Figure 3. A DC bias supply for the W GHz amplifier. The battery should last for a couple of months, 60 73 Amateur Radio Today • October, 1993

power should be held to +10 dBrn maximum to the mixer.

This system, with a 10 GHz FET amplifier (home-brew), can boost output power or transmit to somewhere near the +5 to +8 dBm power level. Thafs a power output of 4 mW to 7 mW (0 0Q7W). Donl scoff at that power as it is quite capable of driving other higher-power amplifiers, such as traveling wave tubes, to full power of 10 watts or more. These do not need to be added now but can be as time and money allow. The simpler low power rig will work quite well with the lower power level. Additional gain can be quickly realized through passive antenna gains.

Realize that 0.007 watts is real QRP power and can give exceptional operation using SSB Comparing operation on 10 GHz QRP levels and system gains in respect to lower frequencies such as 2 meters, you can draw comparisons by noting that with lower frequencies antenna gain is markedly reduced. A 30" dish antenna on io GHz can exhibit 35 dB gain, making for quite a punch, even when using a QRP rig. That's a lot of gain advantage even when compared to a good 2 meter antenna which can run in the 10 to 15 dB range. You would have quite an antenna farm at 2 meters for similar gain as this small dish at 10 GHz.

That's one factor which makes QRP levels workable with reduced bandwidth SSB microwave operation; very high antenna gain and very low power systems. Comparing even further: At 24 GHz the same approach is used, making more power gain in the antenna than is usually in the amplifier at this level. Most rigs today for 24 GHz are limited to very low QRP lev-efs; that is, before antenna gains. This passive antenna gain is what gives microwave low power level rigs quite a boost in radiated power: This gain in power is the most inexpensive to accomplish in short order. The bad news, at least on 24 GHz, is that the equipment Is difficult to obtain even in the land of plenty here in California, if there is inexpensive equipment for 24 GHz then It should be advertised because it would seil.

So much for QRP antennas. The main point is that at a very low QRP level, stations at microwave frequencies are still very effective. The 10 GHz QRP station described above, with or without the preamplifier, is an effective system. Don1! let yourself be locked into the design that I presented in the example given. The local oscillator does not have to be a waveguide type of system to mate with an orihag-onal type of waveguide mixer. In this case use a waveguide to coaxial transition to accomplish the connection in and out of the mixer. There are many different methods that you can use to make adjustments in system construction and still have a great rig.


Steve Roberts N8VKD of Suffield, Ohio, is looking for others in the Ohio

Figure 5. This four-contact transfer relay effectively reverses the amplifier. It is wired with the amplifier in to terminal 1; amplifier out to terminal 4; antenna to terminal 2; mixer out to terminal 3. Norma! (not energized) State: contacts 1—2 & 3—4 dosed When energized, the relay opens 1—2 £ $—4 and 1—3 & 2-4 connect.

Figure 6. Full implementation of switching with 4 SPOT coax relays, switchabie preamp, and 10 watt high power TWT amplifier.

area who are interested in 1.2 GHz and above. He remarked that "if you asked for a Gunn you might get a response like 0.22 or 0.45 caliber" Well, the same mystique abounds here in California. ! have difficulty with the knowledge gap, so when you get on to discussing the electron effect in Gunn devices being negative resistance . . . well, you can just imagine ihe eyes glassing over. It's a hard hilt to hoe without some moral support, especially when you know that you are weli-founded in scientific fact. Trying to convince someone that negative resistance is a true phenomenon is about as easy as teaching a pig to sing. What we did here was to set up a sign and an operating 10 GHz simple wideband two-way system at our local ham swap meets and we were able to get quite a response to our pleadings. Later we formed the San Diego Microwave Group.

I think all that is needed is a little prodding and a lot of advertising, similar to the methods we used. Club meetings and such are a good example and I am sure most are looking for a good program to provide for their members. Sure, you won t interest all the members attending, but a good well-rounded information session on your part of the frequency spectrum as it applies to amateur radio In general can come off well.

Raymond N6RE writes that ever since I published the material on converting the 3.7 GHz LNA amplifiers he has been looking for some to modify. Could we steer him in ihe right direction? Well, I can only offer some iikely sources for you to try. The first is the newspaper—look in the classified ads for satellite antenna installers and also in the Yellow Pages under satellite and TV shops, I made several calls to local shops and most wanted to sell me brand-new units. In bigger shops try to get by the salesman and talk to the repair technician. When I was able to do that I was able to tine up some prospects that might prove interesting. One call to a local high profile installer was met with the initial "sell, sell * but when he found out that I wanted older units for amateur applications and not satellite use, he stated that he was licensed also.

I have yet to visit him, and don't expect to be given a box full of units. But, rather, I have found out thai for a reasonable (smafl) dollar amount he would be wilting to part with some used units. The price discussed was less then $15 each, but somewhat higher than the $5 or so I was accustomed to paying at swap meets. However, his units seemed to be a better bet, as the swap meet prices do not include a guarantee of any kind. It just goes to show that you have to go beyond the normal scrounging attempts and get behind the screens of most bigger companies and talk to the people doing the work to locate parts Give it a try.

Richard KB0MR of Aberdeen, South Dakota, writes; "Where can I buy a TWT amplifier similar to the ones you described in the July 1993 issue of 73 Amateur Radio TodayT Well, Richard, they're not the easiest to locate and when amateurs have found them they were not always careful to get matching power supplies and tubes with them. That's the main reason I wrote the article—to inform our readers about what is available in surplus. From time to time I have a spare unit and they have sold lor less than $200 for a working unit, f feel that the price in some surplus outlets is far too high and they don't even guarantee (hat they will function. I guess they're looking for a customer who is willing to pay the high price for exactly what he wants

At this moment, I don't have a source for these materials. Components used to come out of Surplus Sales tor Collins Electronics in Richardson, Texas, 11 I locate a source HI print it; if anyone knows of one please drop me a line.

Welt, that's it for this month. As always, I will be glad to answer questions concerning microwave and other amateur related topics. Please send an S,A.S.E. for a quick response.




3 4 Energized



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