I have been asked if it is feasible to make a motorcycle accessory that will trigger the traffic ground loop antenna sensors that typically fail to register the motorcycles presence. Reading up on these sensors, I find that they detect a vehicle by driving a signal into a loop and measuring a change in frequency due to the inductive skin effect from the metal in the vehicle over the sensor. This makes me wonder if it might be possible to build a low power transmitter to drive an antenna mounted directly under a saddlebag to force a change in the ground loop operating frequency actively, as opposed to the change registered from the passive inductive skin effect. The only antennas I have made so far have been from following ARRL guides to the letter. So I am hoping that someone might be able to detail what the antenna and loading coil construction would be and its drive requirements to accomplish this. Some of my references are: http://www.editraffic.com/wp-content/uploads/loop_install_guide.pdf http://www.marshproducts.com/pdf/Inductive%20Loop%20Write%20up.pdf http://www.editraffic.com/wp-content/themes/eberle/flash/loopCalculator.swf http://www.fhwa.dot.gov/publications/research/operations/its/06108/02.cfm

  • 2
    \$\begingroup\$ Great idea. You won't need an actual antenna, just a coil, maybe 20 turns, 20 cm. The ground loop detectors are essentially just metal detectors. Problem is there are many ways of making a metal detector, simple ones use steady state oscillator and detect a shift in resonance, but better ones use pulses of AC and observe the decay of the fields. You might need a microcontroller that tries several different modes, to fool it. \$\endgroup\$
    – tomnexus
    Dec 26, 2014 at 5:40
  • 1
    \$\begingroup\$ From my experience, if you simply call the city and complain about faulty sensor (and they are required to be sensitive enough to feel motorcycles, at least where I live), it gets fixed rather quickly. Doesn't cost much money too. \$\endgroup\$ Mar 23, 2015 at 23:39

2 Answers 2


That may work, but a strong magnet low on the bike is more cost-effective.

When I ride my (aluminum and carbon) bicycle, I put a flat neodymium magnet in my shoe and stand on the terminals (where the loop starts and ends). This seems to work well.

  • \$\begingroup\$ I'm not an expert in this, but putting such a magnet in your shoe would be considered dangerous? \$\endgroup\$
    – Paul
    Apr 20, 2015 at 13:11
  • \$\begingroup\$ Not normally. Just remember to take it out before you get an MRI! \$\endgroup\$ Apr 20, 2015 at 23:09

I think the idea to add a coil proposed by Oleg might be the best idea, but a 20 cm circular coil would cause a pretty minor change in the inductance, compared to the skin effect of your average car. Go back to phasors, think of this in terms of z = jwL, and then see if you can find a way to produce a reasonable frequency shift. I don't know how much inductance is required to trigger the sensor, but I bet you could figure something out by running a coil around the engine itself. It would be a huge pain to work around though if maintenance is ever an issue, so you'll have to find a balance.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.