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Range of UHF-links
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Remarks concerning the range of UHF-links

In the context of our Tender Location / Tender Tracking System, composed of Tender-Transponder and Locator, the question concerning the possible range pops up.

Since the TLS-System is configured for 70 cm band by default (2m is possible), the UHF propagation laws apply. Therefore the radio range is a so called quasi-optical range of the electromegnetic waves. This implies that the height of the antennas is the range limiting factor ofshore.

The following table lists the range depending on the Locator- and Tender-antenna heights :

Height of Tender antenna


Height of Locator antenna

1 m
 2 m
 4 m
 8 m
 10 m
 12 m
1 m
 4,5 NM
 5,4 NM 6,7 NM 8,5 NM 9,3 NM 10 NM
2 m
 5,4 NM 6,3 NM 7,6 NM 9,4 NM 10 NM 11 NM
4 m
 6,7 NM 7,6 NM 8,9 NM 11 NM 11 NM 12 NM
8 m
 8,5 NM 9,4 NM 11 NM 13 NM 13 NM 14 NM
10 m
 9,3 NM 10 NM 11 NM 13 NM 14 NM 15 NM
15 m
 11 NM 12 NM 13 NM 15 NM 16 NM 16 NM
20 m
 12 NM 13 NM 14 NM 16 NM 17 NM 18 NM
25 m
 13 NM 14 NM 16 NM 17 NM 18 NM 19 NM
30 m
 14 NM 15 NM 17 NM 18 NM 19 NM 20 NM



Moving along with the theoretical deduction :

Within short range the so called free space loss can be neglected and transmission powers in the range of 1 Watt guarantee for secure connection of tranmitter and receiver. The optical range can be calculated from :

RG=3,57(√HB+√HO)

with

RG the geometrical horizon in km
HB the elevation of the receiver (height of antenna in tender) in m
HO the elevation of the transmitter (height of antenna in locator) in m

The real range of the radio waves exeeds the optical horizon. The reason for this are bending effects of electromagnetic waves in earth atmosphere. This can be compensated by assuming a radio horizon. Here we do not use the mean earth radius (6370 km) but extend the effective radius to a value of 8470 km. This leads to a modified formula for the radio horizon

    RR=4,12(√HB+√HO)

Due to the height of the antennas the radio horizon is extended significantly. Therefore the asumption of a negligible attenuation (free space loss) can not be applied and we have to calculate this loss. Assuming isotropic antennas - gain=1 by definition (0dB) - the free space loss is the ratio of transmitted power to received power at the receivers antenna as follows :

4πR2 / λ , with R=distance and λ =wavelength in m

The usage of high gain antennas (within low elevation angles) will extend the range even further. The gain is now :

a = 20 log (4πR / λ) -aS -aE ,

with
a the overall gain,
aS the transmitter antenna gain a in dB
aE the receiver antenna gain a in dB

The obove said applies to normal conditions. Additional parameters like high humidity, waves or fog govern the range in the negative direction. Some atmospheric effects may influence the range in th positive direction but are not explained here.