RF remote control for PC

[Whitebird001]

Hi all,

I'm building an RF remote control for a PC and have the following constraints:

-The maximum frequency my scope is able to view is 50MHz, so it sets the maximum frequency of the carrier. In my case it is 43MHz.

-The case is 8*10*3cm => the maximum size for the antenna is 10 cm (I don't want any external antenna)

-The supply voltage is 3V. Oscillation amplitude is 1V peak to peak at the antenna feeding point.

-The carrier is ON-OFF amplitude modulated with a PIC.

Actually my problem is that I have very poor range. I need 3-5 meters, but I have only arround 50cm.

If a connect a wire that is 1/4 wave length long, then the range is OK. The problem comes from the fact that I don't know what kind of antenna to build that will fit in the case and will be efficient enough.

-So my question is: what is the best antenna and how to match it?

-How can I improve the range of the transmitter even if the antenna is very ineffective?

 

[MacGyverS2000]

You need to either increase your frequency considerably or increase the effective length of your antenna... kind of tough to get around the laws of physics. Any particular reason you can't use a pre-made kit, something in the 400-500 MHz range?

Dan
Owner

http://www.Hi-TecDesigns.com

 

[VEBill]

Put a detector on your scope and pay no further attention to the carrier frequency. Then you can use your scope to debug the modulated inforation. Use the receiver (the one you have on your PC) to make sure that the carrier is tuned in to the correct frequency. This would allow you to choose a better frequency (shorter wavelength, more efficient antenna).

But 5m is trivial. It's likely that there is something else wrong other than the antenna.

Also, couldn't you purchase Bluetooth user devices?

 

[Higgler]

How about using your electrical power wires as your computer antenna. This should help your coupling probably 20-40 dB since you'd have one good antenna and one bad antenna on your remote.
You could also try connecting your remote antenna to your hand by adding a metal strip onto your remote control box exterior.
As some of you may recall adjusting rabbit ear antennas at home on TV to improve the signal. TV channel 2 is 53 Mhz I believe.
I would think that antenna improvement combo would help the range alot.

kch

 

[Whitebird001]

macgyvers2000, of course if the only solution is to buy a 433MHz kit, I will do it, but first I would like to make this self-made transmitter work.

VE1BLL, because of the relatively small amplitude of the carrier when it is ON, the radiated power if 100% efficiency is achieved will be only 0.5*0.5/1.41/33 = 5mW with a quarter wave antenna.

But because of my very ineffective antenna this power may be only 1% of the above value.

Using the computer power wires as receiving antenna is certainly not a bad solution, but my receiver is external (attached to RS232) and I don't have any problem with antenna on this side because I can make it as long as I want.

What do you think about make the carrier amplitude higher? Instead of having only 1V peak to peak, if I make it 6V peak to peak, the radiated power will be greater because the radiation resistance will remain the same?? Or I am wrong?

 

[zeitghost]

Hope you're not trying this in the UK... you'll have Ofcom after you for using an illegal transmitter... though since you're achieving such low efficiencies, it probly doesn't matter a lot.

 

[VEBill]

One issue may be interference from the PC itself. Making RF reach 5m is trivial. But in my experience, within the area next to a PC it can be difficult to receive even FM Broadcast stations.

 

[Whitebird001]

I'm not living in the UK but even if yes, the target range is so short it cannot disturb anyone.

So I need to build a montage that takes the 1V peak to peak HF signal and by impedance conversion produce a higher amplitude signal.
The input impedance of the system should be 100ohms, the output 5K.

It sounds like a step-up transformer, but how to build it???