All About Omnidirectional Wi-Fi AntennasDecember 24, 2015 by Charles R. Hampton
For a local area network (LAN) to provide good coverage, improved antennas are often a better (and less expensive) option than more access points. Directional antennas are sometimes the better choice, but often omnidirectional gain antennas will work well.
The first article in this series presented some background information on the relationships between Wi-Fi radio frequencies, wavelength, and antenna dimensions. Those particulars are crucial to a full understanding of this article, so if you haven't read part one, or don't remember the details, please review it now.
What is an Omnidirectional Wi-Fi Antenna?
As previously discussed, the only truly omnidirectional antenna is an isotropic source, a theoretical single point source that radiates equally in all directions. Thus, if the radiations could be observed, they would look like a sphere that is very dense at the center point, and less dense as the distance from the center point increases. If such an antenna existed, it might have a purpose in outer space, but would not be very useful on earth.
Here on this planet, antennas that radiate equally well in all directions parallel to the earth's surface are said to be "omnidirectional," and most are vertically polarized. A perfect vertically polarized omnidirectional antenna would emit a radiation pattern in the shape of a torus, with the antenna at the very center. Of course, there are no perfect antennas, and therefore no perfect torus radiation patterns; all real-world antennas have some pattern deficiencies.
Three of the most common vertically polarized omnidirectional antennas are the half wavelength dipole, the quarter wavelength ground plane, and the collinear array. This article will briefly discuss all three in the context of Wi-Fi communications in the 2.4 GHz frequency band, and will show a homebrew example of each.
Half Wavelength Dipole
The most common indoor Wi-Fi antenna is the half wavelength (or half-wave) dipole. Many wireless access points and routers utilize half-wave dipoles contained within plastic cases that protrude from the back or the top of the device. The photo below shows such an antenna removed from the plastic cover.
Laird is a major producer of antennas of all sorts. Their Wi-Fi product line is diverse and extensive, and includes a variety of half-wave dipoles, one of simplest of which is the WXE2400. The specification portion of the datasheet for the WXE2400 is reproduced below. Understanding the specifications is important when comparing one antenna to another.
Some of the entries above were discussed in part one of this series.
- Frequency is the range over which the antenna is designed to perform.
- VSWR stands for Voltage Standing Wave Ratio, which is an indication of the ratio of the amount of power reflected from the antenna back into the coax to the amount of power transmitted from the antenna. The lower the VSWR number is, the better the antenna will perform; perfect VSWR is 1:1. A VSWR of 2:1 indicates that approximately 90% or more of the power delivered to the WXE2400 will be transmitted from it.
- Gain of 3.0dBi is a 3 dB improvement over an isotropic source. The higher the gain number is, the better the antenna should perform.
- Nominal Impedance refers to the input impedance of the antenna. Many antennas have a nominal impedance of 50 ohms and should be fed with coaxial transmission line that also has a 50 ohm impedance.
- Power Rating refers to the maximum amout of power that may be applied to the antenna without risk of damage. Wi-Fi transmitters typically have very low wattage outputs.
- Temperature defines the operating temperature range for the antenna.
- Length is the overall length of the antenna including the cover.
- Drop Test defines the height from which the antenna could be dropped without risk of damage.
A Wi-Fi Dipole You Can Build
Building a Wi-Fi dipole is not especially difficult, providing you have some basic hand tools and aren't too particular about how it looks. A homebrew half-wave dipole for 2.4GHz is shown in the photo below. The actual antenna element should be separated from the vertical portion of the coax by at least 1/4 wavelength (30.5mm.)
Details of the actual element are shown in the following photo. The RG174/U coax is passed through a small hole in the center of the PCB material; the center conductor is soldered to one side, and the braid is soldered to the other side. Normally, the element is mounted with the center conductor on the high side.
A Quarter Wavelength Ground Plane You Can Build
Although rarely used for Wi-Fi applications, quarter wavelength ground plane antennas are popular for other frequencies and are easy to build. A homebrew quarter-wave ground plane for 2.4GHz is shown in the photo below.
Details of the actual element are shown in the following photo. The RG174/U coax is passed through a small hole in the center of the PCB material; the center conductor is used as the vertical element and is approximately 30.5mm long. The coax braid is soldered to the top side of the PCB material.
A collinear antenna is actually an array of dipole antennas stacked one above the other so that they are all in a straight line, i.e., "co linear."
Specification items that were explained in the dipole section of this article will not be repeated here.
- Input Return Loss (S11) describes how much power is reflected back into the coax from the antenna. Note that the loss is expressed as a negative number, and thus the larger the (negative) number, the better the antenna's performance will be. VSWR is similar and may be easier to understand; see the explanation under the dipole section of this article.
- Pole Diameter is the range of pole sizes that the antenna mounting hardware will accommodate.
- Rated Wind Velocity is the maximum wind speed that the antenna is rated to withstand.
- Vertical Beamwidth is the width of the radiated signal in the vertical plane.
- Electrical Downtilt is related to vertical bandwidth, and is used to incline the antenna slightly toward the ground as opposed to directly toward the horizon.
- Weight is the weight of the antenna and mounting hardware; it is not the shipping weight.
- Diameter is the diameter of the cross section of the antenna.
- Wind Loading is the force that will be exerted against the antenna at a given wind speed.
A Collinear You Can Build
(if you're sure you want to)
Plans for Wi-Fi collinear antennas abound on the web. One of the most common designs is shown in the photograph below.
The instructions often provide the exact information shown above and include the suggestion that you can add as many center sections as you want for increased gain. Some builds include a provision for a waterproof housing made from PVC pipe. Apparently anyone can successfully build one...except yours truly.
Multiple different construction attempts did not produce an acceptable result. The tries included the three-element version seen above, a six-element version (with and without insulation,) and finally a nine-element version built with 18 AWG wire, the lower three elements of which are shown below.
Variations were built with and without ground planes. Laird Product Manager, Ted Hebron, provided the following advice regarding ground planes on collinear antennas:
Typically, collinear arrays are designed to be self-resonating structures and will not need a ground plane. In fact, for this type of design a metal ground plane would be seen as a metal obstruction in close proximity to the antenna and as such both the radiation patterns and impedance will be perturbed. The end result may be unexpected nulls in the radiation pattern due to reflections/scattering or mismatch issues because of possible impedance changes. [However,] to make an antenna smaller in length it may be possible to use a ground plane. In this case, the ground plane is intentional and is part of the antenna design.
Antenna Comparison Testing
Proper testing of RF gear...antennas included...requires sophisticated procedures, facilities, and equipment. The author has none of this, and so attempted the following.
- Built a standalone Wi-Fi test access point using an ESP-01 module.
- Positioned the test access point on the opposite side of three plasterboard and one brick veneer walls.
- Connected an ALFA AWUS036NH USB Wi-Fi adapter to a variety of antennas in turn. (See the photo below.)
- Read the signal strength of the test access point on each antenna many times, and averaged the results.
- Created the table shown below.
As you consider the results, remember that because the numbers are negative, lower numbers indicate stronger signals.
Based on the test results, the homebrew PCB dipole and quarter wave ground plane antennas were successful, but the homebrew 9-element collinear did no better than the ground plane. It was disappointing to say the least.
All this does not prove that the popular collinear homebrew design is flawed; perhaps the author's workmanship is to blame, or maybe the testing was was incorrect. Whatever the cause, not one homebrew collinear variant produced good results. Perhaps your efforts will fare better.
In part three of this series, the subject will be directional Wi-Fi antennas. Watch for it.