Thank you for clarity on that point, Steve.
The ANNECKE and JVM both indeed ground the center of the double-differential
capacitor.
This leaves only the BPF feature of the link coupled matchbox as its unique
advantage.
73
Rick, DJ0IP
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Steve Hunt
Sent: Monday, July 18, 2016 10:40 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] OT: Question to the group
I'm new to this discussion, so if the following point has already been made
I apologise:
Link-coupled tuners do _not_ inherently provide good common-mode current
rejection. If the design includes a grounded centre-tap on the "output
capacitor" - as I believe is the case with the JVM - it tends to acts as a
balanced _voltage_ source rather than a balanced _current_ source, so any
load imbalance leads to unbalanced currents. To quote W8JI: "The primary
drawbacks are the fixed coupling link, and the fact they are balanced
VOLTAGE sources, not the generally more ideal current source."
Even when the design is modified to remove the grounded centre-tap, the
impedance to CM current is not necessarily very high - it depends on the
inter-link stray coupling capacitance, which will vary from band to band.
I had a 5kW tuner designed and manufactured by the UK Foreign & Commonwealth
Office for use in overseas embassies - this was one of the plug-in coils:
http://www.karinya.net/g3txq/temp/coil.jpg.
Its CM performance was worse on nearly all HF Amateur bands than my
TT238 tuner followed by a good external 1:1 choke-balun.
Steve G3TXQ
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