Topband: Using Video Baluns on 160.

[ZR]

A couple of things:

What is the conductor size in CAT5 and what is the center to center spacing. 
With that its easy to calculate Z.

Since those TV baluns share a common ground they are likely not transformers 
and more likely a simple 4:1 autotransformer making it useless for common 
mode.

The 90's ones Ive run on a VNA barely make 7MHz on a good day and most 
become a piece of wire by 5 MHz. I dont hold out much hope for improvement 
for the basic TV version, others may be better or just cost more.

Its simple enough to wind a GOOD one using a BN73-202 with independent 
windings using sleeves to further reduce coupling C. And also using seperate 
grounds with at least 20-30' spacing. With a little care Ive measured 35dB 
isolation on 160 and decreasing slightly as frequencies go up.

If there is any crud or signal pickup with the Beverage disconnected from 
the transformer box you need more work on grounding and common mode 
rejection. Or the coax integrity is compromised.

Carl
KM1H


----- Original Message ----- 
From: "Jim Brown" <jim at audiosystemsgroup.com>
To: <topband at contesting.com>
Sent: Monday, May 07, 2012 5:50 PM
Subject: Re: Topband: Using Video Baluns on 160.


> 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
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