The long-wire dipole antenna is effective for low-frequency systems. Adding LC traps makes a single-band dipole antenna into a multiband unit.
The historically significant long-wire dipole antenna may seem like an anachronism in these days of compact, highly mobile wireless devices operating in the gigahertz spectrum, but that’s not the case at all. Due to its many virtues, these antennas and lower RF bands are still widely used by the military, emergency services, broadcasters, and amateur radio enthusiasts (hams) for long-distance, worldwide point-to-point links and wide-area broadcasting.
Among the virtues of the long-wire antenna are flexibility, ease of set-up, adjustable radiation pattern, low visibility to others, and small packing/carrying size. It is primarily used at frequencies below 30-MHz (equivalent to a 10-meter wavelength) in what historically was designated as the high-frequency (HF) band, spanning 3 to 30 MHz, and lower frequencies and their longer wavelengths. Still, it can be used at higher frequencies as well. As a further benefit, a single dipole antenna can serve multiple bands simultaneously by adding simple resonant LC circuits called traps in both arms of the dipole.
Why use a long-wire dipole antenna?
Given that so many of today’s antennas are short (in most cases, on the order of a meter or less) or nearly invisible, such as the chip resonator or planar inverted-F antenna (PIFA) inside a smartphone, the long-wire dipole may seem to be an antique or curiosity. However, Maxwell’s equations and wave theory show that an effective dipole radiator/receiver of such electromagnetic energy must have a primary dimension of one-half the wavelength of interest.
This classic dipole antenna is ungrounded and presents a balanced, symmetrical load to the transmitter power amplifier and to the receiver front-end amplifier, shown in Figure 1. The nominal impedance of an ideal dipole is 73 Ω but is often cited as 75 Ω; the difference is negligible. If the antenna is connected to the common 50-Ω feedline, a modest impedance-matching arrangement is needed between the feedline and the antenna.
Figure 1. The basic, classic dipole antenna has two quarter-wavelength arms and appears as a 73-Ω balanced resistive load at its resonant operating frequency. (Image: MicrowaveTools)
A dipole made from thin wire typically has a bandwidth of around 5% of the center frequency; a thicker wire will increase the bandwidth to as much as 20% but also affect other performance attributes. A balun transformer may be needed if the connection to the transmitter or receiver is via a grounded circuit and uses coaxial cable as the feedline, as is often the case.
Given its simple design, the long-wire dipole antenna is attractive. It requires only two equal lengths of wire and a way to attach them to trees, buildings, signposts, or whatever is handy. The antenna is usually not connected directly to those supports; instead, a length of wire and insulators are generally used as attachment “standoffs,” as shown in Figure 2.
Figure 2. The dipole is usually attached to its supports via insulators(white) and added wire lengths, allowing the dipole arm lengths to be maintained independently of the distance between supports. (Image: Physics Forum)
In practice, the antenna length will likely need adjustment for optimal performance to accommodate the wire’s finite thickness and other deviations from theory, but this adjustment is usually less than 5%. Even if not adjusted, the antenna’s performance is usually quite good, and the voltage standing-wave ratio (VSWR) is usually below a generally acceptable 1.5:1.
Figure 3. The radiation pattern for the horizontal dipole as viewed from a) above in the vertical plane and b) the side in the horizontal plane resembles a torus or donut. (Image: Science Direct)
If there is a significant antenna impedance shift or mismatch, the VSWR will rise to an unacceptable level, and performance will suffer. In these cases, an adjustable antenna tuner in the feedline can compensate for and implement a transition.
The dipole’s theoretical gain is around 2 dBi (dB relative to isotropic). Its radiation pattern is simple and often characterized as a torus or donut in Figure 3.
The user can adjust antenna orientation to direct maximum transmitter energy/receiver sensitivity toward the intended radio transceiver, often located thousands of miles away. Many documented cases of successful communication at these distances using a dipole at 20 m and 40 m with transmit power well under 1 W under suitable atmospheric propagation conditions, as its efficiency and radiation pattern are good.
Multiband operation extends versatility
In many real-world HF communication situations, it is necessary to try to establish contact in more than one band at the same time or switch bands at different hours. That’s because connectivity is a function of many variables, such as sunspots, atmospheric noise, daytime versus nighttime operation, and constantly varying propagation conditions. As a result, a single-band dipole antenna may be insufficient.
The obvious solution is to set up multiple dipole antennas, one for each band/wavelength of interest. However, doing so has practical difficulties in rigging, tangling, and managing, plus switching between multiple feedlines. An RF splitter/combiner could sometimes enable a single feedline to connect to two antennas, but this introduces losses and new impedance-match issues.
Fortunately, there’s a better solution which, like the dipole, has been in use since the earliest days of wireless: the “trap.” A trap is a simple, parallel-connected inductor-capacitor (L-C) combination that is self-resonant between the two bands of interest. It’s unclear when this term was first introduced or by whom; the word is not used in the 1941 US Patent 2,229,865, which presents the technique.
One trap is inserted into each arm of the dipole to make the antenna have two electrical lengths but one physical length. At frequencies below the resonant frequency, the trap’s reactance will be inductive; above the resonant frequency, it will be capacitive. Traps act like a switch, electrically cutting off the rest of the antenna at the trap’s design frequency and functioning as a loading coil below the antenna’s resonant frequency.
Figure 4 shows a simplified electrical model of the trap showing the physical inductor and capacitor and a small parasitic resistance.
Figure 4. The trap is a simple, resonant LC circuit with some undesired, unavoidable resistance that can be modeled a) in series or b) as a parallel RLC circuit. (Image: AntenTop)
Traps can have a reputation for being lossy, which concerns both transmit and receive modes. However, a properly designed and tuned trap will impose a modest loss of about 1 dB, which is usually acceptable in exchange for the convenience it provides.
Selecting the trap component values
Mathematically, an infinite number of LC pairings will result in a desired resonant frequency. Many of these would, however, require an extremely small (or large) inductor matched with an extremely large (or small) capacitor, respectively. Such a pairing would be excessively affected by parasitics and physical-size issues and have a Q-factor (quality factor) that was too narrow or too broad for the band of interest.
Fortunately, considerable literature is available on sizing traps based on theory, implementation, and hands-on field experience. For example, a trap using a 5.55 µH inductor paired with a 100-pF capacitor is a good starting point for an 80/40-m dipole, as seen in Figure 5.
Figure 5. The component values shown and dipole linear dimensions (in feet) are a good starting point for an 80/40-m multiband dipole. (Image: QSL Net)
Selection of trap components is about more than just determining suitable L and C values, as there are some very practical issues of power handling and ruggedness. For receive-only antennas, nearly any inductor or capacitor can handle the very small amount of received power, which is on the order of milliwatts and often less. However, transmitters often provide power levels ranging into the tens, hundreds, and even more watts, so the trap components must be rated for those power levels.
Traps are also exposed to the weather. While some dipole antennas are in benign environments such as an attic or wooden barn, most are outside and thus must endure rain, wind stress, temperature extremes, condensation, and more. Therefore, the trap and its connection must be either completely sealed, have some sort of drainage and venting arrangement, or be constructed with weather-resistant materials. Even if the connections remain intact, water intrusion or corrosion can affect component values and thus shift the resonant frequency.
Trap construction usually requires encapsulating its components by sealing them in a plastic case, using conformal coating, or using some weather-resistant exposed construction, as seen in Figure 6. Low-cost PVC pipe is often used as the core of a wound inductor; in other cases, a PVC pipe with tight end caps is used as the enclosure itself with water-tight wire-access holes.
Figure 6. This home-made 80/40 meter trap uses a hand-wound inductor around a PVC pipe as its core support. (Image: www.vk4adc.com via DXZone)
Tuning and trimming the trap components is another practical issue to consider. While calculating the component values is a necessary first step, these ideal values are often not quite close enough due to parasitics, wire diameter, and inductor winding imperfections, to cite a few real-world factors.
For this reason, most home-made traps and many commercial ones allow the user to adjust the L and C values in the field to achieve the desired performance, usually using a VSWR meter.
The use of traps is not limited solely to a long-wire dipole over two bands. A series of traps can be used to build three — and even four-band dipole antennas. However, doing this requires additional adjustments and some performance compromises and tradeoffs in antenna radiation pattern, gain, bandwidth, and other parameters.
Not limited to simple dipoles
While traps are usually associated with basic long-wire dipole antennas, they are not limited to that antenna design. For example, a multiband, directional, high-gain Yagi-Udi antenna (often simply called “Yagi”) is constructed from an array of active and passive dipole elements. This form of Yagi uses traps in its director, active drive, and reflector elements so it can function across multiple bands, as seen in Figure 7.
Figure 7. Traps can be used for three-band operation on basic dipoles as well as on more complex multiband antennas such as this 20/15/10-meter Yagi design; shown (left to right) are the antenna director, driven, and reflector elements, each with two traps on each arm. (Image: OnAllBands)
Conclusion
The humble, modest, low-tech long-wire dipole antenna has served the wireless world for over a century. It continues to do so due to its simplicity, adaptability, portability, and effectiveness. By using passive traps, its ability to function can be extended across two or even more bands in the high-frequency part of the electromagnetic spectrum.
References
Radio Antenna System, H.K. Morgan, U.S. Patent 2,229,865
Dipole Antenna, MicrowaveTools
Antenna Fundamentals: Radiation Pattern, Science Direct
Antenna Traps—A Way to Cope With Limited Space, On All Bands
Tuned Circuits and Traps, QSL Net
Modeling Trap Antennas, AntenTop
Multirange Trap Antennas, AntenTop
Low Cost Antenna Traps, VK4ADC’s web
Using Antenna Traps, SOTABeams
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