Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!uwm.edu!mailrus!accuvax.nwu.edu!nucsrl!telecom-request From: john@bovine.ati.com (John Higdon) Newsgroups: comp.dcom.telecom Subject: Re: First Time Cellular Phone Buyer Needs Advice Message-ID: <2382@accuvax.nwu.edu> Date: 22 Dec 89 09:20:27 GMT Sender: news@accuvax.nwu.edu Reply-To: John Higdon Organization: TELECOM Digest Lines: 38 Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 9, Issue 591, message 2 of 8 Lars J Poulsen writes: > If 3 watts of power goes into the transmitter, I do not see how the > laws of physics would allow 4 watts to be radiated into the electro- > magnetic field ? Antenna "gain" is most common in the RF world. It involves the amount of energy radiated in a given direction from the antenna. In a given azimuth, a 5/8 wave antenna (as used in cellular mobile phones) radiates more energy than a 1/2 wave reference dipole for a given RF input. An easy way to visualize this is to refer to the case of an FM broadcast antenna (oh, no, Martha, here it comes!). If you take a single "bay" or element and you could see the energy being radiated from it, it would appear as somewhat spherical, coming off in every direction. Unfortuately, the receivers are all out at some horizontal distance from the antenna. There are hardly any listeners in the sky or under the ground, so all that energy is being wasted. If you place a second bay on the tower exactly one wavelength's distance above or below the first and divide the energy between the two, the formerly spherical pattern will now "flatten out", with more energy being radiated out horizontally and less going up and down. The antenna is now said to have "gain" over the standard dipole. No energy was created, just redirected in a more useful manner. The ERP (Effective Radiated Power) from a standard dipole equals the Input Power to the antenna which is close to the transmitter output power. The ERP from a "gain" antenna is equal to the Input Power times the power gain of the antenna. In the case of our FM example above, adding that second element would give the antenna a gain of 2. The gain of a 5/8 wave antenna is approximately 1.3 referenced to the standard 1/2 wave (I think--don't work with those much). I know more about FM antennas than cellular, but the principle is the same. John Higdon | P. O. Box 7648 | +1 408 723 1395 john@bovine.ati.com | San Jose, CA 95150 | M o o !