To receive the relatively weak reflected signals you need an antenna, but it doesn't have to be spectacular. A suitable antenna is a three or four element yagi cut to a frequency within a megahertz or so of the station you've chosen as your "radar transmitter". It only needs to be a few metres above the ground and pointing accuracy isn't all that important either, given the wide front lobe on that size of yagi. Some observers elevate the front of the boom ten or twenty degrees if they're observing transmission from less than 600 km or so, but it didn't make any difference in my case.
There's no shortage of software to help design a yagi. A DOS-based package called QuickYagi is worth looking at. A larger antenna will bring in more signal, but that may not be a blessing if your receiver's front end gets swamped by other transmitters. More useful is a preamp at the antenna to boost the signal-to-noise ratio. For cheapness, reliability, ease of construction and performance it's hard to beat the venerable VK5 Preamp. (Don't buy the relays for it—you're not going to be transmitting!) However, information about this kit is hard to get, even from the WIA's VK5 Division website. An alternative design is described on Ilkke Yrjola's site.
|
|
3 element Yagi cut for 89 MHz, mounted 7 metres above ground level. The preamplifier is protected from rain and sun by a piece of PVC pipe. The boom has been left a bit longer than necessary to make room for experiments with spacing, or maybe another element.
Yagi construction needs only basic metalworking skills, and the only design problem I had was keeping water out of the preamp box since the rainfall at my location is over 2 metres per year and usually arrives by the bucketful. Like anywhere in Australia, during the summer months any exposed object gets very hot and insects can get quite intrusive. Eventually I used a small diecast aluminium box mounted inside a 30 cm length of 90 mm PVC drainpipe, capped by an overhanging "roof" of UV-resistant plastic sheet. The bottom has been left open to help with cooling and to drain any condensation or leaks. The entire assembly hangs from the antenna boom. All connections are coated in self-amalgamating tape and neutral cure silicone sealant.
Use decent quality coax for the run to your receiver, and don't waste money on cheap connectors. Avoid the clunky old PL239 and SO239 types: BNC or F styles work well and are much neater. The expense of type N connectors is probably not warranted at these frequencies.
[Introduction] [Meteors] [Antennas] [Receivers] [Interfacing] [Data logging] [Links] [Email]