Rx antenna switching with the KX3, Wellbrook Loop and a modified MFJ-1707

My new Wellbrook ALA1530PE receiving loop has been performing very nicely, feeding my KX3 and FCD-Pro+ though a combiner, but so far I have been carefully avoiding any transmitting in order to avoid destroying the loop amp and SDR dongle.

I procured an MFJ-1707 thinking that this might be a reasonable solution, as it is mentioned in the RadCom article on the Wellbrook loop.

I decided to try it out with a dummy load connected as the receiving aerial, and my wire antenna as the transmitting aerial and see what happened.

With power not applied to the 1707, the KX3 was hearing through the wire antenna, as expected.  With power applied to the 1707, the rig went deaf, using the dummy load for an antenna.  So far so good.  My plan was to use the KX3's keyline output as a control line to the 1707.

I had also purchased an Elecraft KX3-PCKT cable set for the KX3, so I had the ACC2 module which allowed me to use a mono phono cable to hook up the KX3's keyline output to the 1707 directly.

However, when I connected it to the powered 1707, the relay in the 1707 de-energised immediately, switching back to the transmit antenna despite the fact that the KX3 was not transmitting.  I puzzled over this for a bit, and then got out my DVM.  Sure enough, the centre of the phono socket on the ACC2 module measured 0V with respect to the 13.8VDC supply line whether the KX3 was transmitting or not.  This had me confused.

I started to wonder about writing to the Elecraft list to ask about this, but then I began to fiddle with the ACC2 IO setting in the menu.  I wondered if I could set up a physical transmitter inhibit interlock.  I quickly bodged together a 3.5 mm mono jack plug wired as a short and confirmed that it would enable the transmitter inhibit function.  I went back to check the status of the Keyline output of the KX3 with the transmitter inhibit enabled, and was surprised to note that it seemed to be operating normally now, pulling down to 0V when the KX3 attempted to transmit and back to floating with the KX3 in receive mode.  More interestingly it now did this whether the transmitter inhibit was active or not.

Swiftly forgetting about the anomaly with the formerly always asserted keyline output of the KX3 I wondered about the prospect of a closed-loop solution, using the auxiliary contact closure output on the 1707 to manipulate the KX3's Tx inhibit line via the ACC2 IO.

With the MFJ-1707 un-powered, the Tx antenna is selected and the Aux output is left floating.  When the 1707 is powered, the relay selects the Rx antenna and grounds the Aux output.  When the keyline control from the KX3 asserts, the relay switches back to the Tx antenna and lets the Aux output float.

By connecting the Aux output to the KX3 and configuring the ACC2 IO menu option to "LO = Inh", the KX3's trnamsitter is inhibited whenever the receive antenna is active and the trasnmitter becomes dis-inhibited when the transmit antenna is selected.

On the face of it, this looked like a great solution, but something troubled me.  I realised that if the ACC2 stereo 3.5mm plug was pulled out of the KX3, the keyline wouldn't switch the 1707's relay and the ACC2 IO input would float hi,, dis-inhibiting the transmitter and probably blowing up the Wellbrook loop's amp during transmission.

I didn't like that at all.  The stereo 2.5mm plug for the I/Q output on the KX3 had come adrift enough times during rats-nest manipulation to make me consider the alternative solutions.

I mulled over the notion of bodging in an open collector stage between the aux contact on the relay and it's phono socket, as it would invert the aux-output logic for the Tx inhibit function and it seemed safer than the other alternative I'd imagined which involved strapping the other side of the aux contact to 12V instead of 0V.

A 2N2222A was handy.  It's emitter was strapped to ground and the base pulled up to 12V with a 10k resistor. The base was also connected to the pad where the aux phono plug's centre had previously been connected.  The aux phono plug's centre was now connected to the collector of the transistor.

It looked like this:

Now with the KX3's ACC2 IO set to "HI = Inh", whenever the KX3's keyline out asserts, the 1707's relay switches to the Tx antenna,  Aux output pulls the ACC2 IO input down and the KX3's transmitter is dis-inhibited.  If the ACC2 stereo plug falls out of the KX3, or the keyline and/or Aux/Tx-Inhibit cables become disconnected at either end, or their conductors fail, the KX3's transmitter remains inhibited.  I liked this.

The only unexpected result of the mod was that without power applied to the 1707, the KX3's transmitter was dis-inhibited.  I hadn't expected this, but I suppose there's enough leakage to pull down the  ACC2 GPIO line.  I like this also.  Without power applied to the 1707, the transmit antenna was selected so there was no need for the KX3's transmitter to be inhibited.

I've tested it thoroughly with a dummy load in place of the Wellbrook loop, and it seems good.  I'm pretty happy with this solution.

Now, the biggest risk seems to be the idiot wearing the headphones.  If I somehow modify the ACC2 IO menu option, I could blow up the Wellbrook loop.  I think I'll ask Wayne at Elecraft if he would consider modifying the ACC2 IO option to make it locked, requiring a long press of the RATE button on the KX3 to unlock it.  Then I'd feel really safe.

Now this is all done, I suppose I should get on and make some QSOs.  It has been a while.