Fall 2006 Issue

Adding 3456 MHz
to the Contest Station


At first, adding another microwave band to your contest station may seem daunting. However, N5AC explains how easy it was to add
3456-MHz capability to K5QE’s contest station.


By Steve Hicks, N5AC

 

 

 

 

Photo A. The completed box.

 

Every contester entertains ideas from time to time about how to improve his score . One afternoon, Marshall Williams, K5QE, and I were having just such a conversation about how to improve our VHF-and-up contest scores. Marshall’s station was already impressive, with several tall towers, amplifiers on every band, and multiple operating positions on 50 MHz through 1296 MHz. As a rover with several microwave bands, I encouraged Marshall to add 2304 MHz and 3456 MHz to his station. Not only could he increase his score with contacts with other fixed and rover stations on these bands, but it would also help my score, since I would be able to work him from a number of grids on both bands. Marshall agreed that it was a good idea to add the bands, but only if I helped him get on the air!

We felt as if getting some power on these bands would be good, so we took a look at amplifiers. There are some good surplus solid-state amplifiers for both of these bands, and I felt that they would be easier to keep running at a multi-op station than a TWT. Over the next few months, I built up both 2304 MHz and 3456 MHz for Marshall’s station. This article focuses on the 3456-MHz box, because the design is considerably more complex since we put much of the hardware on the tower. For amateurs the predominant surplus amplifier available for this band is the Toshiba 40-50W amplifier. These are an excellent value at $125–150, are available through a number of sources on the Internet, and are actually designed for the 3456-MHz band and require no tuning. The Toshiba amplifier is also fully protected with internal thermal shutdown and an integrated full-power isolator with a load. This means you can drop the antenna off the end of the feedline and still not hurt the amplifier, although I have no plans test this!

With the amp chosen, Marshall took a hard look at feedline loss. It became apparent that there was going to be considerable feedline loss with the transverter and the amplifier on the ground; Marshall’s microwave tower is 130 feet tall. The cutoff frequency for anything larger than 7/8-inch hardline prevented us from using the larger hardlines to manage the loss. The loss on 7/8-inch hardline for the run we were going to have to the top of the tower (about 200 feet) was 9 dB at 3.4 GHz. This means that with our 50-watt amplifier at the bottom of the tower, we would end up with around 6 watts at the antenna! Not only that, but 9 dB of loss on receive would not be a good thing either. If there was a way of getting the amplifier and a preamp up on the tower, it would probably be worth the effort.

I agreed to get started and see if I could figure out a way to get everything running up on the tower. Since the Toshiba amplifier needs only about 1 mW to produce 40–50 watts of power, we could stand quite a bit of loss in our feedline provided we had some reasonable power at the bottom of the tower. Marshall had plenty of 3/8-inch flexible hardline, which would have a loss of 14 dB at 200 feet. We would need transmit power of 25 mW (25 mW less 14 dB is 1 mW) out of the transverter to drive the PA at the top of the tower, so Marshall ordered a 3456-MHz transverter from Down East Microwave (http://www.downeastmicrowave.com) with 25 mW output power. We would also need to be able to overcome the same loss on receive if we used the inexpensive flexible hardline on receive. We acquired a couple of preamps from Down East as well, one as a preamp and one as a “postamp,” the 9ULNA and 9LNA, respectively. I don’t believe the 9LNA is still available, but two 9ULNAs can be used.

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