steven@ozonebbs.UUCP (Steven Rubin) (12/30/90)
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?
---
Steven Rubin @ @
{netcom, crash!nusdecs}!nusjecs!ozonebbs!steven oo
Disclaimer: I don't even speak for myself! \__________/gpz@ESD.3Com.COM (G. Paul Ziemba) (12/31/90)
steven@ozonebbs.UUCP (Steven Rubin) writes: +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? 1. Yet Another Reason to Avoid Radio Shaft (YARTARS) This radio shlak person, like most, is talking through his hat. I can see the headlines now..._18-foot flying saucer lands on house_. I can not imagine buying, let alone installing, such a collossal piece of metal. Where does radio slock find these incredibly dense people? 2. a. The bad news: the ground plane should be at least a quarter-wavelength in radius, which, for 27 MHz, is about...(slip, slip, slip)...107 inches. b. The Good news: it doesn't have to be solid; in fact, you can get away with three or four pieces of wire instead. Side view: | | (center conductor of coax connects to this) | | ------+------ ground plane (braid of coax connects to this) Top view: | | | -----+----- | | | Sometimes people bend the ground plane radials downward, which raises the feedpoint impedance of the antenna from ~30 ohms to something closer to 50 ohms. This is desirable because the most if not all CBs are designed to operate with a 50 ohm antenna impedance. Note that the radials do not have to be stiff...they can be pieces of wire tied down at both ends. They should be connected electrically in any case. Base station antennas of this sort usually have a full-length quarter-wave vertical element, as opposed to a physically shorter one with a coil. What type of antenna are you currently using? I'm guessing it's a mobile antenna with a loading coil of some sort. If that is the case, you might be better off just buying a regular base station antenna as long as you are going to this much trouble, since the full-length antenna will perform somewhat better (how much better? well, maybe 36.438764356 percent). Wow, it's a good thing I don't read rec.ham-radio, since there will most assuredly be 6.02E23 other replies all saying this same thing, or pointing out that a full-length antenna will really perform 36.438764357% better, or asking if the resulting antenna would be FCC type-accepted or would it be against the rules part 95.7863(a) paragraph 3 section t etc. etc. etc. :-) Good luck! ~!paul Discalimer: I am not a CBer. -- Paul Ziemba api!gpz gpz@ESD.3com.com 408.764.5390 You don't exist. Go away.
dt@yenta.alb.nm.us (David B. Thomas) (12/31/90)
steven@ozonebbs.UUCP (Steven Rubin) writes: >I am setting up a CB in my house, and I am having problems with a ground >plane. I had very good luck with a quarter wave (104") mobile whip mounted on the roof of my house (secured to a short mast), with three (four would have been better still) quarter wave (104") ground radials (#14 wire or thicker) sloping down at about 30 degrees below horizontal. The radials should be electrically connected to the "ground" connection at the base of the antenna, and the far ends should not be touching or near to anything conductive. I used nylon ropes to hold the radials in place, and the radials/ropes became guys to support the structure as well! #14 or heavier wire is best for the radials. The antenna performed nicely with an SWR of 1.1:1 on all 47 channels. (40 if you prefer not to count varactor magic) :-> Another friend did something similar with a center-loaded whip. The results were acceptable, but not as good as I got with the long whip (not surprising). Now, I was just imitating other working designs, without any really good understanding of antenna theory, so there are undoubtedly things I could have done better. Please do post, so we can all learn. [ Note: I left the crossposting since this seems to apply to all the groups. ] little david -- This is my .sig. n?e .sgurd no gis. ym si sihT
mcovingt@athena.cs.uga.edu (Michael A. Covington) (01/01/91)
All you need is four horizontal pieces of wire, going out in different directions, each of them as long as the antenna itself. This is a very common configuration for commercial 2-way radio antennas. .
feg@moss.ATT.COM (Forrest Gehrke,2C-119,7239,ATTBL) (01/03/91)
In article <1990Dec31.232808.18508@athena.cs.uga.edu> mcovingt@athena.cs.uga.edu (Michael A. Covington) writes: >All you need is four horizontal pieces of wire, going out in different >directions, each of them as long as the antenna itself. This is a very >common configuration for commercial 2-way radio antennas. >. As a point of interest, one of the fellows who hold the patent for this antenna is a ham, W2EBS. His name is Bob Lewis, of the classic 1930's paper on radial ground planes (Brown, Lewis, and Epstein). He is 83 and still continues his interest in antennas. He is currently one of the review board of Communications Quarterly. 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
jre@sactoh0.SAC.CA.US (Jim Earl) (01/04/91)
In article <1991Jan3.140815.20900@cbnewsl.att.com>, feg@moss.ATT.COM (Forrest Gehrke,2C-119,7239,ATTBL) writes: [some stuff deleted] > 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. -- Jim Earl - KB6KCP / home: (916) 729-6825 work: (916) 929-0300 x233 INTERNET: jre@sactoh0.SAC.CA.US UUCP: {ames | apple | att | sun}!pacbell!sactoh0!jre or: ucbvax!csusac!sactoh0!jre
feg@moss.ATT.COM (Forrest Gehrke,2C-119,7239,ATTBL) (01/07/91)
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