Dave, K1TTT, gave me the advice to tape the coax to the tower legs. Just
plain, good, electric tape. You should, of course, us some flexible coax at the
point when you need the flexibility for the rotation of the antenna and that
has to be movable, but the rest can just be taped in place. No, I wouldn't
recommend you to have the coax hanging along the boom but tape it there to.
Naturally, you should keep the tape tension "just right" so you don't make
indentations in the coax.
A story has it that when they installed the coax for a TV transmitter in
Gothenburg, Sweden, they managed to put the cable clamps at just the right (or
wrong) distance that the combined reflections from all the clamps came back in
phase with a catastrophic result on the SWR. I don't know if the story is
right but I am sure if you clamp or tape the coax you will not manage to see a
noticeable result on the SWR.
Good luck with your tower and antennas.
Sent: 8/24/2008 10:03:00 P.M. Eastern Daylight Time
Subj: [TowerTalk] Strain releif for vertical coax
I'm putting up my first antennas on a rotator and roof-top tower and I could
really use some help.
Sorry if these questions are simplistic for those of you who have put up at
least one mast. But being this is my first, tower, first experience with a
rotator, etc., I need some help regarding the routing and securing of the
coax for several VHF/UHF antennas sharing the same mast. I'm planning to
use LMR 400 Ultraflex (plain LMR 400 seems too stiff).
The coax will be connected to the antenna connector and then taped to the
antenna boom to strain relieve the connector.
1) Should the coax be taped to the boom all the way back to the mast and
then down the mast? Or should it be allowed to fall vertically somewhere
before it gets to the mast in order to make the rotating loop easier? (Bear
in mind there are 4 antennas on the mast.)
1a) Just in case your answer is to not tape it all of the way back to the
mast, it seems like letting the coax simply hang from the antenna boom would
put stress on the boom and also, eventually, pull the tape free. These are
VHF/UHF antennas which means they are light weight and probably would not do
well with coaxing pulling down on them. Suggestions?
The lowest antenna is planned to be 2 feet above the thrust bearing in order
to have some room for a cable loop for the rotator. It's feed point is
close to the mast. So that cable will probably be taped to the mast for
strain relief. So now I've got to make a loop to allow for the rotator to
Simply letting the coax hang doesn't seem right. As the rotator turns, it
will have to haul the coax up as it reaches full rotation in either
direction and the coax will be rubbing against the mast and guy wires.
Plus, there are four cables so that's some added strain for the rotator. So
I presume I need to hold the coax up so that there is a strain-relieved loop
of free cable to allow the rotator to turn easily.
2) How do you hold the coax up to make the rotator loop? Just tape it to
the tower leg? Seems like if the tape is in the sun and becomes soft, the
coax would eventually slip down.
3) How do you make certain that the twisting applied to the coax doesn't
get translated into twisting at the antenna? Just plenty of tape to
strain-relieve the antenna connector?
Thank much in advance.
Michael - K5MEF
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