Re: QSX radio feature requests

Richard Harris
 

Whilst on the subject of auto-ATUs.....

I've made a home brew auto-ATU for my 20m QCX.  It uses three binary stepped toroid inductors and three binary stepped NPO ceramic capacitors and it has a seventh relay to switch between CL and LC matching, so similar to the Elecraft KAXT1.  The processor is a PICAXE 28X2 - the PICAXE is a PIC with a "PICAXE basic" interpreter and so is slower than an Arduino but plenty fast enough for this application.  The PICAXE is also very easy to program and most of my projects have at least one.

I was concerned about damage to the PA FETs because the impedance seen by the FETs could vary over a wide range when tuning, so rather than a conventional SWR bridge (such the Bruene circuit used in the T1) an eighth relay switches in a 50 Ohm resistive bridge when tuning is taking place and so the SWR seen by the TX never exceeds 2:1.  The processor grounds the QCX keyer dot contact and the relays change during dot spaces and the bridge balance is checked during each transmitted dot.  

There are a total of 128 combinations of Ls and Cs and the processor works its way through these combinations until the bridge approaches balance and then it varies the L and C plus and minus a setting or two to find the optimum the balance.  It takes a few seconds to find a match depending on the keyer speed that has been set.

I don't have a permanent outdoor antenna but the 20m QCX + auto-ATU works well with about 25 feet of wire hung around my ground floor shack and I have worked across the Atlantic with this combination. 

The Elecraft KAXT1 uses 5V latching relays - a very neat design.  My auto-ATU uses inexpensive 12V relays because minimizing current consumption wasn't a requirement.

Richard G3OTK

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