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Old February 21st 05, 10:12 AM
Bob Bob
 
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I would make the grandiose statement that since you are bending the wire
it no longer exhibits the performance of a "standard" 1/4 or 1/2 wave
antenna. I would suggest that if you indeed made a 1/4 wave GP that
protruded from the box surface (with a suitable counterpoise) it would
outperform the 1/2 wave bent one..

Assuming you have to put the antenna inside the box or wrapped around it
I suggest you look into tuning it with some C and/or L. In that case you
would construct the antenna to fit your case parameters and adjust the
matching for best radiation. Keep in mind that the C/L tuning components
could be lengths of coax and open feeder/wire. (because of the high
operating freq)

Cheers Bob VK2YQA (Sydney Australia)



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?

Regards