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- Newsgroups: rec.radio.cb
- Path: sparky!uunet!charon.amdahl.com!pacbell.com!ames!saimiri.primate.wisc.edu!sdd.hp.com!hp-cv!hp-pcd!hpcvaac!billn
- From: billn@hpcvaac.cv.hp.com (bill nelson)
- Subject: Re: Basic Advice
- Message-ID: <1992Nov17.221521.20881@hpcvaac.cv.hp.com>
- Organization: Hewlett-Packard Company, Corvallis, Oregon USA
- References: <1992Nov17.172700.4865@ke4zv.uucp>
- Date: Tue, 17 Nov 1992 22:15:21 GMT
- Lines: 50
-
- gary@ke4zv.uucp (Gary Coffman) writes:
- : >: >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
-
- Isn't that what I said?
-
- : 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.
-
- ... short course in simple antenna theory deleted ...
-
- : 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.
-
- I don't see anywhere that my posting was in disagreement with what you
- posted.
-
- Thanks for providing the extra detail - I was too lazy to do so, unless
- someone asked for it specifically.
-
- : Gary KE4ZV
-