Let's Keep It Simple - Adjusting the Novice Antenna |
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If you have a copy of EZNEC software (now a free download), you can try modeling an end-fed "sky wire" antenna like the one described in this article and see how it functions when driven from a location high above terra firma when the transmitter and receiver are connected to Earth ground via some sort of wire. Author Norman Rowe operated from his attic as did (and still do) many Hams. Although almost any reasonable mismatch can be accommodated with a proper matching circuit, that has no major bearing on what the antenna radiation pattern looks like. However, the characteristics of the ground circuit most certainly affects the antenna pattern. Surely someone like ARRL's Joel Hallas (W1ZR, SK), an EZNEC master, could answer that without even having to run a simulation. Come to think of it, I really don't know what a serious mismatch does to the power factor of the voltage and current flowing in the antenna itself. Does the relative phase of current and voltage always automatically correct itself in the antenna? If not, how does it affect the radiation pattern? Anyone? Let's Keep It Simple - Adjusting the Novice Antenna Simple Voltage-Fed Wire for the Beginner By Norman L. Rowe, K2DFW For various reasons, the simple voltage-fed half-wave antenna has fallen into infrequent use. Nevertheless, as K2DFW points out, it still remains one of the easiest skywires to install and adjust. From this consideration, it is well worth considering by the Novice.
Fig. 1 - Sketch of the simple voltage-fed half-wave antenna. This is written for the Novice who, like myself, has become somewhat confused by too much study. Actually, I'm a very rusty old-timer from 'way back in 1921 when my ham station, 2BKK, blasted a broad, channel somewhere in the vicinity of 200 meters with a half-kilowatt rotary-spark-gap transmitter. In those days, the science of radio, by today's standards, was relatively simple. After thirty-two years of complete indifference to the hobby, I am attempting gradually to crawl back through a maze of electronic theory sufficient to flunk an "Einstein." Take antennas, for example. Occasionally in the literature one can find a casual reference to an end-fed antenna, and a guarded admission that good results can be obtained therefrom, but the experts seem to devote most of their space to the half-wave dipole, or doublet. Then, they go on to 2-wire and 3-wire folded dipoles, impedance-matching transmission lines, flat lines, tuned lines, standing waves, standing-wave ratios, line lengths, etc., etc., etc. What to do? Of course, I do not imply that present antenna theory and practice is not right, but my shack, and possibly yours, happens to be up in the attic and there is no easy way to string or tie into a center-fed dipole. There certainly must be thousands of amateurs whose physical opportunities for antenna construction are more favorable for an end-fed antenna and, who not only need a little assurance that this type, under suitable circumstances, can be made just as efficient as the center-fed type, but who also might use a little help in its design and method of erection. To begin with, it should be noted that the most efficient way to couple a transmitter to an antenna is by direct feed. In other words, move the transmitter up into the air and connect it directly to the antenna. Obviously, this is often a physical impossibility. However, if your shack is located in an attic room, or any floor high enough above the ground to be at usable antenna height, it is very easy to accomplish this desirable objective by the use of an end-fed antenna. Secondly, despite the emphasis in the literature of the desirability of feeding an antenna at a current loop, it is not at all incorrect or inefficient to feed it at a voltage loop. So, if you place your rig close by an attic or upper-floor window, you can direct-feed a half-wave antenna at one end (which you know will be at a voltage loop) and get just as much DX as the ham next door using an equivalent center-fed dipole and transmission line. Moreover, such an antenna is a cinch to erect and can be easily adjusted to the correct length. (See ARRL Handbook Antenna Chapter section on direct feed.) Antenna Length To avoid losses and other troubles, the end-fed antenna should be equal in electrical length to one-half of the wavelength being transmitted. To permit multiband operation or the use of variable frequencies within a band, some compromise in length must be made. However, we novices should not attempt to absorb or experiment with everything all at once, and would do well to leave some of the complexities of the science for a later day. In the beginning, let's try to keep it simple. Accordingly, we might temporarily forget that there are other bands and other frequencies within a band and start out by buying an 80-meter crystal somewhere within the Novice limits, and building a transmitter and antenna around it. So, let's say we pick up a 3.74-Mc. crystal, build a small 2-tube 30-watt transmitter, and now desire to erect an efficient end-fed antenna for connection thereto. The Handbook states that e actual physical length of a half-wave antenna may be calculated by the formula L = 468/f, where L = length in feet, and f = frequency in megacycles. Hence, our antenna should be exactly 125 feet long. However, the proximity of trees, buildings and other structures, and the height above ground - to mention just a few things - have an influence on the electrical length of this antenna, and it is virtually impossible to calculate in advance for all Novice locations the exact length required for best antenna performance for a 3.74-Mc. crystal. Adjustment So, if this situation fits your case, here's what you do. Attach one end of your antenna to a distant tree, barn, house, or pole by means of an insulator, clothesline, clothesline pulley and weight, and bring the other end to a convenient point above or alongside your attic window as in Fig. 1. Start out with about 120 feet of wire between insulators and, without cutting it, lash the balance of the coil of antenna wire temporarily to the insulator outside the window. The stranded type antenna wire is preferred, since it is more flexible than solid wire and therefore easier to handle. However, other types of wire may be used, provided it is bare so that the turns of excess wire are shorted out. The lead-in should be as short and direct as possible, since this actually is an extension of the antenna. It is made by unrolling the free end of the coiled wire.
By trial and error, all very easily performed without leaving the shack, you will finally arrive at the physical length that is electrically one-half a wavelength long. This may be determined when the dial reading of the antenna tank condenser at resonance is the same whether the antenna is connected to or disconnected from the rig. After the correct length has been found, the antenna coupling can be increased so that the amplifier will draw the desired plate current when the antenna tank circuit is tuned to resonance. Now, brother, you're "putting out!" Since an end-fed half-wave antenna is voltage fed, it is important to insulate the lead-in well as it passes through the window, or frame, in order to avoid excessive dielectric loss at this point. No ground connection is needed, although it may be desirable for lightning protection. After the proper length of your antenna is determined, remove the coil of excess antenna wire, and solder the joint to the short lead-in. It is well to leave the clothesline, pulley and weight at the far end, especially if attached to a tree, since this will prevent wind damage. By some slight additional adjustment of length, such an antenna may be readily converted for use on 40 meters by the same trial-and-error method. In this case, it will operate as full-wave, or second-harmonic, antenna, but will still be voltage fed. To make it easier, later on, to shift back and forth between 80 and 40, it is better, when possible, to string a separate wire for 40 meters between your attic window and some other object not too parallel to the 80-meter wire. Start with about 60 feet of wire between insulators and follow the same length-adjustment procedure. At some possible sacrifice in performance, the same idea can be used for operating rooms located below the level of the antenna. Under such circumstances, the length of wire between insulators is reduced to compensate for the added length of lead-in wire, the over-all length, from the far end of the antenna to the antenna tank coil, being the determining factor. For your receiving set, a random length of wire in the attic will usually bring in anything you can reach with a Novice low-power rig. So, up the trees, men, and let's keep it simple.
Posted March 17, 2022 |
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