Extremely directional radio receiver

[madmanzippy]

Hi All,
This seems to be the place to ask for help on a fun project I am playing with. I am wanting to make something like laser tag but with radio. I am no expert on radio so excuse my ignorance. Everyone would where a vest with a simple radio transmitter and when you pull the trigger it will detect the signal if it is pointing at a vest/person.

I am not sure what freq range I should be into do this, however I am wanting to cover a distance of say 200 meters. My thought was that it will all be in the antenna for the accuracy. I am thinking that the antenna could be housed in a tube like a felt pend that is lead lined to eliminate any stray freq and only detect the vest. I understand that at 200 meters the area that would cover would be quite wide, but for this purpose it does not matter too much.

Any help and further reading suggestions would be helpful.

Regards
Nick

 

[the_emi_guy]

Hi All,
when you pull the trigger it will detect the signal if it is pointing at a vest/person.

What you are asking for is known as a directional antenna. How directional would it need to be?

Couple things you need to know about radio antennas:
1 - Degree of directionality is specified as "beamwidth" in degrees
2 - Narrow beamwidth (which is what you want) implies high gain
3- The higher the gain, and thus the smaller the beamwidth angle, the physically larger the antenna needs to be at a given frequency.
4 - The higher the frequency, the smaller the antenna can be for a given gain and beamwidth.

So if the goal is laser like directionality, you would want the biggest, highest frequency antenna you could get.

Problem is, in practice, you probably can't get the directionality would want in an antenna small enough to carry around at a practical frequency. For example a 80GHz parabolic dish antenna 65cm in diameter would have a beamwidth angle of 0.5 degrees. At 200meters distance this would imply a 1.7 diameter target. That would not allow you to aim very accurately.

Compounding this is that the antenna beamwidth is the angle where the power is reduces by only half. You will actually have to move to a significantly more distant angle before the receiver no longer picks up the signal given that the receiver will need to have a large dynamic range (you want to shoot someone right in front of you as well as at 200 meters).

Then there is the problem of multi-path. 60GHz signal will bounce off of any metal object. Every time you pull the trigger you will be scattering "bullet fragments" all over the room (unless you line the walls with RF absorbers).

 

[sophiecentaur]

Radio, for this purpose is really a non-starter. If you don't want the beam to be visible then use an IR system. The beam width of any radio antenna you could carry (at any practical radio frequency) would be far too great and the achievable directional resolution would be very approximate. That's why Laser Warriors etc. do it the way they do.

OTOH, direction finding, based on using a 'null' in an antenna pattern, rather than a maximum, can be much more accurate. I am surprised that field workers (naturalists etc. ) wander round with a Yagi style antenna (like the conventional TV type) when chasing a tagged creature in the undergrowth when a loop could give a more accurate idea of direction. (The sort of thing you see in war films - on top of a van, when they're trying to locate an enemy agent's transmitting site.)

 

[Baluncore]

There are a couple of ways to find the direction more accurately than the traditional
beamwidth = 57° / (antenna diameter in wavelengths)

Firstly there is FM Doppler.
https://en.wikipedia.org/wiki/Direction_finding#Pseudo-doppler_DF_technique
http://www.silcom.com/~pelican2/PicoDopp/ABOUT_DOPP.html
The antenna is about half a wavelength diameter

Secondly there is the Lorenz landing system or Knickebein system.
https://en.wikipedia.org/wiki/Lorenz_beam
https://en.wikipedia.org/wiki/Knickebein

 

[madmanzippy]

Thank you for your responses. I had an idea that it wouldn't be practical. I did think about an optical version, however if you are amongst foliage then you may have emitters hidden behind leaves etc. Without being covered head to toe in IR emitters I could only think of the radio route to make it work.