Frequently Asked Questions about Wireless Antennas

Wireless Antenna Properties
Wireless Client Equipment
FCC Wireless Rules and Regulations
Wireless Antennas in Point to Multi-Point
Wireless Antennas in Point to Point
Pre-Installation and Site Preparation
Radio Propagation
Troubleshooting Wireless

Wireless 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 wireless antenna. The forward gain is the peak gain on the main lobe of the wireless 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 Wireless LAN systems use V-Pol (vertical polarization). This allows the use of inexpensive vertical omnidirectional wireless 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 wireless LAN or wireless ISP has a set of channels or frequency sets that are either vertically-polarized or horizontally-polarized, or some of each. Since the Circular Polarized wireless 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 wireless ISP. Polarization discrimination is generally a good thing, and CP wireless antennas have no discrimination against linear-polarized signals or interference. However, CP wireless 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 2405 circular polarized, ceiling-mount wireless 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 wireless LAN 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 wireless 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 Antenna?

A Yagi-Uda antenna array, commonly called a Yagi Antenna, 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 Yagi Antennas 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 Antenna may be designed for different beamwidths in each plane.

Wireless Client Equipment

For a WISP system, what wireless antennas should I use for my clients (CPE)?

This depends upon the hub antenna, cable type and length, distance, data rate and terrain. You should test your system first before a final wireless antenna selection. For WISP systems using +36 dBm EIRP at the AP and clear LOS, use the following table as a guide:

How can I route coaxial cable into a customer's house?

Whatever you do, do NOT drill through his roof. This will ultimately cause leaks. Most installers route the cable to a wall location below the eaves and drill a hole just large enough for the cable to enter. Ideally, the inside coaxial connector should be installed after the cable is installed. Seal the wall entry from water using a silicon-based sealant. The best installation uses existing holes or a basement wall entry, however sometimes this is not available or practical.

FCC Wireless Rules and Regulations

How much power can I transmit on a 2.4 GHz, 10 dBi omni antenna and still be legal?

The FCC regulations for Point to Multi-Point allows only 36 dBm (4 watts) EIRP. This is 30 dBm (1 watt) into a 6 dBi antenna. If you use a 10 dBi wireless antenna, you must limit your transmitter (or amplifier) to 26 dBm (10 + 26 = 36 dBm). For a 14 dBi panel wireless antenna, this allows a 22 dBm transmitter (or amplifier). Power is measured at the antenna connector, so subtract any cable loss between the amplifier and the antenna. Refer to the following table:

Can I use any wireless antenna of my choice for my Access Point or CPE antenna?

Yes, up to the highest gain antenna specified in the FCC certification information or the product literature that accompanies the device. (See FCC 04-165 adopted July 8, 2004, 15.204(c)) Those people in countries other than the US will need to consult their own regulations. If you are not sure if the antenna that you plan to use is certified or authorized with the radio system, ask the radio or antenna manufacturer/vendor. If you have the radio FCC ID, you can check on the FCC web site for certification information. Some older certifications are not available on this site.

How much power can I transmit with in my Point-to-Point system?

According to FCC regulations, 2.4 GHz Part 15.247 point-to-point transmitters may use a 30 dBm transmitter with a 6 dBi antenna. For a 3 dB increase in antenna gain, the transmitter power output must be reduced by 1 dB. Power is measured at the antenna connector, so subtract any cable loss between the amplifier and the antenna. Refer to the following table.

Is the Customer or Client (CPE) system considered Point to Multi-Point or Point to Point?

If the CPE system (or Subscriber Unit - SU) only talks with the POP/AP and is at a fixed location, then it is considered to be PtP and can use power and antenna gain associated with PtP systems, as shown below. (This has been verified by FCC Certified systems using a 26 dBm radio and a 17 dBi antenna) If a CPE system is part of a mesh network, then it is considered PtMP.

Should I use 2.4 GHz, 3.65 GHz or 5.8 GHz for my WLAN or WISP system?

Currently, most systems use either IEEE 802.11 or 802.11b operating between 2.4 and 2.4835 GHz. As these frequencies become more congested, the U-NII Band 3 at 5.725 - 5.825 GHz (IEEE 802.11a) will be used more. 5.8 GHz also offers data transmission rates greater than 11 MB/s. However, more antenna gain will be necessary at 5.8 GHz for the same distance on 2.4 GHz. 5.8 GHz will have a smaller Fresnel zone, so there may be certain advantages when shooting a signal through a tight space between trees or buildings. The new 3.650-3.7 GHz band is ideal for WISP use as there is no interference from home gateways, microwave ovens or cordless phones. Also, an easy-to-obtain Part 90 license will be required for users in this band. The WCS and MMDS frequencies between 2.1 and 2.7 GHz are also available to FCC-licensed users. (See IEEE 802.16a)

What frequencies are available to WLAN outside the US?

The 2400-2500 MHz band is used worldwide. There are certain channels within this band that are allocated to certain regions, however. The 5725-5825 MHz band is used only in the US with 4 watts EIRP. Europe uses the HiperLAN frequencies of 5470-5725 MHz outdoors with 1 watt EIRP. The indoor band at 5 GHz is 5150-5250 MHz in US/Japan and 5150-5350 in Europe. There are also frequencies between 3.4 and 4.0 GHz which are available in Canada, Asia and Africa and the Far East. (See IEEE 802.16a)

If I obtain a Ham Radio license, can I run more power on my WISP?

NO! Ham Radio is licensed under FCC Part 97 as a not-for-profit service, which provides communications for public service, experimenters and hobbyests.

How much power can I transmit on a 5.3 GHz 10 dBi omni and still be legal?

The FCC regulations for PtMP and PtP allows only 30 dBm (1 watt) EIRP in the UNII-2 band. This is 24 dBm (250 mW) into a 6 dBi antenna. If you use a 10 dBi antenna, you must limit your transmitter (or amplifier) to 20 dBm (10 + 20 = 30 dBm). For a 15 dBi panel antenna, this allows a 15 dBm transmitter (or amplifier). Power is measured at the antenna connector, so subtract any cable loss between the amplifier and the antenna.

How much power can I transmit on a 7 dBi omni on 5.8 GHz and still be legal?

The FCC regulations for PtMP allows only 36 dBm (4 watts) EIRP in the UNII-3 band. This is 30 dBm (1 watt) into a 6 dBi antenna. If you use a 7 dBi antenna, you must limit your transmitter (or amplifier) to 29 dBm (7 + 29 = 36 dBm). For a 15 dBi sector antenna, this allows a 21 dBm transmitter (or amplifier). Power is measured at the antenna connector, so subtract any cable loss between the amplifier and the antenna.

How much power can I legally transmit on a 23 dBi panel at 5.8 GHz?

Wireless Antennas in Point to Multi-Point

How do I know which wireless access point antenna to select for my outdoor WLAN / WISP?

This depends on how your subscribers or clients are located with respect to the access point and what type of terrain is in between. You can place an omni-directional antenna such as our 2439 (10 dBi gain) near the middle of your group of clients at a hub (Access Point) location. This works best if your facilities/customers are no more than 6 miles (9.5 km) from the hub and unobstructed by hills, trees or buildings. You may also select to use several sector antennas at an Access Point location. Our model 2443 (12 dBi 120 degree panel) or model 2444 (14 dBi 90 degree panel) wireless antennas work great for distances up to 12 miles (19.4 km) with clear LOS or up to 6 miles with some trees and buildings in the path. Greater distances may be obtained by using tower-mounted amplifiers with antenna heights above 100 feet HAAT. Whichever wireless antenna you choose, please make sure that it is Industry Canada or FCC certified with your radio!

How high should I place my outdoor wireless Access Point antenna?

This depends upon a lot of factors. If you have a building with roof access, this is usually the best option, since the feedline losses may be minimized if the equipment can be placed near the antennas. A minimum height is usually around 75 feet. This places the antennas above most trees. This height will also give a radio horizon of approximately 12 miles, assuming flat terrain. If you have taller trees, or tall buildings nearby, you may wish to use an antenna height of 200 feet or more. This gives a radio horizon of 14 miles. As towers may fall under local zoning ordinances, you may also wish to consider water towers, grain elevators or utility poles as other options. Placing Aceess Point antennas higher than 100 feet exposes them to greater amounts of interference, more feedline losses, zoning restrictions, FAA lighting requirements, and larger cell areas. Existing towers may be located using these sites - TelecomSiteSource, FCC Antenna Structure Registration and Wireless Radio Tower Locator.

What are the advantages of using Sector Antennas instead of an Omni-Directional Antenna?

There are several good reasons to use sector antennas:

• More capacity - By using 3 sector antennas on DSSS channels 1, 6 and 11 with 3 AP's, you can triple the number of clients in a given area.
• Better signal levels - Sector antennas usually have more gain than omni's and can be mechanically downtilted to focus where the users are. This results in fewer retries and less packets lost. A WIPOP sector antenna will pay for itself if just one customer did not need an amplifier.
• Channel Re-Use - Because the sector antenna can be downtilted, the signals are not thrown out to the horizon. This allows that channel to be re-used several miles away at a different cell site.
• Eliminate interference - Because a sector antenna is directive and usually has good front-to-back (F/B), it can reduce or eliminate interference from sources that are behind the sector antenna.

Example of channel reuse

How do I hook up four 90 degree sector antennas on one tower?

Conventional thought says that there aren't enough non-overlapping 2.4 GHz DSSS channels to put 4 channels on one tower. Usually, panel antennas with high F/B are selected, and channel 1 antennas are placed on opposite sides (e.g. North & South) and channel 11 antennas are also placed on opposite sides (e.g. East & West). If separate access-points are used for all 4 antennas, the isolation may need to be increased between antennas on the same channel by spacing them farther from the tower face or on opposite corners of a building. FHSS systems may use separate frequency sets on each panel without problems.

However, there is new evidence that supports the use of DSSS channels 1,4,8 and 11 on the same tower. A white paper from Cirond Networks discusses this possibility. Also, check out this article from ExtremeTech. Isolation will need to be increased between antennas in this case by spacing them farther from the tower face, or by vertical separation of 10 feet or more.

What wireless antenna should I use to cover a small campus area of a few buildings?

If your coverage area is small with distance to the hub of less than a mile (1.6 km), a small omnidirectional antenna such as our 2437AA (7.5 dBi gain) may be used. If the AP will be located on the edge of the campus, a 120 degree sector antenna such as our 2443AA 12 dBi panel antenna may be used.

What wireless antennas should I use for my clients (CPE)?

This depends upon the hub antenna, cable type and length, distance, data rate and terrain. You should test your system first before a final wireless antenna selection. For WISP systems using +36 dBm EIRP at the AP and clear LOS, use the following table as a guide:

Wireless Antennas in Point to Point

What wireless antennas should I use for Point to Point wireless data transmission?

Directional antennas should be used for point-to-point wireless transmission. The type of directional antenna depends upon the power output, cable type and length, height, distance, data rate and terrain. We recommend the use of a range table to estimate the wireless antenna types. Whichever wireless antenna you choose, make sure that it is FCC certified with your radio!

Is the Customer or Client (CPE) system considered Point to Multi-Point or Point to Point wireless?

If the CPE system (or Subscriber Unit - SU) only talks with the POP/AP and is at a fixed location, then it is considered to be Point to Point wireless and can use power and antenna gain associated with Point to Point wireless systems, as shown below. (This has been verified by FCC Certified systems using a 26 dBm radio and a 17 dBi antenna) If a CPE system is part of a mesh network, then it is considered Point to Multi-Point.

How do I perform a Point to Point wireless site survey?

Initially, create a path profile using one of the various mapping programs. If LOS and Fresnel zone clearance seems good, check for trees and other unusual obstacles to LOS. A good way to check this is to place a person at each end of the path with a high-powered flashlight and a cell-phone. While talking with each other, flash the light so that the other person can see it. UHF hand-held radios (FRS or commercial frequencies) also work well to determine LOS. Use 1 watt radios for up to 4 miles and 5 watt radios for up to 15 miles. If results look promising, place an AP at one end and a CPE at the other and try connecting using 19-24 dBi grid or panel antennas. (Do not swing both directional antennas at the same time!) Look for interference at each end by using a spectrum analyzer and both vertical and horizontal polarized antennas. If you have Teletronics radios, here is a neat site-survey tool. There are also professional consultants (e.g. Cyber-Doctors) that can perform wireless site surveys for a fee. Wireless site survey tools are available on the AeroNet wireless broadband site.

How much power can I use on the new 3.65 GHz band?

What is the Maximum Distance for a Point to Point wireless link?

The maximum distance for a standard 802.11b Point to Point (or Point to Multi-Point) path is approximately 12 miles. This is primarily due to timing issues in the 802.11b firmware. Other operating systems, such as KarlNet TurboCell, Orinoco COR or StarOS can overcome this limit and produce links up to 70 miles, depending upon terrain.

Pre-Installation and Site Preparation

How do I calculate my network link budget?

You should perform a network link analysis for every Point to Point link, and for a sampling of your Point to Multi-Point links. The analysis should be calculated for both signal directions. There are many online calculators for link analysis. Some of these are Wireless Network Link Analysis from Green Bay Professional Packet Radio, Wicklewood & Wymondham Calculators, and RFProp Software by Colin Seymour G4NNA . A basic explanation of link budget calculations can also be found in a white paper from Intersil. NOTE: Some WLAN radio manufacturers use the EIRP output power instead of true "radio output power" in their advertisements! Make sure that you obtain the TRUE or CONDUCTED radio output power from the FCC Test Report to use in these calculations.

What RF cable should I use for my wireless antenna installation?

We recommend Times Microwave LMR-series , Andrew Heliax , Belden RF-series or NK Cables USA cable for the lowest losses. Times LMR-1200 or Andrew LDF5-50A Heliax will produce 2.3 dB loss in a 100 foot run. Times LMR-400 and Belden RF400 will produce 3.3 dB loss in a 15m (50') run at 2450 MHz. Belden RG-213/U cable may also be used for runs of less than 7.5m (25'). Total attenuation should not exceed approximately 3 dB. Cables and connectors may be ordered through TESSCO. Low-cost pigtail cable assemblies are available from ALLCOM and Cable X-perts. Check out the neat Technical Articles on the Times Microwave site. LAN Administrators and ISP's should check with the manufacturer of the WLAN system hardware before adding new cables and connectors!

What towers should I use for my wireless Access Point antennas?

Trylon "Titan" tower models T200-72 and T200-96 are very popular and inexpensive. The new Rohn SCL towers are also available in heights from 40' to 100'. Rohn SSV series are recommended for heights of 100-150 feet. Check out AN Wireless towers also. As towers may fall under local zoning ordinances, you may also wish to consider water towers, grain elevators or utility poles as other options. Placing wireless access point antennas higher than 100 feet exposes them to greater amounts of interference, more feedline losses, zoning restrictions, FAA lighting requirements, and larger cell areas. Existing towers may be located using these sites - TelecomSiteSource, FCC Antenna Structure Registration and Wireless Radio Tower Locator. Grain elevators may be located using this site - Grain Elevator Locator.

Can I mount an Omni Directional antenna on the side of a tower?

Ideally, an omni antenna should be placed on the tip of a mast above a tower. This will give a nice circular radiation pattern. If your tower is 300 feet high and you wish to place the omni directional antenna at the 100 foot level, you will have to attach the omni directional antenna to a stand-off bracket at some distance away from the tower leg. With a spacing of 6" or even 12", you will have many lobes and nulls created by the reflections from the tower. Also with close spacing, there is a greater chance that these reflections will produce an upwards or downwards beam tilt. The depth of these nulls can be reduced by a greater spacing, such as 5 feet. Make sure that your tower can handle the extra wind load of these stand-off brackets, and that the omni directional antenna is parallel to the tower legs at all times.

How do I perform a Point to Point wireless site survey?

Initially, create a path profile using one of the various mapping programs. If LOS and Fresnel zone clearance seems good, check for trees and other unusual obstacles to LOS. A good way to check this is to place a person at each end of the path with a high-powered flashlight and a cell-phone. While talking with each other, flash the light so that the other person can see it. UHF hand-held radios (FRS or commercial frequencies) also work well to determine LOS. Use 1 watt radios for up to 4 miles and 5 watt radios for up to 15 miles. If results look promising, place a wireless access point at one end and a CPE at the other and try connecting using 19-24 dBi grid or panel antennas. (Do not swing both directional antennas at the same time!) Look for interference at each end by using a wireless spectrum analyzer and both vertical and horizontal polarized antennas. If you have Teletronics radios, here is a neat wireless site survey tool. There are also professional consultants (e.g. Cyber-Doctors) that can perform wireless site surveys for a fee. Wireless Site Survey tools are available on the AeroNet wireless broadband site.

What connectors does you Wireless Antennas use?

We can supply antennas with almost any connector, or even without a connector for OEM applications. Standard connectors are Type N plug, Type N Jack, TNC, RP-TNC, SMA, RP-SMA, MC-Card, and MMCX. Cable size dictates which connectors may be used on certain antennas. LAN Administrators and ISP's should check with the manufacturer of the WLAN system hardware before adding new cables and connectors!

What do you recommend for weatherproofing connectors?

We recommend 3M vinyl electrical tape for most applications. Apply one layer of high-quality 3M (88+) tape, then one layer of mastic, then a final layer of 3M tape. (Hint: Apply the first layer of tape with the sticky surface out) Do not use any spray-on or brush-on weather-proofing material, as this is VERY difficult to remove. Times Microwave supplies both vinyl mastic weatherproofing kits as well as 3M cold-shrink weatherproofing kits. See the LMR hardware accessories at the Times Microwave LMR-series web site. Andrew also supplies cold-shrink weatherproofing kits and WeatherShield snap-on connector housings for their Heliax cables. See page 472, 499 and 509 of their catalog at the Andrew Heliax web site.

What do you recommend for antenna grounding & lightning protection?

This depends upon the type of installation. For tower-mounted wireless antennas, there should be a good ground wire (#2/0) attached between the tower base and a single-point earth ground. (There is no need for a separate ground wire running along the tower!) For roof-mounts, the mast should be grounded to the steel structure of the building if possible. If no connection to the building is possible, then a large diameter wire may be run directly to earth ground. Lightning arrestors should be added to the coax cable between the wireless antenna and the amplifier or other radio equipment unless built-in to the amplifier or radio. Otherwise, they should normally be installed where the coax enters a building. For more information, see technical documents at PolyPhaser. Data lines running from the wireless antenna must also be protected from lightning surges. We recommend the Tripplite and APC ProtectNet line of surge suppressors. These should be installed where the line enters the house, in a weather-protected area. If you use PoE, then choose a suppressor model rated for T1 service with a voltage-clamp at 75 volts or higher.

How can I check the VSWR of my wireless antenna before and after installation?

The VSWR (Voltage Standing Wave Ratio) of a 2.4 GHz wireless antenna may be checked with most HP/Agilent or Anritsu RF Network Analyzers that have a maximum frequency of 3 GHz. Lower-cost hand-held units are also available from Anritsu and Bird Electronics. The Anritsu S332B Sitemaster / Spectrum Analyzer combo has both VSWR and Spectrum Analyzer features in one unit. It is also possible to use an IFR spectrum analyzer for return loss (VSWR) measurements. The WLAN expert also has VSWR measurement capabilities for PRISM chipset-based cards. Wireless antennas at this frequency may be checked with an attached transmission line no longer than: 25 feet (LMR-400 & 600), or 5 feet (LMR-195 & RG-58). Longer cables will make the VSWR appear much lower than it really is. When testing an wireless antenna before installation, make sure that the wireless antenna is outdoors and pointing away from the ground and any metallic objects. A VSWR of less than 1.5:1 is excellent, and less than 2:1 is acceptable. Most wireless antenna manufacturers spec their wireless antennas for either 1.5:1 or 2:1 across the bandwidth.

Should I use 2.4 GHz, 3.6 GHz or 5.8 GHz for my WLAN or WISP system?

Currently, most systems use either IEEE 802.11 or 802.11b operating between 2.4 and 2.4835 GHz. As these frequencies become more congested, the U-NII Band 3 at 5.725 - 5.825 GHz (IEEE 802.11a) will be used more. 5.8 GHz also offers data transmission rates greater than 11 MB/s. However, more antenna gain will be necessary at 5.8 GHz for the same distance on 2.4 GHz. 5.8 GHz will have a smaller Fresnel zone, so there may be certain advantages when shooting a signal through a tight space between trees or buildings. The new 3.650-3.7 GHz band is ideal for WISP use as there is no interference from home gateways, microwave ovens or cordless phones. Also, an easy-to-obtain Part 90 license will be required for users in this band. The WCS and MMDS frequencies between 2.1 and 2.7 GHz are also available to FCC-licensed users. (See IEEE 802.16a)

Radio Propagation

What Effect does Terrain or Water have on Radio Propagation?

WLAN signal paths on 2.4 and 5.8 GHz must be line-of-sight. There must not be any hills, mountains, large buildings or obstructions for the signal to pass through. Visual line-of-sight is sometimes not enough. The University of Kansas Wireless Network Visualization Project can help you visualize coverage areas. The radio path should also allow for Fresnel-zone clearance. (See Reference 1) A few trees (0.3 - 0.5 dB/meter) are not usually a problem, however a forest will block the signal (300 dB/km). You can check topographic maps of your area at Topo.Com or Topozone.Com. Also, there a few cool 3D tools such as Keyhole's Earthviewer. You can find your exact latitude & longitude for any address at Geocode. Find distance and direction between two points at Indo. Find the elevation at any lat & long from Widders. Path profiles may be created using TopoUSA or Terrain Navigator. There are also several companies who market propagation modeling software. We recommend Wireless Valley, EDX Signal Pro, MicroPath 2001 , Pathloss , CET GRIP or NIR. Free terrain modeling software may be obtained at Radio Mobile or MicroDEM. WLAN paths over water or extremely flat ground may require optimization of antenna height at one end of the path. This is due to specular reflections adding in-phase or out-of-phase. Adjustment of antenna height by 1 to 3 meters should move the signal from a null to a peak. Antenna diversity (with height separation) at both ends of the path should work great. Hint: Place one antenna in a peak and the other in a null. CP wireless antennas have also shown to work well over water. Also, with vertical polarization, you may use the Pseudo-Brewster Angle to eliminate all reflections.

What is the Brewster Angle?

The Pseudo-Brewster Angle (PBA) is the angle at which the reflected TM wave (from a flat earth or water surface) is 90 degrees out of phase and minimum amplitude with respect to the direct wave. "Pseudo" is used here because the RF effect is similar to the optical effect from which the term gets its name. Above this angle, the reflected signal is in-phase with the direct signal. Below this angle, the reflected wave is between 90 and 180 degrees out of phase with the direct wave. Some degree of cancellation takes place in either condition, depending upon the difference between the lengths of the direct path and the reflected path. The largest amount of cancellation occurs near zero degrees, and steadily less cancellation occurs as the PBA is approached from below.

The factors that determine the PBA for a particular location are not related to the antenna itself, but to the ground or water surface around it. Surface conductivity, dielectric constant and operating frequency all affect the PBA of a particular system. The PBA increases with increasing frequency, all other conditions being equal.

At 2400 MHz, over fresh water, the PBA is approximately 6 degrees. At 2400 MHz, over land, the PBA is approximately 17-20 degrees. The signal cancellation effect is more noticeable over water than land because foliage and buildings normally attenuate and scatter the reflected signal over land.

There are several ways to reduce the effect of signal cancellation. The best way is to adjust the height of one antenna, either up or down until the signal moves from the null to a peak. At 2400 MHz, an adjustment of 1 - 3 meters in height should be enough. Another good method is to place the path midpoint on a rough area of land by moving the path endpoints. Changing the antenna polarization from vertical to horizontal may help some of the time. If the PBA can be determined, then placement of the antennas at prescribed heights for a given distance can minimize the reflected signal amplitude.

How can I get my signals through trees?

Trees are a BIG problem in Fixed-Wireless systems. They absorb and scatter RF energy and can prevent a WISP/FWA system from functioning.

• 900 MHz systems can usually penetrate trees better than either 2.4 or 5.8 GHz systems.
• High-power systems and FHSS work better than lower power systems and DSSS.
• Placing the both the AP and CPE antenna above the tree-tops works the best.
• If there is a small LOS hole through the trees, 5.8 GHz signals may pass through, due to the smaller Fresnel distance required.
• Horizontal and 45 degree polarization has shown to have a slight advantage over vertical polarization at 2.4 GHz.
• Using an Access Point at extreme height (>500 feet) with mechanical or electrical beamtilt also helps clients within 5 miles because the signals pass through fewer trees.
• Wet trees are worse than dry trees.
• Pine trees are worse than leafy trees

What effect does rain or fog have on performance?

2.4 GHz signals may be attenuated by up to 0.05 dB/km (0.08 dB/mile) by torrential rain (4 inches/hr). Thick fog produces up to 0.02 dB/km (0.03 dB/mile) attenuation. At 5.8 GHz, torrential rain may produce up to 0.5 dB/km (0.8 dB/mile) attenuation, and thick fog up to 0.07 dB/km (0.11 dB/mile). Even though rain itself does not cause major propagation problems, rain will collect on the leaves of trees and will produce attenuation until it evaporates.

Troubleshooting Wireless

How do I eliminate wireless interference from a new competitor's unlicensed system?

The best way to reduce the interference is to work with him and agree upon polarizations, channels and coverage areas. One of you should use vertical polarization and the other horizontal polarization for PtMP. If you are using DSSS channel 1 for a certain area, then he should use channel 6 or 11. Both of you should be using sector panel antennas with good F/B. If you are using DSSS and he is using FHSS, then you must rely upon polarization, distance and sectorization for isolation between the systems. You may be able to place a null in your AP antenna pattern toward his nearest AP location. If possible, place your AP as far away from his AP as possible, so that your customers can use the directivity and F/B of the CPE antenna for isolation. If both of you are using FHSS, then you should agree to use separate non-interfering sets. Wireless video (ENG) systems should also be avoided. The Broadband Wireless Alliance has offered to coordinate frequencies for interested parties.

What effect does rain and ice have on wireless antennas and cables?

Rain will have no effect upon wireless antennas protected within radomes. The radomes must also have a drain hole for condensation drainage. However, Yagi antennas without radomes are highly vulnerable to rain, as the rain drops will accumulate on the elements and detune the performance. (The droplets actually make each element look longer than it really is!) Water intrusion in coaxial cable will increase the cable losses significantly and raise the VSWR at the transmitter. (See the next question for weatherproofing suggestions) If the link does not come back up after the rain evaporates, then you probably have a water-intrusion problem in the cable. Sometimes you can open both ends of a cable and measure a very small voltage across the center conductor to shield (< 100 mV) if water is inside the cable. This is caused by galvanic action between dissimilar conductors with water as the electrolyte. Ice accumulation on exposed elements can cause the same detuning effect as rain, however it stays around longer. Radomes will protect the radiator from most of these effects, however if the radome surface is very close to the radiator and/or the ice is very thick, then the VSWR may be impaired. Ice can also damage antennas if it falls on the antenna from a higher structure or tree.

How can I check the VSWR of my wireless antenna before and after installation?

The VSWR (Voltage Standing Wave Ratio) of a 2.4 GHz wireless antenna may be checked with most HP/Agilent or Anritsu RF Network Analyzers that have a maximum frequency of 3 GHz. Lower-cost hand-held units are also available from Anritsu and Bird Electronics. The Anritsu S332B Sitemaster / Spectrum Analyzer combo has both VSWR and Spectrum Analyzer features in one unit. It is also possible to use an IFR spectrum analyzer for return loss (VSWR) measurements. The WLANexpert also has VSWR measurement capabilities for PRISM chipset-based cards. Antennas at this frequency may be checked with an attached transmission line no longer than: 25 feet (LMR-400 & 600), or 5 feet (LMR-195 & RG-58). Longer cables will make the VSWR appear much lower than it really is. When testing a wireless antenna before installation, make sure that the wireless antenna is outdoors and pointing away from the ground and any metallic objects. A VSWR of less than 1.5:1 is excellent, and less than 2:1 is acceptable. Most antenna manufacturers spec their antennas for either 1.5:1 or 2:1 across the bandwidth.

How do I check my coax cable assemblies?

The easiest and quickest way is to use a multimeter (ohmmeter). For each cable assembly, touch each of the 2 multimeter probes to the center conductor at each end of the cable. The multimeter should indicate a "short" or less than 1 ohm resistance. Extremely long cables will show more resistance. Also, touch each of the 2 probes to the sheild conductor at each end of the cable. The multimeter should indicate a "short" again. Lastly, touch 1 probe to the center conductor and 1 probe to the sheild at one end of the cable. The other end must be left unconnected. The multimeter should indicate an "open" or greater than 10,000 ohms.

How can I tell if my Access Point antenna is working correctly?

There are 2 main properties that you can check if you have the proper equipment. The first property is the antenna's VSWR (voltage standing wave ratio). See the separate FAQ on how to measure this. The second main property is the radiation pattern. For an omnidirectional antenna, the received signal strength at a client should be similar for all angles at a fixed radius from the AP. Since the client antenna may have directional properties and terrain & obstacles may affect the AP coverage area, the received signal may vary as much as 6 to 10 dB over different paths at a fixed distance from an AP. For a directional or sector antenna, the received signal strength at a client should be at least 15 dB stronger off the front side of the antenna than off the back side at a fixed distance. If it isn't, then the antenna may be defective or damaged. Another way to check to see if an antenna is working is to unplug the coaxial cable from the antenna. If the received signal off the front side of the antenna doesn't change significantly, then the antenna may be defective or damaged. This may also indicate a problem with the cable or connectors too.