Transponder
I have a simple powered transponder which sends data using binary phase-shift keying (BPSK) on a 5Mhz carrier with a required range of 6-12 inches. It is used to time laps of remote controlled cars and go-karts. It has an inductor on the PCB as the transmitting antenna and looks like the below:
5Mhz BPSK transponder http://edeca.net/temp/photos/IMG_0638.jpg
Receiver
Right now the receiver uses a loop of stranded 24AWG wire. This directly connects to a small transistor amplifier that is powered over coax. The loop has to be tuned using the blue jumpers to adjust the amount of capacitance, making it resonant around 5Mhz. This is dependant on the length of the loop.
Loop amplifier http://edeca.net/temp/photos/IMG_0640.jpg
The signal is sent back over the coax to another transistor amplifier and fed into digital demodulation.
Instructions for a professional equivalent show the below, point #3 is a 470 ohm resistor (presumably a virtual ground). The width of the track is "max 10 metres", which could mean a length of anything from a few metres to >20 metres, with no adjustments required by the operator.
Test loop http://edeca.net/temp/photos/AMB_loop.png
I notice that this is very similar to what radio hams call a beverage antenna. These are usually at least a wavelength long, impractical at 5Mhz because one wavelength is 60 metres. However this made me wonder if it would be possible to use the loop more like an RF antenna rather than a fully tuned resonant circuit.
I have made a small test circuit which includes a balun wound on a suitable ferrite for the frequency, converting from 470 ohms to 75 ohms impedance of the coax.
Test loop http://edeca.net/temp/photos/Test_loop.jpg
My thoughts were to amplify and band-pass filter what comes off the loop, this is my starting point for removing the powered transistor amplifier. This could hopefully reduce the total cost and will make it easier to operate.
So my question: if this is practical, how should I amplify the signal at the receiver?
Best Answer
The clue is in the name - transponder. I suspect that the loop not only powers the normally unpowered transponder (fixed to the car) but also acts as receiver. At 5MHz the transmission process will largely be the magnetic part of the EM wave that is used.
It could be that the transponder (in the car) is normally powered but only transmits when it detects the presence of the magnetic field from the loop.
Either way, I suspect you need the trackside circuit board and (mag field generator and receiver) still to be trackside for this to work. Trying to convert it to an equivalent of a regular RF antenna is, possibly, missing the point. For it to operate as a beverage antenna it's length needs to be one wavelength and at 5MHz that is 60m long. It is a magnetic antenna not intended for EM waves.