Hey Tom,
> In regards to the balun, how does it work when the impedance of
> the line keeps changing? If you use a 4/1 balun, some
> frequencies will be 300 ohm impedance and it will work, but at
> some frequencies, the impedance will be as high as 2000 ohms, and
> it seems that would be kind of hard on the balun if you're
> running any power. 73
> Tom W7WHY
That's right Tom. If the line is mismatched by a large amount, the
last thing you want to use is a voltage balun or any balun that
places a winding directly in parallel with either the input or output
feedline.
With a high SWR on the open wire line, the balun of choice is a 1:1
choke balun.
I use two baluns here. On my long 450 ohm line (1400 feet long) I
use 9:1 transformers in combination with a 1:1 choke balun. I
always make sure that line is matched, so the transformer isn't
stressed.
In my greatly mismatched line, I use a 1:1 choke balun. That way
the balun has minimal stress for any VSWR condition.
Un-un's and other voltage type baluns and transformer type devices
are only good if they see something close to the design
impedance. Choke baluns work best with widely vary load
impedances, but don't do any impedance transformation.
73, Tom W8JI
w8ji@contesting.com
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|