Posted by
W8JI on
Jul 23, 2010; 10:34am
URL: http://elecraft.85.s1.nabble.com/Balun-Questions-tp5328810p5329039.html
> So I've looked at a few sites describing how to make a 4:1 balun... One
> such solution is to take 2 100 ohm 1:1 baluns and connect them in
> parallel on the input side and in series on the output side...
The 100 ohms is the differential impedance, or transmission line impedance,
NOT the choking or common mode impedance.
> I looked at the Elecraft BL1 manual but I didn't see what material the
> core was... However in another article I saw someone post the recommend
> getting a FT140-61 and winding 7 to 8 turns on each side to make the two
> 100 ohm feedlines.
That probably would not be close to enough impedance for lower bands.
> So I ran the numbers and 8 turns on a FT140-61 gives you about 100 ohms
> on 160 meters. Thus two 100 ohm points in parallel gives you 50 ohms in
> and 200 ohms out. 4:1... Great.
Not great. Bad news. That impedance should be as high as possible and at
least 1000 ohms or more at the low end. The conductor diameter and spacing
sets the 100 ohms. NOT the common mode.
> However at say 40 meters... Each feedline is 430 ohms. Thus you've got
> a 215ohm input and a 860 ohm output. This just seems like it would make
> a mess. Why does it still work?
Almost anything will work. It all depends on how well.
> Finally I'll explain my final goal... I've looked at several ways to
> make a 4:1 which involves using two 1:1's. Then there are methods to
> take 2 4:1's to make a 6:1 (the feedlines are 125ohm windings to pull
> this off). My final goal is to try to make a 6:1 and use it to use
> ladder line once I get through the wall with coax. I always just
> figured that a 6:1 would be better as it would have a 50 ohm in and a
> true 300 ohm out.
Why would that be better?? Is your 300 ohm line matched? Most people using
balanced lines do not match the lines, so the input impedance ranges from a
few dozen ohms to a few thousand ohms depending on frequency and band. Any
effort to match the balun to the line is misplaced.
Also the higher the balun ratio, the narrower the balun operating range and
the less likely it is to handle power. Virtually all tuners have a problem
with LOW impedances. With all that in mind, why would anyone want to step
down the line impedance arriving at a tuner when it aggravates almost every
electrical problem in the system including in the balun?
> Is it just that the thing balances the currents on the outputs and I'm
> just over thinking the matching ability?
Yes, but don't feel bad. Some popular books have real zingers in them when
it comes to common mode impedance and balance, and even suggest balun
designs that won't work as baluns at all!!!
http://www.w8ji.com/balun_single_core_41_analysis.htm73 Tom
______________________________________________________________
Elecraft mailing list
Home:
http://mailman.qth.net/mailman/listinfo/elecraftHelp:
http://mailman.qth.net/mmfaq.htmPost: mailto:
[hidden email]
This list hosted by:
http://www.qsl.netPlease help support this email list:
http://www.qsl.net/donate.html