> Speaking of cheap--I built an el-cheapo version of Measures tuner that is
> on his web site. It uses 2 pieces of pvc, some number 12 wire, 1 air
> variable, clip leads, a wood board base, an old ice cream bucket, and some
> 213 coax for a choke balun. It is all out in the open and I can change
> bands in a flash. Easily handles legal limit with no arcing and no
> heating. A friend uses one like it with his pair of 4x1000 with a little
> bigger air variable.
That's how tuners are supposed to be built, with one possible
exception.
I don't know what that exact circuit is, but moving the balun to the
input of the tuner does NOT help balance, or make the balancing
job of the balun easier unless the tuner has a grounded reference
point at the center of the network.
It does reduce differential mode strain, but that generally isn't a
problem with a choke balun.
The reason it doesn't help on matching networks that have floating
centers is simple. There is very little common mode impedance
transformation in a network, so the balun sees the same common
mode voltages it sees at the tuner output, even when it is moved.
Again the balun does work better at the input if the network is a
split voltage-output type system, with a center ground
reference...but now when used with a "floating" matching system
like an single ended T or L network.
Tuners are probably one of the most difficult pieces of gear to
engineer, because the frequency and impedance ranges are so
wide. It seems any solution to one situation makes another
situation worse.
I'm amazed so many people buy tuners instead of building them, or
using stubs.
73, Tom W8JI
W8JI@contesting.com
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|