From owner-qrp-l@Lehigh.EDU Tue Dec 17 23:36:51 1996 Received: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id XAA18695 for ; Tue, 17 Dec 1996 23:36:50 -0500 (EST) X-Received-x: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id XAA18695 for ; Tue, 17 Dec 1996 23:36:50 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35228-24991>; Tue, 17 Dec 1996 23:35:40 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <35166-28322>; Tue, 17 Dec 1996 23:34:49 -0500 Received: from sierra.psnw.com (root@sierra.psnw.com [205.199.144.107]) by nss2.CC.Lehigh.EDU (8.8.4/8.8.4) with ESMTP id XAA192580 for ; Tue, 17 Dec 1996 23:34:43 -0500 Received: from fresno1-15.psnw.com (fresno1-15.psnw.com [206.43.246.15]) by sierra.psnw.com (8.8.4/8.7.3) with SMTP id UAA24352 for ; Tue, 17 Dec 1996 20:34:32 -0800 (PST) Message-Id: <199612180434.UAA24352@sierra.psnw.com> Date: Tue, 17 Dec 1996 20:34:32 -0800 (PST) Reply-To: kd7s@psnw.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: kd7s@psnw.com (Bill Jones) To: "Low Power Amateur Radio Discussion" Subject: High Performance, Small Loop Antennas (very long) Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" X-Sender: kd7s@mail.psnw.com (Unverified) X-Mailer: Windows Eudora Version 1.4.4 X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Status: RO The following is a lengthy description of small diameter, transmitting loop antennas. It is being posted (with permission from Chuck) in response to several requests. If you have no interest in this subject, please delete this message now. ========================================================= Copyright (c) 1996, Bill Jones - KD7S Building High Performance, Small Loop Antennas Soon after my article, A HOME-BREW LOOP TUNING CAPACITOR, appeared in the November, 1994, issue of QST I began receiving letters and telephone calls at the rate of ten to fifteen per day. Hams all over the world wanted more information on how to build small, efficient transmitting loop antennas. Over half the inquiries dealt with modifying the original three-foot diameter loop to tune other bands, especially 30 and 40 meters. Almost everybody wanted to know how to optimize the antenna for maximum performance on any given band. It soon became apparent that although interest in such antennas is at an all-time high, the mechanics behind designing and building small, high performance loops are not generally well understood. For a small loop to perform well, losses must be reduced to an absolute minimum. To reduce losses, three conditions must be met. First, the loop must be properly assembled using highly conductive, low loss materials. It must also be physically large enough for the chosen frequency of operation. Finally, it should be installed as far away from other metallic objects as possible. If any of these requirements are not met, performance will likely be disappointing. If you're wondering why such emphasis is placed on reducing losses in a small loop, consider the following. When r.f. is fed to an antenna, part of the energy is radiated into space by virtue of the magnitude of its radiation resistance. Reduced size antennas (like small loops) typically exhibit very low values of radiation resistance. The rest of the energy is converted to heat. The part converted to heat is a result of ohmic losses and skin effect in the conductor used to make the antenna. It is wasted energy. Generally speaking, we don't have much control over the value of radiation resistance in an antenna of a given size. However, if steps are taken to reduce the ohmic losses to a fractional part of the radiation resistance, even a small antenna can be a very efficient radiator. Reducing losses can be achieved by combining proper construction techniques with the right materials. Because copper is an excellent conductor at r.f., it is generally the material of choice for most home-brew loop builders. Aluminum can also be used, but because it's difficult to weld, it is usually reserved for the commercial manufacturers. Simply clamping sections of aluminum together with hose clamps is unacceptable. Most loop builders use rigid copper water pipe for construction because it's fairly inexpensive and readily available. As you will see shortly, the thickness of the pipe used to construct an antenna plays an important role in loop performance. Half-inch material is about the smallest practical size from the standpoint of efficiency. Larger pipe works better with 1.5" diameter being about the upper practical limit based on cost and ease of handling. A good compromise is 3/4" or 1" pipe. One problem with rigid copper pipe is that it is almost impossible to bend into a circular shape without special tools and equipment. The alternative is to cut eight, equal lengths of pipe and solder them together using 45 degree elbows to form an octagonal shape. If done properly, very little additional loss is introduced by the solder joints. On the other hand, a sloppy job of soldering can result in a loop that is all but useless. Each joint must be meticulously cleaned prior to soldering. Buffing the ends of the pipes and the insides of the elbows with fine steel wool is essential. If the copper is bright and clean, the solder will flow evenly and form a low loss joint. A high quality paste flux in conjunction with a hand-held propane torch will make the job easy. Don't attempt to solder copper pipe with a soldering gun or iron. Neither tool can deliver enough heat. An alternative to rigid copper water pipe is flexible copper tubing. Many larger building supply stores and plumbing specialty shops carry fifty foot lengths of tubing in various sizes. Because it is soft, it can be formed into a continuous circle thereby eliminating the problems with soldering individual pieces and elbows into an octagon. In addition, a round loop is slightly more efficient than an octagon. On the downside, a larger loop made from softer material may need some sort of internal framework to maintain its shape under windy or icy conditions. Finding a suitable capacitor to tune a loop to resonance is probably the most challenging task facing a loop builder. Conventional air variable capacitors with wiping contacts and clamped-together plates are generally much too lossy to be useful. A high voltage vacuum variable capacitor is ideal. Unfortunately, vacuum variables can be hard to find. Worse, they're usually quite expensive if you do find one. As an alternative, a low cost, home brew capacitor can be made with simple hand tools. See my QST article mentioned earlier. Whatever you do, resist the temptation to use the first capacitor you find in your junk box. You'll probably be disappointed with the results. The next thing to consider is how small a loop can be and still operate efficiently. It is generally accepted that the circumference of a small loop should lie somewhere between 1/8 and 1/3 wavelength at the desired operating frequency. Consider a loop designed to work at 7.0 MHZ. About the smallest circumference you should use is seventeen feet. If the antenna were made of 3/4" copper pipe, the efficiency would be around 35%. That means that almost two thirds of your transmitter power would be converted to heat and wasted. Increasing the tubing diameter to 1.5" would improve the efficiency to a little over 50%. On the other hand, if you were to increase the antenna circumference to thirty feet, the efficiency with 3/4" pipe jumps to almost 75%. With 1.5" pipe the efficiency is now slightly over 85%. If you think a thirty foot circumference loop is getting pretty big, consider that it is only about nine-and-one-half feet in diameter. Compare that to the sixty-seven foot span a full sized 40 meter dipole requires. The accompanying table shows the relationship between loop circumference and calculated antenna efficiency for five different sizes of copper pipe. The figures are based on a loop for 7.0 MHZ. You can use this table to plan a 40 meter loop to suit your own needs. Loop Circumference (ft) Tubing Thickness in Inches 0.5" 0.75" 1" 1.25" 1.5" 17 26.61 35.23 42.04 47.55 52.10 20 37.12 46.95 54.15 59.61 63.92 23 47.31 57.39 64.23 69.18 72.93 26 56.47 66.05 72.18 76.43 79.56 29 64.29 72.97 78.26 81.82 84.38 32 70.75 78.39 82.87 85.81 87.89 36 77.50 83.87 87.32 89.58 91.17 39 81.41 86.78 89.75 91.63 92.92 As an example, suppose you wanted to build a 40 meter antenna that exhibited at least 75% efficiency at 7.0 mHz. If you are using 1/2" copper pipe or tubing, it will need to be close to thirty-six feet in circumference. On the other hand, if you switch to 1" pipe, it need only be slightly over twenty-seven feet in circumference. Taking the example one step further, suppose you chose to use 1.5" pipe. Now the antenna only needs to have a circumference of twenty-four feet. A twenty-four foot loop is just over seven-and-one-half feet in diameter. From the table one can see that the conductor thickness has more effect on smaller sized loops than larger ones. So far we have only considered single-band antennas. The twenty-four foot circumference loop made from 1.5" copper pipe just described can be retuned to work on 30 meters as well. Furthermore, the calculated efficiency is almost 92% at 10.150 MHZ. Unfortunately, a twenty-four foot loop is a little too large to work on the 20 meter band. If you reduced the circumference to twenty-one feet, the loop will work on 20 meters with a calculated efficiency of 96%. The efficiency on 30 meters would be 88% and drops to 67% at 7.0 MHZ. The final consideration in getting maximum performance from a small loop deals with its surroundings. Loss will increase quickly when the antenna is installed close to other metallic objects. Items like chain link fences, other antennas, house wiring, automobiles, rain gutters and down spouts can seriously degrade the performance of an otherwise good antenna. Simply stated, loops should be placed in the clear for best performance. If care is taken to build and install them correctly, small loops can be made to work as well as full-sized dipoles and verticals. For additional information on small loop antennas, see Ted Hart's article, SMALL, HIGH EFFICIENCY LOOP ANTENNAS, in the June, 1986, issue of QST magazine. ============================== Bill Jones - KD7S <>< Sanger, California Reply to kd7s@psnw.com ==============================