RX antenna disconnect on TX

Problem: due to the proximity of the RX loop to the 80/160 TX antennas, the AUX RF input of the K3 / KRX3 combo picks up too strong signals. This makes a relay rattle and can’t be healthy for the front end. Problem surfaced last year.

Quick solution: use my old homebrew QRO A/B antenna switch. Feed RX loop through the NC port. The changeover relay opens this line when the PTT is keyed. I use the MK2R+ PAPTT output for this.

Problem with quick solution: beefy QRO relay makes a lot of noise when switching. Click-clack-click-clack every time the PTT switches on or off. This noise should best be eliminated. Furthermore this relay is not built for many switching cycles. It’ll probably wear out soon when used in this application. So I unchecked the PAPTT function in the mircoHAM USB router soft when not on 80/160.

Permanent solution for the quick solution: use a simple reed relay for the job. I already ordered a couple of those many many moons ago. A model with a simple NO contact and a model with a changeover contact. I decided to use the latter because I didn’t have to invert the PTT logic then. I know, it ain’t hard to do but this way I don’t need external components.

I ordered the relays at Conrad. The brand is Comus, their ‘dry relays’ are here. I feed this relay from the 13.8V power supply but max coil voltage is 10V with 5V nominal. The coil resistance is 200Ω so I put two 560Ω in parallel and in turn in series with the coil. This gives a theoretical voltage of about 5.75V over the coil:

13.8V x 200Ω / (200Ω + 280Ω)

Ohm’s law, gotta love it. This type of relay doesn’t have a freewheeling diode on board so I added a 1N4007. Stock minus one, still many diodes to go before stock runs out.

ON5ZO RX-disco-nector
ON5ZO RX-disco-nector

I used a PL and a BNC chassis connector and made an Aircell7 BNC-BNC cable. The RX loop’s power box terminates in RG-58 with a PL plug and the Aux RF input on the K3 is BNC too. The small aluminium die cast enclosure was something I ordered a while ago but I had entered a wrong part number. This box is way too small for anything else but it fits a small perf board PCB. Just what I need.

The BNC chassis part has a plastic threaded part which isolates it from the die cast / ground conductor. I used a small M3 bolt with a soldering eye and used tinned desoldering wick for the connections. I keep all my used desoldering braid for these purposes. Flat wide conductors have less inductance than small hookup wires.

The MK2R+’s PAPTT relay can handle 1A so less than 30mA shouldn’t be a problem.

Testing testing 1-2-3 and there you go: it works. Now if I could only put up the RX loop again and get that K3 fixed…

Final notes:

  • With the doubling of the garden space last summer I can now put the RX loop about 20m or more farther away from the TX antennas. Maybe all this wouldn’t have been necesarry with the extra spacing? Well, I suppose it won’t hurt anyway.
  • I could use some BCD band info to have the relay do it’s work on 160 and 80 and either open or stay closed on the other bands. More parts, more work…

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