My comments embeddded below.
Gene Smar AD3F
----- Original Message -----
Sent: Thursday, March 11, 2010 4:43 PM
Subject: [TowerTalk] tower & antenna project questions
> Good afternoon. Finally, the 3rd sunny day in the 50s here in NJ, and time
> to start the antenna work! Hopefully we'll be pouring concrete in another
> week or two for the new tower. Nothing like having a new tower delivered
> with 24" of snow on the ground! Now,......
> In addition, I need to run grey conduit under ground from the base of the
> tower to where the cables will enter my home. I need to run 9 coax cables,
> and a rotor control line. 10 cables total. Each run will be approx. 210ft.
> 100ft up the tower, then 50ft from the tower to the house, then 50ft along
> the basement and up into the shack, then 10ft or so for movement in the
> shack. Due to the long runs, I was planning on going to LMR600 type cable
> to cut down on the losses, especially at 50MHz and above. I am thinking of
> going with LMR600 right up to the antennas, making sure to oversize the
> rotor loop. I successfully used solid center LMR400 for 10 years, so I
> might gamble this time again. The loss figures for LMR400 keep pushing me
> that direction! Any idea what size conduit I will need if I opt to run 9
> runs of LMR600 size cable and one rotor cable? I was able to easily get 10
> LMR400 size cables in either 2 or 2.5" conduit (can't recall, as the last
> tower/qth was 10yrs ago). I stopped at HD and looked at 3" conduit, but
> damn, that stuff just looks too darn big, especially when it will be going
> into the side of my home. I was planning on entering just above the
> concrete foundation, where there is an approx. 8" sill between the
> concrete and where the first floor rests. Same as the utilities do.
COMMENT: When I was designing electric substatoions years ago, we had a
rule of thumb for designing conduit systems: No more than 40% occupancy.
That is, we calculated the total cross-section area of the cables that would
ultimately be pulled into the conduit and make sure that figure was no more
than 40% of the cross-section area of the conduit. This rule would allow
crews to pull more cables, up to the 40% figure, without too much tension
and wrapping of cables around each other.
With Ham radio I usually go less conservative, say, to 50% occupancy.
With your proposed 10 LMR-600 cables, the area occupied is 2.8 or so sqin.
You'd need a conduit of at least 5.6 sqin cross-section. A 3 inch conduit
would give you about 7 sqin, so that should work.
But whatever size conduit you decide on, go one size larger. That is,
bring a 4 incher to the tower base if you think 3 inches is enough. You
will invariably want to add cables in the future, for example to shunt feed
the tower, half-sloper off the side, etc. (and exceed the 50% occupnacy
figure.) For terminating these conduits, see my comment below.
> One other thing - my plan was to run the conduit from the house out to the
> base of the tower, sink a few 4x4s into the ground near the tower base,
> then mount some sort of grey weatherproof enclosure on them. Have the
> conduit from the house go directly into the bottom of the box, then have
> another elbow come out the bottom, allowing the coax cables to make a drip
> loop before they head up the tower. Inside, I was planning to install a
> copper plate to which I would affix 10 or so Polyphasor arrestors, and the
> rotor control surge arrestor. Then bond the plate and tower to common
> ground rods near the tower. Can anyone recommend a suitable enclosure that
> will comfortably fit 10 or so surge arrestors, plus some room to spare. I
> took a quick look at the DX Eng website, but their stuff only accomodates
> two or three devices.
COMMENT: For my tower installation I selected Hoffman wallmounted Type 4
(watch for word wrap of the URL.) You can usually order them through an
electrical supply house or disributor.
I have one mounted at the base of the tower
and one outside the shack wall near the ground.
The one outside the shack encloses the single-point ground panel (actually,
a piece of 2 inch aluminum angle.) I used unistrut bars and hardware to
mount the boxes at both locations.
To bring the coax cable into the shack, I bored through the siding and
installed the box (I also drilled a hole out the back of the box) over that
bore hole. The cables come from the tower into the box and SPG, then into
the crawl space underneath the shack. I drilled holes for the coax from the
crawlspace into the floor of the shack underneath the operating table and
Finally, don't use elbows in your coax trench; use sweeps. These are
larger-radius turns that are smoother to pull through. Sweeps are made for
any PVC conduit diameter you choose to install.
> I also need to install an electrical outlet near the base for the tower
> motor. So I will be running a 3/4" conduit as well, from the main panel
> out to the tower base. Any objection to dropping the electrical conduit in
> the same trench as the coax conduit?
COMMENT: Good practice would have at least a foot separating the coax and
power cables. With the cables in conduits, I'm a bit murky about what the
actual separation should be. But to be safe, put the coax conduit in the
bottom of the trench, then cover with a foot of soil, then lay the power
conduit and fill in the remainder.
> Finally, as far as grounding goes - is there any advantage to installing
> three 8ft ground rods at the bottom of the tower foundation hole BEFORE I
> pour the concrete, and run the ground wires up thru the concrete base, to
> be attached to the tower once it is erect? Or just stick to three ground
> rods into the dirt around the base of the tower foundation after it is
COMMENT: Placing three ground rods that close together in the bottom of
the excavation is a waste of copper. Lightning energy will have to soak into
the same small volume of earth underneath the foundation. Separating the
rods will dump the lighning energy into DIFFERENT volumes of earth, or at
least MORE earth, so the energy density is less. You will get more
effective grounding by separating the rods in earth OUTSIDE of the
> Thanks for your insight and knowledge. I've been learning a lot just
> reading the postings of the last month or two in preparation for this
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