Troposcatter at 50 MHz - 700 km QSOs any time
Issue 49 & 50, May & August 1996
by Palle Preben-Hansen, © OZ1RH

Do you think "ground wave" does not work well on Six? Is 3-400 km already DX for you? Well, troposcatter is always possible, and provides up to 700 km QSOs day and night. This article explains troposcatter and what it takes to do it on Six. Information on scatter angle includes how to get a low angle of radiation, which is important both on HF and on the higher VHF/UHF bands. Tropo-scatter works better on 144 and 432 MHz and is the main contest propagation mode at these bands, so you may learn something interesting for those bands also.

Foreword

At the Weinheim UKW-Tagung 1994 I gave a lecture on troposcatter for VHF/UHF/SHF. An abstract was printed on page 357-375 in Scriptum der Vorträge 1994. Since last year I have been studying propagation literature further and at OZ5W/OZ9EDR contest team we have included 50 MHz to our activities.

We found that troposcatter also works on 50 MHz, but the QSOs were not at all so easy to make as on the higher bands. In the following I will pass on to you our experience and the theoretical basis for 50 MHz troposcatter QSOs. I will not repeat the text from last year, only some of the facts of troposcatter are mentioned again. So if you want some kind of proof please read the 1994 abstract. I am employed in the computer business, and so this text is based only on my leisure time study of the referenced literature and on my experience from the bands. I know my words are not the whole truth, but do I hope they come closer to "nothing but the truth".

Much of the literature I have read on scatter dates back to 1954-60, where it was a hot item among scientists and radio engineers. I trust we may still use this old information, as the characteristics of the propagation itself has not changed in the past 40 years. Troposcatter was used by commercial services and the military 1950-93, so information on troposcatter was to some extent restricted in those days due to security reasons. Some of the sources I have read mention 'Printed with permission of the Admiralty' or 'Some essential parts are deleted due to US government policies'. Thus troposcatter so far has been quite unknown to many amateurs. Today, troposcatter is little-used commercially, as satellite communication is available, but is still used over the North Sea for communication with the oil platforms. Paying no attention to the lack of commercial use, radio amateurs working on an otherwise dead VHF/UHF/SHF band take advantage of troposcatter communication most often during contests.

Limitations of this paper

This paper is only about 50 MHz propagation by troposcatter, which is the propagation giving you QSOs from 50-700 km day and night regardless of band conditions. 50 MHz has many other propagation modes to offer, some of which are much more easy to use. Most of the argument in this text are to some extent also valid for the higher bands 144 MHz to 10 GHz, but I will concentrate on 50 MHz. Troposcatter is to some extent influenced by the climate. The following only deals with a temperate climate as found in northern/central Europe. In other climates the characteristics of yearly and diurnal variation in signal strength, fading and path loss briefly mentioned in section 3 are different. If you are in an equatorial, sub-tropical, desert, Mediterranean or polar climate zone please read the texts from CCIR Study Groups, ITU, referenced at the end. The body of the following text, that is radiation angles and ground reflections, applies to all parts of the world.

How a dead 50 MHz band looks

For the average amateur with 10-100 W and a 3-5 element beam the 50 MHz band looks something like this when there is no good propagation:

Distant Signals

• 0-50km - 59+ if optical view
  
• 51-100km - weaker signals, but still no problem.
  
• 10 -200 km only QSO if beams are pointed to each other, but then no problem. 201-400 km 51 signals, critical, 100W needed.
  
• >400km - almost no QSOs.

This is in contrast to 144 MHz, where 10W will give you 300+ km and 100W 500+ km QSOs any time. Just try to be active during a contest! The general impression among amateurs is that 'ground wave' is not so good on 50 MHz as on 144 MHz. My experience is however, that if you put up a good antenna on a good QTH and have a linear, 50 MHz will also provide you with 700 Km QSOs anytime. In the following I will explain in theory and practice what it takes to work these semi-DX QSOs on a dead band. This knowledge is quite nice to know, if you want to do good in the contests, where the band at least some of the time is likely to be dead.

General facts about troposcatter

Troposcatter is spreading of radio waves in the troposphere caused by in the atmosphere. The irregularities responsible for the scattering of electromagnetic waves are small changes in humidity, temperature and pressure. The troposphere is the atmosphere below an upper limit called the tropopause, which is at a height of about 10 km. Above the tropopause the temperature is constant, there is no humidity and few irregularities to scatter our radio signals.

Some key points about troposcatter (see Fig 1):

• Troposcatter is always there 365 days and nights a year

• Signal strength is quite reliable, only QSB of some 13 dB on the short distances < 300 km and few dB 5-900 km

• Signal strength often +10 dB during summertime or high pressure, most pronounced for the shorter distances < 300 km

• Path loss increases at about 10 dB per 100 km up to 500 km and somewhat more slowly at the longer distances

• The scatter angle should be as small as possible, each degree of scatter angle costs 9-12 dB in signal strength

• Polarization of the transmitted signal is well preserved

• Most DX stations can make 700 km QSO's on 144 or 432 MHz. EME stations can make daily 900 km troposcatter QSOs

• Troposcatter is independent of frequency from below 144 MHz to 10 GHz.

Long distance troposcatter signals has a special hollow 'tropo sound' some what similar to a little aurora distorted signal. This sound has been on the signal of ON4KST at 700 km many times. It is caused by incoherent scattering as the scatter angle is increasing. At 700 km the scatter angle is about 5º. The same sound is found on back scatter signals on HF or on 432 MHz when the beams are not on each other. All of these characteristics are valid for 50 MHz also. In the following we will investigate what it takes to take advantage of this propagation on 50 MHz. How 50 MHz differs from the higher VHF/UHF bands

Average amateur equipment

Many Six-metre amateurs are happy working DX with 10 W and a 3-5 element beam. Many even try their luck with a ground plane. On Six ,100 W is almost considered QRO. 10 W works well when Es is good, but is far less than a serious DXer would come up with for 144 MHz or higher.

Noise level

The atmospheric noise level is much higher on 50 MHz than on 144 MHz. On 50 MHz ,a quiet location has a noise temperature of perhaps 4,000 K = 12 dB compared to app. 1,000 K = 6 dB at 144 MHz. In a town, manmade noise is even higher at 50 MHz. This means you can not take advantage of a preamplifier at 50 MHz with a noise figure of lower than say 5-6 dB even for EME. On 144 MHz your system noise figure should be below 2 dB for terrestrial QSOs. On 50 MHz low noise receivers will not help you much, unless you have an enormous cable loss. This means your QSO-partner should provide some 6-10 dB more power on 50 MHz than on 144 MHz to provide you with the same signal to noise ratio.

Antenna gain

12-15 dB antenna gain is quite common on 144 MHz, but on 50 MHz a 13 dBd antenna is huge and belongs to the EME-elite. Stacked antennas are rare on 50 MHz, where ground planes are not uncommon for DX. Polarization is preserved on troposcatter. Using a GP this will mean a loss of say 20 dB if you want a troposcatter QSO with a station using a beam and if you both use GPs you lack antenna gain for troposcatter on 50 MHz. If you have tried to work DX on 144 MHz using a GP, you know how bad this is. Troposcatter is generally used with high gain antennas, which have narrow beamwidth. Fig. 2 is the trademark of troposcatter and is found all over the literature. On 50 MHz our antennas are not high gain and have quite broad beam width. Then the picture looks different (Fig 3). With our broad beamwidth we illuminate a large common volume of air, but only the lowest of it contains many blobs or scatter cells with small changes in refractivity. Thus only part of the common volume is useful for scattering. This does nothing good for the signal strength. Narrow beams with 25 dB gain would be nice, but are out of the question on 50 MHz.

Wavelength

It is of course no surprise that the wavelength is longer on 50 MHz than on 144 MHz. The implications of this are several:

• the wavelength is larger compared to the average size of the scatter cells or blobs, which may result in less efficient scattering;

• MHz will provide less ducting, as the required size of the duct of some 50 wavelengths is rare;

• a beam on a 18 metre tower is only 3 wavelengths up, and will give a higher angle of radiation on 50 MHz than on 144 MHz

All together troposcatter is more difficult on 50 MHz than on 144 MHz, but it is not impossible, however the power limit in some countries does not allow full investigation and exploitation of the possibilities of the band.

Scatter angle and path loss

The scatter angle should be as small as possible as each degree of scatter angle costs 9-12 dB in signal strength. The scatter angle is directly proportional to the radiation angle used at each station, and increases with increasing angular distance between the stations.

A troposcatter QSO over distance 'd' is composed of 3 legs:-

• free space loss from your antenna to the horizon: d₁

• scatter loss for the distance: d -d₁ -d₂

• free space loss from the horizon to your partners antenna: d₂

The angular distances must be calculated taking refraction into account; that is, the effect of the diminishing refractivity of the atmosphere with increasing height causing the radio wave to be bent (refracted) towards the earth. This refraction is also known as 4/3 R, meaning that to radio waves the earth looks flatter, as if its radius were 4/3 of its actual radius. As you know from satellite communications the free space loss is relatively small, so you want to have as long d1 and d2 as possible. That is one of the reasons we climb whatever mountains we may find to do /P operations. Using the formula d1=4.1sqrt(h1) where d1 is in km and h is the antenna height in meters over the horizon, you can calculate the distance for line of sight, taking refraction into account. This formula gives you:

As you can see, d1 and d2 are small compared with the wanted 700 km DX even from high mountains. Typically a 700 km troposcatter QSO is done with two legs of line of sight less than 50 km and one scatter leg of 600 km. One conclusion of this is that mountain climbing is not really necessary to work troposcatter DX. As the highest mountains in Denmark are hills of less than 200 m there is still hope for us OZ-contest teams. And the PAØs may go ahead and work DX from sea level. The theory backs up the fact that lowlanders can do well in contests. As the distance between the stations increases, the scattering takes place higher and higher in the troposphere. For the longer DX QSOs, the lowest part of the scattering volume is several km up. One effect of this is that a high mountain at the midpoint between the two stations has no influence on the signal. Thus troposcatter can span the Alps with no problems as long as both stations have a clear view to the horizon. So daily transalpine QSOs are no wonder, just troposcatter QSOs. As troposcatter loss increases with the scatter angle, you will obviously want to have as small a scatter angle as possible.

This means that the two radiation angles should be kept as small as possible since every degree cost 9-12 dB. If there is a mountain in the direction of the other station, you must have a high radiation angle to shoot over the mountain and this will cost you many dB in signal strength. Even though knife edge diffraction may bring your signal around the mountain, the extra 30-50 dB diffraction loss will prevent you from working real DX. You should look for a QTH with a clear view to the horizon at sea level. You may even have a small negative viewing angle to the horizon from a high cliff overlooking the sea.

Let's say you have found such a super QTH. The next problem is to radiate as much power low at the horizon, where the radiation really counts. The second part of this article examines the radiation angle of your signal, and considers how to maximise your chances of making troposcatter contacts.

Radiation angle, ground reflection and ground gain

For DXers on HF it is well known that the radiation angle of a horizontal beam is a function of its height over the ground. Since our beams on 50MHz are horizontal this applies to us, and in the following I only deal with horizontal antennas. The angle of maximum radiation is the angle where the transmitted signal and the signal from the ground image of the antenna are in phase.

Flat Ground

The following radiation angles can be calculated for a QTH on a flat ground:

The radiation angle on 50 MHz from a normal amateur tower is much higher than the (for troposcatter) wanted 1º or less. To have such a low radiation angle it requires a tower of 10-15 wavelengths, which is impossible on six metres. A horizontally polarized antenna many wavelengths up will have a radiation pattern with many loops in the vertical plane. For troposcatter you want to put as much power in the lowest loop, which requires a beam with a narrow free space beamwidth in the vertical plane. This can to some extent be obtained with a long boom length, but much better with many vertical stacked beams or a dish for the higher frequencies. For 50MHz, none of this is easy. You may be able to stack two beams, but a vertical stack of four 9 or 11 elements is almost out of the question, as the stacking distance is 7-10 meters for each. But do stack beams for troposcatter on 50MHz if you can manage it. The angle of radiation for horizontal polarization is only dependent on the antenna height over reflecting ground. Using stacked beams will concentrate the energy at the lowest radiation angle determined by the average antenna height, so stacking is recommended but stacking does not 'pull down' the radiation angle significantly.

Sloping ground

If the reflecting ground is sloping down low radiation angles are possible without a high tower and you may even get a vertical radiation pattern without loops. If you have a hill with a long enough slope of 20º your antenna needs only to be ½ wavelength up and it will illuminate all radiation angles from less than 1º to 10º. If you put your antenna up higher at this QTH, you may get loops and consequently also some minima in the vertical radiation pattern. You may even get a minimum at 0º, not quite what you went to the hill for. So do make some calculations for your QTH. Believe it or not, at some QTHs even a small mast could be too high!

Do you not live on a hill gently sloping in all directions? To get low angle of radiation then you should go /P. Then the dead 50MHz band will open for you if you bring reasonable power, which is at least 100W. Your worst problem may be that no one else knows troposcatter QSOs are possible on 50 MHz, so they are not active or only listen on 50.110. According to the bandplan 50.110MHz is an intercontinental calling frequency, and you obviously do not make your troposcatter CQ for 'only' 700 km QSOs on that frequency. A practical comparison between a 15 m mast on flat ground and on top of a gently sloping 90 metre high hill showed the signal strength of ON4KST at 700 km to be an estimated 10-20 dB weaker at the QTH with a flat ground. 10 dB off a weak signal is very weak and we had to use CW. We have thus experienced, that going /P to a good QTH really pays off on 50 MHz troposcatter, just like theory predicts.

Diversity reception and ground gain

The commercial world often fight fading on troposcatter links with diversity reception, two or more antennas and receivers and selection of the best signal. The two antennas should be spaced 50-100 wavelengths so this is out of the question for amateurs. Also the fading giving problems for commercial service is short term fading, which causes digital links to lose a bit once in a while. As long as we are doing SSB or slow CW this is not our main concern, as we can wait a few seconds in order to copy the signal. If you want to do packet via troposcatter fading may be your concern, but then you most surely are not going to use 50MHz. When we receive a troposcatter signal with our antenna (stacked or not), we actually have two antennas, the real one and its ground image, spaced by twice the height of the antenna over reflecting ground. If this spacing approaches 50 wavelengths, that is the antenna height > 150 metres on 50MHz, then fading on the two antennas will be less correlated. Then you get no ground gain for receiving. For transmitting you will still have the ground gain though.

Radiation angles for Es, ms and aurora

It should be mentioned that the required radiation angle for Es, ms and aurora may be higher than for troposcatter, so a monster vertical stack could end up giving too low a radiation angle for other propagation modes where 5-10º sometimes may be needed. My experience is however that a stack of 2x6 elements works reasonably well on Es.

As long as signals are 59+, many elements do not count for much. If there is a minimum between the loops in the needed radiation angle you may, however, lose 20 dB or more. If the Es signal is not 59+, it may end up being so weak, you never notice what you missed. The simple solution to this is to put up a smaller beam that is low over the ground so it has a high radiation angle. If you are on a hill top make sure the reflection point of the high angle beam is on top of the hill. We have tried to switch between a HB9CV low over ground and the stack. The HB9CV is sometimes better for Es but certainly not always.

One other trick could be to be able to switch between using only the upper or lower beam of the stack. HF-DX freaks know about a BLU-switch (Both, Upper, Lower). This could be supplemented by feeding one of the antennas 180º out of phase by switching in a ½ wavelength phasing cable in one of the feedlines.

*FAI = Field aligned irregularities
*TEP = Transequatorial progagation
*EME = Earth Moon Earth

Table 1 - Summary of 50 MHz Propagation

Summary of 50MHz propagation

Troposcatter is just one of the many propagation modes useful at 50 MHz. Here is an overview from the easiest to the more difficult ones, that is from QRP to QRO (table 1). The next challenge after troposcatter is ionoscatter, which is similar to troposcatter but caused by scattering from irregularities in the ionosphere at 75-85 km of height. This may give you a range of 2,000 km every day, if you can provide 500 Watts to a 12 dB antenna. Ionoscatter has diurnal variation of signal strength, your best chances are at noon at the path midpoint. During daytime the height of the scattering media is lower, perhaps 75-80 km, so range may be lower but with better signal strength. Path loss increases below some 1,200 km due to increasing scatter angle, so a path from 1,200-1,500 km is the easiest for ionoscatter.

If you are capable of EME on 50 MHz and have your antenna at the right height, so you get a suitable radiation angle, you will experience some very interesting ionoscatter QSOs provided you can find someone in the other end with a similar setup. I am looking for sked partners! This diagram (fig 4) shows how many dB the troposcatter path loss is below the free space path loss at various distances from 100 - 3,000 km. For the longer distances ionosphere scatter is included, and this shows you some interesting possibilities on 50MHz for high power stations, as this scatter process is also there 24 hours a day all year round. As you can see the path loss for 700 km troposcatter is about the same as on 1,200 km ionoscatter. Next year I hope to present you with some ionoscatter experience.

Conclusions

The higher noise level on 50MHz requires use of more TX power. The power limit in some countries does not allow full investigation and exploitation of the possibilities of the band. If amateurs are to study propagation we must be allowed to use the necessary power, which is at least the same on 50 MHz as on the other bands. Power must never be limited by ERP, as this prevent us from experimenting with big antennas.

Signal polarization is well preserved during troposcatter transmission. As you need the gain from a beam horizontal polarization must be used in order to work DX using troposcatter. A vertical antenna is no good for troposcatter DX if the other station has a horizontal beam, which is likely to be the case for a serious troposcatter DX partner. Low angle of radiation is very important: 1° lower increases signal strength 9-12 dB. Low angle of radiation requires an antenna high over good reflection ground (>10 wavelengths) if the ground is flat. This is difficult on Six. /P on a quiet hill top helps to hear the weak signals and the downward slope may give you the needed low angle of radiation, if you select your QTH using the information given. All together troposcatter on 50MHz is more difficult than on 144MHz or on the higher bands, but it is not impossible. Practical experience shows that troposcatter has the same characteristics as on the higher bands, if your antenna has a low angle of radiation. The needed radiation angle is different for the various propagation modes, and your antenna and /P QTH should be selected with this in mind.

Now you have experienced the theoretical part of troposcatter. If you want proof, that DX-QSOs can be made regardless of band conditions try looking for OZ9EDR or OZ5W: 4th Tuesday in the month on 50.160 SSB/CW, 19-23h local time

On 50.160 we go /P to a good QTH every 4th Tuesday of the month with an Icom IC736, a Henry Radio Tempo 6N2 linear with 2x8874 and 2x6 elm on a 20 m tower. This is almost a portable EME setup, and if you are within 700 km you should be able to hear us. We mostly have the beam to the Southwest (DL and PA0) shortly after 19h local and to the north east (SM and OH) around 21h. We will also be happy to work you on MS or ionoscatter and skeds may be arranged.

Acknowledgements

Many people helped providing the basis for this text. It was fun to write, but no easy crack. Food provided by Mac Donald's, beverages by the Coca Cola company and the midnight oil burned using Brazilian coffee. Thanks to

• Torben, OZ1BGZ worked many evenings converting my Henry 2002 linear into a 6N2, so we now have a QRO linear also for 50 MHz.

• The OZ9EDR/P contest team especially Børge, OZ4STA have been hilltopping every 4th Tuesday of the month during the whole winter in all weather, and thus provided the necessary practical experience. The team can tell you it is a special experience to raise one 6 ele beam in below zero degrees on a windy hilltop, and also about the beauty of artificial light on gently falling snow in the dark night, when the beam is taken down again. Not to mention the warm summer evenings, where the setup had expanded after much design and hard work to 2x6 ele on a 19 m mast. Again Torben, OZ1BGZ created many wonder mechanical parts, so the 100 mm mast can rotate inside its many guy wires. Not everything can be done by software!

• I will also thank Dave, OZ3SDL/G3SDL for lending us a linear before mine was ready and for providing us with operating tricks.

• PA2TAB has been providing us with a solid signal many times at 474 km. Tnx also for putting us on the cluster. BTW, we like list operations being the DX. '-)

• And last but not least Alain, ON4KST who called us many times since the first in December 94, where I would not believe my own ears and asked "Is it really you" and you answered "Yeah, Yeah it's me: 53 JO20EP". 706 km in the log, tnx. Let us keep on exploring weak signal and almost impossible propagation modes. We all learn from it. If you want to discuss troposcatter or make skeds I can be contacted at:
  E-MAIL: oz1rh@dialip.csc.dk
  Phone: +45 46 78 77 67 GSM during tests: +45 40 71 77 67
  
  73, Palle, OZ1RH (OZ5W - OZ9EDR contest team)
  OZ5W and OZ9EDR QRV JO55UL/JO64GX major contests and 19-23H local:
  1.st Tuesday of the month: 144.280+.305 2x8874 2x18elm M2+2/4x9elm
  2.nd Tuesday of the month: 432.180 3CX800A/4CX400A 4x20elm+2x21elm
  3.rd Tuesday of the month:1296.180 OZ1BGZ 70 W at 4x37 elm JO65AP
  4.th Tuesday of the month: 50.160 2x8874 1kW 2x6elm /P JO55KR
  GSM: +45 40 71 77 67 <- also during tests
  Internet mail to SMS: 40717767@note.sonofon.dk <- only during tests!
  Only Subject field is shown on my phone, so keep it short and expect
  no answer, but a QSO!
  Internet account home: oz1rh@dialip.csc.dk

Literature

• E. Fich and R. Ruddlesden, "The choice of aerial height for ionospheric scatter links", Proc. IEE, vol. 105, pt. B, suppl. 8, January 1958, p. 12-18 (the whole volume is 200 pages of interesting stuff on tropo- and ionoscatter)

• "Report of JTAC on Radio transmission by ionospheric and tropospheric scatter", Proc. IRE, January 1960

• Leang P. Yeh, "Simple Methods for Designing Troposcatter Circuits", IRE Transactions on Communications Systems, September 1960, p. 193-198

• Gordon, "A simple Picture of Tropospheric Radio Scattering", Transactions of the IRE, November 1954, vol. CS-2, Nr 3, p. 97-101

• The scatter propagation Issue: Proc. of the IRE, October 1955, p. 1175-1298 ff.

• Merrill, "Optimum Antenna Height for Ionospheric Scatter Communication", IRE Transactions on Communications Systems, March 1960, p. 14-19

• Roda, G (1988), Troposcatter radio links, ISBN 0-89006-293-5 (quite good)

• Reports of the CCIR, 1990, Annex to volume V: Propagation in non-ionized media, ISBN 92-61-04211-2, Geneva 1990

• CCIR Recommendations, Propagation in non-ionized media, ISBN 92-61-04531-6, Geneva 1992, p. 237-245, Propagation prediction techniques and data required for the design of trans-horizon radio-relay systems (9 pages with almost all you need to know about TS!)

Amateur literature

• Gannaway, J.N., G3YGF, Radio Communications, August 1981, p. 710-714 and 717. (recommended, most amateur literature uses this as source.)

• Gerzelka, UKW-Amateurfunk, 1981, München, ISBN 3-7723-6472-1

• The VHF/UHF DX book, 1992, Buckinghamshire, ISBN 0-9520468-0-6

• ARRL UHF and microwave experimenters handbook

• Jessop, G.R., VHF/UHF manual, 1983, ISBN 0-900612-63-0

• Weiner, K., DJ9HO, UHF-Applikation, UKW-Wellenausbreitung. UHF-Unterlage, Teil V. Scriptum der Vorträge, Weinheim 1994

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