If you want to use your 40 meter dipole on 20 meters, one way is to add two 16 foot wires at the center feed point, putting a 20 meter dipole in parallel with the 40 meter dipole. On 40 the extra 16 foot wires will simply make the radiator look a bit "fatter" to the RF. On 20, the 40 meter dipole exhibits a very high impedance at the feed point (which is what is causing your problem now) while the 20 meter dipole will present a decent match.
Your 40 meter dipole already makes a decent 15 meter antenna because it's 3 half waves long, with a relatively decent match at the feed point. In the extreme you could add a 120 foot wire paralleled wire so the system will cover 80, 40, 20 and 15 meters and a pair of 8 foot wires to cover 10 meters to make it an 80/40/20/15/10 meter system. That was a common multi-band antenna before the WARC bands were opened. The mismatch is much greater on the WARC bands because they are not harmonically-related to the others. Even though your tuner may produce a good load to the transmitter, feed line losses can get rather high. Ideally you want to fan the wires to the ends are as far apart as possible, but even several inches or a foot or so will work. Understand that a 2:1 or worse SWR is common even though the wires are cut to 1/2 wavelength due to interaction between them, but the losses should not be serious over a typical run of good quality coax. 73, Ron AC7AC -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Jerry Weisskohl Sent: Saturday, December 08, 2012 2:59 PM To: Mark Bayern Cc: Elecraft Reflector Subject: Re: [Elecraft] KAT500 Tuning issue Mark, Thanks for the reply. I received quite a few replies pointing out exactly what you said. Because it wouldn't load on 20 meters I was using my other antenna, which works fine with the KAT500 (80 meter vertical). However, I was trying to understand why the 40 meter dipole wasn't working well on 20 and I thank all those that replied. This reflector is a wealth of knowledge! Now I realize my KAT500 is working as it should. Having two antennas comes in handy! Thanks, Jerry - AC4BT On Sat, Dec 8, 2012 at 5:49 PM, Mark Bayern <[email protected]> wrote: > Just to make sure we're all on the same page ... > > When the tuner (at the radio) gets an acceptable match, that just > means the transmitter will _think_ there is a good antenna system out > there. The feedline between the antenna and the tuner will know > otherwise and can have points of very high voltage that could cause > insulation breakdown, and points of very high current that can cause > overheating and will be a source of power loss. This isn't really a > problem in QRP operations, but when you head into the QRO world it can > make a difference. The antenna also has issues. In an earlier post > K3KO had the calculated feedpoint impedance, and it was very far from > the 'nominal' 50 ohms of your feedline or the ~75 ohms of a dipole at > altitude. W6GF also gave this info in terms of SWR to show how bad it > was. All in all a resonant 40M dipole makes an unhappy antenna system > on 20 meters. > > I understand that if that is all you have you're stuck with it, but it > is really time to consider something else. Maybe a fan dipole, or trap > dipole? > > Mark AD5SS > > > > On Sat, Dec 8, 2012 at 3:30 PM, Jerry Weisskohl <[email protected]> > wrote: > > Right now, I have the coax from the antenna directly connected to > > the KAT500. I could try using my Delta-4 switch, have the coax go to > > that and then a small run to the KAT500 to see if that makes a > > difference. The > high > > swr is an issue on 40 and 10 meters, however, ten is manageable, > > it's > about > > 1.9 - 1 and I am working around the issue but using my 80 meter > > vertical > on > > 20 meters. So this is not a "show-stopper" but just wanted to > > understand why it was happening. > > > > Jerry - AC4BT > > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
