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The Incredible Shrinking Antenna
pinkUZI writes "NYTimes ran an article yesterday about a new material, created by a general manager at Integral Technologies, that would enable use of the plastic mold of a cell phone as its Antenna. Pretty neat, as it actually increases the size of the antenna while decreasing the footprint."
It'd be nice to not have an antenna to break off, but I don't want it to be any more effective than the current ones, unless it's more directional, which the article does not indicate it is. More effective cell coverage seems a better solution to me.
On the other hand, some of the other applications sound mighty nice, especially for military vehicles and such.
800 MHz cellular has a wavelength of approximately 37 centimeters, about 15 inches. So an ideal antenna would be half that, about seven and a half inches. This refers to the dipole, the distance from the tip of the antenna to the opposite end of the antenna buried inside the phone somewhere (usually near the bottom). 1900 MHz PCS has a wavelength of approximately 16 centimeters, about six inches. So the ideal antenna dipole is about 3 inches.
The ideal antenna performs best if it is exactly perpendicular to the impinging waveform. In practice the orientation of the phone is somewhat random; the antenna will be pointed approximately upward, but probably at a slant. So cell phone manufacturers generally try to make the antenna 5/8's of a waveform, because if the antenna is at a slant, its cross-section relative to the impinging waveform will be near to the ideal half a wavelength. For a dual-band phone, one which operates at both 1900 and at 800 MHz, it's obvious that determining the antenna length is a bit of a problem. (But not insoluble; it's just a compromise. Since digital is usually more resilient than AMPS, usually the length is optimized for 800 MHz.)
Making the antenna shorter will both decrease the amount of incoming signal the phone receives, and will make the phone's transmitter less efficient. But CDMA operates over a very wide range of effective powers, and it can usually compensate. That's why the phone will usually work with the antenna down. And because it's digital, if it is working it will sound exactly the same. This has lead some people to conclude that the antenna is not actually doing anything for them, which is not quite correct. While the phone can operate with the antenna down, it's easier on the phone if you raise the antenna; it has more signal ceiling to work with and will be less likely to drop the call. Also, it will use somewhat less transmit power, and your battery will last somewhat longer.
Making it longer with some sort of extension is worse than useless; it actually degrades the signal. If the antenna is exactly one wavelength long and is exactly perpendicular to the impinging waveform, it will pick up essentially no signal at all.
When it reaches one and a half wavelengths, signal strength is again maximized, but for physical reasons it's a bit lower than the strength with a half-wavelength antenna. (The physical reason is that the antenna is not an ideal conductor.)
[stolen directly from the CDMA FAQ
Good question!
A resonant antenna, cut to exactly half a wavelength, has some advantages but it's not a necessity.
The advantages are simpler circuit design, because it looks like a pure resistance and doesn't confuse the amplifier, and in theory a simple radiation pattern.
In practice the pattern will depend on everything in the environment, so good antenna testing is Very Difficult. Take all claims with a grain of salt until you hear that the antenna has been tested on an expensive antenna test range by experienced people.
When you're dealing with small fractions of a wavelength longer is better. For very short antennas, only a small fraction of the RF leaves as radiation before it's lost as heat from electrical resistance. "Longer is better" means "longer is more efficient".
BTW you can take advantage of how antennas are affected by nearby objects. If you're in a fringe area, try standing right next to your car, with the base of the antenna level with the roof line. Circle around the car until you're in the right alignment with the cell tower for further optimization.
Fred KC7YRN