Rick,
I was talking about the CM impedance rather than the Differential-Mode
impedance. If you switch in parallel with each other the two 1:1 chokes
which form the 4:1 Guanella balun, you have a 1:1 with only half the CM
impedance of a single choke. It seems a shame to throw away that
hard-earned CM impedance :)
73,
Steve G3TXQ
On 31/10/2013 22:21, Rick - DJ0IP / NJ0IP wrote:
Yes Steve, you are correct.
You have two windings (transmission lines) with the 4:1 Guanella.
Usually the bifilar windings are about 100 Ohms.
Their inputs are in parallel and their outputs are in series.
This gives us 50 Ohms on the input and 200 Ohms on the output.
But if we re-wire the outputs to also be in parallel, then the two 100 Ohm
transmissions lines in parallel give us 50 Ohms on both ends.
Unfortunately the reverse does not always work.
Most 1:1 Guanella baluns simply run a coax cable around a single toroid so
you don't have the two transmission lines to play with.
But, if you build your own 1:1 Balun using two 100 Ohm bifilar transmission
lines rather than coax, THEN you have the possibility to switch it to a 4:1
or 1:1.
Having a switchable balun is a big help when you are trying to cover the
entire HF spectrum with a single antenna and you only have a marginal
antenna tuner, like the built-in tuners in transceivers. This can come in
handy when traveling light on portable expeditions.
73
Rick, DJ0IP
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