[Antennas] WxAU Balun

[Terry Conboy]

On 2011-01-01 7:20 PM, Ron Youvan wrote:
> John Hensley wrote:
>> I recently took down a trap dipole which had been up for a couple of decades.  Cleaning up the balun which is also serving as a "T" shaped center support, I realized I had no idea how to check it.
>> I would enjoy hearing from anyone who has messed with these things.  I have no idea what is inside.  The caps are black, the body is gray PVC about 1.5" diameter.  Grounding strap runs down the side.  I used to see these at every hamfest I went to.
>> Before I forget the performance was lousy but then again, I winged it.
>     A piece of coax (min-8 = 8X) or RG-141 or some such with 20 or 30 ferrite beads around it between
> caps, the inner goes to one eye bolt, the shield goes to the other.  They (usually) can only be hurt
> with super QRO or corrosion.  The 1:1 balun is used to reduce feedline radiation (making your signal
> just horizontal instead of H + V) and reduce the effects of a nearby conductive object unbalancing
> your dipole so much.  I have never heard of them hurting a station.  I use one on my 40 meter dipole.

What Ron describes is a W2DU balun, which is a "current" balun.  It has 
a longer body than the somewhat similar W2AU balun, which is a "voltage" 
balun, which probably has a trifilar winding on a ferrite rod inside.  
The W2AU balun come in both 1:1 and 4:1 variations, which look the same 
on the outside (other than labels).  These are all made by 
http://www.unadilla.com/

With an ohm-meter, the W2DU will appear open-circuit between the antenna 
terminals.  The W2AU models will look like a short.  You need to test 
with a dummy load (200 ohms for the 4:1 model, of course) and measure 
the SWR or impedance at the coax input to see it it's OK.  It's a lot 
harder to test to see if it is actually providing proper balance and/or 
isolation.

In general, a good current baluns are normally preferred, but make sure 
you check out the comments by Walt Maxwell, W2DU, at 
http://www.eham.net/reviews/detail/2300 .

73, Terry N6RY