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
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.