John Brosnahan wrote:
>
> At 05:15 PM 7/6/97, you wrote:
> >>> It wasn't that good and we ended up
> >locating all
> >of the splices with the help of a TDR, digging them up and redoing them
> >. <<
> >
> >Gee... If you really mean hardline (and in radio astronomy I believe
> >that you mean real hardline -- frequently run with refrigeration lines
> >for the low noise head) you would think that there would be a high
> >quality dehydrator circulating and constantly drying the air (or other
> >gas) that becomes the dielectric in the transmission line.
> >
> >Joe / W8SS
>
> Yep, I mean hardline--basic foam-filled 7/8 inch, 100 ohm stuff that was
> surplus.
> Not all radio astronomy is at microwaves. We did a lot of HF and low VHF
> radio astronomy--primarily Jupiter--Io effect and solar emissions.
>
> Sky background was the limiting factor. This was a low budget operation--
> hence the ammo boxes to protect the splices--but it this case the "protection"
> was the problem.
>
> John W0UN
>
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I had a similiar experience at work. A moderate size multi 10 KW HF
site, all antenna feed lines 1 5/8" Andrew foam. Because of orginial
design specs,all feedlines were run in 2 1/2" black iron pipe.
This was for EMP protection, and other considerations.
When we installed a new larger size cable vault and some other upgrades
two years ago I noticed all feedlines were full of water.
Water poured out of the conduits when we broke them open. The water
just migrated through the foam dielectric. Turned the copper inner
conductor and outer conductor sheath in the Andrew cable green.
I have been told by several professionals in the tower trade
that yes even PVC conduit catches and retains water. Even the local
(to me) Andrew Antenna marketing/engineering manger confirms this.
Some even suggest the drilling of weep holes on the underside of the PVC
and to run the conduit with a pitch to it so water will not stand.
I often wondered if using 4 inch perforated black irrigation pipe
(comes in 100' foot rolls and longer) with the pipe back filled in
either sand or crusher run stone would drain? It is cheaper than
electrical conduit. Then you have the best of both worlds, a
path where replacement of feed lines can be done, additions made,
and a system perhaps drier(?) than solid wall conduit.
Even with direct burial of hard line, most vendors (Andrew for one)
suggest that the backfill be sand, not the orginial material removed
when trenched.
Dan Szymanski
K3SKE
Frederick,MD
PVRC/NCDXA
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