Log Periodic Dipole Array: A Wideband Beam for Ham Radio

A log periodic dipole array, usually shortened to LPDA, looks a bit like a Yagi but behaves very differently. Instead of being tuned to a single band, it works over a wide range of frequencies, with fairly constant gain and a steady feed impedance across the range. That makes it appealing to anyone who wants one directional antenna for several bands, and it is why LPDAs appear on television rooftops, at test ranges and on a few well-equipped amateur towers. This guide explains how an LPDA works, how it compares with a Yagi, what it can and cannot do for a ham station and what to consider before you buy or build one.

What a Log Periodic Antenna Is

A log-periodic antenna is a multi-element directional antenna designed to operate over a wide band of frequencies. According to the overview of log-periodic antennas, the design was invented by John Dunlavy in 1952, and the most common form is the log-periodic dipole array, or LPDA. It consists of a number of half-wave dipole driven elements of gradually increasing length, each made from a pair of metal rods, mounted close together along a boom.

The term “log-periodic” refers to the way the elements are spaced. Their lengths and spacings follow a logarithmic pattern, so that the antenna’s electrical properties repeat periodically as the logarithm of frequency. Two design numbers describe it: tau, which sets the ratio between successive element lengths, and sigma, which sets the relative spacing. Together they decide how wide the antenna’s frequency range is, how much gain it has and how long the boom must be.

The radiation pattern is unidirectional. The main lobe points along the boom, off the end where the shortest elements are. If you picture a Yagi with its director end pointing at the target, the LPDA is pointed the same way: the short end at the front and the long elements at the back.

How It Works: Every Element Is Driven

The most important difference from a Yagi is that in an LPDA every element is connected to the feed line. In a Yagi, only one element, usually the second from the back, is driven, and the rest are parasitic. In an LPDA, a parallel-wire transmission line runs along the boom and each successive element is connected in opposite phase to it. The feed line is often seen zig-zagging along the boom, which is one visual clue that you are looking at an LPDA and not a Yagi. Another common construction uses two parallel booms, which also serve as the transmission line, with the dipoles mounted alternately on each.

At any given frequency, only a few elements do most of the work. The element closest to resonance acts as the driven element, and its neighbours behave a bit like a reflector and a director. In that sense, at any frequency the antenna operates somewhat like a three-element Yagi, and the “active region” slides along the boom as you change frequency. At a low frequency, the long elements at the back are active. At a high frequency, the short elements at the front are active. The antenna’s operating bandwidth, the range over which it has near-maximum gain, extends roughly between the resonant frequencies of the longest and the shortest elements.

That is the whole idea. The antenna does not need to be retuned for each band, because a different part of it is doing the job on each band.

LPDA Versus Yagi

The comparison with a Yagi is the heart of the subject. Adding elements to a Yagi increases its gain and directivity, while adding elements to an LPDA increases its frequency coverage. A Yagi with the same number of elements as an LPDA would have far higher gain, because all of its elements are improving the gain of a single driven element.

The numbers from television practice make the trade-off concrete. A common design combined a UHF Yagi in front of a larger VHF LPDA of about equal size. The Yagi section achieved gains on the order of 10 to 14 dB, while the log-periodic section managed around 6.5 dB. For television, that was acceptable because the wide coverage mattered more than raw gain.

For amateur radio, the lesson is the same. If you operate mainly on one band and you want the strongest possible signal, a Yagi or a quad is a better antenna than an LPDA of the same size. If you want a single directional antenna for several bands, or you want to test across a range, the LPDA is attractive.

How Much Gain to Expect

Gain depends on tau, sigma and the number of elements. A narrow-spacing design with many elements has more gain per band, while a design with wide spacing and fewer elements has less gain but a shorter boom. In general, an LPDA offers a moderate and steady gain across its range. It is not the antenna to choose when you need to win a pileup on a single band. It is the antenna to choose when you need dependable directional performance over many frequencies with one feed line and one rotator.

Because the numbers vary by design and manufacturer, read the data sheet and the modelling, not the marketing. Independent references, such as the ARRL antenna resources, are a good place to check design data. Be sceptical of any claim that an LPDA matches a monoband Yagi of the same size. The physics of the design explains why that cannot hold.

Where an LPDA Makes Sense in a Ham Station

Multiband HF operation

Commercial HF log periodics are available for the upper HF bands, offering coverage that includes several amateur bands in a single antenna. For a station that wants a rotatable beam for several bands without the complexity of traps or multiple Yagis, the LPDA is one of the few options. The trade-offs are weight, wind load and cost, so check the specifications carefully against your tower and rotator. Our guide to choosing and sizing an antenna rotator covers the load calculations.

VHF and UHF work

At VHF and UHF, LPDAs are much smaller and lighter, and wideband versions are useful for receiving across a range of frequencies. A wideband LPDA is a convenient way to monitor several bands, and it is widely used for EMC testing and measurement. For weak-signal VHF work, however, a high-gain Yagi built for one band normally outperforms it.

Scanning, monitoring and testing

For a receive-only antenna that must cover a wide range, such as a station that scans many frequencies, an LPDA is a strong choice. The same applies to test setups where a steady impedance and constant pattern across the range are more important than maximum gain.

Feeding an LPDA

An LPDA’s feed point is usually at the short end of the boom, and the transmission line runs along the boom to the longer elements. Many designs present an impedance that is relatively constant across the range, which makes matching easier than on a Yagi. Because the antenna is balanced and the coax is unbalanced, a balun is normally needed at the feed point, and a quality choke is worth the money. Our article on baluns and ununs explains the options.

Check the feed arrangement carefully before you build. The way each element is connected to the boom or transmission line, with alternating phase, is critical to correct operation. An error in the phasing reverses the pattern or destroys the match. Follow a proven design closely, and verify every connection before you raise the antenna.

Mechanical and Practical Considerations

• Boom length. The lowest frequency determines the length of the longest element, and the design ratio determines how long the boom must be. A wideband HF LPDA can be a very large structure.
• Weight and wind load. Many long elements and a long boom mean a large wind area. Your rotator, mast and tower must be rated for the load.
• Feed line along the boom. The zig-zag or parallel line needs good insulation and must not be pinched or abraded.
• Weather resistance. Corrosion at the element-to-boom clamps can change the electrical properties over time.
• Zoning and neighbours. A multiband beam with long elements is highly visible. Check your local rules before you buy.

If space or weight is a problem, a more compact multiband solution may suit you better. A hex beam, covered in our comparison of Yagi and hex beam antennas, trades some performance for a lighter structure.

Testing and Tuning

Unlike a monoband Yagi, an LPDA should not need much trimming, because its performance is designed to be broad. Still, measurement is essential. Sweep the antenna with an analyzer across the whole range, and check that the SWR is consistently low. A NanoVNA is ideal for this, and our NanoVNA guide shows how to calibrate it and run a sweep. If the SWR is high across the entire range, suspect a feed problem or a reversed element. If it is high only at one end, suspect a broken connection to the nearest element.

Common Mistakes

• Expecting Yagi gain. An LPDA of the same boom length has less gain on a single band.
• Underestimating size. Wide coverage at HF means a long boom and long elements.
• Ignoring the balun. A balanced antenna fed through unbalanced coax with no choke has a distorted pattern.
• Getting the phasing wrong. Alternating connections are essential to the design.
• Buying on coverage claims alone. Look at the gain and SWR curves, not just the frequency range on the label.

Choosing Between an LPDA, a Yagi and a Quad

If you are deciding between directional antennas, ask three questions before you look at any catalogue. How many bands do you actually want to cover with one rotatable antenna? How much gain do you need on your most important band? And how much weight and wind load can your tower and rotator take?

• One band, maximum gain: a monoband Yagi or a quad is almost always the better choice, because all its elements work on one frequency.
• Several bands, one feed line: an LPDA or a multiband Yagi with traps are the usual options, and the LPDA keeps its performance more evenly across the range.
• Limited space or weight: a lighter wire beam, such as a hex beam or a Moxon, may beat both on practicality.
• Receive-only or test work: an LPDA is hard to beat for steady behaviour over a wide band.

Whatever you choose, compare the manufacturer’s gain and front-to-back figures across each band you care about, not only the headline number. Ask whether the figures were measured or modelled, and at what height above ground. A fair comparison uses the same conditions for every antenna on the shortlist.

The Bottom Line

A log periodic dipole array is the right tool when bandwidth matters more than maximum gain. Every element is driven, a different part of the antenna works on each frequency, and the result is a directional antenna with steady performance across a wide range. For a single-band contest or DX beam, a Yagi or a quad will give you more. For a multiband beam, a wideband receive antenna or a test antenna, an LPDA is hard to beat. Check the gain figures, match the antenna to your tower and rotator, add a good balun and measure it with an analyzer before you rely on it.

Frequently Asked Questions

What is the difference between an LPDA and a Yagi?

In a Yagi only one element is driven and the rest are parasitic, which gives high gain on one band. In an LPDA every element is connected to the feed line, which gives wide bandwidth with moderate gain.

How much gain does an LPDA have?

It depends on the design, the spacing and the number of elements. In general, gain is moderate and fairly constant across the range, and lower than a monoband Yagi of the same size.

Why are LPDAs used for television reception?

Because the antenna needs to cover many channels across a wide frequency range, and an LPDA provides that coverage with steady performance from a single antenna.

Can I use an LPDA on several amateur bands?

Yes, if the antenna covers them. Commercial HF models cover a continuous range of upper HF frequencies, which includes several amateur bands. Check the specification for the bands you need.

Do I need a balun for an LPDA?

Usually yes. The antenna is balanced and coax is unbalanced, so a balun or choke at the feed point helps keep the pattern clean and the feed line out of the radiation.

Is an LPDA a good first beam?

Not usually. It is heavier and more complex than a small Yagi or a wire beam. It makes the most sense when you specifically need wide bandwidth or several bands from one directional antenna.