Amateur Television (ATV)

I'm not a real ATV fiend, but I have fooled around with it some. I have some gear from P.C. Electronics (http://www.hamtv.com/) that I got to put into a R/C sailplane. I have a 70 cm (440 MHz band) transmitter, a 70 cm to Ch3 downconverter, and a little Ch3 to video demodulator.

TXA5-70b is a 100 mW transmitter, which I've got set up to switch between 426.25 and 434 MHz

TVC-2G is the down converter

The Sailplane

One of my first ATV experiments was I mounted an old CCD color camera, the transmitter, and an antenna on a 2m wingspan sailplane. I've got a page with some pictures of the installation and more description. While interesting, the overall experience was not all that successful. From a flying standpoint, the rig was too heavy. From an ATV standpoint, the linear polarized transmission and a vertical antenna only a meter or so off the ground had huge multipath fades.

Now, I am convinced that circular polarization (CP) is the way to go (that, and a bit of antenna gain and height so the receiver antenna isn't so close to the ground). A az/el (or even just an az) controlled Yagi or Log periodic might be a good way too. For the sailplane version, this would work pretty well, since a beamwidth of 40-60 degrees would nicely cover the entire airspace where the plane would be flying. The Clear Lake Amateur Radio Club has a web page with cheap easy to build medium gain Yagi antennas. I've actually built an 70 cm beam antenna based on their designs.

However, for ground based sources (my current interest, see the robot pages) gain antennas are pretty tricky. The mobile would need to steer the antenna, and multipath is going to be a problem when you are sitting less than a wavelength or so off the ground. So, hence my interest in omnidirectional (in horizontal plane) antennas like the Lindenblad, the Superturnstile, etc. Check out the bucket Lindenblad. For a more crude version, being used for testing, there is the bucketwhip.... A 1/4 ground plane vertical.

Packaging the TVC-2G

Another learning experience with the plane was that a cardboard box with a bunch of stuff in it doesn't make for a good field installation. So, I finally decided to put the TVC-2G into a box so that it doesn't get banged up. A plastic box (metal would be nicer, and I suppose I could coat the inside of the box with copper or aluminum foil) holds 8 AAA batteries for a 12V supply, as well as the TVC-2G. The input is on a BNC, the output on a standard cable TV type "F" connector. I've noticed a few spurious signal problems with the TVC-2G (that is, touching it detunes it), but inside the box, it seems to work ok.

I either run the output to a TV tuned to Channel 3 (using cable) or I couple it to a short whip, which then radiates to the whip of a small LCD TV stuck to the box with Velcro.

LCD TV

When testing it on the bench with the 426.25 transmitter, I noticed an odd thing. The LCD TV will tune the 426.25 signal on the "UHF" position, which is odd, since UHF channels start at 470 MHz, and it tunes a ways up in the band. It's probably an image in the TV receiver. A typical TV IF is 45.75 MHz, so there's an image 91.5 MHz away, or, at around 516 MHz apparent frequency. This is a strong signal case (100 mW into a quarter wave ground plane about 1-2 meters away from the receiver). Typical UHF receiver noise figure is 10-12 dB, a lot worse than the 2 or 3 dB from the TVC-2G.

With a bit of web research, I think I found that this unit (a Radio Shack/RCA 160-3050) is really a Casio TV-880. Thanks to "Frank's Taschenfernsehere: Bunt und Billig (Teil 2)" where he had a similar looking unit (the TV770, for PAL) and a link to a pdf service manual (at a site called www.guenthoer.de/doku/) At that site, I found the TV-880 service manual, which comparing the parts list to the Radio Shack parts list, is virtually identical (right down to package codes on ICs). [ I just found Frank's English language pages: http://www.guenthoer.de/index-e.htm. Thanks again, Frank (Frank Günthör) ]

Photos of the TVC2G and the LCD TV

Front view of the LCD TV on top of the box with the TVC2G
Here are the two components separated, showing the velcro
Here's the end of the TVC2G enclosure. A thin screwdriver pokes in the hole to hit the trim pot on the converter.

LCDTV and downcoverter thumbnail

(44 kByte jpg)

downconverter and LCDTV thumbnail

(48 kByte jpg)

downconverter end view thumbnail

(27 kByte jpg)

 

 

/radio/atv.htm - 31 August 2002 - Jim Lux - radio home page - Jim's home page