[TowerTalk] Replacing antenna jumper
Wed, 11 Nov 1998 11:39:43 EST
In a message dated 98-11-11 08:17:47 EST, firstname.lastname@example.org writes:
> I said:
> > You should replace the antenna
> > to tower jumper (10 feet long)
> > every year.
> > (The piece that winds es unwinds.)
> Steve K7LXC said:
> Why would you need to do that?
> Re: by winding es unwinding the jacket
> gets a workout, eventully water will start
> entering cracks, degrading the inner surface
> of the shield es intering the foam if used
> (a good reason NOT to use foam cable here)
> and increasing the attinuation of this cable.
> This makes an antenna problem (read: loose
> clamps on the ends of the straps) here, harder
> to detect.
> You can reuse the connectors (once you learn how)
> so it only costs you 10' of RG8-U (or RG-213/214-U)
> to remove this `weak point' from the most
> important part of the H.F. station. This should
> be done at the annual tower/antenna inspection
> p.s. any water that has entered into these connectors
> will also befound and removed.
Hi, Ron --
I do agree that preventing water penetration into coax and connectors is
vital to the realiability any antenna installation, however I do not agree
that the jumpers should or even need to be replaced yearly. If your jumper
jackets are indeed cracking, that's a sign of age and perhaps due to your
choice of materials. I can't imagine new coax, even with a regular PVC jacket,
breaking down that quickly. Even exposed to extreme UV it will last several
years with no problems.
Perhaps your rotation loop is too short. A lazy loop of cables will have
hardly any tension on it even through 360 degrees of rotation. I always have a
new installation put through a couple of 360 degree rotations while I'm on the
tower watching it to make sure that everything is clear and doesn't put any
strain on the cables. You can also listen to the rotator to make sure it
doesn't bind while you're at it.
I've worked on over 125 different amateur installations and I personally
haven't run into this problem. This is also impractical for many antennas
since the feedpoint is out of reach of the tower.
An alternative to a big rotation loop is to run the cables down the mast
and then coil the cables around the mast at the top of the tower 2 or 3 times
before they go down the tower leg. There is enough slack in those (also lazy)
coils that the cables actually move very little during rotation and there is
no strain on any part of them since they are not weight-bearing. This is like
the semi-rigid heliax that you see coiled around those hydraulic masts in
electronic news-gathering vans. It can go up and down as well as being rotated
with no effect on the cable.
Cheers, Steve K7LXC
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