ivp wrote:
> Since you mention a dipole, there's an example here (page 11)
>
> http://www.rfm.com/corp/appdata/antenna.pdf  (106kB)

I just scrolled thru that document quickly mostly looking at the pictures,
but it appears to be a nice writeup on compact antennas that can be
integrated on a PC board.

> of a PCB trace 'folded dipole' for 434MHz. In that form it's too
> long for the case I'd like to use, which is unfortunate because the
> author says it has very good performance. Perhaps it can be re-
> arranged to be wider and shorter to suit a small case.

Probably.  That would probably also make the sensitivity in that plane more
even.  Even with fancy antenna modelling software, you still have to
actually try it, then tune it for the performance you want.  You can do som=
e
cool things with small antennas with maybe a L or C in there to resonate at
your desired frequency or to null out the imaginary component of the
impedence.  Just don't expect a nice 50 or 300 ohms or something.  At such
small geometries, the impedences are usually higher.

> For example
> can the top half be rotated through 180 to be next to the lower
> half and still perform ?

Perhaps, but that would be a "major" change such as to be a different
antenna.

> Where he notes 'feedpoint', does he mean
> that the signal is connected to just the lower trace end ?

This is a symmetric self-contained antenna.  It therefore is fed with a
differential signal, just like a ordinary center fed dipole.  Look at it
carefully and you'll see it basically is a center fed dipole with the ends
curled up.

The antennas with ground plane only take a single ended feed because the
voltage at the feed point is relative to the ground plane.  Another way of
saying this is that the antenna is not self contained since the ground plan=
e
is a integral part of the antenna.

Actually all antennas have two feed points, but in some one of the feed
points is implied because it is also the system's voltage reference (like a
ground plane).  There is really no such thing is a single feed point antenn=
a
if you look at the whole system the antenna is a part of.

Hopefully this didn't confuse more than illuminate.


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