[TowerTalk] another subject: coax and cables buried or elevated above ground?

[Roger (K8RI) on TT]

My Opinion:

I vote for underground as induced voltages from lightening strikes are 
likely to be less.
Most of us use PVC because its cheap and easy to work with. The same 
holds true for plastic, thin wall sewer pipe  I no longer glue the 
sections together, but lube them with DC-4 or 5 compound and try to 
install them in cool, or cold weather, under some compression.  I've had 
no problems with water ingress with both ends open (at the tower, 
basement, or shop - basement and shop are heated and air conditioned) 
Current connections and conduit runs at tower end are shown in
http://www.rogerhalstead.com/ham_files/cablebox.htm.  That sewer pipe 
hanging in the air over the trench "was glued".  I've found the NEMA 
boxes to be a much more practical method of entry into the basement and 
shop.

"I think" steel conduit would be better.  It protects from ground 
currents, which can be substantial on rare occasions, BUT it's 
expensive, heavy, requires tools to work with, and requires careful 
attention to deburing. Threads need to be sealed and protected from 
rust.  I don't know of anyone using underground, metal conduit.  I 
worked with it a lot in industry, but we had powered benders and access 
to a wide variety of shapes from vendors.  It's pretty much schedule 40, 
black iron pipe. (Relatively easy to bend if you have the tools.  To say 
the manual benders for EMT do not work well with rigid conduit would be 
an understatement!   We used it in rather hostile environments (very 
hostile), often packed "tight" with wires. So tight we used a LOT of 
wire pulling lube/soap and electricians beating on the conduit with 
engineers hammers every few feet. Not a problem for hams who only half 
fill the conduit for ease of adding or removing cables. OTOH it's not 
exactly practical for the individual, or even club projects. "Normally" 
the ground in this area is either moist, down right wet, or frozen, so 
rust would likely be a problem.
 From a practical approach, rigid conduit is a major PITA to install and 
remove.

EMT is lighter and easier to work with. Connectors, splices, and sweep 
90s are readily obtainable, but rust can/would be a problem.  I use EMT 
in the shop where all wiring (including coax and rotator cable) is in 
surface mount EMT, but PVC exclusively outdoors, buried about a foot 
down.  Below the frost line would be better  (The ground does freeze 
every winter up here), but I lacked the ambition for doing that.  Now 
days it is rare for the ground to freeze more than 12 inches deep except 
under driveways and sidewalks, but according to local code, I believe 
it's still 30 inches, give or take a tad. I'm pretty good at sweeping up 
to a couple inches snow off the sidewalk (with one working hand and arm) 
between the garage and shop, as long as it's not the wet, heavy kind, 
but then I have to get back to pick up my cane.<:-) <sigh>

As there are no coax or cable connections inside the conduit, I no 
longer worry about water accumulation and make no provisions for 
draining condensation.

73

Roger  (K8RI)


On 1/28/2016 Thursday 8:22 AM, StellarCAT wrote:
> My ongoing tower/antenna projects... hopefully I’m not boring others out there...
>
> I had planned on burying all lines to the towers (2, one at 150’ out and one at 300’ out) ... at first I thought in solid joined (sealed) PVC which I can get for $9/10’ ... and I’ve also thought about direct burial which for the coax and control cables has the added benefit of increased capacitance to ground and thus a reduction of energy (lightening event) that reaches the shack... but control cables aren’t normally rated for direct burial... or at least I don’t think so (researching that now)...
>
> Then this latest QST article on coax says DON’T bury in any type of conduit be it plastic or otherwise. It says condensation WILL get inside and ruin (flood) the cables.
>
> So .... if the cables are underground – below the frost line – say minimum 1’ down (upstate SC) ... how will condensation develop? Or will it? I’m an engineer (HW/SW development) and reasonably intelligent but find it difficult to wrap my head around this issue... will it or won’t it – flood that is? if it were always under ground – including both ends then I’d think not.... but it isn’t always under as the ends come up to and above the surface... does that introduce a temperature variant enough to induce condensation? And I don’t like the idea of holes in the plastic – that just means mud will indeed eventually work its way inside.
>
> Maybe use corrugated tubing (again – no cuts or openings) ... this would allow whatever moisture there might be, and I can’t imagine its going to be a great deal, to settle in the lower points of the corrugation.
>
> Comments from others that have been through this - in the south with similar conditions would be appreciated.
>
> Alternatively I could use a leader line and go from say 15’ on the first tower to the second tower and then that same level to the shack with supporting 4x4 poles along the way... but this seems iffy as well as first the cables are ‘leaving’ the tower above ground potential so that might mean a higher voltage (common mode – probably?) on the lines (again: lightening type event)... and the first tower is a rotating tower ... although thinking about it that might work quite well to go from the tower with a ‘swing-arm’ of hanging coax/cables to allow rotation.
>
> thoughts?
>
> Gary
> K9RX
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-- 

73

Roger (K8RI)


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