|
|
|
Tune Around! Topics For Technicians! |
This project should be considered experimental! The "Wave-Missile" HF Antenna Project Copyright May 2011 Free-standing ** THE CURRENT VERSION OF THIS ARTCICLE IS A DRAFT ** A Google search of the Internet May 7, 2011 counted 4,980 websites world-wide that had in them all of the key words: HF, multiband, quarter-wave, vertical, portable, antenna. Apparently there is no shortage of ideas about this very popular type of amateur radio antenna incorporating these concepts. Nevertheless, this article is about an HF, multiband, quarter-wave, coil-loaded, vertical, portable, antenna I've made. I call it the "wave-missile", because it looks, well, missile like. Despite its name the wave-missile is not rocket science. It generally follows the design rules for such antennas with one exception. It does not like radials. The wave-missile is appropriate for operation on the 80, 40, 30, 20, 17, 12 and 10 meter bands. Theoretically, by adjusting the length of the antenna's pole it could operate almost anywhere between these frequencies. The exception is in the range of the 15 meter band, 21,000 - 21.450 MHz. Because of some resonance in the antenna design I could not "tune" the wave-missile on the 15 meter band. I have been playing with the concept of the wave missile antenna over the span of about three years. The description 'portable' with the wave-missile is subject to interpretation. Portable in this case means the parts can be transported in a passenger vehicle and set up, free standing, outdoors. I suppose it could be back-packed into the wilderness but weighing approximately 25 lb.s that would be arduous.
One lives with me in Nova Scotia. The other stays in Arizona where I spend the winter. The Nova Scotia wave-missile has each of its legs and the pole tube in two sections that have to be assembled. The maximum length of any of the parts of the break-down model is about three feet, which means the antenna can be transported in even a small car. The legs and pole tube of the Arizona wave-missile are each in one piece, six feet long. It can be transported in the bed of my pickup truck or in the long storage compartment of my travel trailer. Although the shorter-pieced Nova Scotia wave-missile is somewhat more portable than its Arizonan brethren they both perform about the same. There are other small differences in construction, pointed out later in this article. In Nova Scotia there is plenty of rain and my wave-missile here operates in usually damp woods and fields. The Arizona wave-missile spends its time on the very, very dry desert. In these respects the wave-missile can also be referred to as an 'all climate' antenna.
Figure 1 above shows the antenna circuit schematic. Coil taps select lower frequency bands, and the pole (upper section) is adjustable for finer-tuning and to accommodate higher frequency bands. How well does the wave-missile work? The simple answer is: it is a quarter-wave, vertical antenna. For the lower bands it is coil-loaded. On some days it is fantastic and on others it is so-so. This can be said of many types of HF antennas depending on band conditions. At a small group field day in Nova Scotia the wave-missile worked as well as, or even better than, tree hung dipoles and random wires, and commercially built verticals. On a side-by-side test in the Arizona desert, the performance of the wave-missile was better, in my opinion, than a mobile screw driver antenna properly mounted on a SUV. The design of the wave-missile was literally from the ground up. I knew the base of the antenna had to be big and hefty to keep it from toppling in the desert wind. As it happened I had on hand at home a twenty foot length of nominal 1.5 inch inch diameter schedule 40 aluminum pipe. At one time this pipe supported an antenna attached to my house. This was the obvious material for the legs with the pipe cut into three, six foot sections. I spent an afternoon at Home Depot looking for something to serve as an antenna body. A piece of nominal 4 inch four inch diameter PVC plastic sewer / drain pipe was selected. At the same time I acquired five heavy duty, six-inch angle brackets to attach the legs to the body (three angle brackets for attaching the legs and two more to be cut up for angle bracket bracing). The adjustable pole, on top of the base, was made from nominal 1/2 inch diameter schedule 80 aluminum tubing and 1/2 inch diameter solid aluminum rod, purchased from 'Metals R Us". The rod slides in an out of the tube to change the length of the pole. (At one time, for a pole I tried a slightly modified ten foot extendable paint roller, which worked fine once a portion of the anodized aluminum coating was sanded off so the two parts could make electrical contact.) As it turned out these materials were well suited for experimentation. The wave-missile is the Proto Board of quarter-wave vertical antennas. I've drilled numerous holes and remade pieces over again without any effect on the wave-missile's structural or electrical integrity. Of course the project also required various septic pipe fittings, bolts, nuts, washers, wire, hose clamps, and other and bits and pieces. The essential wave-missile parts and their dimensions. At 20 meters the entire length of the wave-missile, including base and pole serves as a simple, quarter-wave, vertical antenna. At resonance the entire length would be in the range of 16 ft. under ideal conditions. With this approximate length, for lower bands a loading coil is needed to compensate for the antenna being too short. According to theory, when a loading coil is part of the antenna circuit it should be placed mid-point on the entire antenna's length. The nominal 4" PVC pipe (4.21" outside diameter) base provides a form for the coil, located near the base's top end. To be practical the wave-missile's base is 68" long including coil and cap (i.e. PVC clean out plug). Any longer and I would not be able to readily reach up and slide the pole on and off it's mounting holder, a 1/2" threaded rod about 10" long. Theoretically, at 20 meters and below the coil is lower than ideal and a taller person might make the base taller. (On the other hand, at upper frequencies the coil location may be just right or too high.) Determining how to wind the loading coil required considerable attention. The coil would need some form of taps to enable the wave-missile to work on the 30, 40, and 80 meter bands. (The coil is bypassed on 20 meters and higher frequency bands, but is part of the antenna circuit for lower bands.) Since the pole would be adjustable for fine tuning each band there could be leeway in positioning the taps. Nevertheless I hoped to mostly be able to keep the pole at a certain length and change the bands at 20 meters and lower frequencies by simply connecting to different taps. In designing the coil windings I was fortunate to have available Leon Braskamp's computer program, "MOBILE.EXE", that calculates coil specifications given operational frequency and various quarter-wave vertical antenna dimensions. MOBILE.EXE was in a CD accompanying my ARRL Antenna Book, 21 st Edition . Initially I asked the program to calculate the length of a 20 meter, quarterwave vertical antenna for lowest SWR assuming no coil. Total length includes the base, pole, and their attaching wires. Given this dimension, MOBILE.EXE was then used to calculate individual coil winding specifications for the lowest SWR with the 30, 40, and 80 meter bands. The most turns is for 80 meters. There are a lower number of turns for each of the higher frequency bands, which suggested the respective location of taps on the 80 meter coil. MOBILE.EXE provided a good staring point, however, the final coil winding configuration was determined by trial and error. The difference between the coil winding results of MOBILE.EXE and that used in the wave-missile are the program does not take into account the fact that coil segments are wound next to each other nor the dielectric characteristic of PVC pipe. Even in the coil's 'final' form there is room for improvement. Table 1 below presents the approximate coil turns, pole length, SWR, and 2:1 SWR and bandwidth achieved for each of the wave-missile's operating bands. The frequency measurements were taken with the Nova Scotia Wave-missile, in May when the ground was very moist. A quarter-wave, vertical antenna's characteristics can change due to differences in ground conductivity which is a function of moisture. I admit to frustration in taking the measurements recorded in Table 1, because I could not absolutely repeat them from one day (or time) to the next. Thinking there may be a problem with the RF choke I tried four arrangements. Also, when I moved the wave-missile a few feet the measurements were altered even more. The good news, I guess, is the results did not change much, and the benchmark frequencies were within 5% of each other. Although measurements of the Arizona wave-missile, takenin the dry January desert, are somewhat different they remained generally comparable 1 (see footnotes at end of article).
The apparent effectiveness of the wave-missile's legs as a counterpoise is likely due to their relatively large diameter. This would give them a wider bandwidth 3 . Large diameter also provides a greater area to be in contact with the earth for improved coupling. In this regard the wave-missile is not unique in employing thick-legged radials. For example, the Alpha Delta Outpost Tripod vertical antenna holder has a ground coupler consisting of three fairly wide aluminum flat pieces, each several feet long, that fold onto the ground from the ends of the tripod's legs. Alpha Delta refers to this arrangement as "a large capacitor". A tuned vertical antenna, such as in the "Outbacker" series, mount on the tripod.
When operating in Arizona I noticed RF feedback on the coax cable while transmitting on the 40 and 80 meter bands. This was only apparent in digital modes when my netbook computer would freeze with transmitter power above 30 watts. RF feedback can be caused by an inadequate counterpoise / radial system. My solution to the this problem was to kludge together an enhanced RFchoke, between the antenna base and the coaxial cable transmission line, using a spare 50 ft. length of RG8x coax wound around a piece of nominal 4" PVC pipe, left over from making the base. The losses in a coil this big might deter the highest frequencies, but at 20 meters and lower bands it functioned adequately 4 .
The remainder of this article, exclusive of the Appendix list of materials, is a series of pictures presenting aspects of the construction of the wave-missile antennas. My methods and materials may not be optimum, but they seem to work and construction can carried out with common tools. Many of the sizes are based on the materials at hand, practicality, and convenience. It is likely that they could be improved upon for better antenna performance. The pictures are mostly of the Nova Scotia (more portable) wave-missile. Tuning Coil Comparison The Nova Scotia coil on left above, has common wire loop taps, and is held in place by two PVC strips cut from nominal 4" PVC pipe. The strips are easy to remove to trim the coil. The orange disk, (picture 13 below) extends beyond the side of the base and protects the relatively fragile loop taps when the base is laying down. The AZ coil has taps made from small screws going through the base wall, see picture 12 below.
Editor's Note: The following pictures can be used as a guide for your antenna if you decide to build it. You may have different ideas about how you want to build yours.
Footnotes: 2. I concluded that the wire radials of the Arizona wave-missile were acting as if they were elevated, because the ground conductivity of the very dry sand was not much different than that of air. The length of elevated radials must relate to the antenna's intended frequency. 3. Unfortunately, wide bandwidth can also mean high ground resistance caused by ineffective ground radials. 4. A future project may be making an effective, wide-band RF choke. Ver. 14 Editors Note: There are many ways in which you might be able to build a similar version of this antenna using your own modifications. Please feel free to give us your feedback!
|
|
 | |
