Posts Tagged SIG
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 »
I’m back from the third (and fourth) flight, and it keeps getting better! On Friday (New Years Day) I went to the BCRCC (Benton County Radio Control Club) “Polar Bear” event. Basically, the idea is: On new years day, rain or shine, everyone comes and flies something. There is a raffle for all those that fly. I brought the Kadet, mostly because that’s the only plane I have, and the tiny Blade mSR that I got for X-mas.
The Kadet’s second flight was somewhat uneventful. Flying during an event invariably leads to crowded skies, because I didn’t have the runway to myself I just tried to stay out of people’s way. I was still pretty unfamiliar with my plane so it was nerve-wracking. Luckily I did a good job landing with all those people watching :).
Today, I went back to the field with my Kadet, and a new plane that I got at the BCRCC auction for $20 in November. It’s a “Funtana Mini” made by E-flite that has since been discontinued. Since the auction, I’ve been slowly getting the parts I needed to complete it, including a receiver and servos. I reused the motor from another plane that is no longer with us.
I piloted it on its maiden flight today. It is extremely twitchy, even on low rates. Also, something I didn’t expect was how hard it would be to land it. It requires a fairly high airspeed to maintain altitude, and there isn’t much frontal area to slow it down, so the landing was fast. I wasn’t able to stop it by the end of the runway so it spent some time in the mud. Nothing was damaged, but the mud at the airstrip is very stinky. The second flight was more fun, I actually did some aerobatics. I still had trouble bringing it down, though.
I wasn’t satisfied with the prop I was using in the first flight test, which was an 11×7 APC E-series. I bought 2 more, one with higher pitch speed (for the uninitiated, “pitch speed” is the number of inches a propeller would travel forward in one revolution in an ideal fluid) and the same thrust (which how hard it “pulls”), which I think is an 11×8.5 E-series, and one with the same pitch speed and more thrust; a 12×7 E-series, I think. I tried the one with more pitch speed first, and I’m not rushing to try the other one. I’m quite happy with the performance as it is. At full-throttle I can climb-out at 45°, and at 1/4 throttle I can maintain altitude and speed. I’m also very happy with the observed endurance. I ran the stop-watch on my radio during the second flight of the day while doing non-stop touch-and-go’s, and after 12 minutes of flying called it quits. The resting battery voltage on my 3S 2100 mAh LiPo was 11.4 volts. This roughly corresponds to 20% of the pack power remaining. With this knowledge in hand I know that I can set the count-down timer on my radio to 12 minutes and not stress-out my batteries too much.
Altogether, it was a good (long) weekend of flying!