Experiments using cheap, low-power radio

Question

In lecture, I've been using an expensive (\$2000) 10 GHz microwave demo setup consisting of a transmitter and a receiver. It can be used to demonstrate things like the transverse polarization of electromagnetic waves, and the difference in absorption between substances that contain molecular dipoles and those that don't.

Can anyone suggest any ways of doing classroom experiments along these lines using super-cheap off-the-shelf equipment? Considering how ubiquitous radio is these days (wireless computer mice, etc.), it doesn't seem like it should be necessary to spend thousands of dollors to generate a low-power carrier wave and detect it. For example, it seems that you can get RFID readers for about \$50, and tags are extremely cheap -- but I don't know whether they can be used for these purposes. I had thought it might be fun to try to build a crude spark-gap transmitter, but it sounds like it's probably not possible to do so legally, since the signal is very broad-band and "dirty" (and the practical hobbyist setups built by history buffs actually seem like pretty complicated pieces of technology).

My understanding of the legalities is that for devices that fit certain parameters such as maximum power, there is something called a "class license" for a set of Industrial, Scientific and Medical (ISM) bands. This seems to cover a broad range of devices such as wifi routers, garage door openers, and so on.

Answer 1

460mHz "family radio service" walkie-talkies might be worth looking into. they are small, cheap enough (~fifty dollars/pair new, $5/pair at thrift shops) to warrant investigation, easy to use and do not require a license, but I do not know if 460mHz is high enough to produce the effects you wish. The antennas they come with are intended to be omnidirectional (usually coil-loaded vertical) but if you hacked the radio you could wire up a center-fed dipole; this would furnish polarization and directionality which could be explored in your experiments.

Answer 2

Almost any mobile radio aside from satellite links and anything in ISM band that are on the market uses linear polarization and applies a linear monopole where the ground is defined somehow by the body of the radio. But irrespective of all these you can make a dirt-cheap polarizer from a wood frame and parallel copper wires that are wound so that their spacing is $< \lambda/2$. The emission in ISM band is limited to $<1W$ which more than you ever need in a class-room size experiment.

It will be a concern how you actually show that the received power depends on the orientation. You will need to isolate them because you will have all kinds of reflections floating and the receiver will detect them. Any FM radio will have a limiter before its detector and thus it is insensitive to received power except when its SNR drops near its threshold. The FRS @niels_nielsen is talking about is an FM radio and unless it has a signal strength indicator you will not see any continuously demonstrated effect when you are turning one radio relative to the other. But irrespective of all this I suggest that you make an external polarizer to clean the signal and try to place absorbers around to reduce multipath reflections.