AO-40 70cm Uplink Antenna
(created 20 September 2003)
Kent Britain, WA5VJB, has designed some really reproducible Cheap Yagi antennas at http://www.clarc.org/Articles/uhf.htm with parts you can get from your local hardware store. Combine two of the 8 element 70cm AMSAT antennas orthogonally on a single wooden boom offset by 1/4 wavelength, add a power splitter, and you have at least 10dBic gain.
The aluminium stock is 1m lengths of 4mm ali rod from B&Q. I used the inside bits of choc block to make an electrical connection - it's much easier to solder to choc block terminals than to aluminium! Be aware that for permanent outside installation, the electrolytic effect between the terminals and the ali rod is not good news, so you might need to rethink that one.
To make right hand circular polarisation, one antenna plane is located 1/4 wavelength behind the other. Be sure to set them up exactly as in the picture above or you'll end up with left-hand circular! Both antenna planes are then fed from a power splitter made from two 1/4 wavelengths of 75 ohm RG-59 and a good quality T-piece. Try to avoid cheap this Ethernet T-pieces - their performance at 435MHz can be awful.
The power splitter is made from two 1/4 wavelength pieces of RG-59. When calculating this 1/4 wavelength, keep in mind the velocity factor of the coax you're using. VF of solid dielectric RG-59 is about 0.69, and I've seen quoted between about 0.88 and 0.91 for foam dielectric. I used the solid dielectric stuff.
When tuning up, the two planes do couple slightly, so tune up each of the two planes individually with all elements of both planes in place. Also, terminate the plane you're not tuning with a 50 ohm BNC terminator. Once you've obtained a good SWR on each plane, hook up the power splitter, and hopefully you'll be in luck!
Mail Howard, G6LVB