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From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Boosting output of a little walkie-talkie
Message-ID: <17660085@hpfcdj.HP.COM>
Date: 8 Aug 90 18:22:49 GMT
References: <3011@media-lab.MEDIA.MIT.EDU>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 23

>Why high current?  There is very little current travelling through 
>the antenna, so you'd require a higher voltage, wouldnt you?  After 
>all, P=VI...  And air is a very good insulator.   A high current would 
>be impossible, at reasonable  (below 1,000,000 Volts) voltages.
 

Tain't that simple; there are in fact high *currents* present in various
parts of an antenna (exactly where depends on the antenna design).  For 
example, in the simple dipole antenna, the feedpoint (center) is an area
of high current, while the ends are low (supposedly zero) current and high
voltage.  It wouldn't work, otherwise.  (The dipole can best be understood
by thinking of it as a quarter-wavelength, open-ended length of transmission
line, which has been "opened up.")  The feedpoint impedance of a properly
tuned dipole is about 73 ohms, so you can figure the feedpoint current
based on this value and the power.  If we assume 4 watts, then there's about
230 mA at the feedpoint - which is pretty "high current" for an op-amp,
assuming you can find one that'll work at the frequency in question.



Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers@fc.hp.com                          |  sentient life-form on this planet.