[Top] [All Lists]

[TowerTalk] Fwd: Strain releif for vertical coax

Subject: [TowerTalk] Fwd: Strain releif for vertical coax
Date: Sun, 24 Aug 2008 22:47:44 EDT
List-post: <">>
Hi Michael,
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.
Hans, N2JFS
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


TowerTalk  mailing  list

**************It's only a deal if it's where you want to go. Find your travel 
deal here.      

TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>