MT-MR PLATINUM. POINT TO POINT AURAL RADIO RELAY SYSTEM
13
obstructed point,but only in this frequency band. As we said before, at higher frequencies the clearance must be
100% for First Fresnel Zone.
Given these values here obtained, it's very easy for Engineer to visually calculate its optimal antenna height at both
points, A & B, also calculating the total length of transmission line needed for both ends, and its total line losses.
For calculating antenna heights over terrain, some elementary rules should be followed:
1.- In case of dealing with a link to be installed in a completely flat terrain, mainly in deserts, sugar cane
plantations, across big lakes or over sea water, location of both antennas (transmitting & receiving) at the
same height must be avoided, in order to move the possible reflection point from path's middle region. In this
case, an "up-to-down" hop is desirable and convenient in order to increase link's reliability.
2.- Always try to release the First Fresnel Zone to a maximum, blocking as much as possible the Second Fresnel
Zone.
3.- Use the minimum tower height allowable at both ends to satisfy the link's requirements, in order to minimize
transmission line length (and losses), unless you are planning to use periscopic reflectors in your link.
4.- In all cases, try to move the possible beam reflection point from the center of path, arranging antennas' heights to
avoid the reception of reflected signals under normal atmospheric conditions.
1.5.- Transmission Lines.
There are a wide variety of both coaxial transmission lines and waveguides in the International market for use in
any frequency band. Coaxial lines are used particularly at 1.9-2.3 GHz frequency band, and waveguides are
employed at higher frequencies, from 4 GHz ahead.
For links using the 1.9-2.3 GHz band, it's advisable the use of the 7/8" diameter coaxial foam-dielectric line, such as
Andrew LDF5-50A. This also avoids the use of bulky and expensive air compressors or nitrogen tanks to pressurize
the lines. If required transmission line is quite long, lower loss air-dielectric HJ5-50A line should be employed, and
pressurization will be required in this case. But this is advisable only in extreme and critical cases. Generally the
LDF5-50A (or equivalent) will satisfy the attenuation and power requirements in the most cases.
This typical transmission line has the following specifications:
Attenuation at 2 GHz ........................................................................................................................ 6.46 dB x 100 m.
Characteristic Impedance ............................................................................................................................. 50 ohms.
Maximum Allowable Frequency ....................................................................................................................... 5 GHz.
Velocity Factor ..................................................................................................................................................... 89%
Maximum Peak Power ..................................................................................................................................... 44 kW.
DC Resistance:
Inner Conductor ................................................................................................................. 1.15 ohms/1000 m.
Outer Conductor ..................................................................................................................1.18 ohms/1000 m.
DC Breakdown Voltage ........................................................................................................................................ 6 kV.
Capacitance ................................................................................................................................................... 75 pF/m.
Inductance ................................................................................................................................................ 0.187 µH/m.
