[TowerTalk] Radio's/tunnels

[Bill Coleman AA4LR]

On 5/5/00 1:39 PM, K7GCO@aol.com at K7GCO@aol.com wrote:

>The reason for the use of lossy coax with slots was to spread radiation full 
>length of the coax run--not heat.  Cheap RG8 with semi open mesh will do 
>about the same.  The actual resistive loss of this coax is just slightly 
>less 
>than full mesh shield.  80% of the attenuation of coax is in the shield if 
>it 
>is full.  This is why increasing the size of the center conductor in polly 
>foam reduced the loss of RG8 .2 db/100' at 30 Mz.  The reduced loss of using 
>a foam dielectric doesn't start to be an advantage until above 30 MHz.  k7gco

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. 

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.

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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