A path budget is as it implies – a currency pot in decibels that you as radio amateur can ‘spend’ in communicating with others. It’s not quoted in monetary terms – though some might argue that the bigger your purse, the greater the DX. But that’s another story.
The path budget guides the way you choose to spend your system value, and the benefit you get as a result of choices you make.
Take the system value for 144MHz SSB – 166dB.
A typical arrangement of two radio amateur stations is shown below.
Typically, both radio amateurs will maximise the gain of their antennas and minimise their feeder losses. Let’s assume that both have 12dBi antenna gain and 2dB feeder loss.
If the parties can arrange for there to be no diffraction loss between them, the maximum path loss, Pl, between the two is:
166dB + 2(12dBi -2dB) or 186dBi
The maximum distance, d, is given by 20log10 d = Pl – 32.5 – 20log10 f or d = 10((186-32.5-43)/20) or 334,000km.
That’s the Free Space Loss equation, solved for d.
That’s a big distance. So why can’t amateurs communicate at will between any two points on the Earth’s surface on the 144MHz band?
Simply, a Free Space Path is never available. This would require near-infinite antenna height and no Earth bulge. Being practical, diffraction loss always exists. For practical antenna heights of 15 metres, there is rarely First Fresnel Zone clearance – the condition of a Free Space Path. And the Earth’s curvature always intrudes a long path.
Interestingly, of course, 334,000km is about the distance to the Moon – and that’s why moon-bounce or Earth-Moon-Earth communications is possible, but that’s another story. For now, back to Earth.
A more typical pair of stations might have 60dB diffraction loss (comprising loss due to terrain, buildings, and forestation) between them for 50% of time. This would reduce the available distance to a more sensible 300km. Many radio amateurs would concur with this figure – this the distance, for example, between the county of Sussex and the Isle of Man in the UK. Communications between a well sited Sussex station and the mountain of Snaefell is often available.
If, of course, the radio amateurs choose to spend more (money) on antennas – perhaps to achieve 15dBi at each end, the maximum distance rises to around 600km. So, if the radio amateurs can spend more to purchase longer Yagis, they can communicate further. Likewise, they can use better feeder, or install a bigger tower (to reduce the diffraction loss). The path budget can be spent in many ways to incrementally lengthen the distance available!
Such calculations can be repeated for all bands supporting such space-wave propagation – generally upwards from 50MHz. It’s a similar story for ground wave and sky-wave, though the details in the equations are different. You’ll note here the suggestion that this would be available for 50% of time. To be more certain of the viability of such a path, we’d need to add a fade margin. More on that in another blog.