It is certainly a method I like to employ when at all possible. Our
rotor allows 720 degrees of total rotation and that can be sometimes
hard to achieve using the traditional big drip loop of coax taped to the
mast and the top of the tower. The advantages are mostly obvious but
there are a couple less so - it is easier on the coax when turning at
-40 degrees. The rotator works fine at that temperature but I worry
about cracking the coax jacket. It does happen at these temps. I
remember pulling a length of coax into the garage on a very cold day.
The coax turned a fairly sharp corner where it went in, however the
jacket carried on straight. It shed 20 or 30 feet of jacket before I
realized. Anyway, I digress..... The other possible advantage is that
if the pipe is long enough you may get some RF (maybe lightning too?)
choking action running thru the pipe. On the 80m yagi (no I'm not
using the AlfaSpid on this one) I run down the tower thru 150 feet of
pipe.
For Mark and others, the coax has to run up and then bend 180 degrees
(at something greater than the minimum bending radius of whatever coax
you are using) and then run down the inside of the mast pipe. Well
taped at this point to support the weight of the coax and prevent
chafing. The AlfaSpid gearbox has a hollow center (you may need to
remove one small bolt running thru the lower mast designed to keep the
rotator from sitting too low on the lower mast) of just a bit over 3/4
inch. This allows a standard N or PL259 to go thru, or 1/2" heliax (or
similar) if you put the connector on later. I just let the coax drop
straight out the bottom for maybe 10 feet or so and then tape it to the
tower, taking the weight off and allowing slack for it to twist as
needed. Our flat plate mounting adaptor also has a hole in it to allow
the coax to continue thru. With one coax only, purists could employ a
RF rotary joint here. Homebrewers could try a home made rotary joint
from a BNC connector. If you have more than one feedline you will have
to compromise and use smaller coax to fit them all in.
I don't know if any other current production amateur rotors have this
feature (I'm not aware of any so far) but I certainly think it is a
handy feature. It is quite common in the very large rotors for military
log periodics and such.
73 Don
VE6JY
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Mark .
Sent: Tuesday, March 07, 2006 1:25 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Routing coax through the rotator (was dirty trix)
Don, This sounds like an innovative idea. Would you describe in more
detail
the path the coax takes from antenna to tower? Are you also routing it
through the mast?
--...MARK_N1LO...--
Don writes:
<< ...snip..Whenever possible, I run the coax thru the rotor and out the
bottom pipe
so even if something does slip, it is a simple matter to recalibrate.
With ours <Alphaspid>, there is no limit (other than the twist of the
coax)
to how
many degrees you can zero out...snip...73 Don VE6JY >>
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