Posts Tagged LT-25
It has been a while since I finished the transceiver modules, and I how now used them in an actual application. But, before I talk about that, I’d like to show some pictures of the process I used to put them into enclosures. I had some of these cast aluminum enclosures lying around, so I thought I’d use them. They’re a little on the heavy side, as the completed weight is around half a pound, but it’s well within the carrying capacity of my Kadet.
Before diving into the process of cutting the holes, I want to show some images of the transceiver board with the RFI fence installation process.
For use with the spectrum analyzer project, I found some sheet copper at the craft store. It was sold at a local crafts store, and I think it was for etching. I chose the thickest one they had. So far, the best way I’ve found to cut it is using an exact and straight edge. I tried scissors, and it didn’t really work.
Once I had a strip of copper cut, I cut openings for the power and control traces and soldered it onto the PCB. I also soldered it onto the SMA connector. Once all that was finished, I soldered on a lid. Lots of solder flux helps here.
Once the board was prepared, both by soldering on a fence and replacing the pin header with a right-angle one, I began to prepare the enclosure. I was intending to drill a hole for the SMA connector, then cut a hole for the digital connection. The SMA connector hole was trivial to make, though the connector I soldered onto the board was a little short. I ended up having to use an O-Ring from the hardware store (look in the plumbing section) to hold it in. When an antenna or cable is screwed on the O-Ring compresses, having the nice side-effect of sealing it.
For the digital connection, I decided on using a DB-9 connector. I figured it was a prolific connector, so I should have lots of connectors laying around. That didn’t turn out to be as helpful as I had hoped, but I’ll get into that later. To create nice holes for the DB-9 connectors I decided that I could use an old PCI bracket as a template. I lined the bracket up against the side of the enclosure and traced it. On the black box, I traced it using a knife, and on the grey box I used sharpie.
Once the outline was traced onto the box, I drilled holes for the retention screws. Then, I drilled out as much as I could of the trapezoid shaped interior. I most used the drill press, then the dremel with a router/cutter bit. I made sure to leave a margin inside the perimeter to remove with the files. I had avoided purchasing a set of jewelers files for a while, I think I assumed that they were expensive. They’re not, you should get a set.
In the photo above, you can see the finished penetration for the DB-9. I beveled the inside edge to make room for the fillet on the connector that I had. The black box got a male DB-9, and the grey box got a female one. The holes need to be about the same size, as the male shroud always has to fit over the female connector body.
In addition to the RF and digital connectors, I needed a way to securely mount the internal circuit boards. The way I chose to do this was first to drill holes in the bottom of the box, then “countersink” some screws into it. I have countersink in quotes because I don’t have a countersink bit, so I used a larger drill bit. You can see the results of this in the headline picture of this post. Though I think it looks pretty good, I still decided to buy a drill & tap for 4-40 screws after building the black box. For the grey box, I used the tap and screwed directly into the box. This requires slightly less hardware and looks pretty good, I think.
For whatever reason, the board I built for the black box using some veroboard-style construction didn’t work the same as the breadboard. Because I was under time constraint (I was planning on flying one of the transceivers over the weekend. I decided to put it back on the breadboard and use it as the base station. This version uses an FTDI cable to connect to my computer.
For the grey box, I used an extra ATMega48 breakout board I had. This one worked just fine in the enclosure, so I flew it. I also built a power regulation/distribution board, seen on the right of the photo. This concludes this article. Now that I’ve got at least one flight worthy transceiver I can test them in flight. That’ll be detailed on a future post.
I was able to find some designs online that fit my needs. The major constraint that I had to deal with is the impedance issue. Nearly everything amateur radio related is 50 ohms, and nearly everything video related is 75. The cheap Yagis paper written by Kent Britain, WA5VBJ, has a 75 ohm 421 Mhz antenna intended for amateur television (ATV). My transmitter is 434 Mhz, but I figured it would be close enough. The great thing about these designs is that they can be built using supplies from a standard hardware store. The elements are made from #10 bare copper wire, and the beam is wood.
An interesting characteristic of antennas, and RF in general, is that to get a stronger signal you often have to make compromises. A Yagi works by increasing the directionality to increase the signal. Unfortunately, because my plane is going to be flying around, I can’t be too directional. To get better results, without making things worse, I decided to only use the reflector and “driven element”. By eliminating the “directors” I hope that I can get the best possible results. (If you’re confused by this “director”, “reflector”, and “driven element” gibberish, the best place to look is the wikipedia article. But all that is necessary for this discussion is that the reflector is behind the driven element [which connects to the transmitter or receiver] and reflects the signal forward, and the directors go in front and focus the signal into a narrower beam).
There were some problems during construction that I should mention to help others wanting to try something like this. The antenna designs specify that the feed cable should be soldered directly to the driven element. This should work great on traditional 50 ohm radio cabling, such as LMR or RG-type cables. These cables have copper braid shield around the circumference. With the 75 ohm cable used in video, often made as cheaply as possible, a loose aluminum braid is used as the shield. This is a major problem that I had to deal with. It took me a while to even understand why the braid wasn’t soldering. I think I assumed that the braid was made of tin. After a few hours of searching, I discovered it was aluminum. Aluminum oxide forms almost immediately and can’t be soldered to, so even sanding the wire doesn’t help. There are solder pastes and fluxes that help, but I wasn’t interested in waiting for something to be shipped. My solution, if you want to call it that, was to mechanically attach the braid to some other wire that can be soldered.
In the first image of the post, I’m comparing the new antenna versus the others I used earlier. When the Yagi was installed, I rotated the antenna 360˚in azimuth to get an idea for how directional the antenna really is. There wasn’t much of a change in signal quality, so it isn’t very directional. If the transmitter were further away it may have been more dramatic. I am motivated to build a few more antennas, maybe with 1 and 2 directors to see which is better. With that said, I’m pretty satisfied, and I’m hoping for good weather this weekend.
b.t.w: Just to dispel any fears that the shield is shorted to the center conductor, as it appears in the above photo, it was, and I fixed it. Here is a photo of the feed point as it was when I tested it. Also, notice that I got my driven elements and directors confused when I wrote on the board 🙂
The weather sucked, but I was able to get out the the field this morning. I recorded about 17 minutes of video in total. In the embedded clip, I edited out the majority of the static. I’m a little disappointed that the signal quality is so poor. I found a site that has some cheap Yagi antenna designs in 50 and 75 ohms. I’ll probably try to build one this week and see how it goes.
Taking advantage of the crummy weather, I decided to paint the camera module to match my plane. I didn’t just do it for the aesthetics, no really, I swear. 🙂 Actually, the real reason I painted it was to prevent stray light from making annoying reflections on the inside of the window.
I’ve just finished building a camera module for my Kadet. When I was building the plane I knew that I was going to try and put a camera and transmitter approximately where the pilot’s head would be in a real plane. Read the rest of this entry »