This topic comes around on a regular basis. Lots of comments in the
archives.
My approach - 1. water tight sch 40 conduit so ground water and muck
stays out, critters can't (usually) get in, traffic doesn't damage
cables, and I can add/remove cables. 2. direct burial sheathed (usually
polyethylene) cables in the conduit so water can't get inside them. 3.
unsealed conduit ends. Even though pressure and temperature
differentials can allow moisture in, I think (hope?) it about averages
out over a year cycle. 4. I don't worry about condensation. If I had an
above grade low spot for a drain, I would have one but my conduits are
basically level, and with normal trench depth variations there are
probably low spots that wouldn't drain.
If you worry about condensation, then sealed ends and super dry air or
bottled nitrogen or CO2 purging will work, just like very high power air
dielectric coax. $$$$ The air needs to be dryer (lower dew point)
than the lowest temperature ever experienced by the conduit. Sustained
very low dew point purging is needed to make wet conduit and the
contents dry out.
In theory an oil less air compressor and refrigerated shop air dryer
would work if you are willing to buy them and pay for the kwh of
electricity. My shop dryer is good for 40 d F dew point air at 150 psi
which when expanded to ambient 14.7 psi has a dew point of -10 d F (I
found an on-line calculator at
http://www.dew-point.com/dew-point_calculator.asp) (an interesting fact
- why wet things can be dried so quickly with a shop air blow gun, I
didn't know that ambient value).
A good thing to do is check coax for loss every few years. A dummy load
and the same swr bridge measuring power at the near and far end for
precision makes that easy to calculate the db's of loss say at 10m.
Grant KZ1W
On 1/28/2016 5:22 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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