INTRODUCTION

When listening about on the amateur bands I hear many different stories about Tropospheric propagation and how signals on VHF / UHF get from one point to another. Also found on the internet and even in well respected books are theories which I feel have much to be desired. Mostly, I usually refrain from advising these people about their comments unless they are way out of line and/or are willing to listen. There tends to be a number of different theories about how Tropospherics work. For those who happen to be reading this, along with the others, I suggest that you read them all, and then come to your own conclusion.

I have not constructed this paper to sound like I am way up there above everyone else’s intellectual ability by using un-necessary technical jargon and terms of which no one else would understand. I feel this does nothing to help awareness or understanding for people who may not be amateur radio operators and also does not help the general amateur radio operator understanding or involvement in this area.

What is a Duct?

In Southern Australia there is a great deal of ducting activity associated with refractive effects in the atmosphere. Unfortunately, a lot of the instances of these conditions are not well exploited due to lack of operators, lack of beacons and lack of knowledge.

The Duct information presented here is mainly derived from paths across southern Australia, especially across to Adelaide and beyond to the Great Australian Bight path, which can stretch from one side of the country to the other in an east west direction at around 3000 km.

For the VHF / UHF Amateur Radio community on the 2 m band and above, ducts are one of the most common, most widely used and exciting form of propagation modes available. DX contacts can be made in the hundreds to the thousands of kilometer’s range when the conditions are right.

Please, don't think of a Duct as something similar to an air conditioner Duct tube, taking in your signal and spitting it out the other end, as this is certainly not the case. Your signal can not go around corners.

A Duct is usually a layer of warmer air, with less humidity, in comparison to the air above and below it,  which can cover, like a blanket, the whole of a HIGH pressure cell (depending upon fronts and weather conditions around the High cell). It can also be a localised duct in relation to local geography, such as coastal area's. This layer of warmer air is usually called a Warm Air Inversion or a Temperature Inversion layer.

But when does this happen? Just how do we know when the conditions will be right? What are the signs to look for? What tools can be utilized to give ourselves the best chance so we don’t miss the next duct?  

What are Ducts used for?

Ducts are used on the Amateur Radio bands of 2m and above for DX activity. 

While your station on 2m may have a normal range of say, 100 km. In good duct conditions it may extend to 1000 km! 

So as you can see when a duct condition occurs, the radio bands on VHF / UHF become a hive of activity and excitement as the duct does what it may with signals to and from your QTH. You just never know where you may end up. And because a major duct is a fairly rare occurrence (usually once a month or less), it’s a real adrenalin packed session.

How long do Ducts last?

There is no real defined time frame for a duct to last for. Usually, a major duct condition will last for at least 3 hours, on average probably a good 5 hours. 

Time frame will also vary greatly depending upon your QTH. It also varies between winter and summer.

Inland areas in winter can have ducts lasting for a number of days on rare occasions; the weather conditions must be very slow moving and very stable under a HIGH pressure cell.

What are Refractive Effects?

There are a number of Refractive Effects and duct related conditions. See diagram below.

You can see in the above diagram that there are a number of refractive effects, which can do different things to your radio signal. These are: a Standard Atmosphere, Subrefraction, Superrefraction and Ducting or Trapping.

The Standard Atmosphere can cause much boredom in the radio shack! These are line of sight and ground wave paths only. Signals travel and a straight line path and go on out to space. In reality a standard atmosphere is quite rare unless you live in the desert.

Operators with good skills and fine tuned, well equipped stations can utilise these Standard Atmosphere "non ducting" times to make the most of other forms of propagation, such as Meteor Scatter, Earth Moon Earth, etc. 

Subrefraction in the Troposphere is also a cause of concern. Subrefraction in the Troposphere is generally thought to be a rare occurrence, but can happen when conditions exist to support upward bending of electromagnetic waves (EM). Again Subrefraction can also be a cause of boredom as your radio signal is directed into space.

With a Superrefraction effect, we can tend to get a little more excited, especially because Superrefraction is often associated with Ducting. When a Superrefractive condition is present, EM waves are bent downward towards Earth. The degree of bending depends upon the strength of the Superrefractive condition. Some times the EM waves can be bent to an extent where they partially follow the earth’s curvature, but are on a slight upward angle and are eventually lost. At times they can have the same curvature as the Earth and may follow close to ground for some distance.

A Duct or Trapping layer is great strengthening of a Superrefractive layer, to the extent where the signal is curved enough in the layer to then be directed at the lower edge of the layer at a very small angle. Where the layer is sufficiently strong, this signal will then bend back up as it approaches the lower edge of the layer. This is now classed as a Signal Trap or a Duct. If this Duct layer is present for a long distance, the signal trapped within can be propagated along its length and can be received by any other sufficiently equipped station with duct access (closeness to the duct).

What are the different types of Ducts?

There are a number of different types of ducts, which can be distinguished between one and other. 

Surface Ducts, Surface Based Ducts, Elevated Ducts and Evaporation Ducts.

Surface Ducts & Surface Based Ducts - I tend to combine Surface Ducts and Surface Based Ducts into the one category and name. This is simply just Surface Based Ducts. They are virtually the same thing and I wont go into the difference.

When there is an inversion layer which has formed due to night/day and day/night cooling and heating, from the ground up to a certain distance above the ground, (which could be 200 m up before cooler air is reached for example; or the ground layer can be cooler than a warm air layer above) the change in the refractivity gradient causes the signal to bend, this is called a “Surface Based Duct”. Usually the ground and areas closer to the ground cool at a faster rate than the air above it. This causes the standard temperature gradient to become ‘inverted’ or backwards. Meaning that instead of a standard atmosphere where the temperature decreases with height, the Surface Based duct shows an increase in temperature with height to a certain point.

Significant Surface based ducts can be found around coastal areas, and at times extending to other inland locations. I often note surface based ducts between Mt. Gambier and Adelaide and Mt. Gambier and Melbourne (all coastal locations). Very strong Surface Based ducts can also be noted all around the South Australian Gulf areas and many other locations. Surface based ducts are often responsible for stronger local and semi local conditions, which can be up to several hundred kilometers and some times more depending upon the weather stability and consistency along the path.

You can often tell if you are working a Surface Based Duct, because signals are usually strong on a localised scale (up to 400 or 500 Km) and weaker or non existant and sporadic signals up to 700 km (semi local). Also the duct is usually noticed shortly after sun set. Conditions can also pick up after sunrise for a few hours.

Elevated Ducts - Elevated Ducts are the big bangers of the ducting world! This is because they can extend over a great distance due to their elevated origin.

In the first diagram above, you can see that the Elevated Duct layer is much to high above the stations antenna. In this case, no DX is heard from the duct.

In the second diagram above, you can see that the Elevated Duct layer is approaching the stations antenna, the stations antenna is just into the Lower Duct Extremity. At this stage the station is beginning to copy signals traveling in the duct and is able to TX back into the duct, although the signals could be better if the duct were closer.

In the third diagram above, you can see that the Elevated Duct has now dropped sufficiently to have the stations antenna located within the Duct. This is the ultimate situation and maximum signal strengths will be achieved. Maximum TX RF is also being trapped. It doesn't get any better than this, though Duct intensity can vary and so signal strengths vary between one opening and another. Locations the duct is allowing you to 'hear' due to its height and thickness at certain locations, can also vary.

So from this you can see that, if you have a good high pressure cell spanning southern Australia, if the duct is at a workable height at say Albany and also at Melbourne then signals will get through.

Evaporation Ducts - Occur a lot of the time over the surface of oceans and lakes and can sometime occur over storm water saturated land, I have noticed (very rare).

Evaporation Ducts occur because the air in contact with the ocean surface is saturated with water vapor. A few meters above the surface the air is not usually saturated. The rapid decrease in water vapor causes the modified refractivity gradient. A change in the moisture distribution without an accompanying temperature change can lead to trapping of RF.

On a world average, the evaporation duct height is 13 meters.

How do we know when the conditions will be right?

The beauty of the bands on 2m and above is that duct propagation is not controlled by conditions in the Ionosphere, but is mainly controlled by conditions in the Troposphere, which is the layer in the atmosphere closest to the Earth. The main effect in the Troposphere which governs VHF / UHF conditions is, quite simply, our Weather.

Look, Listen and Learn

That’s right, the weather. It’s happening outside right now, so go have a look for yourself. Note to yourself what’s going on out there presently and then make notes about the related radio condition experienced. After each weather front that passes you can expect to look out for what conditions the new weather system (usually a High pressure system) brings.

While I can only really speak from my own observations in respect to local weather, but I believe most areas have the same effects under good duct conditions.

You will have to get to know your own area, but my observations taken near Wagga, NSW, should give you an idea.

These observations indicate there are two distinct types of ducts. Winter and Summer types. There are also Localised Ducts and Major Ducts.

In summer, extremely good duct conditions almost always occur very early morning to near midday, under brilliant clear blue skies, with what I term as “dead calm” wind conditions and warm to hot day time temperatures. Usually the night before will be similar; at sunset there is usually not a cloud to be seen and no wind. All this correlates to a high-pressure weather system if checking weather maps.

In winter, good duct conditions seem to occur more often in the evening, heading towards sundown to around midnight, there should be clear blue skies with “dead calm” wind conditions. Again associated with a high-pressure weather system that has been stable or slow moving for a few days.

A lot of the time there wont be good conditions on the first or even second day of blue skies. But the longer there are clear skies, the slower and more stable the weather pattern the more chance there is of expecting duct formation that is useable.

Remember this is a general rule and there are always exceptions to the rule, especially between coastal and inland areas.

Sometimes, a useable duct can occur directly behind a weaker front and be workable within 6 hours of the front passing.

Once you have established your outside conditions, the next step is to listen. Get those radio’s on and listen to what’s happening and learn just what weather conditions do to distant signals. Note this information down or make a mental note to yourself.

You can listen for beacons, check repeater signal strengths, try T.V audio frequencies close by the bands you are using. Some New Zealand stations even monitor Australian Pager frequencies, which are located just above 2 m. After a while you will get to learn from your beacon signal strengths, what constitutes good conditions and what are poor conditions.

Personally, from here I choose to listen to the Adelaide beacon on 144.450 MHz. Under poor conditions I have no signal at all. I know that something is brewing if I start to hear the beacon, and things are looking good when it reaches about S5 on the meter.

There are many other factors that influence duct conditions. Even though things may appear favorable in your own local area, you also have to take into account where your hopeing your signal will end up. So as well as conditions being OK at your own QTH they have to be good at the distant QTH your trying to work and they also have to be favorable along the whole signal path from point A to point B. Simply what I am saying is, if your in Melbourne and trying to work to Esperance on 432 MHz you wont get through if there is a cold weather front moving through the center of the path at the same time, as the path will be disturbed.

What tools can be utilized so we don’t miss the next duct?

This is how we can work out if there is frontal activity along the center of that path, by utilizing the tools in our everyday lives.

What are the tools? - Newspapers, Television, Broadcast Radio and the Internet.

Ideally, the Internet is going to provide you with the best tools to keep an eye on duct conditions.

But, if you don’t have access to the internet, aside from listening to the Amateur beacon's 24 hours a day, you can also get an idea of what the weather is doing around the country by studying the weather information presented in the Newspaper and on Television and Broadcast radio news and weather reports.

Above: Charts similar to this one are ones to watch for in Australia's south. You must remember though that there are many factors to consider besides MSLP, and charts like this one by itself cannot give you the complete picture. To me it means 'be alert' for openings.

I have seen some newspapers that also provide the Four Day Forecast Charts, so you can see what may be heading your way in the days to come. Television and radio weather reports also give a forecast of what the weather is expected to bring.

Above all, I feel the most valuable tool when checking out current and future conditions on the higher bands is the Internet. Just about every thing you could ever want to look at and study, in relation to the VHF UHF operating is there. You just need to know where to find it.

Below are sites most regularly used to assess coming duct conditions.

Bureau Of Meteorology (BoM) – www.bom.gov.au - Australian MSLP current and 4 day forecast maps, rain radar, state forecasts.

Weather Zone – www.weatherzone.com.au - Local weather conditions and warnings. Colour Synoptic Chart with cloud overlay, colour prognostic chart with predicted rainfall and lightning tracker.

VHF/UHF Tropospheric Ducting Forecast - 5 day forecast duct predictions By William Hepburn. Commonly known as the Hepburn Charts. The "Tropo Forecast" is updated daily at 1700 UTC. It is an experimental forecast of distant VHF and UHF propagation via the mode called "Tropospheric Ducting". The forecasts are prepared on a strictly volunteer basis as a service to the DX community & broadcast stations. Be wary that the forecasts are just that, a forecast, that doesnt mean to take it as gospel as if its going to happen as they are not always correct. It is a educated prediction only.

VK Propagation Logger - The logger site by VK4CP is much more than meets the eye.

• For 2m, 70cm & 23cm enthusiasts in the Australia/New Zealand/New Caledonia region
• Real-time propagation spots & alerts
• Activity reports & announcements
• Weak Signal QSO co-ordination
•  6m Sporadic Es spots welcome as a heads-up for 2 m

Australian National VHF DX Group (ANVDG) Email List – http://au.groups.yahoo.com/group/anvdg/ - Yahoo Email group for those into DX on 144 MHz and above.

This email list has been produced to cater for the interests of Australian Amateur Radio Operators who are interested in DX activity on bands 2m and beyond. Mainly centered around 2m/70cm/23cm and above. The ANVDG caters for all modes FM, SSB, CW and digital weak signal modes like WSJT.

The ANVDG site is also intended to be the gateway to VHF UHF Amateur radio DX in Australia. The ANVDG site is at www.users.bigpond.com/anvdg/

Conclusion

Once you have been monitoring and logging conditions for a while, you soon get a pretty good idea of what to expect with the prevailing weather conditions and when.

Main points to look out for - 

Keep an eye on the MSLP maps.

Keep an eye on the Hepburn Charts (remember today's map is actually tomorrow).

Keep an eye on the actual weather at your QTH. If there is no wind and no cloulds you have a better chance of some DX coming through.

Keep tabs on the Propagation Logger and see what others are logging.

Usually conditions occur early to mid morning or early evening to midnight.

Of course there are exceptions to all these points and this is only intended to give a general idea of what to expect. Propagation can occur differently in different geographic areas.

Most of all...... HAVE FUN..... 73 Leigh Rainbird VK2KRR

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