Chapter 4: Introduction to Omnidirectional Antennas

Let’s begin with a very common antenna called a dipole.  Generally, we mean a “half-wavelength dipole”, but we simply drop the “half-wavelength” modifier.  It is common to describe a dipole antenna as being formed from two conductors (or wires) each of λ/4 length with a small gap between them.  This is often depicted as the ideal structure shown below along with simulated 2D and 3D radiation patterns.

Figure 1.  Ideal Dipole Structure and Ideal Patterns

You can see from Figure 1 that the dipole is an “omnidirectional” antenna.  That is, it radiates energy equally in all directions in one plane.  It is not an isotropic radiator that radiates its energy equally in “all planes”.  The omnidirectional pattern is the azimuth plane pattern, which is a circle.  The elevation plane pattern shows the “focus” that results with the main beam being oriented around the azimuth plane (the floor or horizon).  So, we know the way an omni or dipole radiates its energy just from its orientation and the way these antennas radiate make them an important type of antenna.

A dipole antenna like this, formed by thin wires with the specific length of λ/2, lends itself to closed-form solutions for the radiated fields and for all sorts of operational parameters.  After doing this work, we find that a dipole produces an omnidirectional radiation pattern with an elevation-plane beamwidth of 78-degrees and a gain of about 2.15 dBi.  These parameters are etched on our minds, and they act as foundational specifications for how a real antenna should behave.  They serve as a reference point for the construction and behavior of all antennas.

There are dipoles built in this fashion, often for lab use, but this form factor is difficult to use in a product.  For one thing, the same closed-form solutions for the fields from a dipole tell us that the input impedance (radiation resistance) of this sort of dipole is 73 Ω.  But we typically operate in a 50 Ω system, so there is a mismatch to overcome.  There is the assumption that the dipole is fed in a balanced manner.  That is, the two input terminals are 180° out of phase.  So, a practical dipole would require a balun to be fed properly.  Another issue is that the dipole is λ/2 long and we know that occurs at only 1 frequency corresponding to that wavelength.  We want to be able to use this antenna over a wider range of frequencies – to cover our entire band of operation.  A goal might be to build this antenna into some sort of form factor that is useful.

This is where the design work comes in.  The result is that antennas that attempt to hit 2.2 dBi are often called dipoles whether they are or not and they’ve become a general class of antennas. An example of a PCTEL antenna is shown below in Figure 2.  These types of antennas are known by all sorts of different names, but they closely approximate the theoretical specifications of the dipole antenna described above.

Figure 2.  Example of a PCTEL Dipole Antenna

The dipole antenna radiates energy within a relatively wide beam.  That 78-degree beamwidth describes radiation with respect to the floor or the earth, assuming the antenna is oriented vertically.  What if we want to focus that omnidirectional energy even more?  That would make the elevation plane even more narrow.

We can focus the beam more by deploying an array of elements (dipoles) rather than just a single element.  The idea of an array of antennas can be applied to many types of antennas.  It is frequently applied to dipoles to produce an omnidirectional antenna that focuses the elevation plane further.  Additional antenna elements are “stacked” vertically, and all the elements can be driven together to provide higher gain and more focus.  The result is called a “collinear array”.  Example patterns from a collinear array are shown below in Figure 3.

Figure 3.  Radiation Patterns from a Collinear Array

You can see that the antenna is still an omnidirectional antenna.  All the energy is radiated in one direction in a single plane, the x-y plane.  That might be the plane of the floor or horizon.  The gain is now 5.8 dBi and the elevation plane beamwidth is now 38°.  So by using more elements, we can impact the pattern in an important and useful way making this method very popular.  There are a couple of trade-offs of course.  The first is that the overall antenna is larger in size.  Elements have been added vertically, so the antenna gets longer.  Another trade-off is that there are now side lobes in the pattern as you can see from the elevation plane and the 3D plots.  They can be minimized with element spacing or with more elements (bigger antenna), but they can’t be eliminated completely.  An example of a product that is a collinear array is shown below in Figure 4.

Figure 4.  Example of a Collinear Array from PCTEL

There are other types of omnidirectional antennas too.  Another major class of omnidirectional antennas is the monopole.  Many are similar to a dipole, but they require a ground plane under them to radiate in an omnidirectional manner.  Imagine one of the arms of a dipole deployed over a ground plane and that’s a monopole.  That lends itself to many forms of antennas that begin with the monopole idea.  The radiating element can be bent or augmented to affect the antenna, and the ground plane can too.  The possibilities are endless.  All these things make omnidirectional antennas an extremely important class of antennas.

PCTEL makes many types of antennas, including omnidirectional antennas for several different bands like 4G/5G, WiFi, etc.  There are antennas for different bands, rugged applications, very short omnis, and much more.  Check out all the antennas that PCTEL makes on our website at www.pctel.com.