On 12/03/2012 06:42 PM, houlihan wrote:
> I agree entirely with Frans reply,
> I was told during a conversation with the Trigg development engineers
> some time ago not to use a perfectly circular ground plane at the
> calculated diameter as it could possibly resonate and interfere with the
> signal in some way , the reasoning as to why was well above my pay grade .
>
> It is very easy to avoid this phenomena by making the ground plane
> slightly irregular.
Reason of this is that if it is exactly circular, it resonates at
exactly one frequency, i.e. it is very narrow banded. The transmitting
and receiving of the transponders is on different frequencies, so you
need to avoid extremely narrow banded antenna's. Keep in mind that
resonance not only happens at one quarter wavelength, but also at
multiplies and certain fractions of the wavelength. If you use a true
circular ground plane chances are you are hitting a resonant somewhere.
If it is not exactly 1/4th maybe it happens to be 5/16th?
If you make the ground plane a little bit irregular, each frequency is
able to find a matching resonant size somewhere on this shape, so the
sum of the total lengths possible results in a much wider band width.
Electromagnetic resonance is quite similar to accoustic resonance, I
guess if you try to build a quitar with a circular body it would be
difficult to tune and some notes would play much louder than others,
hence the irregular shape of guitars, which function to make its
frequency response (resonance) much wider than that of a circular shape.
So, put the antenna not exactly in the middle if you use a circular
ground plane, or better yet, use a rectangular ground plane, so the
electrons see various distances to the edge of the ground plane.
I guess a guitar shaped ground plane would serve your transponder also
very well. ;-)
Frans
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