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Old February 21st 05, 10:37 AM
Rich Grise
 
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On Sun, 20 Feb 2005 22:09:15 -0800, Nug wrote:

Hi
I am building an rf transmitter for a short range data link at 433MHZ
and am almost done, but I would like to understand better exactly what
I am seeing with regard to antenna performance.

All technical notes I have read recommend a 1/4 wave whip over ground
plane as offering the best performance, statements like: "Best range
is achieved with either a straight piece of wire, rod or PCB track @
1/4 wavelength over a ground plane", I understand many factors effect
performance however I have found that a "bent" 1/2 wavelength length
of wire offers better performance.

If I use a 1/4 wavelength I need (due to case requirements) to have
two 90 degree bends in it (feed - up, across, up).
If I use a 1/2 wavelength I need to run it once around the (plastic)
case (feed - up, around the case, up).

I hope this makes some sense, anyway I have found the 1/2 wave is less
effected by polarisation and offers generally better performance.
However while more ground plane may help a 1/4 wave it seems to hinder
the 1/2 wave, I guess because it shields the loop around the case?


A 1/4 wave antenna will match to a low impedance, unbalanced. A 1/2 wave
dipole will match to a low impedance, balanced. A 1/2 wave piece of wire
fed at the end will match to a high impedance.

What kind of circuit are you using for your output?

Thanks,
Rich