ITC 220 Base Station, Locomotive & Wayside Radio Transceiver Field Service Guide PN 0046300120 Rev. A
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Base Station:
Common single element fixed station antennas typically exhibit 2.1 to 5 dBi (0 to 2.9 dBd) gain and being vertically
polarized, are usually omnidirectional. Multi-element antennas are designed to concentrate RF radiated power
toward the horizon and away from the sky and the earth and, depending on the design criteria, provide azimuthal
gain directivity that decreases ERP in the direction of other base stations or increases ERP in a specific direction.
They may also be used to make up for large losses between transmitter and antenna. The isotropic gain of a
commonly-used two-element exposed dipole antenna is typically 7 to 8 dBi.
Once the allowable ERP is determined by applying all power-restrictive rules from above and the antenna gain is
known, the transmitter peak envelope power (PEP) output feeding the transmission line is determined by
subtracting the antenna gain in dBi from the EIRP and adding the loss from the antenna feedline and connectors
plus the loss from any combiners, cavity filters or lightning arresters. If the net value is greater than or equal to
48.75 dBm, then the 75 W maximum power of the Base transmitter can be used. If the value is less than 48.75 dBm,
then the transmitter output power should be reduced to the net value. Example for the 50 W ERP case: Antenna
gain = 8 dBi, feedline and connector loss = 2 dB. Assuming no other losses, the transmitter power output limit =
49.15-8+2 = 43.15 dBm PEP or 20.7 W PEP. In this case, the Base RF output power should be adjusted to 20.7 W PEP
or less.
If the calculated transmitter power limit is less than the minimum Base rated power of 10W PEP = 40 dBm PEP, then
an external RF attenuator would be inserted in the feed line to increase the loss between transmitter and antenna
to achieve compliance.
Locomotive
Antennas designed for locomotives at 220 MHz generally use a rugged cast aluminum body and are top-loaded
vertically polarized requiring a metal cab roof for a ground plane. They are necessarily electrically shorter than one-
fourth wavelength due to vertical space limitations. Maximum antenna gain is expected to be at 0 dBd – 2.15 dB.
FCC rule §90.729(b) limits mobile operation at 220-222 MHz to 50 W ERP, which is calculated relative to a free-
space dipole with 0 dBd = 2.15 dBi gain. Since the locomotive antenna has a maximum gain equal to a dipole and
the rated maximum transmitter output power is 50 W PEP, compliance with the 50 W ERP limit is assured.
The actual power will be less than 50 W ERP when the feedline and filter losses between transmitter and antenna
are taken into account. Minimum feedline loss is expected to be 0.5 dB and an inline 160 MHz reject filter adds 0.7
dB loss for a total loss of 1.2 dB. The actual ERP is therefore 47 – 1.2 = 45.8 dBm = 38 watts. The transmitter
installation cannot be operated on FCC channels 196-200 due to the 2 W limitation.
Wayside:
The allowable transmitter peak envelope power output in dBm is determined by subtracting the antenna gain in dBi
from 49.15, then adding the loss from the antenna feedline and connectors. If the result is greater than or equal to
44.85 dBm = 14.85 dBW then the maximum power output of the Wayside transmitter can be used. If the value is
less than 44.85 dBm, then the transmitter output power must be reduced to the calculated value.
Mobile installation:
As an example of a mobile installation, consider a vertical half-wave ground plane on a vehicle
metal rooftop. In an ideal installation, the antenna gain = 2.4 dBd = 4.55 dBi. Ignoring connector losses, feedline loss
is at least 0.6 dB for 10 feet of Times Microwave LMR 195 Ultra Flex coaxial cable. Transmitter output power output
limit = 47 – 2.4 + 0.6 = 45.2 dBm PEP and therefore the system is compliant with the 50 W ERP limit. The actual ERP
in this case is 10^(14.85+2.4-0.6) = 46.2 W. This installation is not allowed to transmit on FCC channels 196-200
because the maximum ERP is greater than 2 watts.