NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/rec/radio/cb/1352

Text File | 1992-11-17 | 4.0 KB | 79 lines

Newsgroups: rec.radio.cb
Path: sparky!uunet!charon.amdahl.com!pacbell.com!ames!saimiri.primate.wisc.edu!zaphod.mps.ohio-state.edu!swrinde!emory!kd4nc!ke4zv!gary
From: [e-mail] (Gary Coffman)
Subject: Re: Basic Advice
Message-ID: <[e-mail]>
Reply-To: [e-mail] (Gary Coffman)
Organization: Gannett Technologies Group
References: <[e-mail]> <[e-mail]>
Date: Tue, 17 Nov 1992 17:27:00 GMT
Lines: 67

In article <[e-mail]> [e-mail] (bill nelson) writes:
>[e-mail] (Brad Staff) writes:
>: Bill Nelson writes:
   [about halfwave vertical antennas]
>: >If it has no ground radials, then it is sensitive to mounting height.
>: >The radiation impedance changes drastically, from a few ohms to several
>: >hundred ohms.
>:
>: Sounds like it might be a vertical half-wave dipole.  Disclaimer:  I have
>: never owned one of these, but I do own a vertical quarter-wave monopole.
>: :-)
>
>Nope. It may be a half-wave vertical, but it is not a half wave dipole.
>It is end fed, not fed in the middle. Look at the sensitivity of end
>fed antennas, you will see that it is high for all lengths.
[deleted]
>: I'll have to take a look at the effect of using radials on a vertical
>: half-wave dipole.
>
>You don't need radials with a dipole antenna. It would only hurt the
>performance.

Bill, a halfwave antenna, no matter whether it's fed on an end or in
the middle, needs no radials to act as a current mirror to make up a
"missing" half of the antenna. That's the purpose of radials on a 1/4
wave monopole. Proximity to ground, real or simulated by radials, does
have effects, *adverse* ones, on halfwave vertical antennas. First,
the radiation pattern will be skewed upward from the ideal. And second,
the lower end impedance will be affected by the loss resistance of the
ground. This decreases the Q of the antenna as well as reducing the
efficiency of the feeder match.

Ideally, an end fed 1/2 wave vertical has a feedpoint impedance of
*infinity* since it's a voltage maximum and a current zero. In the real
world of wire resistance and magnetic and electric coupling to other
objects, especially real or simulated ground, The feedpoint impedance
will drop to a few thousand ohms. This still puts severe constraints on
the matching system since voltages will be very high and flashover is a
constant factor that must be considered. The further above ground, real
or simulated, that you can mount a 1/2 wave vertical, the less the pattern
will be skewed, and the higher the feedpoint impedance will become.

The only significant differences between dipole feed and monopole end
feed for a halfwave antenna are that the feedpoint is balanced in the
dipole case, and that the feedpoint impedances are radically different.
A balanced feedpoint is of little interest unless you are using balanced
feeders, not the case with coax. That's because the balance will limit
feeder radiation. The feed point impedance is significant only in that
it requires different matching networks. In the ideal case, a dipole
has a voltage zero and a current maximum at the feed point giving a
zero feed impedance. In the ideal case, an end fed halfwave monopole
is the reverse situation with a current zero and a voltage maximum
giving an infinite feed impedance. Antennas are never the ideal case
and so the dipole has a low feed impedance while the end fed monopole
has a high feed impedance. Suitable matching networks can be designed
for either case.

Now a *quarterwave* monopole must have ground, or radials simulatiing
ground, at it's feed point because it's only *half* of a resonant antenna
and must simulate the other half in a current *mirror* formed by the
ground plane. Otherwise, it's just a short end fed random wire with an
arbitrary reactive impedance at it's feed point and an arbitrary, but
certainly skewed, radiation pattern. You can't take experience with
quarterwave verticals and translate it to halfwave verticals. They're
working on different principles.

Gary KE4ZV