z needed to be effective in forcing a current minimum at a
point might be quite impractical to implement using suppression sleeves, so
the time honoured insulator looks the better solution.

Owen

Owen Duffy wrote:

Roy, NEC models suggest that lossy chokes (eg suppression sleeves or cores
where Q is very small) don't modify the current distribution much unless
they are of sufficiently large impedance, and that introduction of low Z
chokes just introduces another loss without much impact on the current
distribution or resultant antenna pattern.

The magnitude of Z needed to be effective in forcing a current minimum at a
point might be quite impractical to implement using suppression sleeves, so
the time honoured insulator looks the better solution.

Yes, that's exactly the point I've been trying, apparently
unsuccessfully, to make. It is practical to use ferrite sleeves for
suppression of current at a single point or a couple of points, as Walt
Maxwell pointed out some time ago. Often called the "W2DU balun", it's
done by putting a lot of cores -- typical several tens of cores -- over
the line. But you wouldn't want to do this at a dozen or two points on
guy wires. I personally prefer to use multiple turns on a single core,
because ten turns on one core gives the same impedance a single pass
through 100 cores. But then I don't run so much power that I need to use
RG-8 or larger size cable or go to heroic efforts to insulate the turns
on a single core.

The guy wire requirements would be about the same as for a "current
balun" (common mode choke) -- somewhere around 500 - 1000 ohms is
typically necessary. At that impedance level, it makes no difference
whether the impedance is reactive or resistive from the standpoint of
effectiveness in choking current or in terms of dB loss. But there can
still be enough power dissipated to overheat the cores if they're
resistive and the power level is very high. Then you're stuck with using
ferrites which are more reactive and less resistive (e.g., Fair-Rite 60
series), but they also give you a lot less impedance per core so you
need more cores yet. That makes the ferrite solution even less attractive.

Roy Lewallen, W7EL

Roy Lewallen wrote in
:

The guy wire requirements would be about the same as for a "current
balun" (common mode choke) -- somewhere around 500 - 1000 ohms is
typically necessary. At that impedance level, it makes no difference
whether the impedance is reactive or resistive from the standpoint of
effectiveness in choking current or in terms of dB loss. But there can
still be enough power dissipated to overheat the cores if they're
resistive and the power level is very high. Then you're stuck with
using ferrites which are more reactive and less resistive (e.g.,
Fair-Rite 60 series), but they also give you a lot less impedance per
core so you need more cores yet. That makes the ferrite solution even
less attractive.

Roy,

I have attempted to model the famous Carolina Windom. It takes some
guessing since it contains proprietary (ie secret) components, namely the
"balun" and the "isolator". My flat top is at 10m height.

Since they argue that the vertical feedline is an effective radiator, I
make the assumption that the balun is a "voltage" balun, and that it is
transparent to common mode current, so I have modelled the feedline as a
wire attached to (as it happens) the inner end of the short leg of the
flat top.

The "isolator" is argued to prevent the current flowing on the line above
itself from flowing on the line below itself. It is a naive notion, since
there is mutual coupling... but lets guess that it is a bunch of
supression sleeves on coax.

At 7.2MHz, the isolator is subject to appreciable current, and it does
not effectively force a current minimum until it is well over 1000+j1000.
(BTW, about half the power is dissipated in a 1000+j1000 isolator.) 2000
+j2000 is becoming reasonably effective.

I know there is a proposition that chokes such as the W2DU balun need
only have choke impedance about 10 times the nominal Zo to be effective.
I think that design constraint is effective in limiting the extent to
which such a balun unbalances the load by its own shunt impedance dropped
accross one load leg for a load~=Zo, but I think the criteria has nothing
to do with the choke's influence in forcing a current minimum in the
region of itself. It is the difference between a bench test criteria for
insertion VSWR, and what is needed when plugged into an NEC model where
the intention is to force a current minimum.

Thoughts?

Owen

Owen Duffy wrote:

...
(BTW, about half the power is dissipated in a 1000+j1000 isolator.) 2000
+j2000 is becoming reasonably effective.
...

I have a 4:1 hybrid balun (4:1 voltage balun coupled up with a 1:1
current balun). Now and then I run the full california-kilowatt to it,
I have NEVER seen the thing REAL WARM unless running into a high
standing wave, but NEVER what I would call REAL HOT.

500+ watts would shortly make it so hot it would burn fingers and smoke
even the high temp thermaleze enamel off the windings (if the balun was
forced to dissipate that power!)

I don't see where half the power could have ever been "sunk" into the
balun, no matter what freq it has been operated at, 2-30. The windings
all look in excellent shape and the core in good condition ...

Or, perhaps I misunderstood your previous post.

JS

John Smith I wrote in news:esvehb$f2v$1
@registered.motzarella.org:

Owen Duffy wrote:

...
(BTW, about half the power is dissipated in a 1000+j1000 isolator.)
2000
+j2000 is becoming reasonably effective.
...

I have a 4:1 hybrid balun (4:1 voltage balun coupled up with a 1:1
current balun). Now and then I run the full california-kilowatt to it,
I have NEVER seen the thing REAL WARM unless running into a high
standing wave, but NEVER what I would call REAL HOT.

500+ watts would shortly make it so hot it would burn fingers and smoke
even the high temp thermaleze enamel off the windings (if the balun was
forced to dissipate that power!)

I don't see where half the power could have ever been "sunk" into the
balun, no matter what freq it has been operated at, 2-30. The windings
all look in excellent shape and the core in good condition ...

Or, perhaps I misunderstood your previous post.

Perhaps.

I don't understand the term "California Kilowatt".

I don't know if you are describing the same thing I modelled, I don't
think so.

It is often overlooked that the power of an SSB telephony signal averaged
over a longish time (but not accounting for the duty cycle of "overs") is
around 15dB (a little less for speech processing), so the heating effect
of 500W PEP telephony might be more like 15W.

Owen

Owen Duffy wrote in
:

John Smith I wrote in news:esvehb$f2v$1
@registered.motzarella.org:

Owen Duffy wrote:
It is often overlooked that the power of an SSB telephony signal
averaged over a longish time (but not accounting for the duty cycle of
"overs") is around 15dB (a little less for speech processing), so the
heating effect of 500W PEP telephony might be more like 15W.

Duh, guess who left a minus sign out, should read:

It is often overlooked that the power of an SSB telephony signal
averaged over a longish time (but not accounting for the duty cycle of
"overs") is around -15dB (a little less for speech processing), so the
heating effect of 500W PEP telephony might be more like 15W.

Owen

Owen Duffy wrote:

...
It is often overlooked that the power of an SSB telephony signal
averaged over a longish time (but not accounting for the duty cycle of
"overs") is around -15dB (a little less for speech processing), so the
heating effect of 500W PEP telephony might be more like 15W.

Owen

Hmmm, I just may pull that balun down and run a full KW into it hooked
to a dummy load to look at some temp readings over a range of bands/over
a time span ... maybe the thing has been dumping more heat than I am
aware of, time anyway to check out the system ...

JS

Owen Duffy wrote:
I know there is a proposition that chokes such as the W2DU balun need
only have choke impedance about 10 times the nominal Zo to be effective.

I believe that rule of thumb applies only to matched lines,
i.e. Z0 loads. The other rule of thumb that I have heard
is 5x the antenna feedpoint impedance.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote in
t:

Owen Duffy wrote:
I know there is a proposition that chokes such as the W2DU balun need
only have choke impedance about 10 times the nominal Zo to be
effective.

I believe that rule of thumb applies only to matched lines,
i.e. Z0 loads. The other rule of thumb that I have heard
is 5x the antenna feedpoint impedance.

Ah Rules of Thumb (ROT) abound.

The first ROT relates to what is happening inside the coax at the choke
location (or is it somewhere else), and the choke is on the outside of
the coax.

The second ROT relates to the antenna impedance, wherever that is
(perhaps it assumed feedpoint adjacent to the choke), and again relates
to the differential mode Z (or some transformation) whereas the choke
acts on common mode current.

Take for example the use of a common mode choke at the junction between
open wire line and coax in a G5RV, its behaviour seems to me to be quite
unrelated to the differential mode Z at that point, or at the dipole
centre, or anywhere else.

Owen

Owen Duffy wrote:
The second ROT relates to the antenna impedance, wherever that is
(perhaps it assumed feedpoint adjacent to the choke), and again relates
to the differential mode Z (or some transformation) whereas the choke
acts on common mode current.

The second ROT is of course for cases where the choke is
installed at the antenna feedpoint. The main function of
such a choke is to choke off *conducted* common-mode current.
The effect of the choke on inducted common-mode current is
usually unknown unless measured.

Take for example the use of a common mode choke at the junction between
open wire line and coax in a G5RV, its behaviour seems to me to be quite
unrelated to the differential mode Z at that point, or at the dipole
centre, or anywhere else.

Again, the G5RV choke is trying to prevent *conducted* common-
mode currents and only has a chance of success on certain
bands where the differential impedance is relatively low at
the junction point. The effect of the choke on inducted common-
mode current is hard to quantify without measurements.
--
73, Cecil http://www.w5dxp.com