Sometimes we want to FORCE that voltage balance, sometimes we just want
to limit feed line shield current on the outside of the line. A home
built tuner need not have the secondary capacitors or coil center tap
grounded. Sometimes we need that ground to keep from having the tuner
capacitors arcing to ground from static pickup on the antenna when
charged clouds pass.
73, Jerry, K0CQ
On 11/14/2010 11:13 AM, Steve Hunt wrote:
Of course, the Johnson Matchbox has its secondary capacitors
"centre-tapped" to ground; that makes it a "stiff" voltage source. In
other words it's a VOLTAGE balun, just like the 4:1 Ruthroff found as
standard in many tuners. But usually we want to balance the currents in
the TL, not the voltages on each leg.
That referenced 1:1 balun from Balun Designs is a good choice -
particularly for the lower-frequency bands.
73,
Steve G3TXQ
On 14/11/2010 03:59, Ken Brown wrote:
Question: How does the internal balun in a link coupled tuner
compare with the a 'tri-filer' balun such as used in the
Nye-Viking MB-V-A?
A link coupled tuner IS a balun. It is not a tuner WITH a balun. It is
essentially a tunable air core transformer. Usually (perhaps always) a
link coil of just a few turns is the primary of the transformer, fed by
the UNbalanced transmission line (coax) from the source (transmitter).
The secondary generally has a many turns, perhaps tapped for band
switching (as in the case of a Johnsom Matchbox) and is connected to the
BALanced transmission line to the load (antenna). The transformer
windings have variable capacitors connected to them to make it tunable.
In the Johnson Matchbox only the secondary has tuning capacitors. Other
variations may also have a tunable primary. In the Johnson Matchbox the
secondary is actually two coils, with the primary link in the middle of
them. Or perhaps you could say it is one big center tapped coil, with
bandswitching taps at both ends.
My labeling of primary and secondary assumes that you are transmitting.
Turn the labels around for receiving.
DE N6KB
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|