Tom W3QS wrote: I'm about to order my K2 and I have a load of questions, but just one for now.
I am presently restricted to a random wire (perhaps 60 feet or so - OK, about 20 meters) sloping up to the top of a tree. I am feeding this with 50 feet of RG-8 and have a ground system consisting of two 1/4 wave wires for 40 through 10. Will the K2 and the KAT2 tuner handle this antenna? Anyone have similar setups? -------------------- How are you feeding the wire, Tom? At the end or the center? Since you are using coax, I'm guessing you are planning on center feed. That is, you are erecting a "doublet" fed with coax. If so, then you will experience very high relative losses in your coax. The antenna will show a lot of reactance on all bands except maybe 40 and 15 meters if you can get 66 feet of wire up. A 66 foot wire is self-resonant at 40 meters. That is, it will show very little reactance. The impedance at the center will be resistive and somewhere in the vicinity of 50 ohms - a good match for 50 ohm coax. It will show another resonance at about 21 MHz (15 meters) where it will again show a decent match for coax. On the other bands the SWR will be very high and so your coaxial line losses will be very high. Most operators using a non-resonant doublet like that will choose open wire line or 450 ohm ladder line to minimize these losses. Those lines have much, much lower losses than coax primarily because of the lower SWR on the line. For example, if you have a 60-something foot radiator fed at the center, on 20 meters it will be close to two half waves, presenting a very high impedance at the center feed point. In the practical world, the impedance will probably be something on the order of 2000 or 2500 ohms, maximum. If you connect 50 ohm coaxial line to the center, the SWR will be >40:1. If you use 450 ohm ladder line the SWR will be about 5:1. That's a huge difference that will save you a lot of RF being wasted as heat in the coax. If you simply cannot run the ladder line or open wire line all the way to the rig, use a short piece of coax for the final few feet, but keep it as short as possible. Connect one side of the ladder line to the center insulator and the other to the braid. The coax simply makes a shielded wire to run the last few feet. If your feed line is "in the clear" running up to the antenna, I'd not bother one bit about trying to "balance" it with a balun. Baluns are unpredictable and often lossy in lines with high SWR. Just hook one side of the feeder to the center pin of the antenna connector and the other side to the ground. If you want to try a balun, put it at the end of the open wire section of your feed line. I'd recommend using a 1:1 balun although sometimes if you have a very high impedance at the end of the open wire feeder, a 4:1 balun will bring the value down into the range for the tuner. If you are feeding your 60-something foot wire at one end, don't use coax at all. Just bring the end to the center pin at the ATU output. Avoid running the line any distance inside the house. It may cause RFI and pick up noise from wiring and appliances. To 'ground' your rig effectively, connect a 1/4 wave long wire to the case of your rig for each band you are operating on. The ground wires must be insulated at the far end. They will be hot with RF at the ends. You can run them indoors (along baseboards, etc) or outside. Just be careful to keep the ends insulated and away from people and pets. They can inflict an RF burn if touched while transmitting. You mentioned 40-10. If you can put up 60+ feet of wire, you'll find that it will work FB on 80 meters as well, either center fed or end fed. The Elecraft tuners have excellent matching ranges, with the exception of perhaps the built-in KX1 ATU. The KX1 ATU was designed to fit in a tiny space, so it doesn't offer the wide range of the other Elecraft tuners. It is possible to arrange an antenna they can't match on one or more bands because of the impedance extremes. The usual way to deal with that is to change the length of your feedline. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
