Hello

I wand to build a short range direction finding equipment (say 200m), and I
wonder what kind of antenna would be the simplest and best for that.
the frequency will probably be around 868MHz.
the precision needs not to be very high (30°).
the antenna will be mounted on a car, and should be as small as possible.
The equipment will be mainly used in an urban environment.

Thanks for any help, tip or link

Denis

Snide wrote:

Hello

I wand to build a short range direction finding equipment (say 200m), and
I wonder what kind of antenna would be the simplest and best for that.
the frequency will probably be around 868MHz.
the precision needs not to be very high (30°).
the antenna will be mounted on a car, and should be as small as possible.
The equipment will be mainly used in an urban environment.

Thanks for any help, tip or link

Denis

As the wavelength is fairly low, I suggest you build a 6 * 1/4 wave antenna
array and mount it on a roofrack. You can easily build a direction finder
by phaseshifting the array with a pack of pin diodes.

This method is widely used for tracking vehicles.

Directivity... 868MHz is still a long wavelength for any exotic and
directive antennas. But why not try building a loop yagi in a plastic tube.
Use PCB FR4 with plastic foam as spacer between the radials.

But still, a 4 or 6 antenna array on a roofrack seems to be enough to catch
stolen vehicles in the UK...


Cheers


Dan / M0DFI

thank for that. have you more details about all that stuff ? plans,
schematics, links ?
I'm not an expert...

Denis

Assuming that the signal you're trying to track isn't moving around too
quickly, and is "on" for relatively long times, I'd suggest a simple
Yagi antenna connected to a receiver with a signal strength indicator.
Rotate the antenna till the signal peaks; the antenna points in the
direction of the strongest signal (usually the direction of the
transmitter).

You can find Yagi design info on the web. Do a Google search for it.
I'd suggest something modest, like 3 to 5 elements. The elements will
be about half a wavelength long, which at 870MHz is about 1/6 of a
meter or about seven inches. The whole Yagi might be a foot long. You
can easily make it using PVC pipe and pipe fittings, and elements made
from heavy copper wire. Be careful about having the feedline decoupled
from the antenna so that the directionality of the antenna is not
messed up by the feed line picking up signals.

Another poster suggested what I suppose is a Doppler scanning antenna,
an array of elements switched with PIN diodes, but the complications of
building such a system probably outweigh its advantages for what you
want to do, unless you're going to be doing a LOT of direction-finding.
Besides the PIN-switched elements, you need control/display
electronics and an FM receiver.

Cheers,
Tom

the problem is that the finder will be mounted in a vehicle, and can't be
rotated, unless with a motor. I would avoid such a mecanic system, and I'm
rather interested in an circular antenna or array of antennas.

Denis