[BC] Air-Dielectric Sections

[Rob Atkinson]

Thanks Phil.   I asked because I had the impression that the 50 kw stations 
with ~ 120% pos. mod. used 3" rigid line because a few days ago I read here 
that 3 or 4" air heliax could not handle the full power in the event of a 
vswr change and I always thought feedline runs from the tx house to the 
tuning house were below grade whenever possible.

rob a.

From: "Phil Alexander" <dynotherm at earthlink.net>
Reply-To: Broadcasters' Mailing List <broadcast at radiolists.net>
To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
Subject: Re: [BC] Air-Dielectric Sections
Date: Tue, 06 Mar 2007 08:01:17 -0500

On 6 Mar 2007 at 0:05, Rob Atkinson wrote:

 >
 > Phil,
 >
 > just trying to learn something--how is rigid line
 > normally run below grade?
 > That is, what is it usually placed inside of?
 >

Usually, it is not run below grade. However, when it
is done properly, it would have to be placed in something
like a steam tunnel. Rigid typically comes in 20 ft.
sections and a rigid run does require maintenance. Unless
there are other things in the same run like steam, water,
telephone, utility power etc., or overriding reasons, I
would never consider putting rigid line underground in
this day and age. Today we have air dielectic Heliax
and that can go u/g safely in three different ways, all
of which have been discussed on this list in the past,
and mentioned in this thread to a limited extent.

For an existing AM plant, directional boring is probably
the best, taking the ground system into consideration.
Where that is not a problem, such as a new build-out AM
plant, or for a horizontal FM run, I'd be inclined to give
the fiber duct that Mike mentioned first consideration.
For a new plant, there is also always the old standby of a
PVC liner inside clay or concrete pipe to consider, but
I would never consider bare PVC. That is just asking for
trouble, especially where the ground freezes during winter.

---------------------------------------------
Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology
(a Div. of Advanced Parts Corporation)
Ph. (317) 335-2065   FAX (317) 335-9037



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