Topband: Using Video Baluns on 160.

[Jim Brown]

On 5/7/2012 2:04 PM, Herb Schoenbohm wrote:
> I have yet to be able to
> find out what the impedance of a single twisted pair is on CAT 5 cable.
> Nor do I know if these baluns double up on pairs.

I don't know anything about those "video baluns" you're talking about, 
but I can shed some light on the common mode choke side of the equation, 
and also on CAT5.

Each pair in CAT5 is nominally 100 ohms, and the loss isn't bad at 2 MHz.

In CAT5 cable, each pair has a slightly different "Lay," which is the 
$1000 word for twist rate. The purpose is to minimize crosstalk between 
pairs.

I haven't measured any chokes wound with one pair from CAT5, but I have 
built and measured some chokes wound bifilar-style on 2.4-in o.d. #31 
cores using #12 THHN. In use, they are connected as parallel wire 
transmission line, and have a Zo of about 100 ohms. 16 bifilar turns 
(that is, 16 turns of parallel wire line) yields a choking Z of about 5K 
ohms from 160M through about 20M.  I suspect, but have not measured, 
chokes wound with one pair of CAT5 on smaller (less expensive) #31 cores 
could be equally effective.

I suspect that those video baluns are simple 2-winding 1:1 transformers 
on a core that has low loss below several tens of MHz.  #61 might be a 
good choice. To minimize capacitive coupling across the transformer, the 
windings should NOT be bifilar.

On the other hand, Beverage transformers are a well developed art, so 
any core used for that purpose is also likely to work well for use as an 
isolation transformer.

73, Jim K9YC