In article ,
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.

[.. 1/4 wave and 1/2 wave ...]

An antenna looks like an LC tuned circuit loaded by the radiation
resistance. Your output stage has some impedance that correctly matches
to it (there are exceptions we will ignore) and it is this impedance you
want the antenna system to have. When the correct matching is done, the
antenna works as an impedance mathcing network that matches the output
stages impedance to the radiation resistance.

The normal (90 degrees to) 1/4 wave whip over a ground plane is one half
of a dipole that is 1/2 wave length. The ground plane operates like a
mirror. The electrostatic lines of force follow the same path with the
mirroring as they would if the other 1/2 of the dipole was there. This
lets you use a smaller (1/4 wave) antenna to get the same effect as the
1/2 wave.

In your case, you are not using a whip antenna. If I've read what you
wrote correctly, the antenna spends more of its length parallel to the
surface of the PCB than it does running 90 degrees away from it. You
have some circuit with a ground plane and a limitted sized box to work
with, so the mechanical shape is constained by the box and not the ideal
electronics.

Since the box is small: If you have the equipment to do so, I suggest you
measure (estimate) the impedance of the longest single loop of wire that
will fit within the case. ie: connect to both ends. You have to have the
electronics PCB in the case when you do this. If you are very lucky, its
impedance will not be too hard to match to the output stage.


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