For the record, regarding the original question, I concur with the
suggestion to use Phillystran or another non-conductive guy. There are many
other good reasons to use non-conductive guys (you can tell I'm a convert),
but I won't go into that.
FWIW, I think another reason for minimizing the length to the first
insulator, if you are using conductive guys, is to prevent any non-linear
diode type effects occuring in the guy to tower connection. The point at
which a conductive guy meets a tower is not a hard bonded connection, but is
rather a small surface to surface connection, e.g., typically a thimble or
wrapped guy cable against a bolt. It is prone to developing some kind
corrosion, after which that joint begins to act like a diode and begins
rectifying AND CREATING HARMONICS when RF currents pass through it. The
only answer is to either keep the lengths of conductive elements on either
side of the diode short or install a bonding strap across the joint.
It was a long time ago, but I worked in the field as a civilian Navy
engineer and we had tremendous problems with RF noise created by such
non-linear joints that seemed to be everywhere topside on typical Navy
ships. MIL-STD-1310, at the time and perhaps still today, defined the
bonding and grounding remedies required.
To be honest, I've never seen a documented evidence of harmonics being
created in a ham installation due to this effect. Maybe the typical tension
on the guys we use is enough to maintain a linear connection.
Anyway, that's some related food for thought.
73, Bob - W3YY
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of
donovanf@starpower.net
Sent: Tuesday, September 04, 2007 11:45 PM
To: TowerTalk
Subject: Re: [TowerTalk] Insulator Placement Clarification
Dick,
You question is much more complex than it would appear because of the five
ham bands covered by a log periodic antenna. There's no simple answer, and
an EZNEC model would provide the most accurate understanding of the
tradeoffs.
The simple answer for a 5 band antenna is to use Phillystran or fiberglas
for at least the top 40 feet of each guy. Otherwise, use on
insulator every 8 feet or so for the top 40 feet of each guy.
73!
Frank
W3LPL
73!
Frank
W3LPL
---- Original message ----
>Date: Tue, 04 Sep 2007 19:26:42 -0400
>From: "Dick, W1KSZ" <w1ksz@earthlink.net>
>Subject: [TowerTalk] Insulator Placement Clarification
>To: TowerTalk <towertalk@contesting.com>
>
>Dumb question of the week:
>What if the antenna is a Log Periodic (10-30 MHz?
>
>73, Dick, W1KSZ
>
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