433MHz Direction Finding

abarber

New Member
Hi all!
Having searched around on the internet, I've discovered several sources that suggest that this is possible, and found several diagrams on how to construct directional yagi antennas.

Has anyone here tried this or anything similar? I'll be using the RFSolutions 433MHz TX/RX pair modules, and has anyone successfully used the RSSI (Received Signal Strength Indicator) pin/facility on these modules before?

I'll be using a PICAXE to a) Send out some serial data and b) make sense of the recieved signal strength value, and somehow display it (LED bargraph, Serial display etc)
Thanks!
Alastair
 

hippy

Technical Support
Staff member
Yeeha ! I think the Nesbit thread is brought back to life :)

http://www.picaxeforum.co.uk/showthread.php?t=6074&highlight=nesbit

If it's not in there, search the forum on 'rabbit', 'nesbit' or more generally 433MHz terms.

I've done RSSI with XBee and it seemed to work reasonably well. I had one module in a biscuit tin and the signal strength dropped as I rotated it away from the other end. So the theory's fine.

For 433MHz modules it's probably an analogue signal which should be easy enough to read with READADC/READADC10 and some maths / lookup table to convert RSSI level to a display value. The PICAXE08M is probably fine for what you need - You probably do not want to use a PICAXE-08 or 18 as they have limited analogue input capabilities giving only 16 differening levels.
 

andrew_qld

Senior Member
Direction (bearing) is far more important in direction finding than signal strangth. While you could manually rotate a yagi, a better direction finding technique is to either:

1) use an antenna with very deep nulls. Its easier by ear to hear a null in the signal (ie hear it dissapear) than hear the peak.

2) Use a doppler system. In the old days in Australia you used to be able to buy a "Dick Smith" kit that did this. You use an "electronically roatating" antenna (4 or more antennas arranged in a circle) and switch between the antennas very quickly using diodes. The received audio has an audio doppler shift on it (an audio tone) which drops in pitch as the antenna "rotates" past the signal. This is fed to a display which simply points in the direction of the signal.

Search on google for "Foxhunting", "Doppler" and "RDF".

Andrew
 

manuka

Senior Member
Wireless "foxhunting", although perhaps at it's peak locating spies during WW2/Cold War, is still a VERY serious pursuit. There's even talk of it as an Olympic sport. Aside from locating Emergency Rescue Beacons (Yeah- Steve Fosset style....) or tracking animals, an important RDF need relates to finding sources of radio & TV interference from bizarre electrical problems. These can sometimes be miles away, & due to a rubbing wire on a power pole etc.

At UHF antenna can be quite compact & mildly directional, but a major UHF RDF (Radio Direction Finding) issue relates to the terrain & nearby reflective surfaces, which may cause signal peaks to apparently come from unexpected directions. Roanoke style Doppler gear is now much preferred, but a lot of fun is possible in open spaces with simple receivers & "cotanga" Yagis (or even plain body shielding) suiting scout groups etc. Such youngsters can run off excess energy while also mastering map reading, or even- gasp- applying basic trig! Making their own simple antenna even teaches about resonance & wavelength/freq. relationships. A large part of the fun relates to disguising the transmitter as a plant, or everyday item such as sunglasses, candy bar or a shoe etc! See => http://members.aol.com/homingin/ or Google.

EXTRA: If you are keen to run with this suggest you look at bargain ~470 MHz PRS UHF CBs, as their receiver is usually much more sensitive than 433.92MHz ISM data gear. Additionally their transmitter can allow linkups when fox hunters themselves become lost (it happens...) Stan
 

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demonicpicaxeguy

Senior Member
Wireless "foxhunting", although perhaps at it's peak locating spies during WW2/Cold War, is still a VERY serious pursuit. There's even talk of it as an Olympic sport. Aside from locating Emergency Rescue Beacons (Yeah- Steve Fosset style....) or tracking animals, an important RDF need relates to finding sources of radio & TV interference from bizarre electrical problems. These can sometimes be miles away, & due to a rubbing wire on a power pole etc.

At UHF antenna can be quite compact & mildly directional, but a major UHF RDF (Radio Direction Finding) issue relates to the terrain & nearby reflective surfaces, which may cause signal peaks to apparently come from unexpected directions. Roanoke style Doppler gear is now much preferred, but a lot of fun is possible in open spaces with simple receivers & "cotanga" Yagis (or even plain body shielding) suiting scout groups etc. Such youngsters can run off excess energy while also mastering map reading. A large part of the fun relates to disguising the transmitter as a plant, or everyday item such as sunglasses, candy bar or a shoe etc! See => http://members.aol.com/homingin/ or Google.

EXTRA: If you are keen to run with this suggest you look at bargain ~470 MHz PRS UHF CBs, as their receiver is usually much more sensitive than 433.92MHz ISM data gear. Additionally their transmitter can allow linkups when fox hunters themselves become lost (it happens...) Stan
funny storey i was told by a friend of my fathers that worked as a technition at the big radar array in the northern territory,
they all of a sudden started getting some bad interference and needed to trace it which they did, it ended up being a tv without it back cover on or any of the shielding it was supposed to have, it was in a shed just south of the south australian/northern territory border, the worst bit about the whole thing was that the tv would get turned on and off as each started and finished aparently it was a real pain to track down because of the distance and the fact that the interference wasn't on all the time, needless it took some time to do
 

manuka

Senior Member
For the devoted RDFer,with a decent budget,Doppler RDF is the way to go! However even cheap "cotanga" antenna ( => www.picaxe.orcon.net.nz/yagi433.jpg) can give 6dB gain & good enough peaks/nulls to allow simple triangulation. IMPORTANT POINT- antenna ends are hazardous & can poke you in the eye, so ensure they are protected (hot melt glue tipped etc). I came up with a version made from telescopic whips that better suited confined spaces in fact
 

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abarber

New Member
Hi all - Thanks for your responses!
Hmm, well I'm trying to build a 433MHz yagi, then attach a reciever to it, and swing it around until I find a null, the idea being that the null is pointing in the direction of the source. Then I'll try and get a Picaxe to measure the signal, and hopefully get a computer or PDA to draw out a graphical representation of what it's 'seeing'...
 
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