[TowerTalk] Coax, conduit and toroids
jimlux at earthlink.net
Tue Jun 26 20:00:33 PDT 2012
On 6/26/12 5:30 PM, Rick Kiessig wrote:
> I'm interested in suggestions for how to best segment the coax running
> between my shack and tower-mounted antenna. I'm thinking of something like
> this: (1) from shack to the base of the tower through conduit, tied to
> ground rod via lightning protector, (2) up the tower, (3) from the top of
> the tower to the antenna, tied to #2 using a barrel connector and wrapped
> with weatherproof tape. The runs are fairly short, roughly 20m from the
> shack to the base of the tower and 10m up the tower. Have I missed anything?
that seems reasonable.. you have breaks at the places you're likely to
change something... and, if you get a strange effect in the shack, you
can go out to the base of the tower, disconnect, and check the coax to
the shack separately from the coax to the antenna.. Is it a "dig up the
lawn" problem or a "climb the tower" problem...
> I'm thinking of using LMR-600 for the first two segments (seems easier to
> handle than Heliax)
Very much easier..
, and LMR-400UF for #3, with UHF-style connectors for
> each. For my current setup, I ground the coax before it enters the shack.
> The new tower will be right next to the house, so the tower ground and the
> before-entering-the-shack ground will be the same thing.
If the run is short, just how much do you gain by not using something
LMR600 is $2-3/ft, depending on if you get the DB or fire resistant or
other flavors. 0.4 dB/100 ft loss at 30 MHz
Compare to generic RG213 types at a $1/ft and 0.7 dB/100 ft...
for a 50 foot run, is $50-100 worth 0.2 dB?
You might pull the inexpensive stuff first and see how it works. Spend
the extra bucks on something more "useful"
> What should I do to help ensure that the inside of the conduit stays dry? Is
> applying some sort of weatherproof sealing compound to each end enough?
You will not keep the conduit interior "dry".. water WILL condense
inside because the soil is cooler than the air, and any exchange of air
from inside to outside will bring moisture in. There's all kinds of
strategies for drain holes and dry wells underneath which might work to
avoid huge pools of water
Your best bet is to have a decent vent, and cable that can tolerate
being wet. People who need to keep the inside dry run a positive
pressure to push dry air into it.
> I'm planning two coax runs and two control cable runs, in two separate
> conduits, coax in one, control in the other. What's a reasonable diameter
> and turning radius for the conduit? The Times Microwave
> specifications/limits are one thing, but I also want to do what I can to
> ease/simplify the process of pulling the coax through the conduit once it
> gets installed.
The cost difference between 2", 3" and 4" isn't huge. Make sure you can
pull the *connector* through the conduit if you want to.
BIG sweep elbows.. something like 1 foot radius at a minimum. If you're
using something stiff (LMR600 is pretty stiff), make the sweep
comparable in radius to the spool the coax comes off of.
> Are there any precautions I should take with regard to running the coax or
> control lines near existing electrical wiring for the house? I'm trying to
> avoid proximity as much as I can, but I'm wondering how far I should go.
Code just requires it be "separate".. that is, some barrier between
power wire and your wire.. the conduit wall meets that need.
> Is it a good idea to create a coil of coax in or near the base of the tower,
> run through ferrite toroids? I do this at the base of my vertical, but I'm
> not sure if it's as useful for a horizontal antenna. I am planning to
> continue to use toroids at the shack end of the cable. Should I also run the
> antenna and rotator control cables through toroids at both ends?
I do that, but out of an abundance of caution rather than rigorous
analysis. I have a box of 31 mix cores, so it's easier to just shove
cores in, hither and yon, than to figure it out later.
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