[TowerTalk] Radio's/tunnels

[Eric Gustafson]

Since our company makes a lot of its income using leaky feeder
communication systems in underground mines, I feel somewhat
qualified to pronounce on this subject.

In a leaky feeder system it is _extremely_ important that the
losses in the cable are domnated by radiation losses and not by
losses to heat.  This has nothing at all to do with the power
levels involved.  It has to do with the overall cost of the
system.  Any signal lost to heat as inherent cable loss does not
contribute to the desired loss by radiation from the cable.  The
fact that the quantity of heat is far below the level required
for human perception is irrelevant.

Both the cable itself and the BDAs required to maintain
sufficient signal in all parts of the system are very expensive.
The more loss to heat, the more frequently the BDAs must be
placed in the line.  This drives up costs due to both initial
equipment cost and to higher maintenance and lower system
reliability.  Nobody would spend $3 to $8/ft for 4 miles or
sometimes much more of radiax cable if they could do as well with
low shield coverage RG-58!

As to the power levels actually used, our system drives the head
end with about 0.8 watts per channel.  By the time that the head
end combiner and equalizer puts the signal on the cable it is
a little over 300 milliwatts.  So on a 30 channel system with all
channels occupied and simultaneously operational, the cable
_might_ see something in the neighborhood of 10 watts total
power.  And that level would be seen only at the "upstream" end
of the wire.

So the bottom line is:

1.  Yes, loss to heat is a big problem requiring the use of very
    high quality cable specifically designed for the purpose.

2.  No, the power levels used are not excessive.

73, Eric  N7CL


>From: K7GCO@aol.com
>Date: Fri, 5 May 2000 02:09:36 EDT
>
>In a message dated 04.05.00 12:48:26 Pacific Daylight Time, 
>mconatore@fredmeyer.com writes:
>
><< 
> But with cheap coax, you have a lot more loss in heat rather
> than radiation, don't you?
>  >>
> 
>If heat is a problem the power levels used are excessive.  I
>have no idea what the heat loss vs radiation loss ratio is.  Is
>there a need to know this?  I'm not sure the reason for the
>question? k7gco

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