Antenna Efficiency: Why Gain Is Only Half the Story

An antenna with a perfect SWR can still be a poor radiator. A dummy load has an SWR of 1.0 to 1 and radiates nothing at all. Between the dummy load and a perfect antenna there is a whole range of real antennas, and the number that tells you where yours stands is antenna efficiency: the fraction of the power from the transmitter that leaves as a radio wave, instead of being turned into heat in the wire, the coil, the ground and the objects around. Most amateur discussions focus on gain in dBi and on the SWR, and they forget that a loss of 3 dB in efficiency cancels the benefit of a gain that many operators pay a lot of money for. This guide explains efficiency in terms of two resistances, shows what typical antennas achieve and gives practical ways of raising the figure.

Two Resistances in One Feedpoint

The resistance that an antenna presents at its feedpoint is not a single thing. It is the sum of the radiation resistance, which represents the power that is radiated, and the loss resistance, which represents the power that is dissipated as heat. The transmitter cannot tell them apart, because it sees only the total, and the current flows through both. The power divides between them in proportion to their values, so the efficiency is the radiation resistance divided by the sum of the two.

The Wikipedia article on antennas states the consequence for the gain: for an antenna with an efficiency of less than 100 percent, the gain and the effective area are both reduced by the same factor. In other words, efficiency is not a minor detail in the specification; it is multiplied directly into the figure of merit that the operator cares about. Our article on dBi and dBd explains the units of gain, and you can think of efficiency as a deduction that is made before the gain is stated.

What Efficiency Looks Like in Numbers

A loss of power in dB is easy to calculate: ten times the logarithm of the efficiency. An efficiency of 50 percent is a loss of 3 dB, 25 percent is 6 dB and 10 percent is 10 dB. The table gives illustrative examples of how the two resistances combine in typical antennas. The figures are round numbers chosen to show the principle; the values of a real antenna depend on its height, its ground and its construction.

The pattern is clear. A full-size antenna has a high radiation resistance, and the small losses in the wire are insignificant by comparison. When the antenna becomes small in terms of wavelength, the radiation resistance falls rapidly, while the losses in the coil and the ground do not fall at all, so the efficiency collapses. That is why an antenna that is only a tenth of a wavelength long can show a good match on an analyzer and still be 10 dB down on a full-size one.

Where the Losses Hide

There are four common places where the power disappears, and each has its own cure.

• The ground. A vertical antenna sends current into the earth below it, and the soil is not a good conductor. The current heats the ground, and that power is lost. The monopole antenna article makes the point that low-frequency transmitting monopoles need a good low-resistance connection to the earth for efficiency, which is why broadcast stations bury a large radial system under the field.
• The loading coil. A coil used to shorten an antenna has its own resistance, which grows with the number of turns, the thinness of the wire and the effect of skin depth at radio frequencies. A coil with a poor Q can waste a large part of the power.
• The matching network. A transformer, an unun or a tuner at the antenna has losses in its core and its windings. A transformer on a ferrite core that is run near saturation, or with the wrong mix, can be very lossy. Our guide to baluns and ununs covers the choice.
• Nearby objects. Metal roofs, gutters, wet trees and building walls absorb the near field of a low antenna. The result is the same as a higher loss resistance.

The feedline is not part of the antenna, but it is part of the system. A long run of thin coax at a high SWR adds loss that reduces the power at the antenna and the apparent efficiency of the whole station. Cutting those losses is a separate job, but the principle is the same: every dB counts.

Why Verticals Depend on the Ground System

The most common example of a lossy antenna is the vertical with a poor ground system. The quarter-wave vertical is a half of a dipole, and the earth below it is the other half. If the earth is a good conductor, the picture is correct, but real soil is resistive, so the return currents flow through lossy ground. Radials placed on or in the ground collect these currents and give them a low-resistance path. The more radials, the lower the loss resistance, and the improvement is large at first and slower later. Our article on vertical antenna radials gives practical numbers, and the broadcast standard of a hundred radials or more shows how far the matter can go when efficiency is worth money.

An elevated vertical with a few resonant radials a few meters above the ground is a different case. The radials form a counterpoise that stands in for the ground, and the field below them is weaker, so there is less loss in the soil. For amateurs who cannot bury hundreds of wires, an elevated system of four tuned radials is often more efficient than a larger number of short radials on the ground.

Efficiency on Receive

An inefficient antenna loses signal on transmit, but on receive the situation is different, and it explains why small antennas are used successfully on the low bands. What matters on receive is the signal to noise ratio, not the signal level alone. On 160, 80 and 40 meters the atmospheric noise is often much higher than the noise of the receiver, so a small, inefficient antenna reduces the signal and the noise together and gives nearly the same ratio. That is the reason why a short receive loop or a Beverage, described in our article on low-band receive antennas, can hear very well although it would be hopeless as a transmitting antenna. On the high bands, where the external noise is low, the receiver’s own noise matters, and efficiency begins to count on receive as well.

Short Antennas: What You Can Expect

Every reduction in size has a price in efficiency, and it is better to know it than to be surprised. A magnetic loop for a small balcony, discussed in our guide to magnetic loop antennas, can reach an efficiency of tens of percent when the loop is large enough in relation to the wavelength and built with thick conductors and a high-quality capacitor, but it falls quickly on the lowest bands. A loaded whip on a car can be a decent antenna on 20 meters and an inefficient one on 80. The shorter the antenna in wavelengths, the narrower the bandwidth, and the higher the voltage at the ends, the more every small loss counts.

There is no trick that makes a small antenna as efficient as a large one on the same band. The best compromises make the antenna as large as possible, use low-loss coils with a large diameter and thick wire, place the loading where the current is highest only when the losses permit, keep the antenna in the clear and give it the best ground or counterpoise available. If the result is still low, a few watts more at the transmitter may be the cheapest improvement of all; the difference between 100 and 200 watts is 3 dB, which is the same as doubling the efficiency.

How to Estimate Your Own Efficiency

There is no simple instrument that reads efficiency, but several methods give a good idea. One is to measure the feedpoint resistance on the analyzer at resonance and to compare it with the calculated radiation resistance of the antenna. If the radiation resistance is 36 ohms and the analyzer shows 72 ohms, about half of the power is lost. Another is to compare your signal reports with those of a known reference antenna, such as a dipole, on the same path at the same time; a difference of one S unit is about 6 dB. A third is to model the antenna in a program and include the ground and the loss of the coil, as shown in our article on antenna modeling. None of these is perfect, but a consistent result from two of them is a reliable guide.

Common Misunderstandings

Several beliefs about efficiency keep returning in club discussions, and each deserves a short correction. The first is that a tuner “makes the antenna efficient”. A tuner makes the transmitter happy by presenting 50 ohms, but it does not change the ratio between radiation resistance and loss resistance at the antenna, so the radiated power stays what it was, minus whatever the tuner and the feedline now add in loss. The second is that coax length “improves” the efficiency; it does not, and a longer cable can only add loss. The third is that more radials raise the gain of a vertical. They lower the loss, so the same power produces a stronger signal, but the pattern of the antenna stays much the same.

The fourth belief is that efficiency matters only for transmitting. As described above, it also affects reception on the high bands and on quiet sites, where the receiver noise is the limit. The last is the idea that a high gain figure in an advertisement guarantees performance. A gain figure is valid only if the efficiency behind it is stated, and the honest specification of a small antenna always lists the loss. When you compare two designs, ask for the efficiency or the loss in dB at the band that you will use, and check the figure with a simple test on the air.

It is also worth remembering that efficiency is not constant. It changes with the weather, because wet soil and wet trees add loss and a dry spell changes the ground conductivity; with the season, because leaves on a tree near a wire antenna absorb power; and with the frequency, because a coil or a trap has its best Q only in a part of the band. A station that works well in the summer may lose a few decibels in a wet winter, and the cause is in the loss resistance and not in the radio.

Practical Tips

• Do not judge an antenna by the SWR alone. A low SWR shows a good match, not a good radiator.
• Add radials before you add power. A better ground system on a vertical is the most effective upgrade you can make.
• Use big, high-Q coils. Large-diameter, widely spaced coils with thick wire have the lowest loss.
• Keep antennas in the clear. Distance from metal and wet material reduces the loss resistance.
• Compare on the air. A reference antenna and a few reports tell you more than the best calculation.

The Bottom Line

Antenna efficiency is the fraction of the transmitter power that is radiated, equal to the radiation resistance divided by the sum of the radiation and loss resistances. Full-size antennas at a good height are very efficient, while short, loaded and poorly grounded antennas can lose 3 to 10 dB or more in heat. The losses hide in the ground, the coils, the matching network and nearby objects. Raise efficiency with a better ground system, large high-Q coils and a clear location, judge antennas by reports and measurements and not by the SWR, and remember that on the low bands a less efficient antenna can still receive well.

Frequently Asked Questions

What is antenna efficiency?

It is the ratio of the power radiated to the power delivered to the antenna. It equals the radiation resistance divided by the total resistance at the feedpoint, which includes the loss resistance.

Does a low SWR mean an efficient antenna?

No. The SWR shows how well the antenna is matched, not how much power is radiated. A dummy load has a perfect SWR and an efficiency of zero.

How much does 50 percent efficiency cost me?

It is a loss of 3 dB, which is half an S unit and the same as halving the transmitter power.

Why are short antennas so inefficient?

Their radiation resistance is very small, while the losses in the coil and the ground stay about the same. The loss resistance then takes most of the power.

Can a small antenna still receive well?

On the low bands, yes, because the external noise is higher than the receiver noise and falls together with the signal. On the high bands the difference is more noticeable.

What is the quickest way to improve a vertical?

Improve the ground system by adding radials, or raise the antenna and use a few resonant elevated radials. This lowers the loss resistance and raises the efficiency directly.