Using a thicker wire, or multiple wires in a drop, always helps. Last single thin wire I used was around 1970. :-)
----- Original Message ----- From: Pete Smith N4ZR To: Tom W8JI Cc: [email protected] Sent: Friday, January 25, 2013 8:30 AM Subject: Re: Topband: alternative to vacuum variables Thanks, Tom. I thought I recalled this advice from an earlier reflector post, but couldn't find it. Thanks also to everyone else who replied, and the many interesting ideas My single 97-foot tower has two tri-band yagis and a 2-element shortened 40m yagi at the top, as well as an 80-meter lazy-vee array deployed all around it. When I set up the shunt feed years ago, I did a lot of climbing around on it, trying to find a 50-ohm point. Only after not being able to find one did I install the shunt feed at an arbitrary point (about 50 feet, and yes, it's a single wire) and configure an Omega match with a couple of 3000v 300pf variable capacitors from Henry Radio. I know it is a very sub-optimum system (not enough radials either), but I have it set for a 1:1 SWR at 1825 and it satisfies my occasional needs for a 160-meter transmitting antenna. See you all in CQ 160. 73, Pete N4ZR Check out the Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com. For spots, please go to your favorite ARC V6 or VE7CC DX cluster node. On 1/24/2013 9:59 PM, Tom W8JI wrote: One of the reasons I have mot gone to high power on 160 is the cost of capacitors for my omega match - at least $300. Anyone know of any workable alternative? I remember someone writing about using coiled-up RG-8, RG-213 or maybe Teflon coax. Where can I find more information? Coax isn't the best for two reasons: 1.) Transmission line effects increase voltage at the open end. This aggravates arcing issues 2.) They are lossy Here's something to think about, Pete. The voltage across a capacitor is I^2 * Xc So if you have a shunt feed and use a long, thin, wire....you not only decrease bandwidth and efficiency, you also increase the voltage. A thin shunt feed wire requires higher Xc (lower capacitance), and that makes voltage increase and BW narrow. Use a wire cage and tap the shunt wire down at a 50 ohm point, and voltage goes way down. Do it the lazy way and use an omega match, and you give up bandwidth and need higher voltage parts. 73 Tom _________________ Topband Reflector
