On 1/14/2011 10:52 AM, Paul Christensen wrote:
The link and the output coil do the balancing, I don't force
balance by grounding the center of the output coil, though that often works.
The grounding of the antenna side of the coil (typically between two caps
connected in series, with the the opposite end of the caps connected across
the ends of the coil) as seen in many link-coupled tuners has me baffled.
The EF Johsnon transmatch also uses a ground tap, but the output caps are
configured to create a voltage divider. I have looked for an explanation in
old versions of the ARRL Antenna Book as well as the Frank Jones Antenna
Books from the late 1930s. Cebik discusses it briefly but only in the
context that "it is optional." It seems to me that the tap should stay
ungrounded with balanced feeders With the center grounded, it also seems
that it creates a greater opportunity for unbalance. By removing the ground
tap, doesn't that provide the best opportunity to force equal current into
the line, while also keeping line current perfectly anti-phase?
There is much contention or discussion about baluns where the voltage
balun forces balance to local earth, and the current balun lets the
antenna and feed line pick the balance point. So far since I built my
big "Johnson Matchbox" about 1964, I've not noticed a difference in
performance or hot RF on the key or microphone whether I forced balance
or let the antenna decide the balance. Maybe I've done a good job of
balancing the antenna so the difference of forcing the balance wasn't an
issue. At the worst the difference in balance from the antenna and the
forced balance means some measure of possible unbalance along the feed
line and so radiation from that unbalance which isn't always bad, nor
always good.
Then I tap the feed line to the turns where it matches. And use a
variable link instead of the series capacitor on a fixed link.
I have seen this done to improve current balance, but what is the mechanism
by virtue of the grounded tap that assists with keeping line current
balanced?
The grounded center tap forces balanced voltage. If the load is also
balanced the current will be balanced. If not there's some feed line
radiation which sometimes makes a contact possible, and sometimes puts
RF into a susceptible consumer device or hears noise from that consumer
device. And when there's a charged cloud floating by its really nice to
have a DC ground on the antenna because the noise of the lightning
arrestor spark gaps firing makes it hard to copy desired signals. That
center tap ground is neither universally great, nor universally bad.
Much of the time the antenna seems to work the same with or without that
ground.
My higher frequency tuners tend to have a grounded capacitor center
because I've used a dual section capacitor and didn't want the
complications of an insulated coupling to the knob and an insulated
capacitor mounting in a small package.
Paul, W9AC
73, Jerry, K0CQ
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