South Yemen and the Wadi Hadhramout - A mobile radio system operating in a severe environment.
This article describes the installation of a VHF (band III) mobile radio system, tailored for local conditions, which now forms part of the scheme for the improvement of living conditions in the Wadi Hadhramout, Peoples Democratic Republic of Yemen (hereafter referred to as the PDRY)
First published in the house magazine of Marconi Mobile Radio, 1985
South Yemen has never been an easy place to make a living, and this is as true today as it would be, say, 200 years ago. The oil boom that financed the economies of other Arab countries has not improved the lot of the Yemenis, since, to date, no commercially viable oil prospects have been found. In fact the only natural resource the Yemen possesses, could be said to be the people themselves.
Photo: Clive Warner
In many of the oil states much of the labour required for development is provided by migrant workers from Yemen; those who have worked with Yemenis would, I think, generally agree that they are good-natured, hard-working, talented people.
The interior of the country still remains almost trackless desert, part of the aptly-named 'empty quarter'. It is only recently that new and ambitious development schemes have begun to open up the region.
Most visitors to the PDRY are concerned with business in the capital, Aden, and so not venture into the two other areas of interest: those being, first, the '5 towns' coastal area centred at Mukalla; and secondly, the mountainous central region, known as the Hadhramout.
Both areas were difficult to reach overland until recently, but new roads have considerably improved access.
The Wadi Hadhramout is an irregular canyon, with numerous side branches, stretching for well over 100 Km. My first arrival was by air. Alyemda, the national airline, ran a regular service. It seemed extraordinary, after flying over vast distances of apparently featureless desert, to suddenly encounter the narrow Wadi, with its lush tropical vegetation and farms.
Once on the ground, it became apparent that the Wadi is still barren in many areas. Although the area does receive some rain each year, this occurs at well defined periods, usually lasting a few weeks (or days!) only, and is sporadic.
A solar-powered repeater station in the Wadi. (Photo: Clive Warner)
Installation by the author, shown standing in front of the blockhouse.
I had to survey the entire Wadi and discuss possible repeater locations with the customer.
My first choice proved to be an ancient monument! Generally you find that ideal repeater locations have been military posts in the past.
Number one repeater was to the north, and I found a reasonably substantial mound, about 7m high, on which to place my state-of-the-art and environmentally-suitable antenna: a telephone pole with a vertical co-linear mounted on top.
The repeater hut was built in just a few days from cement blocks.
I mounted a bank of three Lucas solar panels on the flat roof.
These charge a large bank of deep-cycle batteries inside the repeater.
The rain, when it does come, often causes more harm than good, since it is likely to be torrential but quite brief, leading to flash floods that wash away sections of road. Any civil works in the Wadi must take this into account.
This is only one adverse factor. For most of the year the temperature is extreme: reaching perhaps 50C at mid-day, during the summer. It is practically impossible to work in the open between 12 and 2 pm. The humidity is very low; there is almost no moisture content in the air. While this makes the high temperatures more tolerable, it exacerbates the second hazard -- dust storms.
The soil is very friable and, where not held together by vegetation, unstable. The sheer walls of the Wadi channel the natural winds, and as the air velocity picks up, it gathers a heavy load of fine dust, the consistency of talcum powder.
As an example of this problem, I was completing the installation of a solar-powered repeater station, located at the natural intersection of three branches of the Wadi. While this was a great location to maximise communications coverage, it did have one serious drawback.
This became apparent late one afternoon when I became aware that in the distance, what appeared to be a solid wall of reddish material had blotted out the sun, and was advancing rapidly in my direction.
The Yemeni technician with me obviously thought that it would be a good idea to get out as fast as possible; but by the time we had closed up the building and got in the off-road vehicle, the storm was upon us. Visibility dropped to about 20 cm. It was impossible to drive so we sat it out, breathing through several layers of our 'mushedda' headcloths (essential equipment. The Arabs know their terrain and how to survive in it.) When conditions improved we were able, with some difficulty, to make it back to base.
I returned the following morning. Despite the fact that the doors and windows of the repeater building were closed, and fitted well, the entire equipment was buried in a layer of dust several mm thick. In fact, it looked as if the equipment had been there 100 years already! It was at this point that I hoped that the equipment was as robust as MMR (Marconi Mobile Radio) had assured me.
A view of the Wadi.
Photo: Clive Warner
I believe this view is looking south, from the northern repeater.
Notice how the floor of the Wadi is quite flat, hemmed in by near-vertical walls on both sides.
This is quite a challenging environment in which to emplace a mobile radio system.
Map of the Wadi.
Photo: Clive Warner
The grayscales are actually different colours of crayon in the original; different colours represented primary and secondary service areas achieved by the finished system.
Some branches evidently hadn't been visited in ages. The whole area is rather Biblical anyway, but the branches at that time had never had electric power.
Sometimes we were fêted by locals when they saw the 'elektriki' (electricity) sign on the vehicle.
WATER AND POWER
The inhabitants of the Wadi grow the bulk of their own food. provision of reliable electric power is of vital importance to them, since it supplies the pumps that provide water. The government commisioned (in 1981/2) a 16 MVA plant to supply the area.
For generations past, the Wadi folk were able to irrigate small areas by digging wells down into an aquifer lying close to the surface (in some cases, only 3 - 4 metres down.) This was well within the capability of simple lift pumps, usually powered by small, fixed, petrol or diesel engines. Originally, of course, draft animals such as donkeys would have provided motive power (and in some cases, still do.)
The problem is, this shallow aquifer is unreliable, and often fails. The amount of water available from that level is limited, and attempts to increase the area under cultivation would lead to rapid depletion and crop failures.
Fortunately there is an alternate source, a second aquifer, with a far greater capacity. The snag however is that this aquifer is deeper, perhaps averaging between 60 and 90 meters depth. The simple lift pumps cannot raise water this distance. Instead, submersible pumps must be used.
This fact led to the development of small 'village' power stations. Local line networks were installed to supply groups of submersible pumps, immersed in the deep wells bored into the second aquifer.
Sayy'un - where I lived during my stay in the Wadi.
The system was adequate in the early years, but as population pressures increased, researchers found that the deep aquifer could support more extensive irrigation than originally believed. Therefore the government of the PDRY, led (at that time) by his excellency President Ali Nasser, decided to replace the scattered low-power generating plants with a unified distribution system, supplied by one central generating facility.
High reliability and rapid repair of any failure, is prime. Loss of water supply to a farm will rapidly result in loss of crops. A high quality communication system was required to provide links between: sub-stations and power house; between mobile crews and power house; and to provide talk-through between mobiles out of direct range of each other.
While it would have been possible to provide communication between power house and sub-stations using the HV lines, this was ruled out due to expense, and also because it could not provide the mobile requirement.
We decided to use a VHF (band III) system operating in simplex mode, but capable of full duplex operation when required, by simple modifications.
Two identical transceivers with instant changeover were installed in a common cabinet. The two remote substations, housing 11KV and 33KV switchgear, were equipped with base stations.
These were the most interesting part of the system. Designed for unattended operation, they possess dual transceivers, auto control, and full duplex operation withcavity filters. They provide full talk-through and relay functions using a single antenna. The power house is able to monitor at all times, and break in if necessary.
The first task was to decide on locations for the two repeaters. This involved a number of conflicting issues. Since both are identical, it was necessary to locate them so that the topography of the Wadi would prevent direct LOS (line of sight) between them.
Secondly, they had to be as far apart as possible, to minimise the 'common ground' area. (The region where a mobile might be able to energise both at the same time.)
Third: A number of side-branches of the Wadi had to be serviced, and so ideally, the repeaters should be at the junctions of these branches.
Armed with a large scale map, compass, surveying poles and the inevitable lump hammer, I set off with our local techs to find a place for the first site. We had already made a tentative decision from the map, but were surprised to discover near our intended location, a large earthmound.
The additional height allowed us to get more antenna height. When we tested it, we found this repeater gave us much better line-of-sight than we had expected.
A repeater station
Photo: Clive Warner
REPEATER POWER (SOLAR)
A solar array, manufactured by Lucas BP Energy Systems, provided power. The complete kit as delivered to site contained every item required, nothing had been left out, an example of high-quality British engineering. The system requires almost zero maintenance, but since the repeaters are so vital, an alarm system was built in.
Automatic alarms sound in the power house if the repeater batteries are under or over pre-set voltage limits.
The arrays themselves have good fail-safe capability. Formed from several parallel-connected panels, in the event of damage, then as long as one or two remain serviceable, battery charging at reduced rate can still be maintained.
The batteries power the equipment during night time or (rarely) heavy cloud conditions. A generous reserve of five days was allowed for no-charge conditions.
Even under heavy overcast, I was surprised to see a charge rate of 55% compared to full sunlight, showing that considerable energy is still available at the long end of the spectrum.
With construction of the first 'repeater house' under way, the power-house base station and mobiles were rapidly completed, enabling the team to make partial use of the system. Rather like lifting yourself by your own bootlaces...
The first remote substation was surveyed, located near the town of Al Quatn.
Fortunately, construction of the building had not yet begun, since it turned out that the location was shielded from reception by a wall of solid rock. Happily the authorities in the Wadi proved very co-operative and after little more than one hour we received authorisation to buy an alternative site. Field strength tests proved the new location to be very suitable.
Meanwhile, the building for repeater #1 was completed in record time by a local contractor. This was near Buheira, which is close to:-
THE HISTORIC CITY OF SHIBAM
Constructed of adobe (a mix of mud and straw) this city features skyscraper buildings of seven levels or higher. As the visitor rounds a bend in the Wadi and is presented with this sight, it seems as if a mud Manhattan has risen from the desert.
Ironically this magnificent monument to the ingenuity of the Hadhramutis is now in danger of collapse. This is due to the improvements in water supply. Now that piped water is available for the first time, water consumption has risen considerably.
Much of the wastewater filters into the subsoil, which already shows signs of instability. Experts are studying the problem with a view to beginning a wastewater disposal project. Please see the linked websites for further information as to Shibam.
We had now completed the first repeater without trouble. This gave us mobile to mobile transmission for the first time. Using the new facility, we began field surveys to confirm the site for the second repeater.
We had already decided on a tentative site, but in the event, were surprised to discover that at the proposed location, we were able to receive a very adequate signal directly from the power house. This, despite the site being cut off from LOS by the curvature of the walls of the Wadi. Evidently there was some reflection taking place from the far walls, possibly from a mineral deposit or man-made structure.
A repeater may also function as a base station by simply plugging a mic into it, so we decided to install the second repeater in the nearby sub-station, thus giving two considerable benefits:
a) We saved an entire base station equipment set, allowing us to extend the coverage far beyond the original plans. A real bonus for the customer!
b) We saved the time and cost of construction of a second repeater building.
TARIM - CITY OF MOSQUES
We installed the (now spare) base station in the city of Tarim, getting some 30 Km more range than originally planned. Tarim is another superb example of adobe construction, except that this time, the dominant feature is the scores of mosques. Were their elegant towers and minarets to have been built with modern materials, they would be impressive, but to fashion such graceful yet enduring structures from mud and straw is truly extraordinary.
THE CAR CRASH
At this time, feeling slightly complacent, we received news that President Ali Nasser would be arriving to open the power station -- 10 days sooner than expected!
The only work we had outstanding was the completion of the second repeater/base installation, plus final commissioning, coverage survey, and handover.
Returning from the substation the following evening, in company with Ali and Jawad, the two line engineers who had been providing assistance, we had just passed the halfway point.
Suddenly a gust of air blew off the driver's hat. He leant over to grab it, put the wheel over at the same time, and we headed right into a concrete culvert, open topped, more than a meter deep and two meters across.
This is it, I thought. Someone else will be finishing this project. With a tremendous bang the front wheels hit a concrete rim at the edge. While I don't remember flying through the air, I do recall the pain of my head hitting the roof when we landed, miraculously, on the other side. Allah must have been looking out for us. Except for bruises, we had no injuries, but the vehicle was wrecked. The axle had broken and the wheels looked like pop-art sculptures.
One thing still worked: The mobile radio. We called for help, and the power house staff sent a vehicle for us.
With four days to go, we completed the final works. The second repeater was being sustained during the night by a somewhat 'foxed' truck battery, since we had cleaned out the only battery shop in the area when we filled the first set of solar batteries with acid.
This effectively tested the low battery alarm, as the truck battery would run low shortly before sunrise, and the repeater then called the power house.
With two days to go, we located some more acid, and were able to commission the entire system. For me, this was one of the most interesting jobs. It involved traversing every part of the Wadi by 4WD vehicle, to determine the coverage limits. We prepared a map from the results (I still have a photo of it) and wrote a system handbook for the operators.
Finally, we were able to sit back and consider the job very well done. The Yemen is a fascinating land, with kind and hospitable people, and views of magnificent scale and grandeur.
all material is © Clive Warner unless otherwise noted.
The colour photos are not my copyright - they are reproduced with permission from the original photographer.