May 1, 2019

As many of you know, the club is looking to acquire a service console, so that we can start doing our own duplexer check-ups and tuning right on site. In the meantime, we go with the old approach of swapping in spare equipment at our sites and tuning the primary equipment back on the bench.

A duplexer check-up/retune for VE7RSS is about due. So, the technical committee needed to find a substitute duplexer to temporarily swap in. We would get the temporary duplexer tuned up on the bench, take it up to the site, swap it in place of VE7RSS' duplexer and then bring that duplexer back down to the valley for check-up and re-tune if required. Once checked, go back up and swap back. It's a bit more work than just simply doing the check-up and re-tune on site, but if you don't own a service console, that's how you do it.

Fortunately, just as we started looking for a spare VHF duplexer, Bill Crowther VE7VTC showed up with one. Bill and I were chatting before the NORAC HAM Basic Course Spring 2019 certification exam and he just happened to bring a Sinclair Q202G-series duplexer along in the back seat of his car. He had planned to use it for a repeater installation just South of Vernon but never got around to implementing it, so he was bringing it back to the club for re-deployment.

The duplexer was immediately transferred from Bill's car to my car and we were on to the next step: tuning it.

Today, Jane VE7WWJ and I went for a visit to Ralph Olds VA7NU's lab to check the feasibility of the duplexer (could it be tuned to VE7RSS' frequencies) and if ok, get it tuned up and ready to swap in. Ralph is likely the best duplexer tuner around given his mass of experience engineering, building and tuning a variety of RF filtration equipment professionally.

Ralph, Jane and I spent the next hour working on the task. The duplexer is actually only a 2-cavity-per-side model and usually we use a 3-cavity-per-side model for VHF with the classic 600 kHz HAM split between TX and RX frequencies. We needed to tweak the duplexer so that it favored isolation between the TX and RX frequencies over insertion loss. The result: 80 dB isolation with 1.5 dB insertion loss - just enough isolation at the expense of higher insertion loss. It will work as a temporary stand-in.

Big thanks to Ralph for his tuning efforts today. In addition to simply changing the tuned frequencies we also had to adjust the Q of the filters - more work than the usual re-tune, but worth it.

As soon as the gondola opens for summer season at SilverStar, we will head up with a work party and get the duplexer swapped. Keep an eye on the club calendar entries to find out when we plan to go.

For now,
73,
Mike VE7KPZ
NORAC Vice-President 2017-2019, technical committee member and Field Day 2019 coordinator