Propagation UK to New Zealand

Steve Nichols G0KYA, chairman of the RSGB’s Propagation Studies Committee, outlines propagation between the UK and New Zealand in October and explains the concept of “Greyline propagation”.This article was written to support the planning of the October 2024 centenary events

Worldwide communication using the LF and HF bands is dependent on radiation coming from the Sun. But twice a day, at sunrise and sunset, the ionosphere undergoes dramatic changes, giving enhanced propagation in some directions.

In terms of radio propagation, the D and E layers are responsible for most of the absorption of radio waves that pass through them, but the absorption is frequency dependent.

The D layer can completely absorb signals on 160, 80 and often 40 metres during the day, and can attenuate signals on 20m too. Hence the reason you don’t hear much, if any, DX on the low bands during the day as sky-wave signals are absorbed before they can reach the reflective (more correctly, refractive) E and F layers.

The ionosphere undergoes a dramatic change in ionisation at the transition from day to night. The electron (and ion) density in the E-layer decreases by a factor of 200 to 1 and the F1 by nearly 100 to 1. At sunset, the D layer disappears rapidly.

Around the other side of the world, as the Sun sets in the UK, New Zealand is entering into daylight and has yet to form a significant D layer, plus the E layer has not built up from its night-time low. At UK sunrise the reverse happens.

Therefore, for a short period propagation between the two regions simultaneously experiencing sunrise and sunset can be highly efficient. Signals on the lower bands can theoretically travel over great distances with little attenuation.

This is well documented with many examples of such propagation being logged on 160 and 80m over the years. Many amateurs will be familiar with this so-called grey line propagation (the term was coined in 1975) – propagation that occurs along a line separating night from day.

The line is called the terminator but it is diffuse, due largely to the earth’s atmosphere that scatters the light over a large area. In radio terms, the radio terminator is not the same as the visual one. The latter refers to the point when we see the sunrise or sunset at ground level on the earth and the period of visual twilight that either precedes or follows. The former is related to the way the Sun illuminates the ionospheric D, E and F layers.

Most books relating to HF propagation give a brief description of grey line propagation, and how and why it works. What they don’t tell you is the actual frequencies affected, other than a vague idea that 80/160m are definite bands for grey line, and “some” HF bands also exhibit grey line enhancements.

There is an alternative way of looking at grey line conditions, which I favour, and which is connected with the critical frequency (fof2). At frequencies above fof2 a radio wave travelling vertically upwards would pass through the F2 layer into outer space. Below fof2 it would be reflected back to earth.

Now imagine a radio wave hitting the ionosphere at an angle of about 75-85 degrees to the earth – a near vertical incidence wave (NVIS). Below the critical frequency it would be returned. If it is some way above fof2 it will pass into space.

At some frequency close to fof2 it could be refracted through a large angle and end up travelling almost parallel to the Earth, giving a very long first skip distance. This is the condition for the Pedersen or critical ray, discovered in 1927 and characterised as being high angle, long distance and close to and probably above the fof2 frequency.  As there would be no intermediate ground hops the signal strength could be very high indeed.

With the high solar flux this year, due to being near sunspot maximum, the critical frequencies are a lot higher than they were a few years ago.

It is likely that these conditions exist around local sunset/sunrise as the critical frequency passes through 80m and could account for long distance communications under grey line conditions.

For example, the critical frequency above Europe on 18th October 2023 (as measured by the Dourbes ionosonde) at 05:30UTC was 3.5 MHz, but by 05:45UTC it had risen to 4.100 MHz. This suggests that the right conditions for a long-range Pedersen Ray on 80m between the UK and New Zealand might have existed at some point in those 15 minutes.

If we look at 40m, the critical frequency at 06:30hrs UTC was 6.925MHz on 18th October 2023. By 06:40hrs it had risen to 7.4MHz. So if we were looking for a Pedersen Ray contact on 7MHz, we might look around 06:30 – 06:40hrs UTC (07:30 – 07:40hrs BST).  

In 1924, contact between the UK and New Zealand was made initially at 06:15 UTC and signals were lost at 07:30 UTC.

Therefore, on a critical frequency basis only I would say that the optimum time for a contact between G and ZL on 80m on 18 October 2024 is somewhere between 05:30 – 05:45 UTC (06:30 – 06.45 BST). However, given that the Sun doesn’t set in NZ until 0657Z there does seem a rather remote chance of a contact on 80m.

 On 40m I would suggest looking at one hour later at 06:30 – 06:40hrs UTC (07:30 – 07:40hrs BST).

At UK sunset at around 16:59hrs UTC (17:59hrs BST) in 2023 the critical frequency was around 10.2 MHz, so I wouldn’t expect to see any Pedersen Ray enhancement at sunset.

This suggests that the optimum time for the morning (long path) 80m contact is around the UK sunrise, +/- 30 mins, say 06:00 – 07:00 UTC (07:00 – 08:00 BST). And from a critical frequency point of view perhaps 05:30 – 05:45 UTC (06:30 – 06.45 BST).

At the UK sunset (short path) it might be worth looking between 16:45UTC to 17:30 UTC (17:45 BST – 18:30 BST), but we can be less optimistic about a contact being made at UK sunset for the reasons mentioned earlier.

These figures appear to be backed up by the Proppy propagation prediction program (which uses the ITURHFPROP engine).

This is a long and difficult path with all signals going through the polar regions, which will be affected by solar conditions. So ideally we need a low Kp/Ap index, representing quiet geomagnetic conditions. And we haven’t had many of those this year!

Steve Nichols G0KYA

Chairman, RSGB Propagation Studies Committee