[TowerTalk] Pulling coax through PVC pipe

john at kk9a.com john at kk9a.com
Wed Jan 31 14:39:54 EST 2018

FWIW, when I suggested drain pipe I was referring to this:
There are no leaky fittings in my installation.

John KK9A

From:	Grant Saviers <grants2 at pacbell.net>
Date:	Wed, 31 Jan 2018 10:49:21 -0800


I agree, "different strokes" but to understand the principles, the Carrier
Psychometric Chart was pretty helpful to me as it sorts out Rh from dew
point and shows "lbs of moisture per pound of dry air". It was developed
for HVAC uses.

The variables for conduit are its (ground) temperature and air flow water
content. I think that occupied buildings almost always have lower Rh than
outside - heated in winter, AC in summer. That doesn't mean the inside air
dew point is lower than that inside the conduit, which determines if there
will be condensation. If temps inside the conduit are higher than dew
point, no condensation and evaporation of water that is present. When
reversed there is condensation, eg frozen ground and warm outside temps.
Venting to outside is another set of differential temperatures and water
content circumstances.

So IMO, with varying seasons, lagging ground temps and variable inside (or
vented) temps and moisture levels, it seems almost certain to get
condensation at times in the conduit (extreme desert climates excepted).
Sealing may help if done at the right time, but hermetic seals that
prevent in/out air flow are really hard. Broadcast stations sometimes
purge air dielectric coax with calibrated dry gases, bottled N2 or dry air
from a refrigerated dryer or desiccant system or both. Then dew points are
known to be in well into negative F temps (eg -40). Possible, but a bit
over the top for hams.

So my approach, since in conduit condensation is highly likely, is to
always use direct burial cables in conduit with no connectors or breaks in
cables. The polyethylene (PE) covered RG's or Heliax are my choice as are
PE sheathed control cables and UF rated power cables (which go in the
trench for AC, not inside the conduit for me). And water tight conduits
with glued joints. Generally schedule 40 PVC but SDR35 is a good and
cheaper choice for large diameters. Drain pipe IMO doesn't make water
tight seals with the gaskets. Why let in ground water and dirt? I've seen
the water flowing out of conduit (and out of coax!) as you had when there
is a hole in the conduit.

This is an interesting topic, maybe a few hundred threads about it on TT,
but probably 97% of water in coax is due to poor connector sealing.

Grant KZ1W

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