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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