Topband: Using CAT5 in place of coax (was video baluns)

[GeorgeWallner]

On Mon, 7 May 2012 19:17:58 -0500
  Mike Waters <mikewate at gmail.com> wrote:
>it would be interesting to 
>experiment with some
> CAT5/6 sometime, just for the fun of it. :-)
>
I did that. I was testing a small dual-feed DHDL loop (-50 
dBi gain) using twisted pairs removed from a CAT 5 cable. 
On the bench the twisted pair showed 100 ohm of impedance 
and a loss of about 2.8 dB at 1.8 MHz on a 100' run. (I 
also tried CAT 6. The results were the same. The 
difference between CAT 5 and CAT 6 is that the CAT 6 
pairs' twists are more precisely controlled, which results 
in a more uniform impedance and lower losses at the upper 
end of the spectrum.)

I was running two 16 foot lenghts of twisted pairs, one 
from each element, into a phasing/combiner box, which had 
carefully designed balanced-transformer inputs. The 
twisted pairs had #31 common mode chockes every 4 foot. 
While the twisted pairs worked, despite the balanced 
arrangement, there was more common mode (BC and noise) 
pick-up than the same runs using (non-balanced, but 
grounded) RG-179 cables. The RG-179 cables had the same 
#31 chokes as the twisted pairs. I attribute the 
performance difference to the fact that the shields of the 
coax cable were grounded, while the twisted pairs were 
"floating".

I believe that if one wants to use a balanced arrangement, 
twinax may be the ultimate solution. Otherwise, just stick 
with good quality RG-6.

73,

George