ChitlinsCC wrote:
(with respect also to my neighbor to the West, deggie's comments - this is not a debate, but a vigorous discussion and an opportunity for me to learn from my betters)
I see your point - begging the question "What 'receiving/transmitting' antennae already exist in these phones for GPS (satellites are a LONG way off), cellular, etc.? and why would these not be sufficient for FM? (the signals are all around us anyway?)
::sigh:: Once you understand how radios work this answer is easy. And If you don't it's impossible to understand. But I'll try. Radio waves have different frequencies. AM is 550 to 1600 Hz. FM is 88 - 106 Mhz. Cellular is 700 - 2200 Mhz. (short wave, which was mentioned, is 3 - 30 Mhz). Each frequency has an associated wavelength. The wavelength is the inverse of the frequency. Specifically, the wavelength for any given frequency is the speed of light divided by the frequency. The speed of light is 300x10^6 meters per second. To simplify the calculation we work entirely in MHz, so we can divide the frequency in Mhz into 300 to get the wavelength in meters. Thus, the wavelength for FM is 300/100 (the middle of the band) or 3 meters. An antenna, to be effective, must be 1/4 or 1/2 wavelength. A 1/2 wave dipole is the best antenna, but a single wire 1/4 wavelength is the most you can hope for in a portable device. So that is 3 meters/4 or 0.75 meters. A meter is ~39 inches., so 0.75x39 = 29.25 inches is the optimum length for a 1/4 wave FM antenna.
Now consider cellular. And BTW, cellular radio does not use satellites. GPS does. GPS signals are sent at ~1500 MHz, so the same general range as cellular. And, in fact, the antenna can be tuned to GPS as the weakest signal, as you are usually within a few miles of a cell tower, but are several hundred from the satellites. So lets calculate using using 1500 Mhz. 300/1500 = 0.2 meters. 1/4 wavelength is 0.2/4 or 0.05 meters, or about 2 inches. Which you will observe is the approximate length of the band sections on your iPhone.
An antenna can be longer than the optimum bandwidth, but it cannot be shorter. You will receive the signal with a longer antenna, but if it is not a multiple of 1/4 wavelength you will get "standing waves" on the antenna as the signal reflects back from the end of the antenna. This will result in distortion of an FM signal; you will still get it, but it will sound garbled.