The Dipole Antenna

The dipole antenna is the most important antenna in all of radio. Every other antenna design can be understood, analyzed, or derived by starting from the dipole. It is the reference against which all antenna gain is measured in dBd. It is the most common antenna used by ham operators worldwide. And when built correctly from a piece of wire and two insulators, it costs almost nothing and works beautifully. This lesson explains the half-wave dipole from first principles — why it is the length it is, what its feedpoint impedance really means, what its radiation pattern looks like, and how its many variations differ from the basic design.

The half-wave dipole: two equal-length wire elements fed at the center, each a quarter wavelength long. The feedline connects at the point of maximum current.

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What Is a Dipole?

A dipole antenna is simply a straight, center-fed conductor — two equal-length wire elements extending in opposite directions from a central feedpoint. The word "dipole" means "two poles," reflecting the fact that the two halves of the antenna carry current in opposite instantaneous directions, creating the oscillating electric dipole moment that produces radiation.

The most common dipole is the half-wave dipole, where the total length of the antenna is approximately one half of the operating wavelength. Each half of the antenna is approximately one quarter wavelength long. The word "approximately" is important — the exact resonant length of a real dipole is slightly less than a theoretical half wavelength in free space, for reasons explained in the length formula section below.

The dipole is fed at its center — the point of maximum current and minimum voltage in its sinusoidal current distribution. This center-fed geometry presents a convenient, low impedance to the feedline. Feeding the dipole at the center is not merely a convention; it is the point where the feedpoint impedance is lowest and most compatible with standard 50-ohm or 75-ohm coaxial cables.

The Length Formula: Why 468?

The theoretical half-wave dipole in free space would have a total length of:

L = λ/2 = (300 / f_MHz) / 2 = 150 / f_MHz meters = 492 / f_MHz feet

But in practice, a dipole cut to this free-space half-wavelength will resonate at a slightly higher frequency than intended — the resonant frequency will be higher than your target frequency, meaning the wire is too short. Why?

Two effects combine to make a real dipole slightly shorter than the theoretical free-space half-wave:

1. End effect (capacity hat effect): The ends of the wire are cut abruptly. The fringing electric field at the wire tips acts as small capacitors, effectively extending the electrical length of the antenna slightly beyond its physical length. This makes the antenna appear electrically longer than it is physically, so you can cut it a little shorter and still achieve resonance.
2. Wire diameter: A real wire has finite diameter. Thicker wires have a shorter resonant length than thinner wires because the electric field distribution along a thicker wire is slightly different from that of a thin theoretical conductor. The effect is small for typical wire diameters used in antennas (a few percent) but measurable.

Both effects reduce the required physical length by approximately 5% compared to the free-space half-wavelength. Applied to the formulas:

The constant 468 (feet) or 143 (meters) comes from taking the free-space formula (492 ft or 150 m) and multiplying by approximately 0.95 to account for the end effect and wire diameter for typical AWG 12–16 copper wire in the clear. These are starting-point values — real-world antennas should always be cut slightly long and then pruned to resonance based on SWR measurements. The exact resonant length depends on the wire diameter, height above ground, proximity to objects, and even the quality of the insulators.

Half-Wave Dipole Length Calculator

Feedpoint Impedance

A resonant half-wave dipole in free space has a feedpoint impedance of approximately 73 ohms, purely resistive. This value comes from detailed analysis of the electromagnetic fields around the antenna and cannot be derived by simple intuition — but the result is very well established by both theory and measurement.

The 73-ohm impedance is not a perfect match to the standard 50-ohm coaxial cable used in most ham stations. The SWR with 50-ohm coax is approximately 73/50 = 1.46:1. This is a moderate mismatch — it means about 4.3% of the power is reflected back toward the transmitter. Most modern transceivers handle an SWR of 1.5:1 or less without any problem — the internal SWR protection circuits do not activate. In practice, the mismatch between the dipole's 73 ohms and 50-ohm coax is rarely a significant issue.

For those who want a better match to 50 ohms, there are options. A 1:1 balun does not change the impedance — you still have 73 ohms. A 1.5:1 transmission line transformer would be needed for a theoretically perfect match, but this is unusual. In practice, many operators connect a 50-ohm coaxial feedline directly to the dipole, accept the 1.46:1 SWR, and do not worry about it. The cable loss on a short run of RG-213 at 1.46:1 SWR is trivially small.

Height above ground significantly affects the feedpoint impedance. A dipole at exactly one half-wavelength above ground has a feedpoint impedance close to the free-space value of 73 ohms. At lower heights, the impedance is reduced — at one-quarter wavelength height it may drop to 50–60 ohms (fortuitously better matched to 50-ohm coax). At very low heights (under one-eighth wavelength), ground effects become severe and the impedance is unpredictable without detailed modeling.

If you feed the dipole at one end (off-center or end-fed), the impedance changes dramatically. At the far end of the dipole (the voltage maximum), the feedpoint impedance rises to several thousand ohms — the exact value depends on frequency and environment. End-fed antennas require a high-impedance matching transformer (typically a 49:1 or 64:1 unun) to connect to 50-ohm coax.

Radiation Pattern and Gain

The radiation pattern of a horizontal half-wave dipole has the following characteristics:

• Azimuth pattern: A figure-eight (bidirectional). Maximum radiation is broadside to the antenna (perpendicular to the wire). There are deep nulls off the ends (along the wire's axis). If your dipole runs east-west, you radiate maximally north and south, with near-zero radiation east and west.
• Elevation pattern: Depends strongly on height above ground. At low heights (less than λ/4), most radiation goes straight up — good for NVIS, poor for DX. At heights around λ/2 (e.g., 10 meters at 14 MHz = about 33 feet), radiation shifts toward lower angles, improving DX capability. At heights of one wavelength (about 20 meters at 14 MHz), radiation concentrates at low angles with a strong DX lobe.
• Gain: 2.15 dBi (0 dBd) maximum, in the broadside directions at the elevation angle of maximum radiation. The dipole is the reference for dBd gain, so by definition it has 0 dBd of gain.

A horizontal dipole has a figure-8 azimuth pattern (strongest broadside) and an elevation pattern that shifts to lower angles as height increases. At 1λ height, the low-angle DX lobe is pronounced.

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The deep nulls off the ends of the dipole can be exploited deliberately. If you have an interfering station in a specific direction, orient your dipole so that direction is off the end of the wire. You can get 20 dB or more of interference rejection compared to broadside — very useful for competitive operating or in a noisy RF environment. Because of reciprocity, this also reduces your transmitted signal toward the interferer, which is sometimes socially desirable.

Building a Dipole

A basic dipole requires very few materials. You need wire, two end insulators, a center feedpoint insulator or connector, and coaxial cable to run to the radio. Here is a complete parts list and procedure for an 80-meter phone dipole (3.700 MHz target):

The Inverted-V Dipole

The inverted-V is the most popular variation of the dipole for HF amateur radio, because it solves the two-support requirement with just one central support. Both arms of the dipole drop away from the center at an angle, forming a V shape when viewed from the side (or an inverted V when viewed from the front). The antenna only needs one high support point — a single mast, tower, or tree — with the two ends tied off at low points.

The inverted-V changes the dipole's electrical characteristics slightly. Because the arms are not horizontal, the effective height varies along the antenna — the center is at maximum height, the ends are lower. This affects the radiation pattern (the pattern becomes more nearly omnidirectional in azimuth) and the feedpoint impedance (which drops slightly from 73 ohms, toward 50–70 ohms, depending on the apex angle). With a leg angle of 45° from vertical (60° included angle), the feedpoint impedance drops to about 50–60 ohms — a better match to 50-ohm coax than a flat dipole. This is one reason the inverted-V is popular.

The resonant length of an inverted-V is slightly shorter than a flat dipole. For the same target frequency, cut the inverted-V about 2–5% shorter than the flat dipole formula suggests. The exact amount depends on the leg angle. When in doubt, cut long and trim — the same procedure as for a flat dipole.

The main trade-off of the inverted-V versus a flat dipole is the radiation pattern. The flat dipole has sharp nulls off the ends — very useful for interference rejection. The inverted-V's more omnidirectional pattern means you lose that null advantage. For operators who need to work in all directions with one antenna, the inverted-V is actually better. For those who want directional nulls, a flat dipole is preferable.

The Sloper

A sloper (or slanted dipole) has one end at the top of a mast or tower and the other end sloping downward to a low support point. It is essentially a tilted dipole. The sloper has a pattern that is asymmetric — it radiates more in the direction toward which it slopes, with some low-angle radiation in that direction. This makes it a simple, single-support antenna with a degree of directivity.

Slopers attached to a tower have the advantage that the tower acts as a partial counterpart or reflector, which can enhance the radiation in the lower direction. This tower interaction complicates the design but can be used deliberately to create a system with 2–3 dB of gain in the direction of the slope. Installing four slopers from the same tower apex, each sloping in a different direction, creates a switchable directional antenna — a popular design for HF DX work.

The Fan Dipole and Trap Dipole

A fan dipole consists of multiple pairs of dipole elements connected to the same feedpoint, each pair cut for a different amateur band. The elements spread out from the feedpoint like a fan. Because each pair of elements is resonant on its own band, the antenna appears as a low-impedance load on each band it covers. On other bands, the off-resonant elements have high impedance and present only a small load to the feedline, so they contribute little and do not seriously disturb the resonant elements.

Fan dipoles are a practical multi-band solution for operators who cannot afford multiple antennas. A typical fan dipole for 40, 20, 15, and 10 meters uses four pairs of elements all connected at the center. The elements must be spread far enough apart to avoid significant interaction — a minimum of 15–20 degrees of angular separation between adjacent pairs. With careful construction, a fan dipole can achieve good performance on all its design bands with a single feedline.

A trap dipole uses resonant LC circuits (traps) inserted along the dipole elements to allow multi-band operation with a single pair of elements. At the frequency where a trap is resonant, it presents high impedance and effectively disconnects the outer portion of the element, making the antenna electrically shorter for that band. At lower frequencies, the trap has low impedance and the outer element portion is included, making the antenna full-length. Trap dipoles are more compact than fan dipoles but have slightly higher losses due to the trap coils.

The Folded Dipole

A folded dipole consists of two parallel conductors forming a loop, with both ends shorted together and one conductor split at the center for the feedpoint. It looks like a squashed rectangular loop. The folded dipole has the same radiation pattern and the same directivity as a standard dipole, but its feedpoint impedance is four times higher — approximately 300 ohms instead of 73 ohms.

The 300-ohm impedance of the folded dipole is a nearly perfect match to 300-ohm twin-lead feedline, which was the standard TV antenna feedline for decades. Many old Yagi antennas for television used folded dipoles as the driven element precisely for this reason. In amateur radio, the folded dipole is used in Yagi designs where a 4:1 balun transforms the 300-ohm feedpoint to 75-ohm coax, or in special feed systems.

The folded dipole also has broader SWR bandwidth than a standard dipole. The two-wire folded structure behaves somewhat like a step-up transformer that increases the feedpoint impedance and simultaneously broadens the resonance curve, giving the antenna a useful 2:1 SWR bandwidth that is wider than a simple dipole. This broader bandwidth is valuable for bands with wide frequency allocations.

Off-Center Fed Dipole

An off-center fed (OCF) dipole, sometimes called a Windom antenna, is fed not at the center but at a point offset from center — typically at 1/3 and 2/3 of the total length. This offset point has a higher feedpoint impedance than the center (because you are moving toward the voltage maximum), but on multiple bands the impedance may happen to be usable after transformation through a balun.

The most common OCF dipole design uses an 80-meter wire with the feedpoint at about 1/3 of the length from one end. With a 4:1 balun, the impedance at the feedpoint is close enough to 50-ohm coax to work on 40, 20, 10, and 80 meters with reasonable SWR. The attraction is simple multi-band coverage with one feedline. The drawback is that the radiation pattern shifts with frequency (because the feedpoint changes position relative to the element as frequency changes) and some common-mode current on the feedline can be troublesome.