"Sal M. Onella" wrote in news:cc6de1f9-fa1d-4a1c-bf7f-
:


Does the velocity factor of the coax come into play when locating the
second
choke or does the outer shield behave like an open wire, VF ~ 1.0?

Perhaps I am slicing the baloney too thin for an average sandwich ;-)

You will often see emphatic advice as to which side of the thing to
attach the shield, and which the centre conductor.

In fact, the antenna structure lends itself to an effective integral
balun. If you take the feed coax through the bottom wall of the U
section, bonding the shield to the tube, and up one side (doesn't
matter), exiting at the appropriate point. At this exit, trim the shield
and bond it to the tube, and take the inner horizontally across to the
other tube and connect it.

If physical symmetry is very good, the balun will be very good.

Nevertheless, as Roy observes, the feedline and supporting structure
below the U section is coupled and there is still potential for some
level of common mode current.

(To preempt Roy, the velocity factor of the outside surface of the shield
clad with a thin layer of vinyl will be close to 1, close enough for the
purpose at hand.)

Owen

In article ,
Owen Duffy wrote:

In fact, the antenna structure lends itself to an effective integral
balun. If you take the feed coax through the bottom wall of the U
section, bonding the shield to the tube, and up one side (doesn't
matter), exiting at the appropriate point. At this exit, trim the shield
and bond it to the tube, and take the inner horizontally across to the
other tube and connect it.

.... and take *real* care to waterproof the point at which the cable
exits from the tube and the shield is bonded! Buy a tube of
non-acid-containing (metal-safe, "neutral cure") silicone sealant, and
apply it very thoroughly here... squeeze and press plenty of it into
the braid and then form a layer which completely seals the exit point.

If you don't do this, and leave even a small amount of braid exposed
to the air, it will tend to "wick up" moisture into the braid every
time it rains.. The water can even wick its way down to the bottom of
the cable and you can find water dripping out of (or filling up) the
connector at the bottom! Trust me, it really does happen... I found
my lower N connector full of water, and what I swear appeared to be
algae!

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Dave Platt Inscribed thus:

In article ,
Owen Duffy wrote:

In fact, the antenna structure lends itself to an effective integral
balun. If you take the feed coax through the bottom wall of the U
section, bonding the shield to the tube, and up one side (doesn't
matter), exiting at the appropriate point. At this exit, trim the
shield and bond it to the tube, and take the inner horizontally across
to the other tube and connect it.

... and take *real* care to waterproof the point at which the cable
exits from the tube and the shield is bonded! Buy a tube of
non-acid-containing (metal-safe, "neutral cure") silicone sealant, and
apply it very thoroughly here... squeeze and press plenty of it into
the braid and then form a layer which completely seals the exit point.

If you don't do this, and leave even a small amount of braid exposed
to the air, it will tend to "wick up" moisture into the braid every
time it rains.. The water can even wick its way down to the bottom of
the cable and you can find water dripping out of (or filling up) the
connector at the bottom! Trust me, it really does happen... I found
my lower N connector full of water, and what I swear appeared to be
algae!

I'll second that ! Having seen TV's and set top boxes damaged by
ingress of water.

--
Best Regards:
Baron.

Owen Duffy wrote:
. . .
(To preempt Roy, the velocity factor of the outside surface of the shield
clad with a thin layer of vinyl will be close to 1, close enough for the
purpose at hand.)

I'll just add the reason -- the common mode current, which is what
causes feedline radiation and what we're trying to suppress, is on the
outside of the coax. The commonly specified velocity factor (around 0.66
for solid dielectric coax, a bit higher for foamed dielectric) applies
to the field inside the coax where the differential mode current flows,
not to the outside where the common mode current is. So you use a value
near one as Owen says. And it's not at all critical for this purpose.

A standing wave is present on an antenna or radiating feedline -- every
half wavelength there's a current null, and offset a quarter wavelength
from these are current maxima. For example, there are current nulls at
the ends of a half wavelength dipole (or, an even better example, a Yagi
parasitic element) and a maximum at the middle. When you insert a balun
in a transmission line, it causes a current null at that location, so
there'll be a maximum a quarter wavelength down and another minimum a
quarter wavelength below that if current is induced by coupling and the
length to ground supports that distribution. We want to make that
distribution impossible, and inserting the second balun does that.

This is easily observed by modeling, but you have to keep in mind that
the actual path the current takes to the Earth along the outside of the
coax can be considerably more complicated than most simple models
represent. So models can tell you what *can* happen although maybe not
necessarily what *is* happening in a given installation.

Roy Lewallen, W7EL