On Jun 24, 6:16*pm, Richard Clark wrote:
On Tue, 24 Jun 2008 17:41:31 -0700 (PDT), Nick

wrote:
I also don't want to add
what may be a useless choke at the antenna's feedpoint, since that
would decrease the antenna system's efficiency due to the choke's
resistive losses...

Hi Nick,

A choke at the feedpoint does NOT add loss.

The Choke (which may be resistive insofar as the bulk material's
property) is in the Common Mode circuit. *The load is in the
Differential Mode circuit. *You want a very, very high Z (or R, it
makes no difference) in the Common Mode circuit. *The choke is
entirely transparent to the Differential Mode circuit.

If you don't understand the difference between the two modes (and it
is not a common topic outside of electronics design) then, no doubt,
others will fill in (or simply go to wikipedia).

73's
Richard Clark, KB7QHC

Thanks for the further info Richard! I guess I don't really fully
understand why the pure resistive losses in the coax choke wouldn't
affect the efficiency of the monopole. If I understand you correctly
(and I may not!), then even if we increase the impedance of the coax
choke to insane levels, such as by making it 1000 feet long and
winding it on a six inch diameter PVC form, then the huge losses
normally inherent in such a long run of coax would not be seen at all
by the vertical because it is wound on the PVC form? (I'm not arguing
with you at all, I just really want to know).

73,

-Nick

On Tue, 24 Jun 2008 22:12:55 -0700 (PDT), Nick
wrote:

Thanks for the further info Richard! I guess I don't really fully
understand why the pure resistive losses in the coax choke wouldn't
affect the efficiency of the monopole. If I understand you correctly
(and I may not!), then even if we increase the impedance of the coax
choke to insane levels, such as by making it 1000 feet long and
winding it on a six inch diameter PVC form, then the huge losses
normally inherent in such a long run of coax would not be seen at all
by the vertical because it is wound on the PVC form? (I'm not arguing
with you at all, I just really want to know).

73,

-Nick

Hi Nick,

Well, no argument perceived. 1000 feet would come with its own loss.
This is published in dB attenuation/100 feet for almost any type and
grade of coax in the market (and those that are no longer made). This
is truly Differential Mode loss.

Winding too much into a coil would not necessarily achieve the results
expected due to self-resonance (it could act like a shorted choke, and
not very useful). You can achieve astonishingly high Zs with a coiled
transmission line with very little effort (roughly 8-12 turns on a
liter pop bottle - empty of course - for HF). It will peak at some
frequency and provide adequate isolation in nearby bands; however, the
ferrite chokes can be made to cover a decade of frequency - or much
more (3-30 MHz), if you care to really go whole hog (3MHz-3GHz) with
1000 Ohms of Z (principally R) without too much effort either (more
cost than effort I should add).

One proviso should be observed: Whatever problems find their way onto
the shield of the coax (Common Mode circuit) can arrive over the air.
If they do, they can induce themselves into the below the choke point
as easily as at the antenna. The solution here is to repeat the choke
construction 1/4 wavelength from the feedpoint (a second choke, that
is). If you are choking a wide band, you might try distributing the
ferrite beads over a greater length of coax than the 1 foot it would
normally inhabit. I have a 20 foot ferrite choking section that I can
insert, inline, into my feeds. It has one bead every 3 or 4 inches. I
wouldn't advise this as replacing the feedpoint choke, but rather the
second choke some short distance away.

Note that I made a distinction about interference coming in "over the
air." This is not the only way as most home based interference comes
into the receiver by conduction - sharing a poor ground. Most folks
who swear that chokes don't work (or at least theirs don't so none
do), are suffering from ground loops. A choke can help, but not
nearly as well as combining it with conventional grounding solutions.
Poor advice abounds in this topic too.

73's
Richard Clark, KB7QHC

I guess I don't really fully
understand why the pure resistive losses in the coax choke wouldn't
affect the efficiency of the monopole.

A simple way to think of it is: Ferrite beads over the outer of a coax has
no effect on the signals flowing 'normally'
up and down 'inside' it. The beads only effect signals that are flowing on
the outer (treating it like a piece of wire rather
than a coax transmission line). I know that this is not quite the full story
but it is a simple explanation of the situation
under discussion.

Jeff