Dan, WG4S asked: Anybody have any idea how an 80m loop would work at 3m height?
We just moved into a new place with NO ANTENNA RESTRICTIONS, but I fell down hard last summer (3 weeks unconscious in ICU) and am not tempted to get very high again. All this talk about on-ground and under-ground antennas made me think of hanging a W0MHS Loop-Skywire at a low height. That a LOT of copperweld for something that hasn't a chance. I'd hire someone to hang a doublet if I thought it would be much better. But the whole neighborhood is in a hole, about 75 feet below the prevailing terrain, so even a 50' doublet is really 25' underground<g>. The hole is several wavelengths wide, so from my roof, the tops of the trees on the "rim" are up about 25 degrees, so very low DX takeoff just ain't gonna happen. Maybe it's time I learned if I can run one of those Antenna Modelers. Should I look for Windoze or Linux flavors? ------------------------------------------------ What affects the pattern is how the wavefront interacts with the earth. That is a function of the polarization of the wave. A horizontal loop and a horizontal dipole or a horizontal doublet will behave the same in terms of the basic pattern affecting the angle of radiation above the horizon. At 3 meters (10 feet) you'll have a good "cloud-warmer" or what the new Hams call an NVIS antenna all HF bands. (You start to see significant lower-angle radiation when the antenna is about 3/8 wavelengths above the ground and it peaks at just over 1/2 wave above the ground. Above that height you still get FB low-angle radiation, but you lose some of the gain at good DX angles provided by the earth reflections. For high-angle propagation, the ideal height is 0.2 wavelengths effective height. I say "effective" because the effective height is not always the actual height. Since the earth is a lossy dielectric instead of a decent conductor, the actual height is somewhere below ground level. Measuring from the surface of the earth is close enough. Ground losses increase as the antenna is brought closer, but it will still radiate reasonably well even at very low elevations. My doublet is about 25 feet high. That's also very low for 80 meters; it's definitely a "cloud warmer". I work out to about 1,200 miles with it on 80 with usually excellent signal reports out to at least 700 or 800 miles. That's pretty typical range for short-skip propagation. You might actually have another effect at play at your QTH that may gain you some unexpected results: knife-edge effect. When a radio wave passes the edge of the earth, such as the edges of the "bowl" you live in, the part of the wave in contact with the earth slows down in the earth. That tilts the wave to a lower angle. It also costs signal strength, since that earth is still a lousy, lossy dielectric, but the end result may be some surprising low-angle propagation from a "cloud warmer" antenna. Something else you might consider is an "inverted V" arrangement with one tall pole at the center, since you have no antenna restrictions. Years ago I rented a house and was loath to clamber about on the roof, so I picked up a push-up TV mast. It collapsed to about 10 feet and extended to a full 50 feet. It held the center of my inverted V antenna whose ends came down to the eaves at one corner of the house in front and to the fence in the back. No climbing, no high altitude work, but a nice high HF antenna. My radiator was 130 feet long and fed with open wire line. It did a decent job as a Cloud Warmer on 160 and got out very well for DX contacts on the other bands. If you use open wire line, keep it away from the pole. To avoid messing around with standoffs, etc., I ran the open wire line away from the pole at an angle equidistant from the legs to a point along the fence, then to the shack. Just keeping it two or three times the width of the feeders will do. Of course, with coax you have no worries. Of course, I guyed the pole at full height. It was sitting on the ground and attached to the eaves about 8 feet up on a side of the house that had no gutter. Mounted like that I could extend it without guys on a calm day with plenty of guy wire with excess lengths attached to rings as they were hoisted. I then attached the guy wires with only minimal climbing on a short stepladder to reach tie-points on the fence and eaves as needed. With good marine pulley and halyard at the top, I didn't have to move the mast again. I could take the antenna down as needed standing on my patio! For modeling, I use EZNEC from W7EL (www.eznec.com) It runs under windows. Just remember that a modeling program is only as good as the assumptions you make for it. So it is, at best, as guess based on assumptions. (Did I really say that?) 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
