Xref: utzoo rec.radio.shortwave:4765 rec.ham-radio:29344 sci.electronics:16755 Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!mips!pacbell.com!att!cbnewsl!moss!feg From: feg@moss.ATT.COM (Forrest Gehrke,2C-119,7239,ATTBL) Newsgroups: rec.radio.shortwave,rec.ham-radio,sci.electronics Subject: Re: Ground Plane for my CB antenna Message-ID: <1991Jan7.150956.480@cbnewsl.att.com> Date: 7 Jan 91 15:09:56 GMT References: <1990Dec31.232808.18508@athena.cs.uga.edu> <1991Jan3.140815.20900@cbnewsl.att.com> <4576@sactoh0.SAC.CA.US> Sender: @cbnewsl.att.com Reply-To: feg@moss.ATT.COM (Forrest Gehrke) Distribution: na Organization: AT&T Bell Laboratories Lines: 48 In article <4576@sactoh0.SAC.CA.US> jre@sactoh0.SAC.CA.US (Jim Earl) writes: >In article <1991Jan3.140815.20900@cbnewsl.att.com>, feg@moss.ATT.COM (Forrest Gehrke,2C-119,7239,ATTBL) writes: > >> BTW, the proper configuration is an upright quarter wave element >> and quarter wave length radials. Since in this case the radials >> are tuned, they must be exactly quarter wave length. >> >> Forrest Gehrke feg@dodger.att.com k2bt > >I believe that the statement about the radials being the same >length as the vertical element is incorrect. The ground radials >should be 10% *longer* than the vertical element. If you are >building a 1/4-wave ground plane for 27 mHz, the length of the >vertical element should be around 108". That makes the radials >come in around 119". I'm no expert, but that is what I seem to >remember reading about them. > >I also seem to remember something about making sure the radials >are "drooping" at a 45-degree angle, because that makes the ground >plane closer to the desired 50-ohm impedance. >-- Incorrect? That depends upon the objective. First of all, making these radials longer than a 1/4 wave will introduce reactance and generally screw up the radiation pattern. Would you advocate making a dipole with one side longer than the other? Drooping the radials and adding to their length will raise the impedance of the vertical from approx. 37 ohms. The downside is a higher takeoff angle--which seems to me to be counter-productive, particularly since the SWR caused by a 37 ohm load on 50 ohm coax is trivial. (Another instance of the obsession so many hams have with SWR, carried to the point of shooting themselves in the foot). Actually, with only 4 tuned radials, a plot of the vertical angle radiation of this antenna will look like a scalloped candy dish, with the best (i.e. lowest) angle occurring over each radial. Ideally, one would want a solid disk, but it can be approached quite closely by attaching a circular peripheral wire to the ends of the radials. This is why top hats on shortened verticals, so treated, have to be only half the diameter to achieve the same capacitance effect. Forrest Gehrke k2bt feg@dodger.att.com