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Antenna Detail, Path Analysis and System Information

Antenna Background

To get the carrier signal from "here" to "there" involves an antenna system. The height, type and efficiency of this system will determine the distance between transmitting and receiving stations. Wireless and light are alike is many ways, especially in propagation. They each tend to travel in a straight path called "line of sight" (but radio travels futher than light due to atmospheric bending). Because the earth is round, the line of sight distance will vary with height Above Ground Level (AGL). At 8 feet AGL, the radio line of sight is a little over 4 miles. At 10 feet AGL, it is about 4.7 miles. At 25 feet AGL, it is a bit more than 7 miles. The formula for calculating antenna height or distance is:

h =d2 /2 or d =Ö 2h(h=height in feet, d=distance in miles)

 Fresnel Zone Another consideration is keeping the lower 0.6 Fresnel (pronounced Fra’-nel) Zone in the clear to prevent echoes or multipath from reducing the received signal. Multipath is to wireless as "ghosting" is to television. The lower part of the 0.6 Fresnel Zone is like a "sag" or widening of the radio beam at the middle of the path. Its width (900 MHz) is about 44 feet at 2.5 miles for a 5-mile link and about 72 feet at 7 miles for a 14-mile link. The lower 0.6 Fresnel Zone, as well as the radio center line between the antennas must clear all obstacles for best results. The antenna should be about 97 feet AGL at each end of a 14 mile unobstructed path over "flat" ground.

The 900 MHz formula for calculating the lower 1st Fresnel Zone is:

F1 = 72.1 Ö d2/2fd F, =lst Fresnel Zone radius in feet

d=midway distance in miles, and

f= frequency in GHz.

Then the 0.6F = F1xO.6

 Space Attenuation Wireless signals get weaker as the distance increases. For 900 MHz, the attenuation is
-96 dBm for the first mile and increases by 6 dBm each time the distance doubles. Two
(2) miles would equal -102 dBm, 4 miles would equal -108 dBm, 8 miles would equal
-114 dBm and 16 miles would equal -120 dBm. These numbers are important in
determining how strong the received signal will be and if a proposed link is practical. Decibels The term dB is a logarithmic ratio that compares (in this case) two power levels. An easy way to appreciate the magnitude of the ratio is to remember that a loss of I dB equals a loss of 0.2 (xO.8 of the whole); a loss of 3 dB equals 0.5 (xO.5 of the whole); a loss of 6 dB equals 0.75 (xO.25 of the whole); and a loss of 10 dB equals 0.9 (x0.1 of the whole). An increase is the reciprocal: I dB equals times 1.25; 3 dB equals times 2; 6dB equals times 4; and 10 dB equals times 10. The term dBm is the same ratio but related to .001 watt. An example: a transmitter with a 1-watt power output is +30 dBm.

Examples:

0.001 watt = " 0"dBm
0.01 watt (x10) = +10 dBm
0.1 watt (xl0 again) = +20 dBm
I watt (xl0 again) = +30 dBm

 Types of Antennas There are many options in choosing a proper antenna. The most common external antenna is a, Yagi-Uda or simply Yagi. The Yagi is a directional antenna that has a relatively narrow transmitting and receiving angle. It can be mounted for either vertical or horizontal polarity and is offered with several gain figures. The higher the gain, the narrower the angle. Another popular antenna is a panel type used mostly with cellular systems. Then, there are times when it is necessary to install a non-directional or omnidirectional antenna. These also come in various gains, with the 6 dB a good choice A system gain (transmitter output + antenna gain - transmission line loss) limit Is usually imposed to reduce range and interference. This limit is termed Effective Isotropic Radiated Power or EIRP. In the U.S. the value will be +36 dBi or about 4 watts. In some European countries this limit is +20 dBi (100 mW). It means the total power level in dBm of the radio plus the antenna gain must not exceed +36 dBm, so the antenna gain figure will be important in meeting the EIRP technical requirement. Coaxial Cable How the connection from the antenna to the transceiver is accomplished can be as important as the antenna. If much of the wireless signal is lost before it reaches the antenna, or the radio, the system will function poorly, if at all. The transmission line of coaxial cable must be chosen to match the 50 ohm antenna impedance and to limit the losses due to the cable length. A reasonable choice for up to 100 feet of cable is BeldenTM 9913 with a loss figure of about 4.5 dB/100 ft. (NOTE: The loss in dB is linear with distance, so the drop for 50 ft would be 2.25 dB.] Data Quality The radio requires -95 dBm of "clean" received signal to provide a minimum Bit Error Rate (1 error out of 1,000,000 bits sent). It is wise to have some "insurance" or fade margin so the goal to have adequate signal strength at the radio of at least -80 dBm allowing 15 dB of receiving margins. System Calculations Putting all of the above calculations together, it is possible to determine the viability of a wireless link before installing the equipment. The process is simply to add the gains, subtract the losses with the goal being a received signal better than -80 dBm.

EXAMPLE

Suppose that it is necessary to send data 5 miles over a "flat" terrain. The AGL of the receive and transmit antenna should be between 45 and 50 feet for best results. NOTE: 15 ft AGL can work but signal losses and ground reflections will reduce the overall performance.

The radio transmitter RF power is +30 dBm. The receive and transmit antennas each have a gain of +6 dBi. By adding those numbers the total system gains +42 dB. Five miles of space loss is approximately -110 dB. With 2.7 dB of coaxial cable losses (60 feet) at each of the transit and received ends, the total loss is about -116 dB (rounded up).

Then -116 dB and +42 dB combined will equal -74 dB, which is the calculated received signal strength. This -74 dBm is better by 6 dB (four times better) than the minimum design -80 dBm signal, therefore the path should work well.

If the AGL is lowered to 15 feet at each end, then an obstruction loss of at least 10 dB will occur. Now the received signal is - 84 dBm or about 60% weaker than the design minimum. The system will still work but it is more susceptible to interference and fade.

 Site Details There are some important details that must be considered before the antenna is installed. Check the proposed antenna site for other radio transmitting antennas, your antenna must be as far as possible from them. If the antenna is a directional antenna, Yagi or panel type, it should be pointed away from it. If an omnidirectional antenna is used, it should be mounted above or below the field of other transmitting antennas. Stayaway from locations that are sites for TV stations, RADAR stations, paging systems which are high powered, pulsing modes of RF energy and other spread spectrum systems if possible Taking these precautions into account can save a lot of time and effort. The external antennas and fittings are vulnerable to many problems and they must be installed properly for easy repair. If the antenna system fails, the wireless system fails. Mast/Tower If it is necessary to provide a tower to elevate the antenna, then the structure must also safely support the weight of any service personnel. When a mast is used, it is desirable that the top be reachable with a self-standing ladder for antenna orientation. Lightning protection is very important and the National Electrical Code and/or other codes ought to be consulted for proper grounding procedures, especially on tall buildings. Consider a coaxial cable lightning arrest as well as an antenna that has a grounded matching network for added protection. [NOTE: The PolyPhaser Corp. offers an excellent application guide regarding lightning protection.]

The antenna is usually mounted by "U" bolts, with polarization either vertical or horizontal. Vertical is typical for wireless, although sometimes horizontal is used to reduce interference. Either will work, but the polarity must be the same at each end of the link for successful operation. Point the antenna as near as possible toward the far end of the path.

When routing the coaxial cable, leave a service loop at the antenna so there will sufficient length of cable to replace a faulty connector, when necessary. Secure the coax so that there is no mechanical stress at the antenna connection. Follow the super structure with the cable to its base to the building, If the cable requires suspension from the base to the building, use a stranded wire to support the cable weight. (The support will prevent a migration of the cable’s inner conductor to the shield.) Upon entering a building, leave a drip loop so that water will not follow the cable inside, If the cable lays on a roof or the ground; protect it with a conduit to guard against crushing. Inside, at the radio, use a short flexible patch cable, with the appropriate fittings, from the radio to the main coaxial run for stress elimination at the radio’s SMA connector. An electrical test should be performed.

After the test, waterproof all outdoor fittings, the ground connections, and the cable entry points into the building.

 Grounding Make sure that the antenna system is grounded to protect against voltage surges, built up static charges, and lightning strikes:

Use No. 10 AWG (5.3 mm) copper or No. 8 AWG (8.4 mm) aluminum wire or larger as a ground wire. Secure antenna lead-in and ground wires with insulated standoff insulators spaced 4-6 ft. (1.2 - 1.7 m) apart. Mount the antenna lightning/static discharge unit as close as possible to where the lead-in enters the building.

 Testing When the mechanical installation is completed, a Voltage Standing Wave Ratio "VSWR" test should be done to determine that all electrical connections are correct and that the antenna is properly matched. A test instrument called an inline wattmeter is connected between the radio and the coaxial cable going to the antenna. The transmitter is turned on and its output power is measured on the meter. Next, the reflected power or "VSWR" is read, it must be less than 5% of the transmitter power to be acceptable. Each radio site will be checked in this way. If the "VSWR" is acceptable and the antennas are; pointed correctly, the link is ready to be tested using data. The data can be two computers, two terminals, or any device that will send back "known" information when polled". [Or just put the system on line and see if it works!] Interface RS/EIA232 data port is wired DCE to accept DTE with the transmit, receive data and RTS lines being active. The data flow and transmit functions are controlled internally by the radio, in a half duplex mode.

Using the VOICE OPTION, if it is installed, is a push to talk and release to listen operation, the same as in other two-way radio communication. A bonus with this option is the convenience of easy transmitter keying for RF testing.

 Test Equipment and Accessories To achieve a satisfactory performance with an external gain antenna, it is necessary that the accessory pads, transmission line, connectors, filters, etc., and the antenna function properly. A common RF test instrument for testing is the inline VSWR wattmeter. A suitable low cost meter is the Comet 900N.

A Bit Error Rate Test set maybe required when a high quality data, circuit must be guaranteed. Also a Volt Ohm Meter, VOM, is an important item on every test equipment list.

Miscellaneous "gender menders" and data cables are also necessary. The radio has a DB9F RS/EIA574 interface and most async data equipment use a DB25F connector to an RS/EIA232 port.

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