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.

Monday, January 28, 2019


In the center, the regulator that was spraying
water over my equipment. Note the ceiling
tile above it has disintegrated from the spray.
It's a terrible feeling. You're walking into the basement to grab some hardware right before going to work, and you are greeted with huge puddles of water on the basement floor. Looking for the source, you find the pressure reducing value (also known as the regulator) spraying water all over your ham radio gear. Oh no!

The next several minutes, I hardly knew what to do. I shut the water off immediately, then called my wife to see who our preferred plumber was. Then I mislaid my phone so I couldn't call anyone for a while. I had to get my daughter to look for my phone.

In any case, I got busy clearing out the shack, mopping up the shack floor and vacuuming up water from the basement. The first plumber arrived, I told him the water was spraying from the regulator. He said he'd have to see if he had a replacement. An hour later, another plumber arrived, an associate of the first. He looked at it, and said he'd have to check his truck if he had the right unit. He didn't. So, he disappeared for another hour and a half....

During this time, I'm working hard. I'd cleared out the shack and mopped up the water, vacuumed the water off the basement floor, thrown out the few soaked cardboard boxes.

Then it came to determine how to salvage my soaked equipment. I decided to take an ancient tip and placed my Elecraft K3/100, plus a few other pieces of gear in the kitchen oven for an hour at 150 degrees F. This temperature wouldn't hurt the electronics, but it would drive out any water that had gotten into the gear.

By about 1:30 PM, the plumber replaced the regulator, and lightened my wallet by $325. Later in the day I tested the K3, and it seemed to be working fine. I suspect, other than some papers I had on the desk, nothing will suffer any lasting damage.

That was Friday. Monday night, I'm putting the pieces of my shack back together. So far, everything seems to be working. Whew!