[BC] Air-Dielectric Sections

Fri Mar 9 07:45:59 CST 2007

Andy,

Fiber duct is made up to 10".  But they come in 50' straight sections 
beyond 5" and special coupling equipment is needed to properly couple the 
sections.  Hence why I suggested 5".

If you are going with rigid conduit, some key points....

Order 60" wide sweep elbows if you need to make any bends.  They are 
special order elbows and are not cheap at about $300 per elbow at that 
size. They come in 45 and 90 deg. only.  This is commonly used on utility 
service feeds.  So a GOOD electrical supply house can get them.  But you 
must specify 60" wide sweep.  OTOH, a really big electrical contractor 
might have their own bending gear and can make one to spec for you if 
needed.  Like 45 deg. bends which would take up a whole 10' section and 
thus increase the radius to something around 75" for a 90 deg. 
bend.  That's HUGE..

I would also strongly advise Cadwelding #2 or bigger conductors to the 
conduit at the ends and every 100 ft. (for 1000 Khz or lower) and bond them 
to co-located ground rods driven along the conduit's length as well as the 
adjoining system ground system strap.  Sitting in sand, the conduit could 
become an insulated inductor of sorts because of it's length.  Add should 
it contact the ground at either end, there could be substantial potential 
differential at the end not bonded to the ground system.  This would be 
especially the case in a 90 deg. length of steel.

Additionally, the grounds on the conduit would aid in shunting of 
electrical faults as well as lightning strike energy. Further, the grounded 
conduit will also tend to aid in diverting energy from a lighting strike 
back to the ATU's systems.  Again, it has to do with the torroidal nature 
of a long run of conduit on the line and common mode inductance.

4" line will not alter the electrical design of the phasor.  Just the 
mechanical aspect as the attaching point will need to be quite rigid and 
strong.  The best thing would be to go the EIA route like FM's and run the 
flex line to a point just above or below the connection point at the 
phasor, then transition to the phasor via a mitered elbow and section of 
rigid line. That way, you can pressurize the line at the connector in a 
manner identical to FM's as well.  I really do line to have the tubing run 
into the phasor if I can avoid doing such.

OTOH, 3" line will be fine.  WGN, WLS, WSCR and WBBM all run 3" and have 
for years on their big 50's.  I see no reason to spend that much more on 
4"...both to buy and install.

Cheers...

MM

At 08:08 PM 3/8/2007 -0800, Andrew Skotdal wrote
>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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