jhs%Mitre-Bedford@d3unix.UUCP (03/18/85)
------- Forwarded Message > Need a cheap and quick 2m groundplane antenna? > > Recently I needed such a thing and laziness and lack of time > prevented me from 'doing it myself.' I took a walk to the > local Radio Shack and purchased a 20-176 ($14.95) "All band > base station monitor antenna." It just so happens that the > elements are the correct length for 2m fm work and the antenna, > at the end of 50 feet of good RG/8U coax shows an SWR of about > 1.2 to 1. > Of course, you could just solder 5 pieces of 1/8" brazing rod into an > SO-239 connector and get the same thing for about $1.50. Trim the rods > to about 19" and bend the radials down at about a 30 degree angle, and > voila! Screw the coax on it, and drop it into the end of the antenna > mast. Or use a dual pipe clamp from the hardware store. (Another 75 > cents). > > Bend the radial up and down if the SWR isn't right. > > If you're going to run over 250 watts, maybe a teflon connector is worth > it. So now the whole thing costs $3.00. > > I'm cheap. > Brian Kantor, WB6CYT UC San Diego > > decvax\ brian@ucsd.arpa > akgua >--- sdcsvax --- brian > ucbvax/ Kantor@Nosc ---end, forwarded message Unfortunately, these nifty ground planes often end up "sort of" working, but not nearly as well as they theoretically could. The pattern tends to spray upward, leaving a null at the horizon where you really want to radiate. Another configuration which tends to work much better, is cheaper and easier to build, and is MUCH more portable if you want to take it on vacations, etc., is the COAXIAL DIPOLE. Take a piece of RG-58/U or RG-59/U and strip off the outer insulating jacket from about 30 inches of it. Then work the shield braid back along the coax until it loosens up and then fold it back over the outer jacket that remains. This is a little tricky until you get the hang of it, but eventually you will find it not hard at all. The trick, as I recall, is to let the shield bulge out at the point where it is held by the jacket, and it will start folding back over the jacket quite easily. Anyway, when you get it folded back, pull it up tight over the jacket, stretching it out in the reverse of its original direction. Trim it off to about 18 inches. You may eventually trim it down to more like 16 or 17, but it is easier to shorten than to lengthen it. You can adjust the effective length somewhat by adjusting how tightly it is stretched. You can tape or glue the end in the final position that you like best. I have found the cyanoacrylate "crazy glue" products to work well for this purpose. Note that in stretching the 25 or 30 inches of shield over the jacket, it loses a lot of length, and there isn't as much to trim as you would think given the above instructions. As for the center conductor, you will want to cut it off at about 19 inches. I have done this in several ways, but the best was to work the conductor and insulation back and forth until the conductor broke off and could be pulled out of the insulator leaving a few inches of extra insulation. Then I attached an alligator clip to the free end of the INSULATION ONLY, far from the end of the center conductor. The alligator clip then serves as a convenient fastener for hanging the antenna on a curtain rod or whatever. Another method I have used is to clip off the whole thing at 19 inches, pull the conductor out of the clipped-off piece, fasten the clipped-off piece of insulation back to the end of the antenna with a crimp-type "butt connector" and then crimp on the alligator clip as before. Put a BNC connector on the other end of the coax and you have a very effective 2m antenna. The whole assembly looks as shown below: |Z]==============//===============ZZZZZZZZZZZ=============xx< / \ \ BNC Folded back insulated \ connector coax feedline coax shield center \ (10 or 12 feet) conductor alligator clip (not connected to center cond.) The folded back shield must be somewhat shorter than 19 inches to resonate properly. I would guess about 16 inches, but by experimenting you may find a better length. The free end of the center conductor should be about 19 inches, maybe slightly less because of the increased dielectric constant of the insulator as compared to air. I have had no trouble getting a VSWR of around 1.5 to 1 with RG-58/U coax. This is about par for the course, since the antenna feedpoint impedance is around 72 ohms theoretically. For 2-meter FM operation, of course you want to hang the thing vertically. The clip on the end makes this convenient. You will want the overall length of coax to be about 10 to 15 feet to permit accessing the antenna from a convenient spot in the room when it is hung near a window or something. This is a mighty handy accessory for your 2m Handie-talkie if you spend much time on trips -- it coils up compactly and fits easily in your suitcase or briefcase. ============================================================================== Now, if you want to get SERIOUS about 2m antennas, get one of Dick Austin's Model 264 "DFQer" units for about $60.00 list, and I guarantee you that you will notice a dramatic improvement over the kind of antennas discussed above. Dick's phone number is (603) 887-2926 (Sandown NH). ============================================================================== 73, John S., W3IKG ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ (Deliberate choice of separator character) Today's word of antenna wisdom: Remember that a 50-OHM RESISTOR gives a perfect 1:1 match at the end of 50 feet of good RG-8/U coax. But it doesn't do much for your signal! ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ