extending range of cheap 315 MHz radios

I have a project where I am using some cheap Seeedstudio 315 MHz radios in a fixed installation. I have these: http://www.seeedstudio.com/depot/315mhz-rf-link-kit-p-76.html I want to get as good reliability as possible, likely at the extreme end of the range that they can handle. These are the SAW-based radios that have virtually no documentation, so it's mostly guesswork how to get the most out of them.

  • the transmitter power is higher with higher supply voltage. I will use unregulated power from a VRLA battery -- around 12 to 13.5V -- which is basically the limit for these parts
  • I am using the VirtualWire library which uses a reliable protocol well suited to the capabilities of the radios.
  • I am using as low a bit rate as possible -- I'm trying 1000 baud, but could go as low as 300 or 500 baud

I am looking for guidance from folks who maybe know these radios in more detail than the documentation gives.

The receiver has an AGC circuit. Anyone have an idea how responsive that circuit is? Is there going to be a baud rate below which the AGC gets confused? Is 300 or 500 baud too low?

I have a fixed installation and could design an efficient antenna. Anyone have a clue what the matching impedance for the transmitter or receivers is? Anyone actually designed a working antenna?

I just don't have either the RF experience or tools to figure this stuff out for myself.

Thanks

It might be too obvious but height of TX and a clear line of sight between TX and RX are good starting points

Well, yeah. It's a fixed installation, so I have some constraints, but I will try for the best LOS I can, and if I use external antennas, I can position them for the best link. The overall link distance is not too bad -- ~100m or a bit less. But there is a building in the way that I just can't avoid. And it has a steel roof.

A 'pringle can' antenna is highly directional and they're reputedly very effective at boosting the range for fixed locations.