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Re: [TenTec] ant

To: tentec@contesting.com
Subject: Re: [TenTec] ant
From: "Dr. Gerald N. Johnson" <geraldj@weather.net>
Reply-to: geraldj@weather.net, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Fri, 14 Jan 2011 13:27:21 -0600
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>

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 

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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