I have an application that requires RF to be propagated for several
hundred metres behind a vehicle at 433 MHz.

I am looking for ideas for an antenna that could be built into the metal
enclosure that houses the electronics. (ie. A single square or round
enclosure).

Ideally the field would fan out behind the vehicle and not have any side
lobes. (Only want RF behind the vehicle).
I am not worried about vertical propagation, Mainly just parallel to the
ground about 1.5m.

I am not sure if maybe I could get a corner reflector, patch antenna,
PCB loop, parabola or slot antenna into a fairly small surface area
(around 200mm x 200mm).


The transmit power is 25mW.

Any ideas much appreciated.

Thanks

Regards

David

Hi David

There are lots of possibilities here. Generally speaking though
confining the antenna to a metal enclosure of 200mm x 200mm is going to
severely impact its efficiency. I'd suggest that if you really want to
use this size constraint make part of it out of something transparent to
the signal (eg plastic/fibreglass)

My first thought (and there are bound to be many more) is to setup a
single full wave circular loop fed through a 1/4 wavelength balun such
that the shield on the opposite side of the loop from the feedpoint is
"grounded" there. Then mount it in front of a metal plate of gauze
inside the plastic thing that contains it. If it is a square shaped box
make it a square full wave (quad) shape. For horiz polarization the
antenna would be fed at top or bottom of loop.

Assuming the TX/RX device needs a 50 ohm antenna you can then adjust the
loop to plate/gauze distance to get the best match or received field
strength.

You could of course mount the antenna off the metal box face some, and
maybe use plastic of the same metal box dimensions if it is important to
keep it enclosed.

My gut feel for approximate dimensions are as follows;

Antenna to metal plate/gauze - 50-100mm

Antenna diameter (if circular) - 200-240 mm

Antenna side length (if square) - 170-190 mm

This antenna would probably give you a large single horiz lobe in the
vicinty of 120 degrees half power beamwidth. This vertical pattern will
of course be affected by the nearby ground and much of our radiation
will be angled upwards and broken into a few lobes.

If the 200mm is a hard dimension number I'd suggest you consider lightly
loading the loop with some inductance to make it smaller. This will of
course impact performance. I still maintain though that it should be
mounted clear of the metal box in plastic or air.

Keep in mind that at 433MHz there is going to be a lot of multipathing
and the phase cancellations may be an issue if the distance becomes too
great. I havent bothred to figure the path loss for the circuit you have
described. It might be worth doing so to make sure its doable. Another
consideration is do you expect it to work over the several hundred
metres if there is a hill in the way?

Hope you find this useful.

Cheers Bob VK2YQA

David wrote:


I have an application that requires RF to be propagated for several
hundred metres behind a vehicle at 433 MHz.


Any ideas much appreciated.