[BC] Direct Burial Transmission Line...

Mike McCarthy Towers
Wed Mar 7 08:07:24 CST 2007


Be really careful about above ground installations.  Especially of sample 
lines.  Unless the sample lines are all equal length, thermal changes can 
and will impact the line's physical, as well as resultant electrical 
length.  Even on phase stable lines. I consult one station which has same 
and the pattern's readings shift about 1-3 deg. when there is spike in 
temps.  Not so much the ratio, as much as the phase moves. But it's 
noticeable.

It's less an issue in areas where the real temperature range is not the 130 
deg. or so experienced in the northern tier of states.

I've also used rigid conduit in some applications and usually for the last 
30 or so ft. from the pull vault to the ATU box or shelter.  I would use 
rigid where I could except at $50/stick of 3" or $100/stick for 5", plus 
10-15 minutes per stick to install, 3000 ft. gets really costly REAL 
fast.  That and conduit of any type can sheer and kink...though the bigger 
pipe is more resistant to sheer.  But not to kinking.

Many people under-estimate the durability of fiber duct.  It's MUCH tougher 
than PCV conduit and yet more flexible.  It won't crush or split  readily 
(there is a spec for crush resistance and burst pressure) and can be 
ordered by color so you can easily identify the duct when digging in later 
years. (I always order orange.)  Interior ribbed duct also  pulls MUCH 
easier than smooth wall conduit due to the reduced points of contact with 
the cables being pulled.

While far more costly than direct buried cables, only one instance of 
repair/replacement will pay for the adder.  Been there....

MM

At 08:40 PM 3/6/2007 -0800, Burt I. Weiner wrote
>Not to long ago I had to replace all of the directly buried transmission 
>and sample lines at a station.  I installed the new lines above ground, in 
>conduit.  The manufacturer was surprised as the cable was all specified 
>for direct burial.  It may be have been water proof but it certainly 
>wasn't tooth-proof.  If you must go underground at least put it in good 
>conduit that's tooth-proof.
>
>Burt
>
>
>At 07:44 PM 3/6/2007, you wrote:
>
>>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
>
>Burt I. Weiner Associates
>Broadcast Technical Services
>Glendale, California  U.S.A.
>biwa at earthlink.net
>K6OQK
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