Actually all coaxial cables exhibit this migration over time. It is called cold
flow.
The dielectric type and how tight the bend or circle is will affect how quickly
this
occurs as will, of course, the temperature variations. This is the reason for
minimum
bending radii provided by the manufacturer and why as an example cable
television
went from rounded drip loops to the squared-off style.
Many, many moons ago, I was involved in some tests measuring cold flow using
CATV hardline and drop cables. We played games with bending radii and
temperature extremes (-40C to +60C) which would rarely be seen in the real
world. If memory serves, it would still be at least a year or two before the
migration was severe enough to cause problems.
I believe the impedance bump caused as a result of cold flow may or may not
cause a noticeable change in VSWR but would be quickly found using a TDR. Of
course, there are too many issues to make too many blanket statements, but I
think Cushcraft was just hedging their bets by stating never to use this type
of cable.
73, Bob AA0CY<@contesting.com>
----------
From: K7LXC@aol.com[SMTP:K7LXC@aol.com]
Sent: Thursday, June 19, 1997 12:23 PM
To: Dhueben@aol.com; towertalk@contesting.com
Subject: [TowerTalk] Re: Coax Balun and weatherproofing
In a message dated 97-06-19 18:54:41 EDT, you write:
>We are installing an A4S with 40M kit this Saturday...Question is this:
> The existing feedline is a foam dialectric Superflex cut to the existing
> required
> length. In other words, no excess to wind a coax RF choke. Cushcraft says
> NOT
> to use foam coax for balun. I have a new piece of RG-213 that we can use
to
> wind a choke. Would it be OK to join the two UNLIKE types of coax with a
> barrel
> connector (properly weather sealed, of course) or are the differences in
the
>
> two types of coax such that I would have problems. If I am going to have
> issues
> with the coax, I will have to buy some new RG213 ASAP. Thanks.
Howdy --
The reason that Cushcraft doesn't want you to use the foam coax is for
simple mechanical reasons. Over time, the center conductor may migrate
through the foam dielectric because of the winding of the coax. Obviously
this will gradually degrade the antenna performance so they make a flat
statement against using it. You can use it, just be aware of the potential
problem.
Yes, you can use a barrel connector and join just about any coaxes
together. The key to success is making it weatherproof. Use pliers to seat
the connectors - hand tight isn't good enough.
Next take some Scotch 33 or 88 electrical tape (use NO substitutes under
penalty of water leakage) and wrap 2 layers around the joint. Next, take some
vapor wrap butyl rubber (available from TOWER TECH) or CoaxSeal (if you don't
have anything else) and apply it over the tape. Finally wrap 2 layers of the
aforementioned tape over the vapor wrap, applying the final layer in an
upward direction. That way, the tape will shed water running down the coax
like the shingles on your house. Tape running down will COLLECT water in your
joint.
Gently tear or cut the tape from the roll. Let the tape relax before
applying the final couple of inches; do not apply under tension or it will
"flag".
BTW, some professional installers apply the vapor wrap directly on the
joint. DO NOT USE COAXSEAL DIRECTLY ON ANY CONNECTION!! It'll never come off
and the connector will have to be replaced. Commercial vapor wrap will come
off with a razor knife.
Any additional coating on the electrical tape (Scotchkote, spray
urethane, etc.) is up to you. If you don't use it, you'll still have a
longterm reliable commercial-type joint that'll last for years. Skipping any
of the steps will be hazardous to your connection.
The A4 with the 40M add-on will give you lots of fun and flexibility.
It's a good bang for the buck.
73 and good luck, Steve K7LXC
TOWER TECH -- professional tower supplies for amateurs
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