Constructing a programming interface

A little while ago, I was given a pair of EF Johnson UHF mobile radios. These put out 35 Watts of power, which is nice, but like most radios not specifically designed for amateur use (and several that are, as I understand it) you can't set the operating frequency directly from the radio. Instead, one must use a special programming cable to store a number of frequencies (in this case, about 100, in banks of 16) in the radio. And these cables aren't free - aftermarket versions run about $40 a pop. And as my motto with ham stuff has lately been "why buy a thing when you can build it?", I decided to make one of my own.

There are a couple of schematics out there for this cable, all based around a MAX232 IC (integrated circuit). I used one designed by Kyle Yoksh, K0KN.

Basically, what the MAX232 does is convert RS-232 signal voltage levels to TTL signal levels. So a lot of the work has already been done in the IC. All I had to do was solder the IC to a circuit board, attach some capacitors, attach a female DB9, and attach an appropriate connector to interface with the radio.

I was able to obtain all the necessary parts from Electronics Supply Company here in town. I probably paid a little more than I would have if I'd bought from, say, Mouser Electronics, but I like supporting local businesses.

Soldering the entire circuit together was a bit of a learning experience, which I won't recount here as it's extremely boring in the telling. I know that if I built this circuit again, I'd do it slightly differently, but I'm still pleased with the way I did it.

Remember how I said "appropriate connector" above? This whole "appropriate connector" business is easier said than done. It's an 8-pin connector that looks like an RJ45, but it's more narrow. I wasn't able to find one online, so I decided to adapt a bit of Cat 5 that I had laying around instead. This also had the advantage of not requiring me to find a crimper for that weird connector. So, instead of using the Johnson connector, I'm filing down (sloooowly) the RJ45 on the end of the aforementioned Cat 5 stub. Once I've done that, I just need to secure the board and try it out.

For a project box, I decided to use an old Altoids tin that I had laying around. I drilled a hole in it for the Cat 5 stub, but for the DB9, I ended up poking a hole in it with my needle nose pliers and then just working at it with a metal file and clippers for a while. I made the holes for the screws to secure the DB9 to the box using a sheet metal screw I had laying around. A bit of pressure and it poked right through the tin.

OK, now for some pictures. :)

Here's the project board inside the Altoids tin. As you can see, it's really not that complicated of a circuit - a few capacitors, a voltage regulator, and the appropriate connectors on either side, and that's it.

Close-up of the project board.

And this is what the thing will look like once it's finished.

Kansas City Dilettante is now Kansas City Polymath!

Please update your bookmarks to reference http://kcpolymath.blogspot.com/

Changing my name

It occurs to me that "Kansas City Dilettante" does not exactly convey the impression that I want to convey. I originally chose "Dilettante" to reflect the fact that I was interested in a great many things (despite the fact that most of the blog content, lately, has been about radio in one form or another, I am interested in a great many things). But "dilettante" also conveys someone who only toys with things briefly, then moves on. I'm not that person.

I have often said that when I grew up, I wanted to be Benjamin Franklin. I've always admired Franklin's wide range of skills and knowledge. And a word for someone with a wide range of skills and knowledge, like Franklin, is "polymath", and that's something I aspire to be. So, effective as soon as I can figure out how to make Blogger do it, I'm changing the blog's name to "Kansas City Polymath".

Adventures in J-pole construction

So when I said that the J-pole was soldered together, I neglected to mention that the coax - the thing that I connect my radio to - had not been connected to the antenna.

To begin with, I used a borrowed antenna analyzer (a MFJ-259) to find the point on the antenna where I got the lowest SWR. I then marked those points (there's a point on the 3/4 wave section, and a point on the 1/4 wave stub, and in theory they should be at equal height) on the antenna with a Sharpie. Then I screwed a chassis mount SO-239 connector (photo) into the 1/4 wave matching stub, and added some copper wire wrapped around the base of the connector. This was intended to improve the electrical connection between the 1/4 wave stub and the side of the connector that connects to the shield of the coax. I'm not sure it was strictly necessary, though. :)

I had previously soldered a piece of solid core copper wire that I had laying around to the center conductor of the SO-239. After I got the connector screwed into the 1/4 wave stub, I tried soldering the end of the wire to the 3/4 wave section of the antenna at the low SWR point I had previously marked. My soldering iron wouldn't get the copper pipe hot enough for the solder to stick - not surprising, since copper is an excellent conductor of heat - so I ended up using the blowtorch I had used to solder the antenna together to begin with. This worked much better, and when all was said and done, I had an SWR of between 1.1 and 1.2 at 146 MHz.

Today I covered the wire and everything but the threads on the SO-239 with electrical tape to protect them from the elements, and to make sure that nothing could short the matching stub and the 3/4 wave section (apart from the bottom of the "J"). I then (with my upstairs neighbor's permisssion) attached a couple of U-bolts to the railing of the back staircase. They will hold the mast that the j-pole is mounted on to the railing. I might fabricate another mast out of something or other later to get some additional height - I haven't decided yet. I didn't connect any feed line to it, as I don't have a UHF -> F adapter.

Soon, I will have a working outdoor antenna!

The J-pole is soldered together!

Tonight I soldered together the J-pole I mentioned in my last post.

Conditions were less than ideal for this. Not having a garage, and it being awfully cold outside, I had to work in a basement. On the plus side, the basement was relatively warm. On the minus side, it was awfully dirty down there. I kept things as clean as I could, though.

Another less than ideal thing was the flux I used. I purchased a kit from the hardware store that came with a small roll of solder and a small tube of flux. The tube of flux had a thick plastic lid, and I didn't have anything to puncture it with, so I ended up heating up a piece of scrap copper wire and using that to puncture the tube. I only managed this once, however, so the flux came out in a thin ribbon.

But, all problems and less than ideal conditions aside, the j-pole is together! Now all I need to do is find the 75 Ohm match point and I'll be good to go. Now, experienced hams might have noticed that I said 75 Ohms in my last sentence and be a bit confused. After all, most ham equipment is designed for 50 Ohm loads, and you generally want the best impedance match you can get, so as to maximize your power output. But as it happens, I have a bunch of leftover 75 Ohm RG6 that various cable installers have left with me, and I don't want it to go to waste. So I'm going to try and use that as feedline, and try and set something up to match that impedance as best I can. It was suggested in this thread on QRZ that if I make sure my 75 Ohm line is a multiple of 1/2 a wavelength, I can then connect a bit of 50 Ohm line at the radio end. We'll see how that goes. :)

Adventures in ham radio

In my quest to become a broadcast engineer and general polymath, I've started getting into ham radio.¹. I got my Technician-class license in August of last year, and started operating on EchoLink, which provides VoIP connections to other ham operators and those repeaters connected to EchoLink. I made a couple of "Slim Jim"-type antennas, one out of twin-lead, the other out of ladder-line, but didn't actually get a radio until fairly recently.

My current radios are a Midland 13-505 that I bought at the last hamfest, and a pair of EF Johnson 9800 series radios that were given to me by a fellow ham. The Midland radio operates on the 2 meter band, which covers (for hams) 144 - 148 MHz. The Johnson radios operate on the 70 cm band, 420 - 450 MHz.

Neither of these radios is exactly ideal. The Midland is locked (unless I can find or build a synthesizer) to those frequencies for which it has crystals installed. As the Midland dates to the early 1970s, crystals to fit it are fairly rare. The Johnson covers the entire 70 cm band, but requires a special programming cable which I do not currently possess. I have, however, found two sets of plans on the Internet for building such a cable (here and here), so I think that I shall soon have that problem sorted.

As of yet, I have not been able to make a QSL (a contact) with either radio. Several times I have heard traffic on the Midland radio, but I have yet to be heard. I have heard nothing on the Johnson. I'm hoping all that will change soon, though, as I'm building a new antenna. It's another J-Pole (note that the "Slim Jim" is a special case of the J-Pole), but this time, I'm making it out of 1/2" copper tubing.

I used plans by G.E. "Buck" Rogers, K4ABT to get the proper lengths of copper tubing. I went to Westlake Hardware in Westport to buy the tubing, caps, tee, and elbow, and they were kind enough to cut the pipe in the lengths I needed at no charge. I haven't assembled it yet, as I don't know how to solder copper pipe together, but once I have, I plan to mount it on the railing to the stairs at the back of my house. That will give me a reasonable amount of height, especially compared to where I have my antenna now - hanging on the wall above a window in the rear of my house.

That's all for now, Dear Readers. I'll keep you posted (sporadically, of course) on my further adventures in ham radio. And I've still got all manner of things to talk about at KCUR. :)

¹Before you ask, Dear Reader, no, I still haven't learned where the "ham" in "ham radio" comes from. I'm sure it's on the Internet somewhere.

Recently at KCUR

Even though I haven't been posting as much of late, I'm still active at KCUR. Two weeks ago Robin and I installed a fix to the HD transmitter to prevent RF interference from the power supply. I'll note that Robin never mentioned noticing any kind of RF interference coming from the HD transmitter, but it's still a good precautionary measure to take.

This fix involved putting a ferrite choke around an internal power cable¹. The linked article discusses chokes in depth, but basically the point of a choke is to present a high impedance to RF currents. Any such currents that are generated along the cable will either be reflected up the cable or will be absorbed by the ferrite choke and dissipated as a negligible amount of heat. Also, we attached a grounding strap (a braided strip of wire) to the outside of the crossover pipe (rigid coax - where the generated RF signal comes out). Since the outside of the crossover pipe acts as a shield, I think this means that any stray currents on the outside of the pipe will be shorted to ground and won't get outside the transmitter cabinet.

This whole business took something like 15 - 20 minutes. The rest of the day was spent helping Robin out with various things around the station, including installing a T-1 surge protector in the TOC. Last week I did the same thing at the transmitter.

¹ Looking at the manual, the purpose of the cable is "current share", and I'm not sure what that means. If there's the potential for RF interference, I think it must have AC on it.