A LITTLE KNOWLEDGE OF PROPAGATION CAN REALLY HELP!

Back in the 70s and 80s, AFAN McMurdo Station in Antarctica had its own shortwave broadcast station, which operated on 6012 kHz with a power of 1 kW.

A LITTLE KNOWLEDGE OF PROPAGATION

CAN REALLY HELP!

(This post is an article I wrote for "The World of Shortwave Listening" column of The Spectrum Monitor magazine - April 2018 issue. Further details on this excellent publication are available at www.thespectrummonitor.com)

I am a member of many Facebook groups that focus on DXing and shortwave listening. These are fantastic sources of information and hot DX tips. And, some wonderful friendships are often formed through these groups.

However, it is interesting to observe that many enthusiastic SWLs are not familiar with the fundamentals of how shortwave radio signals arrive at their receivers. This is seen in the occasional misidentification of stations made in their reported loggings. Often we see someone posting that they have heard a station on a particular frequency when propagation at that time would be either very difficult or near impossible to achieve under normal conditions. This is especially evident of low frequencies where, for example, propagation during the day in summer on the 60-meter band is virtually non-existent for all except semi-local transmitter to receiver scenarios.

There are many helpful resources available on the Internet that can assist in the identification of shortwave broadcasters. You can access some great tools to aid you in identifying a signal on a particular frequency at a certain time of the day. Such lists include the EiBi Database - http://www.eibispace.de/ and Short-Wave.Info - http://www.short-wave.info/. But just because a station is listed on some database to operate at a particular time on a particular frequency does not mean that you will be able to hear it at your location.

So, to increase your enjoyment of and success in shortwave listening and DXing, some reading into how medium, high and very high-frequency signals travel from the transmitter to the listener can really prove helpful.

The science behind high-frequency propagation is complex and beyond the scope of this month’s column. For those readers who are hazy on how HF signals bounce around the Earth’s upper atmosphere, you would do well to check some of the free online resources that cover this subject, both at an introductory level and in-depth. One excellent introduction comes from the Australian Government’s Space Weather Services. Also worth reading is an article that first appeared in the September 2002 issue of QST magazine entitled Understanding Solar Indices by Ian Poole G3YWX, which you can download from the ARRL website.

And, of course, you can’t go past the highly informative and educational Radio Propagation column by Tomas Hood NW7US in The Spectrum Monitor magazine! Each month, Tomas looks at specific aspects of propagation and offers his monthly predictions. It’s always a good read!

However, it is essential to understand that the ionosphere (the atmospheric layer responsible for refraction of radio signals) is always in a constant state of flux. It changes diurnally (throughout the day), through the seasons, with the solar cycle, with the location of ionospheric refraction points, and with daily variances in solar activity.

Some less experienced shortwave listeners can sometimes become frustrated by the constant variations encountered in propagation while trying to listen to their favourite radio stations on a daily basis. So an understanding of the what, how and why behind HF propagation will lead to a more satisfying experience with this part of the radio hobby.

The successful reception of a shortwave signal can be influenced at the transmitter site by the choice of frequency used, the angle of elevation at the transmitting antenna, transmitter power, height above sea level, and type of antenna utilised for service. Furthermore, at the receiving location, there will be influences such as the quality of the receiver (sensitivity, selectivity, etc.), the noise floor, the quality of antenna, and the degree of environmental man-made electrical interference being picked up by the radio.

Signals from Antarctica

In Antarctica, there is a United States research centre called McMurdo Station, on the shore of McMurdo Sound in the New Zealand-claimed Ross Dependency. The site was established in 1956 and is coordinated through the US Antarctic Program, a branch of the National Science Foundation. It is said to be the largest community in Antarctica, and in summer can support more than 1200 residents. The station is in the same time zone as New Zealand.

Last December, there was much excitement when the American Radio Relay League (ARRL) announced that there would be a broadcast of Christmas carolling on December 23 from the McMurdo Station. This event is held each year at the same time and involves singing and sharing Christmas Carols via HF radio with those scientists and researchers at South Pole Station and in remote field camps. McMurdo acts as the net control to coordinate transmissions from each of the camps.

In an email to me after the event, Nathaniel Frissell W2NAF (assistant research professor at New Jersey Institute of Technology) advised that both McMurdo and South Pole Stations were running 1 kW with permanent antennas, which he believed were probably conical monopoles.

These monopoles are base-fed vertical antennas having an omni-directional pattern. But their elevation pattern is low, and thus ideal for long-distance transmission because the energy is kept down close to the horizon. However, the field stations and remote camps were using much smaller power outputs, potentially down to as low as 20 watts (and probably employing less-sophisticated or more temporary antennas).
The frequency chosen was 7995 kHz (above the 40-meter ham band) at 2300 UTC (midday on December 24 local time at McMurdo).

In the 48 hours following this event, the various online SWL and DX forums included many expressions of disappointment by listeners globally at not being able to hear this broadcast. But really, this should not have been a surprise. The broadcast had little chance of reaching most listeners beyond the Antarctic region. Why? Well, let's take a look at the circumstances surrounding why this broadcast didn't make it to North America.

1) Fundamental propagation studies tell us that during daylight hours, the ionosphere comprises of basically four layers: D, E, F1 and F2. At these times, high levels of absorption occur when signals pass through the D and E layers, especially at lower frequencies. At night, the D, E and F1 layers disappear leaving a single F layer to provide suitable conditions for the reflection of radio signals on lower frequencies.

2) As mentioned already, the broadcast was scheduled at midday local time at McMurdo Station in high summer. The frequency of 7995 kHz was sufficiently low enough to be subjected to significant D and E layer absorption of the signal once it had left the transmitter.

3) To make matters worse, the maximum amount of daylight - the southern hemisphere's summer solstice - had occurred only several days before (on December 21). The McMurdo Base is located at latitude 77 degrees South, so at that time of the year, this station is in daylight for a full 24 hours of the day.

4) Figure 1 (below) shows the short path track between McMurdo and Texas for midday McMurdo time on December 24 (Dec 23 UTC). It is an all daylight path at a distance of some 13,600 km (8500 ml) between McMurdo and Dallas, TX. The signal would have “a snowflake’s chance in Hell” of making it to the continental US at 8 mHz. Indeed, even at my location in southern Australia, a distance of only 4,500 km (2800 ml), not a whisper was heard from McMurdo because of the all-daylight path between the transmitter and my receiver. Recognising that most stations in the net were operating on low power would have made this broadcast a still more challenging DX catch. Even a big 500 kW broadcast transmitter would have struggled to be heard on this path!

Fig 1 - The great circle path shows a continuous daylight journey across the southern Pacific Ocean between McMurdo and Texas at 2300 UTC on December 23. Maps courtesy of VOACAP. (Click on image to enlarge)

5) Interestingly, Nathaniel W2NAF advised that he had received a few reports of reception from Europe, specifically from the Austria/Germany region but listeners there had indicated that the signals were quite faint. Figure 2 shows the short path between McMurdo and Germany at that time. Although about two-thirds of the route is in darkness, the first part of the journey from McMurdo would still have been subjected to much D and E layer absorption.

Fig 2 - The short path between McMurdo and Germany shows much of the route in darkness but the first part of the track still suffers from D and E layer absorption at the transmitter end.

6) We could reasonably expect that the field and remote stations up to an 800-1100 km (500-700 ml) range from McMurdo would probably have maintained good communication with the base station. Those readers who regularly listen to the 40-meter ham band will be aware that this is a very good daytime band for transmissions over those distances. No doubt, this is the reason why the frequency of 7995 kHz was selected in the first place. The objective was to establish and maintain contact with the field and remote stations for local broadcasting of this special event.

So, given the above points, it is clear that this was an event for the Antarctic workers themselves. These annual transmissions are not intended for an international listening audience, especially with the low output power being utilized. I suspect that the ARRL picked this as a unique interest news item rather than as a realistic possibility of global reception. It was subsequently widely promoted on various Facebook and online SWL websites.

But even a little knowledge of propagation and signal paths would have shown that an Antarctic transmission during broad daylight on that frequency in the southern hemisphere’s high summer was going to be most unlikely to reach the United States.

I encourage you to take the time to read and understand the science of radio propagation. It will undoubtedly enhance your enjoyment and success in listening to new and hard-to-hear radio stations on the shortwave bands.

73 and good DX to you all,

Rob Wagner VK3BVW

This AFAN (American Forces Antarctic Network) QSL was received by the author for a broadcast in May 1981. However, due to Antarctica suffering from long periods of shortwave blackouts during times of high sunspot activity, the station moved exclusively to FM.