Jamie,
First of all, you are not being overly cautious!! What you
are wrestling with are the normal engineering tradeoffs in
a complicated project.
Next, visit my Website and look at the design approach I outline here:
www.erols.com/n3rr
I suggest a cable loss of less than or equal to 1 dB from your XMTR to the
antenna. You have a chance to tradeoff cost
for dB by using the W3LPL attenuation tables I have provided on my Website
under the "Cabling" link.
If you are concerned about breaking a center conductor due to raising and
lowering the tower, don't use
a solid center conductor cable. Use RG-213 instead. That's what I use on
my crank up, though I rarely
raise or lower it.
As far as the base of the tower is concerned, you need to consider:
lightning protection
coax cable transition from one type to another
connector weather proofing
rotor cable lightning protection and cable transition from one type to
another
ease of cable additions (for more antennas) and cable deletions/replacements
(for repairs).
All coax cables should be grounded at the antenna and at the bottom of the
tower.
To not do this means that the lightning voltage would ride the coax to your
SPG on the house.
My rule is: move the lightning to ground at your first opportunity.
Again, look at my Website for ideas. Use the ones you like and leave the
rest.
Question: why have 120 VAC at the base of the tower? This usually requires
an additional permit
(electrical) from the municipality and may introduce hum into your RX
cables. We recently had a discussion here
on how to minimize this issue, so you may be aware of it.
Lots of things to consider.
Bill, N3RR
-----Original Message-----
From: owner-towertalk@contesting.com
[mailto:owner-towertalk@contesting.com]On Behalf Of James C. Hall, M.D.
Sent: Tuesday, August 21, 2001 12:53 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Coax recommendation
Gentlemen:
I am trying to decide on a proper cable arrangement for my 72' crank-up
tower which is 125' (approx.) from the shack that will have Force 12 Mag
620/340 and 4BA on the mast with 10' separation. My thought is to use
LMR-600 from the SPG to the tower junction box and much more flexible stuff
to the coax switch, from the switch to the tower leg grounding point, and
from the SPG to the shack. My question pertains primarily to the run up the
tower to the antennas (4 of them). It has been suggested to use LMR-600
with its low-loss properties up the tower to a point (at the tower top
grounding point) and then use flexible stuff (9913 or LMR-400UF). My
problem is that it's a crank-up tower that has the potential to crack the
solid center conductor when it is cranked down. Am I being overly cautious
here or is this valid ? Concrete will be poured this Thursday !! (Yes, I AM
excited ).
I plan on placing a small junction box at the tower where there will be
received 120 VAC as well as a place for strain relief and weather protection
of the connectors. Now , I'm kinda new at this. Please explain why this is
necessary. Originally I had planned to blow all these cables down the PVC
to the SPG where the lightning protection gear would receive them and then
on to the shack. Am I missing something here ?
Jamie
WB4YDL
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
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-----
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Problems: owner-towertalk@contesting.com
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 96 feet for under $1500!! http://www.anwireless.com
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
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