Initial tests last year, just using the simple RF-level squelch from the HF transceiver to trigger transmit PTT on the VHF port proved rather poor, as fairly readable HF SSB signals were not causing the squelch to open. What was needed was a syllabic squelch - as found on professional HF SSB radios - for the HF port. Using the VHF transceiver's normal FM squelch is perfectly okay, of course.
I found various designs for "Voice Activated Squelch" circuits and then stumbled on a kit on eBay:
I bought a couple and built one to play with, in series with the audio from the HF receiver. It certainly worked much better on weak SSB than an AGC/RF level squelch.
The Barrett device also used DTMF (or SELCAL) to activate the talkthrough, so I thought I'd try and emulate that, and found another kit that would fit the bill:
I built the kit, but never tested it. I consigned both the VOS board and the DTMF board to the "in progress" pile, and forgot about them for 12 months!
Today I thought I'd resume the project......
After modifying the VOS board to trigger a relay, which could be used as a PTT source for the VHF radio, I wired it all up on the bench, and it works, I even get the revertive CW signals from the DTMF board sent back over the VHF port when the commands are issued to turn the relay on & off.
Adding a few switches to manually inhibit talkthrough, or to override the DTMF driven active/inhibit relays and it's all boxed up and working. I need to add some LEDs, to show what's happening inside. The one main difference between this box and the Barrett 2062 is that the Barrett device allows remote channel changing of the HF transceiver via the VHF port - something that's not immediately possible in this simple version. I guess a PIC and some programming and it might be possible to send CiV signals tothe IC7200 on receipt of specific DTMF sequences, but that's starting to get more complicated!
Using the IC7200's rear-panel accessory socket also has the bonus that the Speech Processor is available, unlike the old IC718 (where the processor only worked on the front panel Mic socket).
Now I need to try it out on the air, 5MHz would seem the ideal place to test these things - so I need to set up a sked with a willing volunteer...
After much local testing, and quite a bit of listening, I heard MM1MAJ/P on 5403.5kHz operating from a SOTA summit near Loch Leven. Weak, barely moving the S-Meter on 5MHz but triggering the syllabic squelch in the x-gate reliably so I thought I'd try a call to him. He came back instantly and gave me a 5/7 report. The only minor issue is the few seconds the HF squelch takes to drop at the end of an incoming transmission, but that's no real problem. I'm quite pleased it worked on the first real on-air QSO.
I've thought a bit about HF channel changing, and the only simple solution seems to be to use one of the DTMF relays (there can be up to 6 relays) to trigger an up/down channel change via the Mic socket (pin 3). Unfortunately the "pulse" option of the DTMF board toggles a relay for 1/2 a second, which is too long for a single channel step. I will think about a pulse shortening circuit (differentiating the 1/2 second pulse and using that to click a relay for a brief instant.....?).
Or just leave the HF transceiver on one frequency.
I will try to monitor 5298.5kHz as this channel tends to be lightly used, and the regular occupants tend to mess about with odd setups too, so perhaps it's a good place to try some tests.
I've found it necessary to leave the DSP Noise-blanker turned on to remove the occasional (weak) clicking from a distant electric fence, which would occasionally trigger the syllabic squelch. I also find that night-time lightning static crashes can also be a nuisance, as they tend to occur at roughly speech pulse rate, I guess. Using the DSP Noise Reduction and (for good measure) leaving the Auto Notch Filter engaged might just be enough to keep things usable.
A 5-minute TimeOutTimer on the V/UHF transceiver (an FT817) means it's possible to recover control should something (a long...long...over for example) hold the x-gate open in the RXHF->TXVHF direction. Once the V/UHF rig "times out" it's possible to send the DTMF "OFF" sequence and kill the x-gate until the long QSO on HF is over, or the HF channel can be changed.