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Re: Topband: (no subject)

To: <>, <>
Subject: Re: Topband: (no subject)
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Sun, 31 Aug 2003 07:44:50 -0400
List-post: <>
> "No tower is zero ohms from top to bottom, other wise it would be a
> superconductor"

Even if it were a super conductor, it would have considerable voltage on the
gamma wire.

With high frequency energy (like lightning) skin effect pushes current to
the outside. It's the changing magnetic field that causes this. The better
the conductor, the more shallow the skin depth at a given frequency. The
rule of percentage of tap point and resistance doesn't work like at dc.

This is why wide smooth conductors, even when very thin like foil, can be
much better than stranded or woven wires for lightning and RF. At 30 MHz
loose braid removed from RG-8 coax has about the same resistance per foot as
a #14 or 16 solid wire.

> this 15 foot section.  The voltage across the coaxial cable to the station
> reduced by the air gap in the wide spaced capacitor and due to spark gaps
> both ends of the buried coaxial cable. The equipment failure Tom describes
> more like problems at the station end grounding allowing unequal voltage
> build up across the earth system at the shack end.

Once a gap ionizes the resistance is very low, so a series gap (like a
series cap in a lightning protector) has virtually no effect in a direct
hit. Shunt gaps and series inductance are big inhibitors, series caps aren't
worth talking about.

In my case the path was easy to trace. Lightning blew a 7.5kV vacuum cap on
the gamma wire. It melted the contacts of a relay connected to that gamma
that switched antennas, and a portion followed the coax center conductor
into the house. It blew the amplifier TR relay NC contacts, and blackened
the TR relay board in my T4XC and the front end coils of the R4C connected
to that T4XC.

Everything "powered up". I first noticed the "problem" as no RF output, a
loading cap that wouldn't turn, and an insensitive R4C.

Gamma matches are not protection. I doubt anyone could accurately predict
voltage, since it  is certainly not equal to percentage of the "tap point"
vs. full tower height. The energy frequency spectrum is too wide, and the
effects are complex.

What this really boils down to is a few people here (and hundreds of BC
stations) report no problems when using proper gaps on series-fed towers,
and you have no problems using gaps on a shunt fed tower.

It's the gaps, not the feed.

73 Tom

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