FAQs - Antenna Properties

What is F/B?

F/B stands for Front-to-Back Ratio. It is the ratio (in dB) between the forward gain to the gain off the rear of the antenna. The forward gain is the peak gain on the main lobe of the antenna. The gain off the rear may be defined as the gain at exactly 180 degrees from the main lobe, or it may be defined as the average or peak gain from 90 degrees to 270 degrees from the main lobe. The second definition of rear gain is the best to use. A F/B of 10-15 dB is considered fair or poor. A F/B of 15-20 dB is considered good, and F/B of 20-30 dB is very good. F/B above 30 dB is superior!

What Antenna Polarization should I use?

Most Point-to-Multipoint WLAN systems use V-Pol (vertical polarization). This allows the use of inexpensive vertical omnidirectional antennas. Higher-density areas are beginning to use more H-Pol (horizontal polarization) antennas for PtMP. Point-to-Point (backhaul) systems may use either vertical or horizontal polarization as long the same polarization is used at each end. Horizontal polarization may perform slightly better when transmitting through a forested area, otherwise there is very little difference in propagation effects. Most standard Telex Wireless antennas are vertical polarization except -H versions of the dish antennas and the 2445AA sector antenna. The 2401 patch antenna may be mounted for either polarity.

Will CP (circular polarization) help my system?

Normally, a WLAN or WISP has a set of channels or frequency sets that are either vertically-polarized or horizontally-polarized, or some of each. Since the CP antenna responds (theoretically) equally to either polarization at a level of 3 dB down from maximum signal, there is not much reason to add CP to a system that already has vertical, horizontal or both polarizations. This won't gain additional spectrum for the WISP. Polarization discrimination is generally a good thing, and CP antennas have no discrimination against linear-polarized signals or interference. However, CP antennas do work well in situations where the polarization is not pure vertical or pure horizontal, such as in downtown areas with lots of multiple reflections from buildings, airborne applications, over-water systems and indoor applications where the client antenna can be either vertical or horizontal or anywhere in between (such as a laptop or PDA antenna). The Telex 2405 ceiling-mount antenna works great in these indoor situations.

What is the Half-Power beamwidth?

In a radiation pattern cut containing the direction of the maximum of a lobe, the angle between the two directions in which the radiation intensity is one-half the maximum value". The Half-power beamwidth is also commonly referred to as the 3-dB beamwidth. Beamwidth typically decreases as antenna gain increases.

What is VSWR?

VSWR stands for Voltage Standing Wave Ratio. It is the ratio of the maximum/minimum values of standing wave pattern along a transmission line to which a load is connected. VSWR value ranges from 1 (matched load) to infinity for a short or an open load. For most WLAN antennas the maximum acceptable value of VSWR is 2.0. VSWR of 1.5 or less is excellent. This is approximately the same as a Return Loss of 14.5 dB. What this means is that most of the signal from the transmitter to the antenna is being radiated. (96% radiated and 4% reflected) A VSWR of 2.0 (return loss of 9.5 dB) means that 90% is radiated and 10% reflected.

What is a Yagi antenna and how is it different from a panel?

A Yagi-Uda antenna array, commonly called a Yagi, is made up of linear wire or rod-type elements, each having a length of approximately 1/2 wavelength. These elements are arranged in a row, with each element parallel to each other. The rear element in this array is called the reflector. The second element is the driven element, which is connected to the transmission line, and all other elements in front of the driven are called directors. The gain of a single Yagi antenna ranges from about 6 to 20 dBi, depending upon the length of the array. Multiple Yagi antennas may be connected together side by side in larger arrays, which may have gains from 10 to 26 dBi or higher. A single Yagi antenna has a long, narrow profile and UHF yagis are usually enclosed in radome tubes to protect them from the environment. Gain, sidelobe and F/B performance of a Yagi antenna is very similar to a panel antenna. The main differences are the appearance and that single Yagi antennas have approximately the same beamwidth in each plane, while a panel may be designed for different beamwidths in each plane. For more information on Yagi antennas see D. Jefferies' web site.

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Technical FAQ Categories

Basics, Access Point, Antenna Properties, Canadian RSS-210, Client Equipment, FCC Part 15, Point to Multi-Point, Point to Point, Pre-installation and Site Preparation, Radio Propagation, Reference Material, Troubleshooting.