[Hallicrafters] ANTENNAS and OLD WIVES' TALES

[John Schmitz]

This has been a great thread, I don't jump in often but I'm going to throw
my 2 pennies in on this one. I taught grounding and bonding for Motorola
some years ago and here's some stuff to ponder.

IEEE standard defines a typical stroke current waveform as having an 8us
rise time from 10% to 90% of peak stroke current, a 20us pulse width (10% of
peak point to 50% decay point), and a typical peak current of 18,000 amps.
So what we are working with is a very rapid, 8us, rise in current to
18,000amps. The voltage developed across an inductance is E=L(delta
current/delta time). 18000 amps divided by 8us gives us 2250000000, so,
E=L*2250000000. It's pretty apparent that even a very small inductance, a
bend, will produce a very large voltage across it and yes, it will want to
arc to ground if it's the lower impedance path present at the bend.

We ran an example that used a 150 foot tower with a 135 foot coax run down
the tower where it made a 90 degree bend to the shack. The run to the shack
from the tower was 20 feet. Anyway we calculated out all the inductances etc
and determined that at that bend to the shack there would be something like
28KV (ref to ground) at that bend point. A real eye opener! This is why you
should put a ground bond strap between the coax outer conductor and the
tower right at the beginning of that bend. And for that matter, anywhere you
have a bend you should have some type of ground bond from the outer
conductor to ground. Keep in mind these are tower direct hit numbers. But it
is evident that even small inductances can have huge voltages developed
across them when large current flows and rapid rise times are involved.

Next, my understanding of coiling the coax had nothing to do with lightning
but is done to form a choke that prevents RF currents from flowing in the
outer conductor of a coax transmission line. Current flows in the outer
conductor of the coax transmission line would cause radiation from the
transmission line which we don't want. We want all the radiation at the
antenna. Radiation from the transmission line represents loss. We only want
the transmission line to transport, not radiate. If I remember right
transmission line outer conductor currents occur when connecting a single
ended coax directly to a balanced tuned antenna such as a dipole (I be wrong
about this point).

Anyway, the interesting thing about this is the coiling of the coax does NOT
look like a choke inductor to the center conductor, only to the outer
conductor. This is because the lines of force produced by the center
conductor are contained within the coax due to the outer shield and
therefore are unable to interact with each other as windings on a coil do.
On the other hand, fields developed by current flows in the other conductor
are not contained by anything and free to interact with each other like a
coil. So it appears as though there is an RF choke on the outer conductor
but not on the inner conductor, thus no RF current flows in the outer
conductor. Outer conductor currents are choked but not inner conductor
currents.

Feel free to let me know if I'm not right on this.

John

-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net]On Behalf Of Todd, KA1KAQ
Sent: Thursday, February 24, 2005 8:47 AM
To: Mr. and Mrs. Magoo
Cc: hallicrafters at mailman.qth.net
Subject: Re: [Hallicrafters] ANTENNAS and OLD WIVES' TALES

Bill -

It sounds more like a multi-purpose fix: balun and lightning
protection perhaps?

It is true that lightning does not like abrupt bends in wire and will
generally exit and find its way to ground. It's also common practice
to make a few turns of coax (last time I saw it done a piece of large
PVC pipe was used as a form) right near the feedpoint of a Yagi or
other systems as a simple form of impedance matching. I have no clue
why you'd need to wind another coil where the feedling enters the
building, but a single turn would give the lightning an additional
exit point and probably wouldn't change things much with respect to
the antenna, provided you are using well-shielded coax.

de Todd/'Boomer'  KA1KAQ


On Wed, 23 Feb 2005 08:00:44 -0500, Mr. and Mrs. Magoo <magoo at isp.ca> wrote:
> While we are on the subject of "old wives' tales" and ham folklore about
> antennas, when I first became involved in the 2-way radio business back in
> the 70s an "old hand" told me that when installing a base antenna place a
> coil of about 5-7 turns of coax diameter about 8 inches or so just below
the
> connector on the antenna and affix securely to the tower.
______________________________________________________________
Hallicrafters mailing list
Home: http://mailman.qth.net/mailman/listinfo/hallicrafters
Help: http://mailman.qth.net/mmfaq.html
Post: mailto:Hallicrafters at mailman.qth.net
----
List Administrator: Duane Fischer, W8DBF **for assistance**
dfischer at usol.com
----
Hallicrafters Collectors International: http://www.w9wze.org