From: myers@hpfcso.FC.HP.COM (Bob Myers)
Newsgroups: rec.radio.cb
Subject: Re: Re: Let's talk about skip (was Re: AM or SSB)
Date: 16 Apr 92 20:04:10 GMT
Organization: Hewlett-Packard, Fort Collins, CO, USA

>I guess I did not use the right words in my posting....  What I was trying to
>say is that the antennae has *two* parts, the stinger and the ground plane. 
>Normally, you use the stinger as your main element of transmission.  The
>stinger accounts for about 91% of your radiated power.  The other 9% is put out

I'm sorry, but this post is just about COMPLETELY wrong regarding the
operation of a quarter-wave (or even a 5/8-wave) vertical antenna.

First of all, let's get the terminology straight - the "stinger" referred to
here is actually the "vertical element" (assuming conventional mounting),
or, since the vertical is normally considered an unbalanced antenna, the
"driven element" might also be appropriate.  Where the "91%" and "9%" numbers
come from is beyond me - this would imply that such antennas could get along
just fine without a ground plane, and that simply isn't the case.

The operation of a quarter-wave vertical is best understood by first
understanding the good old half-wave dipole (which in turn is best
understood if you understand a little about transmission lines, and know how
open-ended quarter-wave stubs behave).  If you "split" a half-wave dipole
by a perfect, conducting plane (a "ground plane") you can take away one of
the quarter-wave elements that make up the dipole, and the radiation pattern
from the remaining element will be unchanged.  Notice, though, that the
"ground plane" is ESSENTIAL to this behavior.

In the case of the typical "ground plane" CB antenna, the ground radials are
an attempt to simulate this "perfect conducting plane" - and they do a 
reasonably good job of it.  (The more radials, the better, though.)  Note
that EQUAL currents MUST be flowing in the vertical and the ground system, 
and so we cannot say that one contributes more to the overall radiated power
than the other.  (They do have different jobs when it comes to the *pattern*
of that radiation, but again they both have equally important roles.)  If
the original dipole were split by this hypothetical perfect conducting plane,
the feedpoint impedance of the resulting quarter-wave vertical antenna would 
be exactly half that of the dipole (the dipole is about 73 ohms, so the
vertical should come in about 37.5 ohms - and it does!).  Most "ground plane"
antennas, though, "droop" the radials a bit, which has the effect of
raising the feedpoint impedance slightly for a better match to 50-ohm
line.  (Think of it as moving slightly back toward the dipole!)

A 5/8-wave vertical has "gain" over a quarter-wave, ONLY because it has
a "flatter" radiation pattern, so more of the signal is squirted out toward
Them in the Listening Audience.  (No antenna truly provides "gain" in the
sense of adding power - they just put more of the signal in a given direction,
at the expense of other directions.)  However, the 5/8-wave vertical does
NOT provide a "nice" feedpoint impedance (i.e., it's not purely resistive),
and so requires that some form of matching device be added to the design
(one of the compromises you accept in going to this antenna). 


Bob Myers  KC0EW   Hewlett-Packard Co.      |Opinions expressed here are not
                   User Interface Tech. Div.|those of my employer or any other
myers@fc.hp.com    Fort Collins, Colorado   |sentient life-form on this planet.