Basics of radio for a focused transmitter

  • #1
Hi there! I'm looking to create a small (max 20 foot range) radio transmitter, to direct a focused radio beam at a receiver, for the purpose of remote switching. Ideally, I would like the device to work at a relatively "exotic" frequency, as I would think there is a lot of radio noise where I live.

I have a basic knowledge of electronics and physics, but have only recently begun to delve into radio - I thought this might be a good experiment to this end. My problem is that while I understand how the transmitter itself is supposed to oscillate at high frequency/power and thus induce an RF field in an antenna, I do not know how I would focus the radio waves into a beam.

If it makes a difference, the device is intended to transmit at one receiver at a time. I have heard mention of using satellite dish-like contraptions to focus the beam, but I honestly have no idea.
Thanks in advance for the help :)

P.S. - I live in the US - as for the law, I am willing to attain a radio license to use this transmitter, if need be.
 

Answers and Replies

  • #2
berkeman
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Welcome to the PF. :smile:

For that short of a range, you could use a simple kit in the ISM (2.4GHz) unlicensed band. You would only need to "focus" or direct the transmission if you were trying to transmit over a large distance with low power. Have you looked at ISM band kits?
 
  • #4
Please excuse my ignorance; I am infinitely far from being an expert. Perhaps I should have mentioned - I am basing my idea off of a set of electronic candles my mother has: they are able to be turned on and off by one radio remote. I was inspired to create a similar device, to switch things other than candles (lights via relay, etc.). would an multi-directional antenna have the same effect? I assumed that unless I focused the beam, the signal would just switch any device that happened to be in its transmission radius.
 
  • #5
berkeman
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I assumed that unless I focused the beam, the signal would just switch any device that happened to be in its transmission radius.
In practical communication systems, you use addressing to uniquely identify the intended recipient of the message. Kind of like how your garage door opener works. There is a unique code for your opener that does not match the other openers in your neighborhood (hopefully).
 
  • #6
anorlunda
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Please excuse my ignorance; I am infinitely far from being an expert. Perhaps I should have mentioned - I am basing my idea off of a set of electronic candles my mother has: they are able to be turned on and off by one radio remote. I was inspired to create a similar device, to switch things other than candles (lights via relay, etc.). would an multi-directional antenna have the same effect? I assumed that unless I focused the beam, the signal would just switch any device that happened to be in its transmission radius.
Think of the key to modern cars that can unlock the doors from 20 feet away. They are pretty simple. Chech the link @berkeman gave you.
 
  • #7
berkeman
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And think of the IR TV remote control paradigm -- there you aim it at your TV, and there is little chance for it unintentionally changing your neighbor's TV settings (because the IR light doesn't penetrate your walls to get to your neighbor's TV).

But with radio, even with a directional antenna, your signal will be received by any receivers in the general area of the intended receiver. Hence the need to encode a recipient's address in the data that is transmitted. Makes sense?
 
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  • #8
Borek
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I would like the device to work at a relatively "exotic" frequency
Beware: these things (frequency usage) are heavily regulated, you have to use one of the allowed bands, not something "exotic" of your choice.
 
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  • #10
Yes, I believe it does. Initially I intended to use laser light for this purpose, but figured that natural light interference would make actually using the device impossible. Your point about addressing makes a lot of sense - for a general example, I could have a button A for receiver B and button C for receiver D [...].
 
  • #11
jim hardy
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There are any number of approaches light, ultrasonic, radio...

The easiest is just present-absent your signal, like CW radio .
Next step up is to modulate your signal somehow , as in AM or FM radio, with a second signal .
That's how TV remotes work - most modulate an IR beam at 40 khz, that 40khz signal carrying the code specifying what button was pushed. You can buy those transmitter-receiver pairs at hobbyist sites very cheap.

Keyless entry and garage door openers use embedded computers to change the entry code on the fly for extra security. That gets complicated in a hurry.

If you're just starting out i'd suggest the IR route. Point your TV remote at your cellphone camera lens and see if it shows the IR . Electronic cameras see into infra-red and that's a quick check whether a TV remote is transmitting.

Try a search on IR communication hobby kit.
my first four hits
https://snootlab.com/lang-en/parts/131-ir-communication-kit.html
https://www.sparkfun.com/products/13235
http://www.rainbowkits.com/kits/ir-2p.html
http://www.hobbytronics.co.uk/hx1838-infra-red-remote
 
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  • #12
Good idea. I have recently considered using some sort of laser, and it seems much more doable for a person of my stupidity :) I think if I have multiple frequencies (1 for each receiver) and then put an LC circuit (complicated stuff :/) between the receiver and the switch, that should work.
Thanks!
 
  • #13
davenn
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I think if I have multiple frequencies (1 for each receiver) and then put an LC circuit (complicated stuff :/) between the receiver and the switch, that should work.
I doubt you will get the selectivity you need with basic LC filtering, they will be too wide (broad in their frequency coverage) and will
respond to anything freq close to the preferred.
You would really need good a receiver design that has sharp filtering

BUT if you go for digital codes superimposed on the single freq transmission, then it is much easier to recover the appropriate code
to activate a specific function at the receiver end of the link


Dave
 
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