I'm looking to build a one directional signalling system with a 1000m range and bi-directional reliability - i.e. either end can see if the link is lost. Only about three states need to be carried. The path is completely clear and unobstructed. The JeeNode would seem to be a sensible basis with a lightweight signalling protocol.
This is (I assume - I haven't bought the JeeNodes while I'm doing some other work) going to need more than a wire quarter-wave antenna. I don't want to spend £££ on a commercial antenna. Has anyone got relevant experience?
1. Does a television antenna for the UK's Group E have any gain left at 868MHz? It is supposed to cover channels 35-68 (586 - 850MHz) but I have always been a bit sceptical about wideband yagis.
2. Does anyone make a printed circuit antenna for this frequency? I've seen them for 2.4GHz but with a half wavelength around 170mm it ought to be possible.
3. Might a helical be the hacker's answer? The calculator at http://www.vk2zay.net/calculators/?body=helical.php suggests that a diameter of around 100mm should be OK and 3.5 turns would be 1.25m long and have a gain of 10dBi. That seems a plausible. I'll have to look for PVC pipe at the building store.
Happily I can mount the electronics at the antenna so there are no feeder losses to worry about.
I suppose 433MHz might have a more managable path loss but I worry about car keys etc.