Friday, February 15, 2019

Mark V.5 Shunt Feed Matching Network (75 & 80m)

Additional capacitor for 80m at lower
right, relay is black box just above.
Three years ago, I improved my shunt-feed matching network for 75m by upgrading to a T200A-2 core and a way more beefy variable capacitor. It tuned my shunt-fed tower quite nicely to a 1:1 match around 3800 kHz. And it could easily handle a little bit of power.

However, when using the antenna on the low end of 80m, the match isn't quite so nice, reaching nearly 3:1 at the bottom of the band. I needed to be able to re-adjust the network when operating there.

It occurred to me that additional capacitance would do the trick, along with a way to switch it in. I found good capacitor of about 25 pF and suitable plate spacing. Switching it was more of a problem.

I wanted to use a large, 12 volt, open-frame relay. Unfortunately, I didn't have anything in my junk box, and couldn't find anything I liked at a hamfest. Eventually, I settled on using a PC board style relay. I figured it had enough of a contact gap to avoid arcing.

Mounting it, however, was problematic. I ended up just having it floating in the box, suspended by the wiring connections.

Side of box showing the control voltage wiring, the 75m
tuning control (upper right) and the 80m tuning.
After re-wiring, the network tuned up nicely on 75m, but there wasn't quite enough capacitance to bring the match down in the CW portion of the band. I ended up putting to 33 pF 6 kV capacitors in series across the 25 pF variable, essentially adding 16.5 pF to the circuit.

That did the trick, and I got a 1.1:1 match around 3570 kHz. VSWR at the bottom edge of the band was under 2:1.

The existing four-wire bell cable control lines were re-purposed to drive two relay circuits. The existing frame relay selects between the 75/80m and the 160m matching networks. The additional relay is energized to bring the additional capacitance online for 80m.

This works so well, I'm surprised I didn't try it earlier. With a better match at the antenna, losses in the feed line and antenna tuner are reduced.

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