This is one of several articles Ron has done on 2304 MHz equipment conversions. He describes his initial intrepedation at attacking the problem of commercial surplus equipment and how he went about the process.
After some 40+ years as a Ham and 20+ years with 23 cm as my highest band I decided to try my luck at something higher. My choice was between 3 cm and 13 cm, the next step up in frequency. Due to the lack of current nearby activity, 3 cm was ruled out and 2304 MHz was to be my next band.
Planning for this new item followed my usual criteria; Ham Radio is a HOBBY and is dealt with as a minimum expense item. As you all know, equipment is always available, but many times with a hefty price tag. Since no transceivers are available for this band, a transverter was the only alternative. Just connect it to your 2M radio, an antenna and you are on the band. Simple enough. Transverter, antenna and feedline, still less than a kilobuck. But wait, that is more than any other single ticket item in my hamshack, other than my HF transceiver.
I have some rules I apply to Ham Radio that have been a useful rule of thumb for me.
Test equipment is also a problem when you get to the 'upper' bands. A good microwaver has a room full of test equipment to tell if and how well their equipment is working. Well I did have a HP 431B power meter, that along with my trusty DMM would have to do.
The antenna needs to be high above all obstructions, simple enough, but my 75 foot tower already had antennas for 144, 222, 432, 902 and 1296 MHz. Where will it fit? A loop yagi takes little room and should do the job, just take down the mast and add this to the other 5 antennas. Sounds like a lot of work.
The feedline would need to be low loss. Another 75 foot run of 7/8" Heliax up the tower should do. Just add that to the other three runs. Even then the loss would be about -3 dB, that would drop power at the antenna down to half and increase the system noise figure 3 dB. I could add a mast mounted preamp and a 15 watt amplifier, that would add just another $500 or so. That made a good argument for tower mounting the equipment and just run RG-8 for the 2M IF.
The transverter was the next item to resolve. SSB Electronics and Down East Microwave both make good 13cm transverters, ready built as well as kits. These kits are no "Heathkit" type of kits, they take pretty good expertise in surface mount type soldering as well as testing, very little adjustment is necessary as they use "No Tune" type circuitry. After procuring a 1 watt transverter and getting it working, the other minor items then became the holdup, a watertight box, a power supply, some RG-8 and power cable and I would be ready to connect the antenna.
A word for the wise, unless you have plenty of time and enjoy building equipment, the additional cost for an assembled and tested transverter is well worth the small charge, it saves gnashing of teeth and pulling of hair.
Pic #1 Transverter During Assembly
The loop yagis looked like a good choice but the cash outlay was more than I wanted to spend on this 'low cost project' so I looked at alternatives.
A friend gave me a MDS antenna a couple of years ago, and like any true Ham, I stashed it for later use. Checking into the details of this antenna, I found that was used at 2.6 GHz and should be good at 2.3 GHz with minor modifications. I threw it into the truck and took it to 1999 Microwave Update along with a 4 foot dish with WA5VJB LP feed.
Kent Britain, WA5VJB, who ran the antenna gain measurement, would tell me how good they were. To make a long story short, the four foot dish measured 21.9 dB and the 'junker' MDS antenna measured 20.0 dB, without even tuning to 2.3 GHz! It was smaller than the four foot dish and a lot easier to handle.
The first thing I needed to do was to get the feed down in frequency for a reasonable SWR or 'return loss' as it is called in the microwave bands. If the dipole is optimized at 2.6 GHz and I want it to work at 2.3 GHz., a little math told me that the dipole will need to be extended about 0.1 inch or 0.05 inch on a side. Doing this is another thing, I did not even know what the feed looked like, it was enclosed in a plastic clamshell box. No problem, just get out the trusty xacto knife and open it up.
The dipole was made of flat brass stock and has a quarter wave matching stub of the same material, they were soldered together. It was a simple matter with the soldering gun to heat up the joint and move the element out approximately 0.05 inch, by eyeball measurement, no less. This was repeated for the other element and a measurement made to confirm the additional 0.1 inch in dipole length. The plastic 'clamshell' fit back together and Scotch electric tape sealed it up from the weather.
Click on pictures for zoom.
Pic #2 MDS Antenna
Pic #3 MDS Antenna Feed
Pic #4 Inside Feed Enclosure
The Belton Texas fleamarket (our equivalent to Hamarama) came up the weekend after Microwave Update and the Roadrunner Microwave Group had their monthly meeting there. One of the guys brought his 2304 station and Daiwa wattmeter which presented a good time to check SWR. The MDS antenna was connected and showed an impressive 1.3:1 SWR. I do not know how many dB that is in return loss but it looked good. That is one thing you learn after many years of being a Ham, stop when you are ahead. The antenna was now ready.
Now just where to put it, at 24 inches wide by 36 inches high, it was smaller than the four foot dish but there was still no room to place it on the mast along with the long Yagi supports for the Boomers. Back to the drawing board. I Guess I could have mounted it on the tower and rotated the tower, or maybe used a ring rotator but that would be just more money for a minimum budget project. See rule #1.
One benefit of being in South Texas (EM10) is that there is minimal activity above 2M to the South. How about mounting the antenna on the North side of the tower and rotating it there? One should be able to get 270° of usable rotation which would cover Houston to the East (and FL), Austin to the West and Dallas to the North. That's about 90% of expected 13 cm activity within reasonable range. Even San Antonio to the Southwest should be OK. Fine, now just do it.
What rotator to use? I have an extra Ham IV. No, it is too heavy and bulky. What about an old Alliance U110? No, that's a step type rotor with 10° increments, too large a step for high gain, narrow beamwidth antennas, but it sure was nice and small. There is an analog version of the rotor with a linear readout, that's what I needed. I'm sure I had one somewhere but I am still looking. I even had the lower thrust bearing.
The June contest was coming up and I needed to be on the air and ready to see if I could get my 10 grid squares for VUCC. Checking the Alliance rotor showed me that by judicious jockeying of the knob, I could move and stop the antenna in between the 10° steps. Good enough, I decided to use it and if I found the analog version, I could just swap it out. Now, just how do I mount it?
The local ACE Hardware store surprises me with the items available, a couple of pieces of galvanized 1 inch angle iron four feet long and four 1-1/2" U-bolts looked like what I needed. I drilled the angle iron at one end for the U-bolts to fit two legs of the Rohn 25G tower. The U110 rotor had 4 bolts for the bracket, I decided to use just two and drilled the other end of the angle iron for it, that also worked for the thrust bearing, which used the same hole pattern.
I had to be sure to get them the same distance from
the tower so the mast would be vertical. I mounted all of the parts,
took a five foot section of TV mast and mounted the brackets to the
tower near the base to check it out. Surprisingly it worked! Then I
mounted the antenna and tried it again. It actually allowed full 360
degree rotation but the antenna pointed directly into the tower to
the South (we expected that).
When I set my tower, I unknowingly did two things
right! The first was that two of the legs were aligned to the North.
Mounting the brackets to these two legs would hold the rotor due
North of the tower. The second thing was that I guyed the tower at
the bottom of the top section. This gave me about 8 feet of mounting
room above the guy wires, who says we don't plan ahead.
The junk box also yielded a NEMA-4X box about 8"x10"x12" and a 3 amp, 15 volt Lambda switching power supply. Maybe it would be noisy, but it sure was small. I knew that it used 115 VAC, which probably is contrary to good engineering practice to run up the tower, but I connect it with a fused plug into a ground fault interrupting (GFI) outlet. The risk should be no more than shaving with an electric razor plugged into a GFI outlet.
all fit in the box with only a minor modification to the DEM
transverter, which brings up a question: Does anyone really use the
power switch on the transverters? I unscrewed it and taped it up
before placing it inside the transverter (in the 'on' position)
which saved about ½ inch, making the other connections on the rear
easier. A N-type feedthru for 13 cm, a BNC feedthru for the 2M IF
and a small MS connector allowed the box to be closed with all
necessary connections on the bottom.
While on the subject of building and testing, I do not like to use a separate keying line for the transverter, I prefer to use either a carrier operated relay circuit, or DC up the coax. The IC202 I use as IF has a DC voltage on the coax during receive which is enough to switch a small relay in the box via a transistor, voltage = receive, no voltage = transmit. So far it hasn't let me down. The voltage was used in the box to switch a relay and change the transverter from transmit to receive.
Results: With the box mounted at about 70 ft and the antenna mounted within 5 feet, the RG-214 feedline was only about 6 feet long. How about that for low feedline loss. Power output was approximately 1 watt, receive NF about 2 dB. I consistently work Bill, W3XO/5 in EM00, who uses a similar setup at a distance of 120 miles, usually on CW but often on SSB.
Several times I would have qualified for Rag Chewers Club (old timers remember what that is) so heavy usage has not been a problem. The switching power supply does not generate any noticeable noise and so far is working fine, not generating a lot of heat or adding a lot of weight. We have actually made contacts on 13 cm when we could not on 23 cm even with more power on that band.
Click on pictures for zoom.
Pic #5 Interior of Box
Pic #6 MDS Antenna On Tower
Did the 10° steps on the rotor create a problem? So far no appreciable benefit is noticed by working in between the steps, maybe the antenna was just designed to have a broad pattern. I never did check antenna gain after modifying the feed as I don't have the equipment or a suitable antenna range. There was no chance to take it this year to Central States VHF conference, as it was in Winnipeg, or to Microwave Update, in Pennyslvania, so I guess I'll just have to be ignorant of the real gain until next year.
Total expenditure for this new band was the cost of the transverter and the mounting hardware for the antenna, less than $500. Of course, it sure helped to have a well stocked junk box. The flea markets at all the hamfests and conferences give an opportunity to stock up the junk box for all these new projects, all you need is a 30 X 50 foot barn to stash the parts away, the only thing then is to remember what you have and where it is.
I am still looking for the other U100 rotor.What more would I consider? Probably a 10 to 15 watt power amplifier, if it will fit in the box and run off the existing power supply. I have heard people who could not hear me (for example Rex, W5RCI in EM44) and that might help resolve the problem.