What’s the difference between an LPDA antenna and a Yagi antenna?

Posted by Mike Parker on Oct 18th 2017

Radiation pattern of a directional antenna Figure 1: Radiation pattern of a directional antenna, in dB gain.
Rooftop television antenna Figure 2: A rooftop TV antenna is a dipole antenna.
50-Ohm and 75-Ohm LPDA antennas Figure 3: A 50-Ohm LPDA (top) and a 75-Ohm LPDA (bottom)
LPDA antenna Figure 4: The dipoles on an LPDA antenna increase in width down the length of the boom, forming a triangular shape.
Yagi antenna Figure 5: The dipoles on a Yagi antenna are the same (or nearly the same) width down the length of the boom.

This blog entry is going to get a little technical, so let’s nerd up and examine how a directional cellular building antenna works:

First, a little review

There are four parts to a cell phone signal booster system:

  1. The booster, which amplifies cell signal.
  2. An outdoor antenna (also known as a donor antenna) that sends and receives signal from a cell tower.
  3. One or more indoor antennas (also called broadcast antennas) that send and receive signal from a phone or other cellular device.
  4. Coax cables that connect the antennas to the booster.

Right now, we’re interested in outdoor antennas. Broadly speaking, there are two types of outdoor antennas for a cell phone signal booster system:

  1. A directional antenna, which sends and receives signal to and from a specific cell tower.
  2. An omnidirectional antenna, which sends and receives signal to and from any and all surrounding cell towers.

In this article, we’re going to discuss directional antennas.

How a directional antenna works (in the plainest English possible)

As “directional” indicates, this type of antenna works by being pointed (or focused) toward the source of the signal.

A directional antenna’s signal pattern can be compared to the light from a flashlight: It’s strongest at the center of the beam, and tapers off toward the edges, only illuminating a small area where the flashlight is pointed. Similarly, a directional antenna has a horizontal beam around 40 to 80 degrees wide, depending upon the antenna type. The center of the beam is always the area of strongest signal. (See Figure 1.)

A directional antenna consists of a long boom, to which are attached a series of conductive metal rods, called dipole elements. A dipole element must be the same length on one side of the boom as its counterpart on the other side (getting technical, this means they’re bilaterally symmetrical). The combined span of two dipole elements is half the width of the wavelength of the radio frequency on which they transmit and receive (a half-wave dipole).

If you grew up before the mid-1980s, you may remember when every house had a huge television antenna mast on the roof (see Figure 2); that’s an example of a dipole directional antenna. Rooftop TV antennas actually include two different types of directional antennas on a single mast, which brings us to the point of this article:

Directional building antennas

An LPDA (log-periodic dipole array) antenna is an assembly of dipole elements, with each dipole element on the boom longer than the one in front of it, giving the antenna a triangular shape (see Figure 4). Because it has many dipole elements of different lengths, an LPDA antenna can transmit and receive across a wide range of frequencies.

Many people (including Wilson Electronics themselves) refer to LPDA antennas as “Yagi antennas.” A Yagi–Uda antenna (named for the two Japanese engineers who invented the design) looks similar to an LPDA, but there are important technical differences between the two. The biggest difference is that the dipole elements on a Yagi antenna are the same (or nearly the same) length along the entire length of the boom (see Figure 5). With a Yagi antenna, only one pair of elements transmits and receives radio signals; the other elements amplify the signal and reflect it to the active pair of elements. Because only one pair of dipole elements is active, the Yagi antenna design works on a single frequency or a very narrow range of frequencies.

So, to sum up:

  • A Yagi antenna is designed to operate in a very narrow frequency range with higher gain. It’s a narrow-bandwidth antenna.
  • An LPDA antenna is designed to operate in a wide frequency range. with lower gain. It’s a wide-bandwidth (or “wideband”) antenna.

A cellular Yagi antenna can only operate in a narrow frequency range. This makes a Yagi antenna ideal for public safety systems or other applications where the user only needs to amplify a specific set of cellular frequencies.

Most commercial and residential cellular signal boosters, however, need to be able to operate across a wide range of frequencies (700–2200 MHz). If a directional antenna is called for, it’s best to use an LPDA wideband antenna.

Oh, remember the old TV antenna, back in Figure 2? If you look at it again, you’ll see that it combines a Yagi antenna for picking up higher UHF frequencies (the narrow, “sawtooth” boom in front, with equal-length dipole elements) with an LPDA antenna for picking up lower VHF frequencies (the wider dipole elements spreading out behind the Yagi).