Ok I have another question. Im looking to build a waveguide for some car roof mounted antennas, though am unsure as to whether i require gain or not. The transmitter and receiver modules im using will give me a range of 100m, which i intend to maintain. I want to use a waveguide to increase the directionality of my antenna. If i had a simple aluminium box design surrounding my antenna that had dimensions above half the wavelength would that cause any loss in signal strength recieved or would it maintain the 100m range i require but over the specific direction dictated by the waveguide?
I need to keep the design as compact as possible, and given that im using 868Mhz UHF any specific flaring to increase gain, would make the guide far to big. Would a small amount of flaring increase the gain in any way?
Hope someone can halp.
Many thanks
Matthew Webster

I can't imagine what you're trying to accomplish, but please post a picture when you are done.

With multiple horizontally polarized UHF antennas connected with waveguide, your car is going to look like a mobile air conditioning plenum.

It does sound a bit crazy, I know. Im hoping to keep any waveguides used to be as compact as possible. I just need to guage how big it would be to be applicable to 868Mhz UHF.
I really hope my car doesn't end up looking like a mobile air conditioning plenum. I really do!