End-Fed Half-Wave Antennas
by Joe Everhart, N2CX firstname.lastname@example.org
NOTE 1: This original N2CX article was first published in the 72 newsletter back in 1997 or 1998.
See the Halfer kit page for additional details.
Hams in general, and QRP'ers in particular, are always on a quest to find the "ultimate" antenna. Of course there is no single skywire that fills every ham's needs, but there *is* one type that belongs in the casual portable QRPer's bag-o-tricks. What I'm talking about is a classic historical aerial, the End-Fed Half-Wave Antenna (or EFHWA, pronounced "EFF-WAA"). It is extremely simple to build, erect and use. In spite of its simplicity the EFHWA has the benefit of giving repeatable, efficient and effective performance.
Many antennas used for portable operation suffer from lack of effectiveness. That is, they don't produce the number of QSOs expected, as compared to operation from a home location. Most of the time this stems from the fact that the antennas used when going portable are intentionally simple, and because of this they don't put you few watts of where they belong.
One type of "simple" antenna used for portable operation is the vertical antenna. If used with a good ground system a quarter-wave vertical antenna can be very effective. But often portable use dictates an antenna that is not a full quarter wavelength long and a ground system that is far from optimum. Consider two things: 1) According to the ARRL Antenna Book, a quarter wave vertical antenna with fifteen 1/4 wave ground radials may have an efficiency of only 50%; with fewer radials the efficiency suffers even more. 2) Efficiencies of short mobile whip antennas listed in the same reference are often only 10% or so - and that is with a car body as a ground plane. With a poorer ground it is even worse! Unless you have a full-size vertical antenna with an extensive ground system you are converting most of your transmitter power to heat, not tickling the ionosphere.
Another popular portable-use antenna is the random length wire and tuner combination. If properly implemented this antenna can be very good. Unfortunately it can be hard to tell just what "proper" means. If the antenna is shorter than a quarter wavelength, it suffers from the same efficiency problems as the vertical antenna. Further, unless you choose the length correctly, the radiating part of the antenna may be at your operating position, wasting power radiating into ground-level surroundings, rather than being up in the air squirting out RF where it belongs. Because of the unknown nature of its impedance, the random length wire antenna needs a tuner capable of matching over a wide range of impedances.
One of the best choices for portable operation is the center-fed half-wave dipole antenna. It has the benefit of being very effective and not too difficult to properly erect. On the down side, it does require end supports at the right spacing and height. You need to contend with a heavy feedline at the center which has to be brought away from the dipole at right angles or performance will be degraded. Multiband operation is possible but the best way to do this with a dipole requires an open-wire feedline and a relatively complicated tuner.
If you take a half-wave dipole, eliminate the feedline and feed it directly at the end, you have an antenna that has many of the advantages of the dipole with few of the limitations of other portable antenna methods. This antenna has been described for years in the ARRL Antenna Book and other amateur radio publications but it has received little attention lately. Both K1SWR (ex-NN1G) and W1FMR have made reference to it in 72 within the last couple of years but it doesn't seem to have "caught on" very well.
Without the feedline the antenna is a snap to put up. More about configurations later, but freed of the restrictions of the center feedline, the EFHWA fits into situations that would be difficult for the dipole to handle. When erected well of the ground and clear of surrounding objects, it is as efficient as the dipole and it is effective because radiation from it is predictable so that the signal goes where you want it to go. Although it needs a tuner, the half-wave antenna has a predictable impedance and a simple tuner is usually sufficient.
Also, because it is only a single wire with a couple of insulators and a simple counterpoise, the EFHWA is lightweight and small so it is easy to store and transport - things to consider for portable use. It doesn't have to support a center feedline so physical strength is not an issue. A temporary portable antenna can use ordinary small diameter stranded hookup wire and insulators made of scraps of Plexiglas or small sections of PVC pipe. The whole "shootin' match" will fit inside a zip-lock plastic bag and fit in a coat pocket!
The total overall length of the EFHWA is an electrical half-wavelength, calculated from the formula L (Ft) = 468/F(MHz) where L is the overall wire length in feet and F is the desired operating frequency in Megahertz. This is the length of wire right from the tuner terminal to the insulator at the far end. The formula is approximate, taking into consideration the "end" effect which makes the antenna shorter than a half-wavelength in free space. Extreme accuracy isn't necessary, since even the simplest tuner will make up for inaccuracies of five percent or so. Additionally, a wire that is any multiple of a half-wavelength has the same impedance characteristics; for example, a wire cut for 40 meters will also be useful on 20, 15 and 10 meters with a suitable tuner.
As mentioned earlier you can put up your wire in a number of different ways. Probably the most common method is to make it an inverted L as shown in Figure 1. The antenna wire is stretched vertically as high as possible from the operating position, then extended horizontally to make up the remaining length. This is usually the most practical way to put up the antenna for 7 MHz and below because of the lengths involved. If supports are available, one useful way of arranging the "L" is to make half the length vertical and the remainder horizontal. Since radiation takes place in the middle of a half-wave antenna, this gives a combination of vertical and horizontal polarization to cover both local and distant propagation.
Another method of erecting this antenna is the inverted "Vee" configuration, as shown in Figure 2. This is really the same as the inverted Vee dipole often used with center-fed antennas. As with center-fed inverted Vee dipoles the center should be as high as possible, thus limiting its usefulness to the lower frequency bands. The center should be no less than 20 feet or so above the ground or the antenna will waste its power on heating earthworms.
The sloping wire illustrated in Figure 3 is very convenient to use, particularly on the amateur bands above 7 MHz, where lengths are shorter. It needs only one high support, and if that one is tall enough it puts the radiating center portion of the EFHWA fairly high in the air. The more vertical the antenna becomes, the lower its radiation angle is, thus making it useful for distant contacts. Radiation tends to favor the direction of the slope.
Taking the sloper to the extreme limit results in a vertical antenna (Figure 4). Used vertically, the EFHWA suffers few of the shortcomings of shorter vertical antennas. With its high feed-point impedance, its efficiency is not dependant on an extensive ground system. And since the radiating portion of the antenna is well above ground the transmitted signal is not wasted by absorption in foliage or buildings. The vertical half-wave has a very low angle of radiation so it is most effective for distant contacts.
The EFHWA can be used as a "stealth" antenna for use in motels or in houses or apartments where antennas are discouraged or prohibited. The wire can be sneaked out an upper story window and run horizontally to a convenient tree or other structure. Small diameter wire elevated to 20 feet in the air can be very difficult to see unless you know just where to look. W1FMR has reportedly reduced this art to a science.
Making use of the fact that a half-wave wire is also resonant on its harmonic frequencies, an effective 7 and 14 MHz antenna can be made. A 14 MHz half-square antenna (see Figure 5) can thus be used on 7 MHz as well. On 14, it has a low angle radiation characteristic at right angles to the long side of the horizontal section for DX. Onn 7 MHz, it is an end-fed half-wave with a horizontal mid-point for high angle close-in contacts. With a good ground system the antenna could also be used a compromise quarter-wave "Marconi" antenna on 3.5 MHz, yielding three-band coverage.
The EFHWA needs a ground connection, but it does not need to be very extensive. The ground or counterpoise connection simply acts to decouple the tuner and rig from the antenna system by providing a path for ground current to flow. A quarter-wavelength wire, which is half the antenna length, laid out along the ground or tucked out of the way is usually sufficient. Outdoors either the counterpoise or a short jumper connected to a large metallic structure such as an automobile or camper also works fairly well. When one end of the antenna is indoors, such as in a motel room, a heater radiator or air conditioner can be pressed into service. Try whatever ground you have to see if it works. If you find that hand capacitance affects your tuner or if you have a "hot" chassis on your rig, try the quarter-wave counterpoise wire to cool things off.
Tuners for EFHWA use are a snap. Common commercial wide-tuning range units can be used, but a simple parallel tuned circuit is usually adequate. Figure 6 shows the schematic diagram for the tuner portion of the Rainbow Bridge/Tuner, co-winner of the 1996 NorCal design competition. It tunes both 30 and 40 meters using a single toroidal inductor with an inductance of about 6 microhenries and a mica compression tuning capacitor with a minimum/maximum range of about 20 to 100 picofarads.
The antenna and ground are connected across the tuned circuit while a 50-ohm coaxial cable is connected to taps on the inductor. The tuned circuit presents a high impedance to the antenna and the tapped inductor steps this impedance down to 50 ohms. Adjusting the tuning capacitor tunes out slight reactance variation if the antenna is not an exact electrical half-wavelength.
The tuner is adjusted by adjusting the capacitor to obtain the lowest SWR reading. The correct tap position is determined by changing taps until the SWR is below 1.5:1. With these values the tuner will cover 30 meters with a 46-foot antenna or 40 meters with one about 67 feet long. The tuner has been operated at power levels up to 5 watts, and it would probably be adequate for several times as much. Simple scaling of the coil and capacitor values will allow use on other bands as well.
Other tuners for the EFHWA have been described in amateur radio literature with a variety of construction techniques ranging from large air-wound inductors and high voltage capacitors, to capacitors housed in a plastic 35-mm film canisters using coils attached on the outside of the enclosure.
I urge you to try the EFHWA for portable QRP hamming. It is truly a minimalist antenna with maximum performance.
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