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

[Jim Brown]

On 5/7/2012 6:12 PM, GeorgeWallner wrote:
>   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".

Hi George,

Thanks for sharing your work on this.  Your observations make sense, 
especially the comment about the coax shield being grounded minimizing 
common mode.  We know that a multi-element common mode filter can be 
formed by a ferrite choke followed by tying the shield to ground at 
additional point, and even by a second choke.

In private email, Herb raised the issue of common mode chokes on full 
size cores getting fairly expensive. One way I was thinking of using the 
CAT5 was to use one pair removed from the cable to wind chokes around 
smaller, less expensive cores, and inserting that choke into coax lines 
that are grounded as noted above.  #31 material is still likely to be 
the best core material, and you still need a lot of turns.  While one 
could certainly get anal about it and make a lot of measurements, for a 
choke intended for 80 and 160M, I'd start with 15-20 turns, and consider 
"more is better" a pretty good rule of thumb.

One other point.  While the loss in coax feeding a Beverage is no big 
deal, shielding is important, and shielding effectiveness is directly 
related to shield resistance at the frequency of interest.  Coax 
designed for use in UHF CATV systems generally has foil shields with 
flimsy braid, and the resistance at 2 MHz is fairly high.  Poor 
shielding effectiveness converts common mode current on the shield to 
differential voltage inside the coax.  Thus, a better choice for feeding 
Beverages is RG58 or RG59 with a heavy copper braid shield.  On the 
other hand, if we're effectively killing common mode current with chokes 
and additional ground rods, it may not matter. :)

73, Jim K9YC