[Elecraft] [K2/KX3] Tuning a magloop

[Gary Hawkins]

The QEX featured circuit designed by WB0OEW takes a slightly different 
approach.  It monitors serial communications on the ACC1 of the KX3.  
 From this it pulls the KX3 operational frequency (AUTO INF must be set) 
that it compares to the frequency the loop is currently tuned to.  If 
these frequencies are sufficiently different and the loop needs 
retuning, the KX3 is disconnected from the Alex Loop using a coaxial 
switch and a Fox Delta DDS then excites the loop at the KX3 frequency 
and an algorithm varies the angular position of the variable cap to 
minimize VSWR - similarly takes about 10-20secs.  When the loop is 
returned to resonance the coaxial switch reconnects the KX3/disconnects 
the DDS and a logic line allows the KX3 to go into TX if required.  This 
circuit does include a balanced bridge at 50 ohms and can't place any 
strain on the KX3 as it's not connected during the tuning process.  I'm 
currently implementing the serial read of ACCI, after I've got that done 
I'm going to look at DDS options.   All the components are controlled by 
the Arduino Uno - which is a fun device to play with.

On 12/4/2015 9:56 AM, Edouard Lafargue wrote:
> Hi Gary,
>
>    I need to set aside some time to finalize it, but I have designed a 
> fully automatic AlexLoop tuner that is designed to work with the KX3: 
> you just need to tune to the frequency you want to use, then press the 
> "tune" button on the KX3 (long press on XMIT), and the Alexloop will 
> find the correct tune in about 20 seconds worst case. It uses a tiny 
> stepper motor as you described.
>
>    I have demonstrated a proof of concept for it at the Palo Alto 
> Amateur Radio Association, and I hope to have instructions ready by 
> the end of the year. The idea is to release complete instructions for 
> whoever wants to build one themselves, and maybe also offer it as a 
> partly soldered kit for people who do not want to bother with SMD 
> soldering. Contact me direct if you are interested!
>
> Ed, W6ELA