[BC] Air-Dielectric Sections

Thu Mar 8 22:08:35 CST 2007

Thank you to everyone who took the time to respond to my initial post
with the question on this topic.  You are correct.  This is for a 50kW
omni-directional operation at a brand-new transmitter site.  We intend
to bury 3" air line (heliax?) at least three feet deep (not rigid line),
in 6 inch conduit, with lots of sand around the conduit (42 inch deep
trench, 6 inches of sand, conduit, and 6 additional inches of sand
on-top of the conduit with warning tape and dirt backfill over that).  

It now appears we may be able to get one complete run of transmission
line that extends 750 feet or so (I incorrectly believed that 400 feet
was the manufacturer's max length and thus asked about the joint where
the sections of transmission line would attach to each other.)
Fortunately, this means we don't have to deal with a joint for two
pieces of transmission line, if it's true that a pure length of
transmission line is possible.  We'll seal up the conduit at both ends
to avoid rodents.  As for fiber flex conduit, I'm open to the idea.  4"
Heliax is a new wrinkle, though.  I hadn't considered that. I'd probably
cause the phasor designer to have a heart-attack over changing the size
of transmission line and thus the velocity factor.  I guess it never
hurts to ask! 

The advice you've provided allows me to ask questions of the people
helping me, which is also very helpful.  I've never done this before,
and it's really my only opportunity to ever build a 50kW plant, so I
want to get it right.  I appreciate your help.

Andy Skotdal

-----Original Message-----
From: broadcast-bounces at radiolists.net
[mailto:broadcast-bounces at radiolists.net] On Behalf Of Glen Kippel
Sent: Thursday, March 08, 2007 4:42 PM
To: Broadcasters' Mailing List
Subject: Re: [BC] Air-Dielectric Sections

Actually, you could fill the tunnels with carbon dioxide -- nothing
could
breathe in that, and it's heavier than air so it won't go anywhere.

I was reminded of that when I heard about a pair of campers that decided
to
fill their air mattress from a CO2 fire extinguisher.  Well, it sounded
good
at the time.  But, the mattress sprung a leak, the campers settled to
the
bottom of the tent and suffocated.

On 3/8/07, Rob Atkinson <ranchorobbo at hotmail.com> wrote:
>
> <<4 Inch Heliax is pretty common for buried AM TX line at 50 KW. I
think
> the
> discussion you are referring to was the one where someone wanted to
use 1
> 5/8" Heliax at 50 KW which is not OK.>>
>
> yes, that was the one.  I did not realize the power handling of 4 inch
was
> so far above that of 3 inch.
>
>
> <<4 Inch Heliax (HJ11-50) has an average power rating of over 600 KW
at AM
> frequencies and a peak power rating of over one megawatt. This is of
> course
> at moderate temperatures and low SWR.
>
> One big technical advantage that proper cable burial offers is that if
> buried deep enough it stabilizes the temperature, keeping the cable at
a
> better operating point than if it is exposed to high above ground
ambient
> temperature and solar load. Also buried cables are inherently more
> electrically stable when used in directional systems.For a new plant
(no
> ground system) a 10 foot trench with lots of sand to protect the cable
> works
> well for the larger air dielectric lines. The larger lines can be
quite a
> challenge to pull through ducts unless they are more like tunnels and
have
> manholes at strategic intervals. There are a few 50KW stations with
such
> an
> arrangement. The problem is that you have also created a nice safe
home
> for
> critters, unless you can pressurize it all with nitrogen which is
> unrealistic.
>
> R
> >>
> Thanks so much for taking the time write such a thorough and easy to
> understand explanation Robert.  Much appreciated.
>
> rob atkinson
>
> Rob Atkinson wrote:
>
> >  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.
> >
>
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