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


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?

  • \$\begingroup\$ Do you have any current photos/ updates on your transponder stuff? \$\endgroup\$
    – Drewster
    Mar 10, 2021 at 20:41
  • 1
    \$\begingroup\$ @Drewster I haven't worked on this for a number of years, but if you email me (david at edeca dot net) then I'd be happy to share anything that is useful to you. See also my blog post about the first version, which worked but needed significant improvement! edeca.net/project/kart-timing-mk1 \$\endgroup\$
    – David
    Apr 10, 2021 at 11:26

1 Answer 1


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.

  • \$\begingroup\$ If I'm using terms wrong then apologies, but the portion fixed to the car is active - it is powered from a 9v battery in the one I have built. The professional models (made by AMB) transmit all the time and do not need activating in the presence of a loop. This can be checked very simply with a shorted scope probe. Mine (pictured) does the same. However the professional receivers do not seem to need adjusting or "tuning" for different length RX loops. Therefore they are either compensating somehow or they work in a different way. \$\endgroup\$
    – David
    Oct 8, 2013 at 5:09
  • \$\begingroup\$ @David OK so the car transmitter isn't technically "transponding" i.e. responding to a transmission! That make it simpler conceptually. Maybe the pro receivers have broad tuning to suit quite a few loops then after initial amplification they use IC filters to extract only those frequencies around 5MHz. If the signal is reasonable this can be done. Interesting. But I'd forget about trying to mimic a proper RF antenna. BTW how does the system cope when two cars pass over the loop at the same time - how does the system differentiate between the two signals? \$\endgroup\$
    – Andy aka
    Oct 8, 2013 at 7:17
  • \$\begingroup\$ I have no idea about the professional models but the one I built (pictured) does a simple avoidance technique with random delays (1 to a few msec) between transmissions (which are a few usec). In a field test over the summer I only missed one detection. Would it be feasible to use the sort of balun/coax above fed into a 75R input impedance opamp to amplify? \$\endgroup\$
    – David
    Oct 8, 2013 at 15:21
  • \$\begingroup\$ I liken this to a metal detection type system that I've expanded on rotating machines to transmit data to and from a rotating part. Sometimes if the data rate is low (less than 1Mbps I modulate at 80MHz and tune the receive loop. If data is much higher I rely on it being picked up albeit very differentiated. Yours sounds like the former so I'd be looking at tuning the receive coil at the carrier frequency ie 5MHz. No trying to make it like an rf antenna because it is near-field data. \$\endgroup\$
    – Andy aka
    Oct 8, 2013 at 16:17
  • \$\begingroup\$ Are you using a modulation scheme and why are you trying to change it. Always best applying an amplifier at the nearest point to the received signal and you don't need to worry about it being balanced unless there is an interference problem from something else. \$\endgroup\$
    – Andy aka
    Oct 8, 2013 at 16:22

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