[MLB-WIRELESS] Radio Wave Theory

Tony Langdon (ATC) tlangdon at atctraining.com.au
Fri Nov 5 08:53:33 EST 2004

>         At best, firing a 2.4 Ghz signal a a flat plane 
> "survey marker" (as the original message was about) from 
> probably not only a horizontal angle but also some angle with 
> a vertical component would  result in all sorts of reflected 
> waves being seen and probably not much of a useful signal.

This is true.  There will be a scattered component, but it will be smaller
than the main reflection.  
>         Ok well get a friend to park a trailer nearby to the 
> Trig Station with a more suitably sized reflector thats 
> tilted at a better angle, for your bounce, just to prove a point

That'd work. :)

>         Then once you have it working with the large 
> reflector - then shift across to the marker, you never know your luck
>         Another angle (oops) to try - is if there is another 
> hill or building between you and the reflector then lower 
> your antenna so that the signal has to diffract over the 
> first obstacle, and then across to the reflector - this 
> diffraction will lift your angle of incidence to the 
> reflector and if you get it right, give you some signal gain 
> as well on the way

Knife edge diffraction.  Can work very well.  I may be dealing with this one
myself on 438 MHz in the not too distant future (sans the reflector). :-) 

>         I install 400Mhz radio in the Metro area of Sydney 
> and environs and I go for the diffraction gain - not that it 
> happens very often that the geometry is right - but when you 
> do get it the signal lift is dramatic

Yes, if you get it right.  The details get down into Fresnel zones and the
like, but it can work well.

As for myself, I use mostly VHF/UHF between 50 MHz and 440 MHz, with a
little 1.2 GHz, plus the HF bands.  Also heavily involved in direction
finding, where accurate knowledge of RF propagation in complex terrain can
be a decided advantage (so many novices end up chasing reflections on VHF).
>         Cheers and thanks for the input,

You're welcome.

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