[BC] Air-Dielectric Sections

[Robert Meuser]

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. 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

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.
>