Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!sdd.hp.com!hplabs!hpfcso!hpfcdj!myers From: myers@hpfcdj.HP.COM (Bob Myers) Newsgroups: sci.electronics Subject: Re: Ground Plane for my CB antenna Message-ID: <17660137@hpfcdj.HP.COM> Date: 3 Jan 91 23:39:13 GMT References: Organization: Hewlett Packard -- Fort Collins, CO Lines: 38 >I am setting up a CB in my house, and I am having problems with a ground >plane. The equipment works fine in my car, so I know its not broken or >anything. I want to put the antenna on my roof, but I am having trouble >finding a good ground plane. The manager at Radio Shack suggest getting a >peice of sheetmetal for a ground plane. How big should I make the sheet >metal? At a minimum, 1/4 wavelength radius, centered on the antenna. (At CB frequencies, that's about 109", or a bit over nine feet.) But you don't need a solid sheet; several "radials", or grounded wires of about this length stretched radially from the grounded mount at the base of the antenna, will work just as well and be a whole lot simpler. You've probably seen the CB "ground plane" antennas, with a 1/4 wavelength vertical whip surrounded by 3 or 4 drooping horizontal elements of about the same length - those sorta-horizontal elements are the ground radials. For a home installation, you'd be much better off making a full-length 1/4-wave whip than using a loaded mobile antenna (the kind with a coil in the middle or at the base), and at least 3 or 4 ground radials (the more the merrier; keep 'em symmetrically spaced about the vertical element, i.e., if you have four, they should be 90 degrees apart. "Drooping" the radials slightly - as you see on the commercially-made base antennas - raises the feedpoint impedance slightly for a better match to 50-ohm line (a vertical whip with a "perfect, flat" ground plane has a feedpoint impedance of about 37 ohms). Another option would be a half-wave dipole (about eighteen feet overall at the CB frequencies) mounted vertically; this'll only work, though, if you can keep the bottom end as well as the top well off the ground and away from any conductive structures such as towers. (Obligatory plug for ham radio - have you heard about the new no-code Technician license? Now may be a good time to consider "stepping up" from CB!) Bob Myers KC0EW HP Graphics Tech. Div.| Opinions expressed here are not Ft. Collins, Colorado | those of my employer or any other myers@fc.hp.com | sentient life-form on this planet.