Re: [Elecraft] 33.25ft vertical with KXAT1 - Why did it work?
No reason for any forgiveness. We are all learners. I am awaiting an answer to this good post also. john-n3drk - Original Message - From: Martin Gillen [EMAIL PROTECTED] To: elecraft@mailman.qth.net Sent: Monday, September 12, 2005 12:27 PM Subject: [Elecraft] 33.25ft vertical with KXAT1 - Why did it work? Hi. I was trying to build a field portable quick to install antenna and I really liked what I read about an end fed halfwave having a very low angle of radiation and only requiring a very short counterpoise however I remembered that the KXAT1 manual stated not to use lengths near a halfwave as they would be outside the matching range. So... I decided that I would cut an antenna to 33ft (20m halfwave), and then I would cut it back a foot at a time until the KXAT1 found a match. I started at 33.25ft vertical but I did leave a pair of 18.5ft counterpoise wires running at about 2 feet above the ground connected to the ground terminal. I tuned up and ... 1.2:1 match on 20m! I then proceeded to work KF6GC with 2W (almost 4000km) in the middle of a K Index = 8 solar storm! So I know the antenna worked well. I modelled it in EZNEC and the impedance at the base of that antenna should have been 970 + j 1700 ohms, so I am surprised that the KXAT1 was able to match it. Any comments? Was it because I left the counterpoises connected that it worked so well? If I remove the counterpoises in the NEC model the impedance increases and becomes inductive to 1456 - j 1819 ohms. Comments? PS. I am only learning in this stuff so forgive any overly simplistic analysis!! Thanks, Martin. VA3SIE. ___ To help you stay safe and secure online, we've developed the all new Yahoo! Security Centre. http://uk.security.yahoo.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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 ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
RE: [Elecraft] 33.25ft vertical with KXAT1 - Why did it work?
Martin VA3SIE wrote: I really liked what I read about an end fed halfwave having a very low angle of radiation and only requiring a very short counterpoise however I remembered that the KXAT1 manual stated not to use lengths near a halfwave as they would be outside the matching range. So... I decided that I would cut an antenna to 33ft (20m halfwave), and then I would cut it back a foot at a time until the KXAT1 found a match. I started at 33.25ft vertical but I did leave a pair of 18.5ft counterpoise wires running at about 2 feet above the ground connected to the ground terminal. I tuned up and ... 1.2:1 match on 20m! I then proceeded to work KF6GC with 2W (almost 4000km) in the middle of a K Index = 8 solar storm! So I know the antenna worked well. The counterpoise is part of the antenna. (Think an off-center-fed wire without feeder.) So you started out with a nearly 52 foot long radiator as far as the KXAT1 was concerned. Longer is usually better. First, you do have lower ground losses. And, yes, when the radiator is exactly 1/2 wave long the impedance will be very high, beyond what the KXAT1 can handle, so approaching it as close as possible is a good plan except for one thing. That issue is that the rig will end up at a voltage loop meaning that touching the rig will detune the system badly. The counterpoise exhibits a fairly low impedance to keep the case of the rig near RF 'ground'. You mention a low angle of radiation. The pattern is entirely dependent upon the antenna's position with respect to the earth or other conductors. The manner of feeding the antenna has nothing to do with the pattern. An end fed half wave (dipole) has a pattern identical to a center-fed half wave. In free space a half wave radiator has maximum radiation at right angles to the wire. Stand it on end over the earth and I'll show very low angles of radiation out toward the horizon, BUT very little radiation down below 20 degrees in most cases. That's because the earth is not a good conductor. Most of the RF at low angles is used up in earth losses. Those losses occur at several wavelengths from the antenna, so even a hundred radials won't stop that action. Put the antenna horizontally over the earth and its height above the earth has a dramatic effect on the radiation pattern. Keep it low - about 0.2 wavelengths - and it forms a two-element beam using the earth as a reflector producing a huge lobe straight up. That's what's often called an NVIS (near vertical incidence system) nowadays. Raise the horizontal up to about 1/2 wave above the earth and that huge vertical lobe splits out and drops closer to the horizon, producing great low-angle radiation for DX and, because the radiation is horizontally polarized, the ground losses are less than with a vertical. Such a horizontal antenna can show BIG gains - about 6 dB. That means a K2/100 feeding such an antenna can put out a signal equivalent to a 400 watt rig feeding a unity gain antenna like a ground plane. The problem is that most of us can't get a horizontal up 1/2 wave: 130 feet on 80 or even 66 feet on 40. So we live with a compromise. Sometimes a vertical is better for DX than a low horizontal wire. Were radials come into play is when the antenna is too short to be self-resonant. The shortest self-resonant antenna is a dipole (1/2 wave long radiator). When a radiator is shorter than 1/2 wave (a so-called Marconi antenna because that's what he used), the ground system has to take over the duty of providing a current sink that would normally have been provided by the missing length. The lower the impedance of this ground system, the more efficient the antenna. That's why people using radiators shorter than 1/2 wavelength, no matter vertical or horizontal, try to do everything they can to provide the best possible ground. AM broadcast stations, for example, have for years used 120 radials 0.2 wavelengths long for this purpose. It's also why ships have such great success with short antennas. On the old 600 meter marine band (500 kHz), a half wave antenna would be almost 1,000 feet long! No ship I ever saw could provide room for a wire even half that long; usually much, much less. Still they got out very well, and on the short waves their signals were downright astounding compared to shore stations. That's because salt water is just about the best natural ground plane one can expect to find on the planet, and a ship at sea is sitting in the middle of one that extends for thousands of wavelengths in all directions! Ron AC7AC ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
Re: [Elecraft] 33.25ft vertical with KXAT1 - Why did it work?
Martin: Antennas work by witchcraft and magic. NEC is about as good a modeling program as you will find, but all antenna calculations are only a gross indication of expected performance. Actually, the problem is that the near field has extreme sensitivity to boundary conditions; these are impossible to model realistically, but they do affect performance measures such as feedpoint impedance. All antenna analyses are overly simplistic compared to the real antenna that they represent. Thus, comparing NEC studies to observed results often gives the appearance of witchcraft and magic. Three things might account for your result. First, the idealized description of the antenna in NEC tends to depart from reality when you are trying to look at impedance for a feedpoint at the voltage loop. (Technical detail: Near the current loop, where dipoles are usually fed, the voltage along the antenna varies slowly with displacement. Thus, NEC's assumption that the voltage is uniform along a segment is approximately correct, and estimates of impedance are pretty close. Near the voltage loop, where you're feeding your antenna, the voltage along the antenna varies quickly with displacement. Thus, NEC's assumption that the voltage is uniform along the segment is far from correct, and estimates of impedance can be way off. ) In my experience, the NEC estimate of impedance for voltage loop feedpoints is often overstated by a factor of 2 to 3. Second, your counterpoise is very near the ground, and the description of ground in EZNEC is (unavoidably) very crude. Third, you did not say how long your feedline is. A lossy feedline will have a non-trivially lower SWR at transmitter end than at the antenna end. If you're getting 2000 km/watt in a K=8 storm, you've got a good antenna/feedline system. Congratulations and 73, Steve AA4AK At 05:27 PM 9/12/2005 +0100, Martin Gillen wrote: Hi. I was trying to build a field portable quick to install antenna and I really liked what I read about an end fed halfwave having a very low angle of radiation and only requiring a very short counterpoise however I remembered that the KXAT1 manual stated not to use lengths near a halfwave as they would be outside the matching range. So... I decided that I would cut an antenna to 33ft (20m halfwave), and then I would cut it back a foot at a time until the KXAT1 found a match. I started at 33.25ft vertical but I did leave a pair of 18.5ft counterpoise wires running at about 2 feet above the ground connected to the ground terminal. I tuned up and ... 1.2:1 match on 20m! I then proceeded to work KF6GC with 2W (almost 4000km) in the middle of a K Index = 8 solar storm! So I know the antenna worked well. I modelled it in EZNEC and the impedance at the base of that antenna should have been 970 + j 1700 ohms, so I am surprised that the KXAT1 was able to match it. Any comments? Was it because I left the counterpoises connected that it worked so well? If I remove the counterpoises in the NEC model the impedance increases and becomes inductive to 1456 - j 1819 ohms. Comments? PS. I am only learning in this stuff so forgive any overly simplistic analysis!! Thanks, Martin. VA3SIE. ___ To help you stay safe and secure online, we've developed the all new Yahoo! Security Centre. http://uk.security.yahoo.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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 ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
