If I use another transmitter (I have a keyfob one) it works 100%. I would think the antenna I am using is far better than the non existent one in the keyfob?
Soooooooooo many variables in antenna design and it would seem that you’ve overlooked arguably the most basic one…tuning an antenna.
And it would also seem you are breaking 2 of the most basic rules of that tuning…using an arbitrary length of copper wire, and worse, an exposed arbitrary length of copper wire.
The antenna in the keyfob is likely a tuned length of copper on the PCB, far from non-existent.
Just the fact that you said you can touch the antenna and make it work says it all right there.
Google it…hand capacitance, antenna tuning, and so on.
Ya can’t just take a chunk of copper wire and expect it to work as an efficient antenna. Just doesn’t work that way.
I wonder if that $4 wonder (($0.50 of parts) has a crystal or other controlled frequency source? Perhaps your body capacitance is changing the frequency a bit, to favor the receiver.
Also, try varying the length of the wire one way or the other by 6 in. or so. Could be you accidentally have a length that yields a high VSWR = lots of wasted transmitter power. These random wire antennas don’t match 1/4 or so wavelength (1/freq), nor do they well match the 50 ohm need of most transmitters. At this price point, it’s hit and miss. The key fob probably has a decent PCB copper antenna based on at least some RF engineering.
stevech:
I wonder if that $4 wonder (($0.50 of parts) has a crystal or other controlled frequency source? Perhaps your body capacitance is changing the frequency a bit, to favor the receiver.
Also, try varying the length of the wire one way or the other by 6 in. or so. Could be you accidentally have a length that yields a high VSWR = lots of wasted transmitter power. These random wire antennas don’t match 1/4 or so wavelength (1/freq), nor do they well match the 50 ohm need of most transmitters. At this price point, it’s hit and miss. The key fob probably has a decent PCB copper antenna based on at least some RF engineering.
I think these are RC Op-Amp oscillators. Keep the cost down. Also the RX unit should have a pot. Try twisting that to tune the unit to the transmitter.
These modules are crystal controlled (SAW, or surface acoustic wave mode) and in my experience are quite accurate with regard to transmit frequency. It is possible that the module is defective and hand capacitance is somehow kicking it into oscillation. The antenna should be a straight wire about 14 inches long, but is not needed at all for short range (a few feet) tests.
The receivers have a tunable tank coil but if the keyfob works, a detuned receiver coil is unlikely to be the problem.
thanks for the responses guys. I have other links just like these that are working and have been for years. Maybe I just got a bad batch of transmitters.
The transmitter has a 6 inch wire, and the receiver has a 3 inch wire. I have tried varying the length of wire from just over two feet down to 17 inches on the transmitter. Sometimes it works however it is not 100% reliable unless I touch the antenna wire in which case it works every single time.
I have no knowledge or expertise when it comes to designing an antenna for these units. I was hoping for an easier solution.
The receiver module does not have a pot. It does however work very well (from 300 feet away) with a keyfob transmitter I have to test with.
These modules are a few years old. I bought them while sparkfun was changing suppliers for this product, they don’t look like the units in the link above. Perhaps they are duds?
I will try an older transmitter from about 5 years ago. I will post my results later today.
I removed a transmitter module from another circuit that has been working reliably every day for about four years now. Just as I thought, the older transmitter modules work perfectly, every single time about 60 feet away. The antenna is nothing but a 6" piece of copper wire.