Communications, Radar & Electronic Warfare (201.. - Index of

Planning and Optimising Radio Links 223Table 12.1AspectConsiderations in determimng parameters to be ued in a path predictionConsiderationsRadiated powerTransmitter platformTransmitter clutterMid-path clutterReceiver clutterReceive antenna pointing lossReceiver platformFade marginReceiver sensitivityDetermine power radiated in the direction of the receiver. Fordirectional antenna, thi may be lower than the main beam if thereceiver 15 not in the direction of the horizontal boreightDetermine whether the platform affects the radIated power. Forbody-mounted antennas, body loss may need to be conideredAssess whether the transmitter is tranmlttmg through clutter. If so,additional losse5 may need to be added to account for losses closeto the transmit antennaDetermine whether clutter m the mId-path needs to be taken intoaccount. In Figure 12.2 only clutter present at the two obstructionpoints need to be considered. Clutter obstructions may accountfor in a simplified fashion by addmg height to the obstructionheight above local terrain. Thi i not normally included in thelink budget but is included in the propagation loss predictionDetermine whether additional loss need, to be added to account forclutter in the location of the antennaDetermine whether there is any loss down from the mam beam in thedIrection from the transmitterConsider whether the receiver platform influences the receivedpower, again, factors such as body los may need to be included.For ships and other mobile platforms, pointing loss may need tobe consideredDepending on whether the receiver sensitivity is faded or unfaded, aloss factor may need to be added. Even If the receiver sensitivity isquoted for the fading conditlOn, there may need to be additionalcorrections to account for differences between the conditionsunder which the faded sensitivity was calculated and the actualconditionsThe required input power mto the receiver to achieve the requireddegree of performance must be determined so that the energyneeded at the antenna can be calculatedTable 12.3 shows some parameters that can be changed by the operator, and theeffects that they can have.We will now look at the effect of adding a re-broadcast terminal to an otherwiseunviable link between two physical locations.12.3 Re-Broadcast LinksA re-broadcast C're-bro') or relay terminal is used to overcome problems caused byterrain or distance. The principle is that an additional radio is added to the link to accepta transmission, amplify it and re-direct it to the other terminal. A re-bro can be used forexample to route communications around a difficult hill. An example is illustrated in