Re: QCX tuning


Glen Leinweber
 

In defense of Hans,
I don't recall seeing a brag about QCX able to handle infinite-SWR conditions. (ie: antenna short or open-circuit).

 I can see a dangerous scenario....
if you're using an open-circuit type outdoor antenna - one that measures open-circuit resistance between driven
element and ground. An extreme example might be a kite-elevated long wire. This type of antenna can attain
a considerable charge, and rise to very large DC voltage levels.
Then you attach this antenna to your radio. Could even be an un-powered radio....
Ka-blooy!
The QCX has an output filter that nearly floats as far as DC is concerned. It has a path through R43, a 120k resistor
to one of the four 74ACT00 logic gates - most likely in a logic "high" state. If you measure the DC voltage at the
antenna connection terminals with a high-Z DC voltmeter, you'll measure close to +5V.
I would ensure that any antenna has a DC path to ground, either through a RF choke, or a resistor, to bleed off any
static charge. The QCX's 120k resistor path is too feeble to serve as a reliable discharge. A charged antenna can
blow the finals nearly instantly when connected. There is no over-voltage antenna protection in the QCX.

A loop-type antenna is safer from this scenario, but I'd still include a discharge path from the loop to earth.

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