[TowerTalk] Radio's/tunnels

[Tom Rauch]

> I thought that the majority of the losses in typical coax were dielectric
> losses, not conductor losses. Witness that the conductors of say, Belden
> 9913 aren't much differerent in size or composition from RG-213. But the
> dielectric in 9913 is largely air, where 213 it is solid. Consequently,
> 9913 has a LOT less less than 213, especially at VHF and higher. 

As frequency is increased, the dielectric becomes more important.

At all frequencies, the conductor size and style is very important. 

Most of the loss is related to resistive loss in the conductors. 
Smooth surface conductors have less loss, as do larger diameter 
conductors.  

> Similarly, for RG-58A solid and RG-58A/U foam -- the addition of air in
> the dielectric reduces its losses. The conductors don't change at all.

I think that is incorrect. If the conductors didn't change size, and if 
the velocity factor increased through the addition of "air" in the 
dielectric, the impedance would also increase. To maintain the 
same impedance, the center conductor MUST be increased in size 
or the shield must have a smaller ID, or both. 

I think if you look at the cable, you'll see the center conductor got 
larger in the foam cable. Some cables also add a layer of foil below 
the shield to decrease shield resistance. Most low loss cables 
have smooth surfaces on the shield and the center conductor.

  

> 
> Also, consider that open wire line has less loss than twin-lead, even
> though they are the same basic construction -- twin-lead generates loss
> through the dielectric between the conductors.
> 
> I believe the point here is that a cheap, lossy coax doesn't cut it in
> this service -- while signals can certainly migrate in and out of the coax
> through the partial braid, signals in the coax are disappated by the coax
> losses -- which defeats the purpose of moving weak signals from one area
> to another.
> 
> 
> Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@radio.org
> Quote: "Boot, you transistorized tormentor! Boot!"
>             -- Archibald Asparagus, VeggieTales
> 
> 
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> 


73, Tom W8JI
w8ji@contesting.com

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