Playing around at 10 GHZ

The family moved to new location in rural New York, which happens to be on a hill with a clear view to the west. While the sunsets are nice in the three months that we have sun, it’s good for VHF and up all year ’round.

It had been more than a decade since the last time I had communicated with another human being on the 10 GHZ band, but look at that horizon:

K2UA was kind enough to drop off a couple of old dishes from the 1990’s, and said, “You should be able to make a good one out of these.” He also mentioned that a tree fell on one of them. That said, he was right–after hosing out the waveguide to remove some “organic dielectric” I was able to cobble together the fine apparatus pictured above. The photons don’t care that he fiberglass has yellowed around the edges.

Now, you have to remember the last time I had this on there was no such thing as an “iPhone”. I connected the IF radio (an FT-290R/II that I used in high school), and since I didn’t have a 50 ohm cable of a few feet in length, I used a 6ft RG-6 cable to connect it to the transverter….yet another use for F-to-BNC adapters!

This happened to be the morning of the second 10 GHz Contest, and I got a call from W2FU asking if I had any plans to be on. This really surprised me. How did he know I had been skulking around in the moving boxes pining for small wavelengths? I didn’t even think he’d been active on 10 GHz since I was. Something was in the air. Now I was motivated!

I connected the power source…..and I had conversion gain! Or at least a lot of noise in the receiver!

Now, to transmit….I had a bit of a problem. I had no sequencer. Now, for anyone reading this who doesn’t know why this is important, I will explain…. A transverter has tow jobs: “downconvert” a high freuqency to he low freuqnecy of a normal 2m receiver, and “upconvert” the 2m transmitter to the high frequency. Thus there are two ports inside the transverter–one for transmiting and one for recieving. Since this happens over a single connector on the 2m transceiver, it’s very important that you don’t put a transmitted signal into the very-sensitive recieving port, otherwise you will blow up the recieve chain. This means that when you want to transmit you have to switch the transverter to “transmit mode” before you turn on the actual transmitter and send energy up the cable. This is what a sequencer does–it switches things in the proper sequence to prevent blowing up the receive chain.

I found an old DPST knife switch that came from the AWA convention flea market. I put the key in series with one side, and the ground-to-transmit line of the transverter on the other side. I literally cleaned off the coal dust and applied sandpaper to the contacts to make sure it would be reliable. This arrangment made sure that the transverter would be switched before the key could be closed.

See that big knife switch in the middle?

This meant I was CW only, but being an “old timer” I figured anyone who would work me would simply have to struggle with Morse code. I removed the microphone just so that I wasn’t tempted to defeat my switch. Saftey first!

That fine Saturday afternoon W2FU and WA2TMC were in FN02vu, about 50km west of me. In fact, their location is almost line-of-sight through the edge of that tree line that is just visible on the right side of the dish in the picture. I had W2FU send my way with 20 watts and a 2ft dish, and I tuned around but heard nothing….what was wrong? Was my LO flying? I tightened some connections and thought we’d try again. Jeff suggested that the LO could be a few hundreds of kHz off with all time that had passed, so I tuned wider on the second try. I found Jeff about 250 kHz higher than I thought I would, and boy was he loud!

I quickly worked both him and Bruce. Both said I was very loud, so I figured all was well.

The following week I tried with VA3ELE at his home station in FN03dm (near Toronto), about 189 km away–real DX on 10 GHZ. I heard him fine, but he didn’t hear me. Hmm….

Analysis in the lab revealed a critical mistake….I was severely underdriving he transverter: my power output was about 0dBm, or about one milliwatt! Now, at 10GHz, where antennas have 30dBi or more of gain, this is fine for line-of-sight, or near LOS paths like the one with W2FU and WA2TMC, but probably not for Peter who is four times as far away.

I made the adjustment to get something in the neighborhood of a watt or two out (I don’t trust my power meter), and we tried again. This time Peter said he heard me 579, but I couldn’t hear him! He checked his transmitter, while I checked my receiver. Peter found the issue was on his end and we tried again after a repair. On October 7, I set up in my garage and we managed to complete a QSO.

Then things got fun….

It turns out Peter had been pointing at my old address in Rochester, about 5 degrees off! When we got things line up, signals were in the neighborhood of 50dB out of the noise, maybe more. Since I was operating in my garage, I thought I would close the garage door to see what the result was. Here’s Peter’s view of things:

Given that the garage door is worth about 40dB, this is on the order of mixer power over almost 200 km. Wow!

Now, it’s clear from this video that Peter is hearing me better than I’m hearing him, so I think I need some work on the receive side. I have a preamp ready to be integrated, but I think the transverter should hear a little better on its, so I start there.

Given that the path is so good, I’m thinking it should be possible to work on the other bands with just a bare mixer and a small horn. Time to visit the junk box…

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