Long Path Propagation
An Introduction for Shortwave Listeners

This is a scripted version of a popular YouTube video I published in early July. For those who don't use YouTube, prefer reading to watching, or who are not English speakers, this text version is for you. It can be translated into many languages using Google Translate or a similar facility.

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Here at Mount Evelyn, every year around June and July, Radio Japan appears on the 49mb frequency of 6105 kHz in the early afternoon. It’s only here for 3 or 4 weeks, either side of the winter solstice, the shortest day. After that, it disappears and we don’t see it again for another year.

What’s so special about this? Well, it’s a great example of long path propagation. I’ve made a video of this reception. Let’s watch it first, and then discuss it.

OK, so what are we looking at here? Why is this so interesting? Why did I head out into the bush on a cold and rainy afternoon specifically to record this station?

Radio Japan’s Japanese service to Central America is broadcast daily on 6105 kHz between 0200 and 0400 UTC (midday to 2:00 pm local time here) via the Issoudun transmitter site in France. The station has been on 6105 for many years now, practically owning that frequency every day at that time. But, as I’ve already mentioned, down here near the bottom of southeastern Australia, we only ever hear it for a few weeks each year. What we are witnessing is long path reception of Radio Japan when it’s summer in Europe and winter in my part of the world.

What is Long Path Propagation

A great circle path, also known as an orthodrome, is the shortest distance between two points on the surface of a sphere (in this case, the Earth), represented by an arc of a circle that divides the sphere into two equal hemispheres. When talking about radio propagation, we can have both a short and a long path.

Long path propagation occurs when a radio signal travels along the longer, opposite great-circle route between a transmitter and receiver, rather than the shorter, more direct great-circle path (which we call the short path). This route involves the signal circumnavigating a significant portion of the Earth—often over 20,000 km—before reaching its destination.

For example, if you’re in London and contacting a station in Sydney:

Short path: ~17,000 km (eastward via Asia, on that solid green line you see here)
Long path: ~23,000 km (westward via South America and the Pacific, which you see as the red dotted line)

The Great Circle short and long paths from London to Sydney.

Despite the greater distance, sometimes the longer route provides a stronger or more stable signal due to better ionospheric conditions along that path. Most of the shortwave broadcasts we listen to are via the much more common short path. However, in certain cases, we can observe long path reception.

Here are some things to think about when considering long-path propagation:

👉 Multi-Hop Reflections

HF radio waves are refracted by the ionosphere—particularly the F-layer, which is roughly 200–400 km above the Earth. These waves can then reflect off the Earth’s surface and bounce back into the ionosphere, forming a multi-hop signal path.

👉 Favourable Geographical Features

Signal paths crossing large oceans generally encounter less absorption than land routes because of lower ground losses, which can make long-distance signal propagation more effective in certain cases.

👉 Lower Absorption at Nighttime

This is the big one when we are talking about our Radio Japan reception. The D-layer, which forms during daylight hours, absorbs HF signals at certain frequencies and specific take-off angles, but then dissipates at night. However, the F layer remains at night, providing better propagation conditions over long distances, especially at lower frequencies. That’s why, on our radios, the lower frequencies come alive at night.

Now, when the short path passes through sunlit areas with high absorption, but the long path remains in darkness or twilight, this can result in clearer signals over the longer route than the shorter one.

When Does Long Path Propagation Most Often Occur?

Long path propagation occurs more frequently under specific conditions. These include:

Near sunrise/sunset and in the twilight periods: at either the transmitter or receiver location. The so-called “grayline” effect also comes into play, enhancing ionospheric reflectivity.

High Solar Activity: A long path on higher frequencies above 15 MHz is more likely during periods of high sunspot numbers and solar flux levels above 150.
At lower frequencies: Lower frequencies often exhibit long-path propagation during favourable conditions, especially during solar minima.

What does this mean for users of the shortwave spectrum?

For amateur radio operators, long path propagation is very useful for working distant stations, especially when the short path is weak or blocked by high absorption.
For shortwave broadcasters, long-path propagation can occasionally be used to reach target audiences where short-path propagation is unreliable.
And for shortwave listeners, long-path propagation offers a chance to pick up some genuine long-distance DX signals.

When discussing long path propagation, we're only just beginning to explore the topic. And, I’ve assumed some familiarity with concepts such as great circle maps, signal paths, ionospheric layers, maximum usable frequencies, absorption-limiting frequencies, and many aspects of propagation. There's so much to cover that it can't all fit into this blog post. If any of this seems a bit unclear, don't worry—it's a good idea to learn more about how propagation works and the factors that influence it in the shortwave spectrum. The more you learn, the more fascinating it becomes!

So, back to Radio Japan’s 49mb signal:

That video you saw earlier was taken at Kurth Kiln Regional Park. It’s a large forested area only about 25 km from my home. It’s a fantastic spot for DX because there is hardly any man-made noise there, as you could see from the low noise floor on the RSPduo receiver’s waterfall. And Radio Japan was the only signal on the band at that time.

Let’s take a look at the maps that show the circuit that Radio Japan employs.

I’ve put together this map (below) to help you understand the short path, marked by the solid green line, and the longer path, shown by the dotted red line. It also features the terminator — the moving boundary caused by Earth’s rotation that separates daytime from nighttime on our planet. I created this map specifically to show where the terminator was on the recording date, July 1, 2025.

Short path from Issoudun to Kurth Kiln Regional Park (near Mount Evelyn)

Let’s look at the short path (above) first at 0345 UTC, or 5:45 am, which was sunrise at the Issoudun transmitters in France on July 1. You can see that the signal from Issoudun would travel almost entirely during daylight across Europe and Asia, particularly in the summer in the Northern Hemisphere. On 6 MHz, the absorption of any signal travelling that route would be very high, causing it to be absorbed in the D layer of the ionosphere before the first hop could be completed. The optimum working frequency would be around 17 MHz or higher, depending on that day's ionospheric conditions. So, there was no way I could hear Radio Japan at that time of day on that frequency using the short path.

The winter long-path route on July 1 at 0345 UTC.

Now, let’s look at the long-distance trip (map above) at the same time, 0345 UTC, or 5:45 am in France. The signal travels a nighttime path over the Atlantic Ocean, crosses the top of South America, and moves across the southern Pacific Ocean to reach my location. More than 80% of this distance is over seawater, and it’s only the final signal hop that travels in partial sunlight. With the Sun at a very low angle, this is typical for a deep southern hemisphere winter location like Mount Evelyn. There won’t be much absorption of the signal along that path, despite it being a distance of 23,130 km.

The December 21 short path trip from Issoudun to Kurth Kiln.

Let’s examine the summer map (above) as it would be on December 21, the summer solstice in the Southern Hemisphere and midwinter in Europe. Here, the terminator is flipped, indicating more extensive darkness in Europe. The short path signal would pass through one or two hops in darkness before reaching bright daylight for the remainder of its journey to Mount Evelyn. Once again, the 6 MHz signal would be absorbed by the time it passes India.

The December 21 long-path shows too much daylight for the 49mb signal to make it to Kurth Kiln.

Looking at the long path (above), although Issoudun is in total darkness at that time of the morning in December, the last 40% of the trip occurs in daylight with the Sun nearly overhead at Mount Evelyn, which means that the 6 MHz signal would be lost entirely long before it reached my location.

So this is the reason why I have the opportunity to hear the Issoudun transmitter on 6105 kHz for a few weeks in winter, but absolutely no chance in Summer (or any other time of the year!).

But notice one other thing! Remember I said the service on 6105 kHz was being beamed to Central America, all year round? In both winter and summer months, the signal travels in darkness from Issoudun to Central America. That provides great reception throughout the year for those listeners in that target zone! And, of course, the trip between Issoudun and Central America is the short path for those listeners (map below).

The short-path route from Issoudun to Central America.

Now, there's another reason why, here at the bottom of southeastern Australia, we hear Radio Japan in winter on the 49 mb in the afternoon. It is because Issoudun is using its big, beefy beam antennas. Those antennas are beaming at 290 degrees towards Central America. Mount Evelyn is only about 20 degrees off from the beam's heading (approx. 270 degrees), so we are almost directly in the firing line for a signal aimed at Central America. Pretty cool, eh?

The big beams of Issoudun.

Now, there are many other factors and variables that influence why we hear certain stations at particular times of the day and year. But that’s not the focus of this little video today.

And remember, we are more likely to hear these signals in areas that are quiet or have a very low noise floor, as we saw in the video. That’s why I often head off into the bush, looking for noise-free locations that make it easier to detect weak signal propagation.

So, if you’ve stayed with us right to the end of this blog post, congratulations! You’ve done well, my friend! Remember, a basic understanding of propagation and how it works will help you make your DX listening pursuits much more interesting and successful.

73 and wishing you good DX on the shortwave bands.

Rob Wagner VK3BVW