[BC] Air-Dielectric Sections
Robert Meuser
Robertm
Fri Mar 9 04:24:07 CST 2007
A few suggestions and comments.
First, 3 feet is too shallow, consider a minimum of 6 feet. You
definitely want to be safely below the frost line and then some. Second,
I don't see how you can easily pull 3 of 4 inch line line through 6 inch
conduit. That conduit is way too small for the cable and at the minimum
you will have bending radius problems. If you use conduit I would
suggest something at least around 1 foot. Depending on soil type, dig a
deeper trench and bury the cable directly in sand. I would also
suggest you you check out exactly how much weight you are dealing with
with 750 feet of line on a reel. This greatly impacts upon the handling
of the line during installation. Finally the is little or no difference
in the electrical specs of 3 or 4 inch line other than power handling.
You said this was an omni station so there is no phaser, just an ATU
changing line size will make no difference.
R
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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