Core concepts

Fresnel Zone

The Fresnel zone is important in radio propagation because any path obstruction that impinges the First Fresnel zone causes obstruction loss. Most paths between amateur radio stations are heavily obstructed (unless both are on hilltops) but it doesn’t do to tolerate avoidable loss.  Make sure that your foreground is as clear as you can make it at all azimuths round your station. Often the single variable here is antenna height. Here’s an analysis and explanation.


When one radio amateur sends a signal to another, there’s a chance that the signal will be received above the threshold of reception. If the signal received is indeed above the threshold of reception, an exchange of voice or data messages may be possible. Chance governs communication between amateur stations. Many amateurs embrace this, and chance gives the well-established concept of ‘DX’. Read why chance is so significant to radio amateurs.

System Value

The system value is the maximum loss between the transmitter output and the receiver input for a given response in the receiver. The system value is a fixed value for that system technology (comprising modulation and coding). There are differing system values for narrow band FM voice, SSB and each narrow band data mode such as FT8. Here’s how it’s calculated.

Path Budget

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 path budget and resulting 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. Here’s how it’s calculated.

The Normal Atmosphere

It’s difficult for radio amateurs to understand all the various propagation mechanisms without an understanding of the core benchmark of the normal lower atmosphere or troposphere. The density of the troposphere reduces with height. This is hugely important in propagation and in understanding the more unusual propagation mechanisms. Radio waves are bent towards the surface of the Earth and hence propagation distances are typically enhanced. Here’s more.

Reflection, Refraction or Scattering?

There’s huge ambiguity across the radio amateur books, magazines, and sites when it comes to describing how it is that the ionosphere returns radio waves to Earth. The ambiguity is this. Many writers routinely talk of reflection. Others call it scattering. And some refer to the mechanism as refraction. So, which is it? Here’s an argument and conclusion.

Ground wave, space wave and skywave

Hams routinely identify skywave propagation, and, to a much lesser extent, talk of space wave and ground wave. But how do these modes come about? It’s to do with the launch angle of the antenna in use and the frequency of operation. Generally, ground wave is used for LF and MF propagation, sky wave is used for HF communications and space wave is for VHF, UHF, SHF and above. Here’s a description and summary.