Total hop attenuation during fading-free time
The following parameters are included in the calculation:

• free-space basic attenuation
• additional attenuation(s), consisting of attenuations due to
  
  • obstacles
  • partial clearance
  • r.f. attenuators
  • periscopic antenna
  • passive repeater in the near field (where the repeater’s contribution is considered as an additional attenuation)
• attenuation due to atmospheric gases
• attenuation in the antenna feeder at the transmitting and receiving end
• attenuation in the RF-branching assembly of the radio-relay equipment
• antenna gain at the transmitting and receiving end
• gain in a passive repeater  (If the passive repeater is operating in the far field, its contributing to the total hop attenuation is considered as a gain)

Hop distance
For almost all hops, the length of the radio path, ie the distance between the transmitter and receiver antenna, is practically the same as the geodetic distance, as read from the map or calculated with the help of the co-ordinates. On short, but steep paths, however, the difference between antenna-to-antenna distance – the beam path length - and geodetic distance can be significant. When eg, in a 500 m wide valley, the RL in the down station is connected to a repeater on the close-by mountain top, and the difference in height is 900 m, the beam path has a length of 1030 m, and not 500 m. This corresponds to a free-space attenuation, which is 6.3 dB higher than it would be for the geodetic distance.

RLTool feature:

• RLTool uses always the beam path length when calculating the free-space attenuation, taking the difference in altitude between the sites into consideration. This is also the length shown in the performance report.

Radio frequency
The free-space attenuation is calculated applying the centre frequency of the r.f. band utilized, irrespective of the frequency given as input data.

The radio path will always operate with two r.f., one for the go and one for the return direction. The difference between these r.f. is in the magnitude of 154 MHz (at 7.4 GHz) to 1120 MHz (at 19 GHz). This would correspond to a difference to the free-space attenuation calculated with the centre frequency of ±0.08 dB … ±0.25 dB. However, these deviations from the centre-frequency attenuation will be compensated by the antenna gain, which varies with about the same magnitude, but in opposite direction. Remaining differences are negligible.

Tests:

• If an r.f. lies outside the range of the ITU-R stated r.f. bands, RLTool will perform all calculations with the stated frequency. However, a warning message will be displayed.

Antenna feeder attenuation
RLTool offers 3 options to consider the antenna feeder attenuation:

• The correct length of the waveguide between RL equipment and antenna is known.
• No waveguide length is known for the moment. RLTool takes the antenna height above ground level (agl) as the waveguide length, extended with 10 m to consider a probable horizontal distance between RL room and antenna bearer.
• No antenna feeders are used at all, as it is for tower mounted RL equipment with integrated antenna.
• If tower mounted RL equipment is separated from the antenna, and a short waveguide interconnects these two devices, eg an ultraflexible, twistable waveguide, this case has to be considered as correct waveguide length is known.

RLTool feature:

• The typical waveguide attenuation in dB/100m can be obtained from an internal database or given by the operator.

Antenna gain
As for the free-space attenuation, the antenna gain used is that for the midband.

RLTool feature:

• The typical antenna midband gain in dBi can be obtained from a database or given by the operator.

Loss in atmospheric gases
The upper frequency limit for the calculation is 350 GHz. For the following input data:

• average lowest ambient temperature during the worst month
• lowest air pressure during the worst month
• water vapour density

default values are preset, but the operator can edit them. They will be valid for the entire project.
No gas absorption loss will be considered for r.f. < 15 GHz, neither will it be shown in the print out.


Gain or loss in a passive repeater
RLTool handles both antenna-back-to-back repeaters and plane reflector type repeaters. When considering their attenuation and their interference contribution, they are treated as an integrated part of the radio path, which is defined as a connection between two active RL stations. (Passive repeater stations are integrated in the interference calculations.)


Antenna-back-to-back repeaters
This type of repeater is considered the following way:
The radio path is split into two "legs", one between site A and repeater, the second one between repeater and site B. Each leg having its own free-space attenuation. The antennas in the repeater contribute with their antenna gain to the compensation of the cumulated free-space attenuation.


Plane reflector type repeater in the far field
If the plane reflector operates in the far field of the closest antenna, its contribution to the total path attenuation is considered the same way as that for the antenna-back-to-back repeater. The plane reflector contributes with a gain.


Plane reflector type repeater in the near field
The above way of considering passive repeaters cannot be applied when the plane reflector operates in the near field of the nearest antenna. In this case, that site, which is closest to the repeater, is virtually moved to the repeater site, and the free-space attenuation is calculated for the longer leg. The shorter leg's attenuation and the reflector contribution are considered as an additional attenuation. No formulas are known to perform that calculation, only a diagram that represents measured data for such cases.

RLTool feature:

• The diagram is incorporated in RLTool, ie the program finds out the additional attenuation from the digitalized diagram.