williaty wrote:Bunsen wrote:There's more to it than just compensating for the de-tuned antenna. His arm is acting like the center conductor of a transmission line, and is introducing a lot more RF into the bottom of the can than would be there without it. If you've got a big metal can around, you can try it yourself using a radio that doesn't need its antenna extended. It should even work on AM.
Married to a physicist and majored in it myself, so you can actually give me a good answer or link me to one (aka no need for the obligatory car analogy )...
Why is the arm acting like a transmission line? I'm probably missing something obvious, but I don't see why arm would do a better job than air.
This would be better with diagrams, but mspaint sucks compared to a chalkboard. Try looking at the difference between a waveguide and coax. A waveguide has only one conductive surface (i.e. it's just a hollow tube), and any signal with a wavelength longer than about twice the diameter can't propagate in it. Below the cutoff frequency, any field that's present at an open end of a waveguide is exponentially suppressed as you look deeper into the tube. Coax has two separated conductors, and has no cutoff frequency. It can have fields all along its length right down to DC, where it just looks like a capacitor.
The empty can is like a short section of waveguide. Low-frequency EM waves are suppressed as they try to travel down the inside, because the walls are effectively a short circuit at low frequencies, so the radio hears a rather weak signal. Shortening and detuning its antenna makes it weak enough to get scratchy.
The can with they guy's arm stuck into it now looks like a section of coax, with the can acting as the shield and the arm as the center conductor. Low-frequency signals can propagate freely, because there are now two separate conductors (i.e. there's no low-frequency self-shorting effect). The coupling from the arm to the radio is capacitive, but there's more to it than pulling the circuit back into resonance. The same behavior should still be observed if you use a compact radio without an extendable antenna.