From qrp-l@lehigh.edu Thu May 25 19:47:40 1995 Received: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.6.10/8.6.6) with ESMTP id TAA01510 for ; Thu, 25 May 1995 19:47:37 -0400 Received: from fidoii.cc.lehigh.edu ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <40266-4>; Thu, 25 May 1995 19:44:44 EDT Message-Id: Reply-To: rohre@arlut.utexas.edu Originator: qrp-l@lehigh.edu Sender: qrp-l@lehigh.edu Precedence: bulk From: "rohre" To: Multiple recipients of list Subject: (long) Ants. for QRP depend on goals X-Listprocessor-Version: 6.0c -- ListProcessor by Anastasios Kotsikonas X-Comment: Low Power Amateur Radio Discussion Date: Thu, 25 May 1995 19:44:44 EDT Status: RO Some questions on qrp antennas have turned into more general issues, and there was a statement that should be clarified: Someone thought it was stated to stay away from verticals. Now that is a an exaggeration, for it really depends on one's objectives, and the implementation, as to how well any antenna is going to work for you. If you want to talk to your ham friend in the next town 20 mi. away, a HF vertical is NOT the best antenna. If you want to talk to any ham friend 2000 or more miles away, a vertical, PROPERLY installed may be the best low cost antenna, if you can't put up a gain antenna such as an array or beam, or a very high horizontal antenna several half waves up. I think at a home QTH a mix of antennas is needed for different bands and distances, and FOREMOST you have to evaluate the reflectivity of local earth before choosing an antenna. That will affect how you install the antennas you choose. A vertical is definitely a primary long skip (DX) antenna, especially if it is modestly elevated, (six feet) off the earth, and installed with a built in counterpoise, or resonant radials. When elevated, only radials in four directions may be sufficient. Under some propagation conditions, I have had short skip and skip zone success at the same time with an elevated "half-wave" vertical, of the commercial models. A ground mounted quarter wave vertical is very dependent on the earth reflectivity, or requires an elaborate radial system to work well. The standard for this are the 120 radials under AM broadcast antennas. Folks with better "RF" earth grounds can use fewer radials. A dipole antenna, center fed as an inverted V, has worked well to the short skip and in its favored two directions, to long skip. An inverted V of dipole or G5RV style may work in the skip zone where you want near vertical incidence to reach nearby to a few hundred miles. A 40 Meter horizontal dipole 30 some feet up has been found to not work well in the skip zone, ie. within 200 miles. ( Signals were readible but weak, and likely were being dominated by longer skip signals.) The angle of the antenna to earth and its elevation affect the "horizontal antenna" performance. Sloping half wave dipoles have been used for DX work, and quarter wave slopers worked against a metal mast or tower "ground" have had good reports. A fine DX antenna mainly in two directions is the half wave dipole for each band, erected horizontally, at least an electrical half wave or more above earth. But, this requires two supports of sufficient height, and the requisite distance between them. It will not give you optimum results off the favored directions. It will not fulfill the short range needs inside the skip zone in an optimum manner. For Near Vertical Incidence Skywave (NVIS) coverage of one state on bands like 80, you may find the best antenna is a low dipole, or an insulated one laid upon the ground! A tuner would be advisable to match the impedance variation imposed by local ground conditions in these cases. Received noise: The off center fed, near half wave vertical dipole, all band commercial antennas have been found to be quieter than base fed verticals by reviewers. (Gap- reviewed by Lew McCoy and others). This may be a combination of the "center" feed, and the run of coax inside the vertical forming an RF choke for local noise that would be heard on base feed, or even the capacitative grounding of the feed to the vertical top end, at the end of a matching stub. When installed as an elevated vertical, they work well as a compact antenna to work all bands, and all DX locations. They have been found to work well to both coasts from the central part of the U.S. If it was practical to erect true elevated full size vertical dipoles for all bands, I think they would be a most popular antenna for low cost DX work. They would work best with a clear space around them in all directions. Now we come to single conductor horizontal wires: the quarterwave Marconi or untuned long wire antennas. Some have reported good success in the field or at home with them. However, they also have failed when used in a desert location, and for a long wire, (400 feet) in a tropical location. Why? Earth conductivity. In the desert you have dry sandy soils, a poor conductor. In the tropics you have near daily rain, but again the soil is a poor conductor because frequent heavy rain leaches the conductive salts out of the soil. I suspect the areas where these have worked are those with good "RF" soil. Now how to get them to work elsewhere? The counterpoise is a neglected accessory to wire antennas. Like the radials of the verticals, a counterpoise provides the other half to go from an unbalanced system like the Marconi or long wire to the balanced system that works more like the dipole in that unattainable "free space". A counterpoise could be laid upon the ground under a portable horizontal wire and parallel to it. This would be the optimum reinforcement for the elevated portion of your antenna. However, you may find offsetting the counterpoise, or elevating it above ground will give better results in some directions over others, and this can be exploited to advantage. Much remains to be done in exploiting and finding the tricks of the counterpoise. The counterpoise was much used under early antennas, but added to complexity, and gave way to dipole designs for installation ease. It could not really be used in the air, thus the WW II trailing wire had to work against a metal plane fuselage for HF transmission. There probably is some good old information on using counterpoises that has been neglected since the dipole became popular. Much of the data on using reflectors for array or beam type antennas should apply when using counterpoises. A counterpoise can easily be made from insulated hook up wire. Anything that increases the area of our antennas will improve our qrp success. However, blindly selecting an antenna that worked in a very different location from yours without attention to local conditions, will likely bring frustration or belief that "X", "Y", or "Z" antennas don't work. The counterpoise can even have a dramatic effect at VHF where compact helical antennas are used. This can be demonstrated by taking a 2 M Handi Talkie on low power, and trying to hit a distant repeater. Often, when unsuccessful, the addition of a quarter wave counterpoise clipped to the ground of the connector will spell success. For mobile qrp antennas, the short whips or helicals can be improved by adding insulated quarter wave wires to the base ground of the mobile antenna, and threading it even zig-zag fashion through the frame of the vehicle. This can be done no matter how long your whip antenna may be, for it likely is short for RF and does not work all that well against a non resonant vehicle body, which today may be mainly plastic. In summary, pick an antenna whose area and angular patterns are suited to the communications you desire, and your available space. Be aware of the RF consequences of local earth, and supplement an unbalanced antenna with radials or a counterpoise. A balanced antenna works better for DX the higher you erect it. (A way to evaluate local earth as a reflector is to erect a test dipole cut by the standard formula for 10 or 15 meters, in the clear. Raise or lower its elevation parallel to earth until you see it read 70 ohms on an antenna bridge, at the center feedpoint. See if the elevation is close to the handbook values for a 70 ohm antenna. The amount off the chart values can give you insight into whether RF Ground is below physical ground at your location. This also can help you determine a correct formula for cutting dipoles for your location, in the future.) The results for a higher frequency dipole in the clear should scale to lower frequency dipoles, if the area is still in the clear for the longer dipole. Looking at the recent two part article on tuners in QST reminds one of the losses a tuner can inflict, even while providing an impedance transformation. It seems reasonable that to get the best result with qrp power, one would do well to use resonant antennas not requiring a tuner for the band in use. Carrying this thought along, perhaps one should design a high impedance link coupling to the final tank circuit to allow use of lower loss high impedance open wire line to feed dipoles, in the interests of having the least loss between the final and the antenna. Has any qrp operator installed the rig at the antenna feed point, and keyed it remotely? Now that would be RF efficient! 72 and good Antenna experimenting! Stuart K5KVH rohre@arlut.utexas.edu