Since the early days of radio, dipoles have been the go-to antennas for hams. They’re effective, inexpensive, and simple to make. But there’s more to building and installing one than calculating the right length, cutting some wire, and throwing it into a tree.
Here’s a guide to help you choose the right materials and the best configurations to make your dipole work as well as possible. Included are some answers to commonly asked questions about this popular ham antenna.
Wire Selection
Copper is the most common material for dipole antennas because it is highly conductive and durable. Three main types of copper wire are widely used, each with its own advantages.
Copperweld® Wire:Steel wire coated with a layer of copper offers a good compromise between durability, weight, and conductivity. It’s less likely to stretch, meaning its resonant frequency won’t change much over time.
Stranded Copper Wire: Often the first choice of antenna builders, stranded wire is the best choice for portable use where the antenna will be erected and frequently taken down. It has less tendency to kink than solid wire.
Solid Copper Wire: Solid wire is strong and rigid. Those qualities are great for applications that require strength, but not so much for pliability. Electrically, solid copper is comparable to stranded wire. Alpha Delta has used 12 AWG solid wires for years on its antennas, and they hold up well for permanent installations.
- Does it make a difference whether I use insulated or uninsulated wire?
In practice, there is no noticeable difference. Regarding tuning, insulated wire antennas are electrically shorter by about 1.5 to 2% at resonance than their bare counterparts—something you’ll adjust anyway. If you choose insulated, be sure it’s UV-rated for outdoor use. Stranded wire may stretch some as compared to hard-drawn solid copper or Copperweld, but it’s not a deal breaker.
- Why should I spend more on copper?
I’ve been told you can get a quarter-mile of galvanized fence wire for about $20 online. Galvanized fence wire is inexpensive and can technically be used as an antenna in a pinch. However, it’s generally not considered an ideal solution due to its lower conductivity and potential for corrosion.
Beginnings and Ends
Dipoles need a center connection for the feedline, whether you use coax or ladder line. A dipole center insulator should be used to absorb the strain caused by the tension on the wire to prevent damage to the feeder over time.
It is always best to use insulators at the end of the antenna wire. The ends are points of voltage maximum, and insulators enable the voltages to be properly managed without breaking down. Avoid using rope only. It can hold moisture and detune the antenna.
Sturdy plastic cutting boards and PVC plumbing parts are popular materials with DIY builders on a budget. Both provide the necessary parts for insulated ends and a center connection. 3-D printers are a new option for making custom parts.
A large selection of ready-made center and end insulators is available from several online sources. There are heavy-duty kits made by Alpha Delta and DX Engineering, as well as pairs of end insulators. Often, 1:1 baluns are recommended for dipoles, which come in enclosures that can also act as center insulators.
- When do I need a balun instead of a center insulator?
A balun matches a balanced feedline to an unbalanced antenna. If you feed your dipole, which is balanced, with unbalanced coax, you might experience a problem with coaxial cable shields radiating. A 1:1 balun (also called a common-mode choke or line isolator) solves this.
Common Configurations
Factors that will help you choose the configuration of your dipole include the space you have, the availability of supports (towers, trees, poles, etc.), and the bands you plan to use.
Flat Top: A flat top refers to a type of dipole antenna where the horizontal elements of the antenna are kept parallel to the ground, creating a flat, horizontal radiator. A flat-top dipole can offer slightly better gain in the broadside direction but can require more horizontal space for installation. Using the 468/frequency in MHz, a dipole tuned to the middle of the 80M band would be 124.8 feet. For comparison, a dipole tuned to the middle of the 20M band would be 33 feet. What would fit in your backyard?
Sloper: A sloper is a standard dipole antenna mounted at an angle, sloping downward from a single support point such as a tower, pole, or tree. It’s a good way to utilize an existing tower. Most contacts will be in the direction opposite the slope of the antenna. The slope angle is usually between 45° and 60°. It has a considerably smaller footprint than a flat-top dipole—great for smaller spaces.
Due to its low-angle radiation pattern, a sloper performs well for long-distance contacts. It is typically fed with a coaxial cable in the center. For best performance, at least 1⁄4 of the wavelength of the feedline should be at a 90° angle to the antenna.
Inverted V: This type of antenna resembles a standard dipole antenna but has two sides bent downward toward the ground, creating an upside-down V shape. It’s commonly used in situations when space is limited, making a smaller footprint while maintaining decent performance. An inverted V only needs one central support point for mounting. The radiation pattern is more omnidirectional than that of a horizontal dipole due to the legs being angled downward.
The inside angle between the sloping wires must be at least 90° and preferably 120° or more. Sloping of the dipole wires causes a reduction of the resonant frequency for a given dipole length, so about 4% will be subtracted from standard dipole dimensions. Compared to the 80M flat-top example, each V leg would be 60 feet; at a height of 40 feet, the footprint would be about 90 feet. The flat-top dipole would be nearly 125 feet.
Sloping the dipole wires will also change their radiation resistance. The center feed impedance of the inverted V dipole is just 50 ohms, less than the typical 75 ohms of a horizontal dipole. This makes a good match for a standard 50-ohm impedance coaxial cable.
Bend It, Shape It
You can bend the ends of a dipole antenna to fit it into a smaller space while still maintaining its overall electrical length and functionality. Typically, the ends drop downward at a 90° angle. Don’t just drop the ends near or parallel to a metal end support pole—it could detune the antenna. Although it may slightly affect the radiation pattern and impedance, it should still work.
In many ways, horizontally bending or zig-zagging a dipole is superior to vertically bending the antenna. Horizontal bends can preserve gain and directivity, but feedpoint impedances can decrease. When the ends of the zigzag version exceed 20 feet each, the pattern tends to tilt away from the bent ends.
Decreasing the length of the main wire run below 60% of full size decreases gain, especially for antennas with the ends bent in the same direction. The zigzag bend is more immune to this effect because its bends produce less field canceling.
Wrap or Cut?
- It’s time to tune, but should I wrap or cut?
You can wrap back insulated or bare antenna wire around itself at the ends of a dipole antenna to effectively shorten its length. This makes it less likely you’ll come up short. Be sure the wire is routed through the insulator before wrapping. If you have more than a few inches wrapped, you may want to trim the excess after you’re satisfied the antenna is tuned correctly.
Once you have your antenna trimmed satisfactorily for your desired operations, secure the wire so it doesn’t unravel. UV-rated zip ties and self-fusing silicone tape are good options. Tie up a permanent installation with Dacron® or polyester rope. If you’re using trees as anchor points, provide some slack and strain relief to avoid snapping a wire when tree branches move around in the wind. Some hams prefer to hang a small weight over a pulley attached to a tree limb with the cord attached to the dipole’s end insulator. When the tree moves, the cord and weight will keep the wire taut without overstraining it.
What on Earth?
Keep in mind that the antenna orientation, height, objects in the vicinity, and soil conductivity are among the variables affecting performance. Nearby metallic structures and objects may detune the antenna—keep them as far apart as practical. There will be some degree of interaction between the dipole and ground, decreasing as the antenna is raised above ground level.
- How high does the antenna need to be?
A dipole closer to a good ground will produce a more vertical antenna pattern, like a Near Vertical Incidence Skywave (NVIS) antenna. This is best for local nets, emergency communications, and rag-chewing. For DX work, higher above ground is preferred for producing low-angle radiation (less than 20°).
Then there’s the half-wavelength rule. Place the antenna at least one-half wavelength above ground, ensuring a good balance of radiation angles and signal strength across most frequencies. A half wavelength at 20 meters is about 34 feet—within reason for many hams. However, it may not be practical on lower bands—for example, 246 feet on 160M and 64 feet on 40M. Place the dipole as high as possible, within any limitations you may have.
Fan Out
Purists will say only resonant antennas should be used. But what if you want to operate on several bands with a single antenna? Resonant dipoles can be combined to make a multi-resonant antenna, also called a multi-element dipole, parallel-wire, or fan dipole. Each element is individually tuned to one of the desired bands. It’s an easy way to provide multiband capability using a single feedline.
So, how does it work? The multiband fan dipole will appear to have a number of resonant frequencies, each corresponding to the resonant frequencies of the different dipoles. Each dipole has a low impedance at the feedpoint on its resonant frequency. As the signal frequency moves away from the resonant frequency of one dipole, its impedance increases, and it does not radiate power. But at the resonant frequency of another dipole in the fan configuration, the impedance drops and will allow the signal to radiate.
Tuning is done much like any other dipole. As mentioned earlier, elevation affects Standing Wave Radio (SWR), so hoist the antenna up for measuring, then lower it to make any necessary adjustments. Adjust both sides equally. The order makes a difference with fan/parallel dipoles. Begin with the lowest frequency band (longest element) and work progressively to the highest frequency band. Interaction between the elements may have some effect but shouldn’t cause serious problems.
Parallel wire configurations, where all elements are equally spaced along their entire length from each other, will require more effort to tune than one where the elements are spread out. Sometimes, adjusting the lengths can be a challenge if the fan dipole has many sections covering different bands. Two or three dipoles connected to the same feeder can be easily adjusted, but four or more dipoles can test your patience.
Care and Feeding
Choosing the right type of coaxial cable for your application based on its signal attenuation and power handling characteristics is essential, especially for longer cable runs. The greater the length of coax cable you use, the more signal loss you will experience. Despite copper being an excellent conductor, it still has some resistance. This means that some electrical energy will be lost as heat.
Many hams choose 50-ohm coax like RG-8X due to its availability and reasonable cost. For HF, RG-8X is great for all bands with cable lengths up to 100 feet and will handle 1 kW up to 30 MHz. For longer runs, RG-8, RG-213, and 400MAX handle higher power and have reduced losses.
A popular myth says that specific lengths of coaxial cable, such as multiples of 1/2 wavelengths, will improve SWR. Not true. The length of a coaxial cable does not directly affect an antenna’s SWR. SWR is a property of the antenna itself and is measured at its feedpoint. Changing the cable length will not change the antenna’s inherent SWR. In general, the right length of coax for an HF antenna is the distance it takes to connect the transmitter to the antenna.
To properly care for a dipole antenna, be sure it’s installed at an optimal height. Use a good quality feedline rated to handle the amount of power you plan to use. Regularly inspect the antenna for damage, clean connections as needed, and properly seal the feedpoint to prevent moisture ingress, especially in harsh weather conditions. Back at the shack, always check for proper SWR before transmitting to optimize performance. Your radio’s finals will thank you.
