Chance

Last Updated on April 17, 2026 by John Berry

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.

The threshold of reception is different for each modulation mode, frequency band, local noise level and transmitting and receiving equipment. I describe elsewhere the notion of system value – the maximum loss permissible between transmitter and receiver. The threshold is a description of the receiver limit in the system value calculation.

Even if they are stationary, the propagation loss between the two stations is not fixed. The environment between the two is constantly changing. The result is that sometimes the threshold is exceeded, and sometimes it’s not.

There are two principal dependent variables in success – time and locations.

Time and locations

When attempting communication with between two points on the Earth’s surface there is a chance that a viable exchange might take place. This chance is expressed in percentage time and locations for each path.

This state is fundamental to understanding propagation. This is particularly the case where propagation is via a sporadic medium. Examples include via an aurora or via sporadic ionisation of the ionosphere’s E region.

To elaborate, consider call attempts between two radio amateurs with a pair of VHF handhelds, each located anywhere on a football pitch. The chance of an exchange approaches 100% of locations for 100% of time. That’s to say that if the two users wandered at random around the pitch, they’d almost always be in contact. Likewise, if they stood still at any two points, they would be able to be in touch almost for ever (or until the batteries expired!).

Consider then if the two move apart. The further apart the two stations move, the lower the percentage time and locations.

Consider if one station is in Scotland and the other in Spain. Imagining each wandering at random in their country. There would be some locations where they could communicate. And there would be others where the received signal would be lower than threshold. In this latter case, the path loss would be too great.

Then fix the stations at two locations and have them attempted to communicate over a long period of time. Communications would be possible for some of the time.  The rest of the time, the signal would fade below the threshold.

All of this depends of course on a whole load of technical parameters, but as a thought experiment, it sets out the idea of chance.

Chance in terms of percentages

So, whatever the scenario, communications are defined in terms of ‘threshold exceeded (or communications possible) for x% of time over y% of locations’. Both x and y can approach 100% (and this is the case with mobile phones in cities using extensive operating company infrastructure). Significantly, they can also be very low numbers and then things become interesting for radio amateurs.

This applies across all frequencies and propagation modes.

Often propagation prediction software fixes the locations and reports results in terms of path or circuit reliability as a percentage of time.

Short term and long term effects

To complicate things slightly, the time variate element has two parts: short term and long term. Path loss varies second by second, and minute by minute. Communications can be established, and then fade out minutes after. And in the long term, signals vary hour by hour, week by week, month by month, and year by year.

This leads neatly into the idea of a margin. If the average received signal at a receiver is just on threshold, the signal will, in the short term, be usable for half the time. To be more confident about communicating between two stations at given locations, the received signal needs to be well above the threshold – by increasing the transmitter power or changing some other equipment characteristic at either end to yield a margin.

If the technology changes to FT8, the system value, the maximum loss between transmitter port and receiver port, improves hugely and overcomes the fade, effectively re-establishing a fade margin, albeit for a lower information rate.

So, what can we conclude?

Chance in ham radio

Simply, chance governs communication between amateur stations.

The idea is shown graphically below.

Every path, between pairs of radio amateurs across the globe, can be described by a path loss for a given propagation mode and frequency. For a given system value (technology, power, gains), the path loss will allow communications for some percentage of time. In the example shown, communications will be available for 10% of time. For 90% of time, the path loss is just too high. The path may improve, and path loss reduce, later in the day, tomorrow, next month or next year – or indeed it may never improve enough.

Defining DX

Many amateurs embrace the tail of the path loss distribution. Low chance gives the well-established concept of ‘DX’.

DX is:

Establishing a communication, when propagation conditions suggest that there’s a small chance (in percentage time and locations). Many radio amateurs chase low probability events like meteor trails simply because the low chance excites them.

DX is also:

Establishing a communication over a path that’s a first for the hobby, or perhaps working a path that is a first for you. You are the lucky one that happened to be there at the right time and place to exploit the chance.

Good luck and enjoy the QSO when and wherever it happens!

[1] Path loss is typically normally distributed about a median. Hence the use of a normal or Gaussian distribution. Typically in radio communications, the y-axis would be a logarithmic scale. Often received signals depend on multipath arrivals which are Rayleigh distributed about a median. When a path contains both Gaussain and Rayleigh components, it’s described by a Ricean distribution.