Topband: Re: [TowerTalk] Winding Beverage RX Balun de K0FF

[Tom Rauch]

 Hi George,

> Anyway this one is four windings of 16 turns each to make both the primary
> (3 windings) and secondary (1 winding....the impeadance ratio is the
> SQUARE of the windings ratio), and the little paper tags you see are to
> help me remember which phase is which. The core is an FT50-75 and I've had
> good results here with similar layouts. There are two schools of thought

If you use the correct core, two turns are enough for the primary 
and five for the secondary for 75 ohm. I hate winding a bunch of 
turns, plus the winding is more prone to failure.

For 50 ohm cable, make it a two turn primary and 6 turn secondary.

If you need more turns that that, you are using the wrong core. I 
use a dual hole core with 73 material, about 1/2 inch or so square 
and 3/8 inch thick. You need about one inch of core hole depth to 
use 73 material with this many turns. 
.
> on the windings that go to the coax cable. On has the transformer in a
> plastic box, or otherwise isolates the coax shield from ground at the
> antenna end. The low side of the antenna winding is the only thing going
> to ground, and the two wires on the coax side go to the connector. I
> myself don't like this way, and invite discussion as to why it may be
> preferred. The arrangement shown here uses a metal box, and the shield is
> automatically grounded too. Actually I've tried it both ways but fail to
> be able to discern a difference, and given a choice, I always ground
> everything that can be.

The idea of floating the primary is to reduce common mode current 
on the feedline. It doesn't work well with twisted windings and lot's 
of turns, because you have high capacitance.

That separate winding method also allows you to swap phase if 
you are combining Beverages. It also **slightly** reduces the 
chance of lightning damage away from the Beverage. You might 
loose the transformer, but with the coax grounded to a separate 
ground several feet away from the Beverage ground you at least 
have a double wall of protection.  
 
73, Tom W8JI
w8ji@contesting.com


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