[TowerTalk] Balun Recommendation

[Jim Lux]

On 4/14/12 10:42 AM, Michael Tope wrote:
> Guys, I was testing their Unun. Although I do agree with what K9YC says
> about testing common-mode chokes, I just want to be clear that is not
> what I was testing.

I would think that an UnUn is probably using a different material than a 
choke?


   Also, I think thirty seconds key down at 1KW is a
> pretty good test for something with small thermal mass. I would  have
> gone longer, but my amp is a Drake L-7 and I would be leery of running
> it longer than that (even 30 seconds is probably questionable).  A loss
> of 0.2dB would be ~50 watts dissipation in the Unun. I claim that much
> dissipation would produce a noticeable temperature rise in the unun
> core/windings after 30 seconds.

I think you're right. Basically, you're looking at the Easy-Bake oven 
kind of thing.

How much does that thing weigh?  I just weighed a single 2.4" core and 
it's 110 grams, and if we assume it's thermally like iron, which has a 
specific heat of 0.45 J/gK..

50W for 30 seconds is 1500 Joules

1500/(0.45*110) = 30 degree rise... I think you'd notice a 30C rise, or 
even a 15C rise.


However, if there's 3 or 4 of those 2.4" cores, and a bunch of copper 
wire, you might only be seeing a couple or three degrees, which I don't 
think you'd notice.




The goal of the test was to give some
> indication of whether the unun was going deliver the bulk of the input
> power to the antenna. On that score it passed IMO. I also didn't see any
> drift in VSWR while doing the CQ 160 CW contest, where contrary to what
> W6RMK would have you all believe (he's heard to many of my "fish
> stories"),  I do CQ a lot with no answers :-)
>