I started out to address this through Dennis's answer, but then though I'd
start with the original.
> Dear Colleagues,
>
> I'm in the final stages of arranging antennas on top of this tower
> (project)
> that you've been reading about since last May. I have run against a
> dilemma
> that I'm not sure is real or imaginary.
>
> Situation:
>
> I have a 72' crankup tower with side-arm "cable loop guides" on each of
> the
> tops of the 4 tower sections. At the top is a 20 foot mast supporting
> several
> antennas. The lowest antenna is a HyGain TH-11DX to be mounted about 1
> foot
> above the thrust bearing. Above are 4 antennas, including 432 and 1296MHz
> yagis
> at the very top.
>
> My plan is to feed LMR-400 cable directly to all antennas 2M and below
> and
> use LDF4-50A hardline to the top of the tower, joining it to a more
> flexible
> LMR-400 jumper feeding the 432 and 1296 antennas, some 16 - 20 feet above.
> This
> makes a total of 5 feeds of LMR-400. This combination was selected keep
> the
> losses to a minimum. The difficulty of working with hardline & LMR-400
> with a
> crankup tower has been carefully considered. The conclusion is that
> lowering
> the tower is a two man project.
>
> Problems, questions, dilemma:
>
> 1. Is LMR-400 flexible enough to be used with a rotator? My rotator has
> a 450
> degree travel.
>
Not normally.
Under *some* conditions you can use relatively stiff cables like LMR400 and
even 600 for rotator loops, but I doubt it'd work in this specific
situation.
I've gotten away with LMR-400 for rotator loops and no problems, but that is
with a three turn loop that lays on top of the tower (flat plate) for each
pigtail. That's an 18" diameter, three turn coil that is at it's maximum at
one end of the rotator travel. At the other end of the travel it just gets
to be a bit smaller in diameter and there isn't a lot of flexing going on.
However, bear in mind that is a bit over 14 feet of coax just for the
rotator loop in each pigtail. Although I've done it I'd certainly not
recommend it and in the case of the crank up I'd stay away from doing so.
Nor would I use hard line on a crank up without making some specific
provisions as Dennis mentioned.
> 2. How do you physically arrange the LMR-400 to allow enough slack for
> the 450
> degree rotation, once out of the top loop? Remember, the HyGain TH-11DX
> HF beam
> antenna is just about 1 foot above this upper loop.
You could provide a flat disk shaped area at the top of the tower to support
a 3 turn loop. With the beam just a foot above it that would *probably*
work. (Please note weasel word, "probably") Even using the 400UF or
equivelant I'd use at least a 1 1/2 to 2 turn loop.
It's a bit more expensive but you don't need a lot and you are talking some
432 and 1296 operation so I'd go with LMR-600 UF or a Davis Equivelant for
those bands. I'm using (or will be)LMR-600 for all VHF and UHF. As you
already know I have several hundred feet of LDF4-50A (1/2" Heliax) but I
chose to go with the LMR-600 even with slightly more loss. LRM-600 can be
found for some pretty reasonable prices, but I've not found much of a
discount on the Ultra Flex (UF) version which is in the neighborhood of two
bucks a foot.
>
> 3. How do you terminate/support the LDF4-50A to make the transition to
> LMR-400
> cable?
LDF4-50 does not lend itself well to crank up installations. It's more
fragile than the LMR-400 and 600, has a much larger minimum bend radius, and
is much less crush resistant.
I realize you've put a lot of thought into this but I'd respectfully suggest
a couple of changes.
I'd use LMR-400UF for the HF feed lines up the tower and LMR-600 UF for the
440 and 1296 bands.
I said LMR, but Davis and others have some good lines and good prices. I'd
stick with the flexible stuff, reguardless of size, for the tower and pig
tails even though they are more money.
73 and good luck,
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com
>
> 4. Anyone have photos that might help with this or perhaps a webpage URL
> showing details?
>
> All thoughts and suggestions are all welcomed.
>
> 73,
>
> Mike DiGirolamo, W4XN
> W4XN@arrl.net
> Charlottesville, VA
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|