Antenna Installation

The hardest part of setting up a microwave network is finding a location for the antennas for all the nodes in the network.  Your radio signal path must have a clear, line-of-sight path and a clear Fresnel zone in front of antenna.  This is because microwaves travel essentially in straight lines through the atmosphere due to their short wavelengths.  You can think of Fresnel zones as a series of imaginary cones with their base pointing in the direction of your antennas.  The greatest Fresnel zone losses generally occur when a diffracting object lies within the first 0.6 Fresnel zone.  The losses and gains for all other Fresnel zones are small enough (1 dB or less) to be ignored.  Fresnel zone losses of up to 6 dB can be avoided by ensuring that there are no object large enough to act as diffracting edges within the first 0.6 Fresnel zone.  If a large, rounded object is in your path, losses may exceed 20 dB through several Fresnel zones.  This will force you to mount your antennas on towers or buildings at a significant height.  Unfortunately, microwave frequencies can also be affected by too much antenna height, and the signal can be degraded due to ground reflections canceling out the signal.  Signals will propagate through a few obstructions such as trees or small buildings, and the radio signal will slightly extend over the line-of-sight horizon, but you shouldn't always count on it.

Attenuation By Trees

It's safe to assume that if light can't penetrate a stand of trees, microwave losses will be unacceptable.

Frequency (MHz)       Approximate Attenuation (dB/meter)

432                   0.10 - 0.30
1296                  0.15 - 0.40
2304                  0.25 - 0.50 
3300                  0.40 - 0.60 
5600                  0.50 - 1.50 
10000                 1.00 - 2.00

Microwaves are not effected by the ionized layers in our atmosphere because these layers are higher than the normal line-of-sight transmission of the signals.  Temperature inversions can be a problem though.  This is because as hot air rises, moisture rising with the air causes attenuation of the signal. Lower microwave frequencies are also not as affected by water vapor and oxygen as you might think. Refer to this table, or this Navy site for more information on atmospheric absorption and ducting.  Also consider the temperature effects on paths such as: reflections, refractions, diffractions, transmission "ducts" and even tropospheric reflections and scattering.

Refer to the VHF/UHF/Microwave Radio Propagation: A Primer for Digital Experimenters for a very good overview of radio propagation on microwave radio networks.

Other sources of performance degradation in frequency hopping systems are spectrum background noise, received signal fading, interference from other services in that frequency range, random FM components in the signal, "click" noise resulting from the phase discontinuites between frequency hops, errors in receiver synchronization, or even the wind moving your antennas.

Signal Polarity

The antennas will also have to have the same RF signal polarity.  The polarity of the signal will depend on the direction the actual antenna is positioned.  If it's up/down then the polarity is vertical, if it's left/right then it's horizontal, if it's diagonal (45° usually), then you'll have diagonal polarization.  By not having the same polarity on your network's antennas, you can receive a 20 dB loss of signal strength.  This is an enormous loss, but can also be very useful.  By changing antenna polarization you can help eliminate certain types of radio interference, or allow many antennas in one location.  Horizontal antenna polarization at microwave frequencies will generally provide less multipath and may also provide lower path loss in non line-of-sight situations, but you should always experiment with different polarizations.

Try to also avoid installing your antenna in areas that are located near MMDS or ITFS transmitter sites.  You can query FCC or PerCon frequency databases for the coordinates to transmitter locations in your area.  You can then look up the sites via these coordinates at the Tiger map server.  This is the type of data you will be searching for.  You should also note that MMDS uses horizontal antenna polarization, so if you need to locate your antenna site near one, use vertical polarization.  Other things to look out for at your antenna site are high power PCS wireless cell phone transmissions in the 1.8 - 1.9 GHz band, broadband noise from high power co-located transmitters, harmonics from mobile radio and paging transmitters, and other nearby microwave links.  Don't forget about the intermod problems...

Antenna Tower Grounding

The proper Earth grounding of your antenna tower is essential for lightning protection and static discharge.  Many towers are inadequately grounded by using only a few grounding rounds and large gauge round copper cables.  This is not correct.  The small number of grounding rods are inadequate, and round copper cable has a relatively high impedance to an instantaneous rise in electric current (lightning hit).  Extremely high voltages will develop across these cables and instead of going to ground, these charges will go directly into your building equipment.  A minimum of four ground rods per tower leg with some sort of chemical grounding material should be used.  The chemical grounding material will help to lower the ground rod resistance.  Copper straps should be used to connect the ground rods to the tower due to their low inductance.  In areas with sandy soil and/or excessive wind-generated static, it's advisable to use a more elaborate grounding method.  Most likely a radial grounding system like that found in AM radio.

You should also try to have all you transmission line runs inside your tower column.  This will also help shield them from lightning when it hits the tower.  It's also advised that you securely bond the lines to the tower every 15 meters or so.  Use the recommended bonding kits that your tower manufacture approves of.

Refer to this PDF file from PolyPhaser for very good information on grounding and lightning protection. You can purchase lightning protection from Harger Lightning.

Antenna Beam Tilt and Fill

Antennas mounted on very high towers may need to take into account beam tilt.  Beam tilt is needed when a radiating signal's vertical beamwidth is narrowed (by using high-gain antennas), and the areas near the tower location lose service because most of the signal is wasted by broadcasting into open air.  The beam must be tilted either mechanically or electrically to steer the signal back into its proper location.

Mechanical beam tilting is relatively easy.  The antenna can be mounted slighty less than 90 degrees from the horizontal plane so the tilted beam illuminates the desired service area.  However, in the opposite direction, the signal will be pointed toward the sky, reducing the effective service area in that direction of the antenna.

If the signal needs to be "bent" downward in all directions around the antenna site, an electrical tilting method must be used.  This is commonly refered to as "null fill".  Electrical tilting is produced by controlling the current phase in the antenna itself.  Thus, must be done during the antenna's design stages by an engineer with expensive equipment.

Ice and De-icing

ICE IS BAD!  Ice buildup on antenna elements will result in a increased SWR (impedance mismatch) that will de-tune a transmitter system, significantly reducing its output power.  Ice can also can also cause severe transmission line damage, and failing icicles can kill.  The easiest way to prevent ice buildup is with special antenna heaters or by covering the antenna system with a fiberglass radome.  Radomes will increase the wind load on the tower and antenna heaters can be expensive, so move to California.

Frequency Reuse / Cells

The following is from Intersil.

To maximize system utilization it was desired to keep the spread rate (in DSSS systems) the same in order to maintain at least three non-overlapping channels in the band.  This is the minimum necessary for co-located network because it allows for frequency reuse in the 2.4 GHz ISM band.  The following diagram illustrates the concept of frequency reuse in co-located networks.


The figure shows three DSSS channels occupying the ISM band.  We see how having a minimum of three channels in the band allows the network planner to implement a cellular network via frequency reuse.  Each adjacent cell is assigned a different channel and therefore interference between adjacent cells is minimized for the two dimensional network shown.  This is the same concept used in cellular phone networks.  Of course adding more channels within the ISM band would allow for increased system utilization by allowing the network planner to fit more users per unit area in smaller cells.  The drawback of adding more channels is we must contend with Shannon's capacity law.  By narrowing the channel bandwidth in order to put more channels within the ISM band, we reduce channel capacity.  In this case it was desired to increase the data rate by a factor of at least 10 over the basic access rate so decreasing the channel bandwidth conflicted with this goal.  In addition, a bandwidth reduction would make it difficult or impossible to meet the 10 dB processing gain requirement of the FCC since processing gain is related to the DSSS spread rate.

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