Sunday, February 13, 2011

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.

Monday, February 7, 2011

Kansas City Dilettante is now Kansas City Polymath!

Please update your bookmarks to reference

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".

Saturday, February 5, 2011

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!