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From: feg@bonnie.ATT.COM (Forrest Gehrke,2C-119,7239,ATTBL)
Newsgroups: rec.ham-radio,sci.electronics
Subject: Re:  Theoritical Equations for Antennas
Keywords: Antennas
Message-ID: <3693@cbnewsl.ATT.COM>
Date: 17 Jan 90 16:12:53 GMT
References: <481303aa.1423f@godzilla.UUCP>
Sender: nntp@cbnewsl.ATT.COM
Reply-To: feg@bonnie.ATT.COM (Forrest Gehrke)
Organization: AT&T Bell Laboratories
Lines: 40

In article <481303aa.1423f@godzilla.UUCP> dalyb@godzilla.UUCP (Brian Daly) writes:
>
>Subject: Re: Need Theoritical Antenna Info
>Newsgroups: rec.ham-radio,sci.electronics
>Keywords: Antennas
>
>     Note that if the antenna length is slightly less than 1/2 wavelength,
>     the reactance will go to zero. For l of a halfwavelength, the reactance
>     is approximately zero and the resistance is close to 50 ohms. (42.5 ohms
>     for wire of small radius).
>


I think this statement is a bit mixed up. The resistance is
going to be more like 72 ohms.

For an electrical 1/2 wavelength the reactance will be 42.5 ohms and the
resistance will be 73 ohms.

>
>    The analysis of a quarter wave ground plane antenna is essentially
>    a vertical electrical dipole above a ground plane.
>
>    When the length of the antenna is a quarter wave, and the height above
>    the ground plane is zero, then:
>
>      Input impedance = 36.5 + j21.25 (note that this is one half the input
>                                       impedance of a half wave dipole)
> 
As I noted above for a halfwave, your parenthetic comment now agrees
with the halfwave case.  Again, if the monopole is slightly less than
an electrical 1/4 wavelength the reactance will go to zero.

BTW, I think most of these cases assume the current loop
location, e.g. for the half wave dipole we are looking at the
impedance at the dipole's center.  If we were looking at the
current node the impedance would be quite different.

Forrest Gehrke feg@clyde.ATT.COM
k2bt