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HamTV Power Levels

The HamVideo power levels in the table at the bottom of this page result from worst case computations, intended for the  evaluation of system efficiency. See Link budget computation by I0KPT.

As shown in fig 2, the ISS orbits at about 400km above the surface of the earth. That is the distance between the ISS and the « subsatellite point ».

The tangent from the ISS to the surface of the earth (Fig 2, point B) describes a « circle of visibility » with a diameter of about 4700km (see fig 3). At places on the earth at a distance of 2350km from the SSP, the ISS rises over the horizon at the start of a pass and disappears over the horizon at the end of the pass. For these locations and at that moment, the elevation angle versus the horizon is zero degree and the « slant range » (= straight line distance to the satellite) is maximum.

At the middle of the pass, the elevation angle is maximum and the slant range is minimum. For an overhead pass of the ISS, the elevation is then 90 degrees and the slant range 400km.

In theory, VHF communication with the ISS is possible as soon as the location of the ground station is within the circle of visbility. At that moment, the radio waves travel a considerable distance through low atmosphere, highly affected by manmade radioelectrical pollution. Absorption of radiated power and noise interference is maximum. Solid VHF communication becomes possible when the ISS is about 5-10 degrees above the horizon. The slant range is then about 2000km.

For HamVideo reception, the slant range should not exceed 1000km. This corresponds to a « circle of HamVideo reception » with a diameter of about 800km.

Remark: the orbital plane of the ISS has an inclination of  51 degrees versus the equator. See fig 4.



Visibility circle – Drawing Gaston Bertels ON4WF



ISS footprint comparison – Drawing Gaston Bertels ON4WF



Orbital plane – Drawing Gaston Bertels ON4WF



Link Budget computed by Piero Tognolatti I0KPT


Element Power per element

(loss = negativ)

Total Power Remarks
Ham TV PA output 10.0 dBW 10.0 dBW Transmitter output power measured with spectrum analyser – equivalent thermal power
Feeder loss -8.0 dB 2.0 dBW
Antenna gain in direction of maximum antenna gain 7.6 dBi 9.6 dbW EIRP toward sub satellite point (SSP)
Antenna gain toward location 800km from SSP -4.4 dBi -2.4 dBW EIRP for 1000km slant range
Free space loss -160.1 dB -162.5dBW Loss for 1000km slant range
Receiving antenna gain 25 0 dBi -137.5 dBW
Receiving antenna pointing loss -1.0 dB -138.5 dBW
Feeder loss -1.0 dB -139.5 dBW
Downconverter gain 40 dB -99.5 dBW
Coaxial cable loss (25m cable) – 6 dB -105.5 dBW -75.5 dBm presented to receiver input


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