Emergency situation for Doctor ZO

This morning around 7AM utc I was calling an HC2 and a 6Y on 80m. I was especially listening to get used to diversity mode. I really like it. But when I was keying with a kilowatt, I heard some cracks in the headphones. Not audio but RF cracks coming from the TRX. Of course it went away when using only the TX antenna. I immediately knew what was happening because I had witnessed it before a few editions of WAECW ago. I had built myself a crappy beverage-like thing in yet another attempt for better ears on 80/160. I tried to listen to it on eighty but since the RX wire ran through the elevated TX radials…

Similar situation right now: the RX loop is too close to the TX antenna. In fact one elevated radial is about 3m away. Make that two meters, I just looked out the window. The RF is picked up by the loop and taken to the rig, but the TX/RX switching on the aux RX antenna input isn’t as bullet proof as the main TX/RX changeover. If I remember well (not sure anymore), the first is a mechanical relay, the latter is done with PIN diodes. The problem is also documented in a few postings on the Elecraft reflector so I soon had my thesis confirmed.

This left me with several options:

  1. Don’t use the RX loop for now, fix problem later.
  2. Ignore the problem, it’s annoying but maybe it won’t damage the hardware?
  3. Make a front end saver that shorts the aux input on PTT.

Moving the RX loop further away is not an option since it is already placed against my lot’s fencing.

I didn’t feel much for option 3 because I had planned a lot of resting today and not frantic homebrewing. Option 1 is a waste of time and money since I bought the loop for this and I spent a few hours last weekend to get it operational. I didn’t want to bet my money on option 2, furthermore it is an annoying phenomenon and I will be spending quite some time on the low bands.

So I took the option 3 ‘light’ approach. The simplest way for now was to take the unused PA PTT output on the MK2R+ device. I key the amp from the K3 so the keying output there is taken and I don’t want to run multiple devices from this without extra buffering for which in turn I don’t have the time right now. And for some reason the PTT out on the amp didn’t work. I soldered a small 12V light bulb to a 9V battery clip and a RCA female socket. I tested the amp’s PTT output with this 9V-powered device but the light didn’t glow. Did the same on the MK2R+ and it didn’t work. Oh wait, I need to activate that feature by checking a box in the USB router panel. There you go: keying the rig made the light glow as long as the amp was keyed. This was the way to go.

The control circuit was done in a few minutes. It’s very convenient to have a modest warehouse with all sorts of parts. Now I needed to take a relay. I have plenty in all flavors.

Then an enclosure. I have some small die cast aluminium boxes within reach. PL chassis sockets? Got ‘m! I was ready to go to the garage where my workbench is and then it hit me: I already have a similar device. My old A/B switch box I used to switch the 40m slopers. I just need to hook it up to the control ciruit. It has a relay that… oh wait a minute. It’s just a NO/NC changeover relay that switches, no port is shorted. I decide to use it anyway since it provides plenty of isolation and the relay has a few millimetres between the contacts. It worked when blowing a kilowatt through it on 40m so it’ll probably do its job for this application. The coil resistance measures 150Ω. With 12V that’s 80 milliamps and the specs say the open collector output is rated for 45V/0.8A. Headroom galore.

So after almost two hours of thinking, testing, soldering, looking for parts all over the place I was done. The only thing that annoys me is that the relay is switched on all bands. For this weekend I will disable the PTT output in the microHAM routing software when not on the Low Bands. I will make a genuine shorting relay circuit later on (yeah right) and maybe use the band data from somewhere to (de)activate it depending on the band.

WX has been VERY windy since yesterday. That got announced only 24hr BEFORE. Why do I check a bunch of online forecasts a dozen times a day for ten days before a contest then? I don’t think I have ever left the tower out under these conditions. The winds have settled and all is OK. Deep inside I know the tower and its ten tons of concrete are rated for much much more but still… I was especially afraid of the low band wires. When the wire for 160 and the one for 80 are shaken in the wind and approaching each other, the resonance point changes which may trip the amp. I really do NOT want that happening this weekend!

Before typing this I snatched EL on 15 RTTY (yes I can do that mode too) and PT0S on 10m SSB where there were calling CQ in vain. First call 🙂

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