VHF/UHF radio communications using the troposphere (1950 – 1967)
At the beginning of the 1950s, the radio group of the TNO Physics Laboratory started researching the propagation of VHF-UHF waves over large distances beyond the horizon. That is, distances on the order of 30 to 50 times the radio horizon distance as viewed from the transmitter or receiver antenna. This research was set up because it would be important for future military radio communications. Inherent to this research was the development of very low-noise receivers for the VHF and UHF transmission bands. The horizon was at a distance of 12.8 km [Gratama] as seen from the laboratory’s 10 to 20-metre high transceiver antennas.
In the beginning, field strength measurements would be made using 3-meter transmitters at a distance of 100 to 200 kilometres. At that time there were no official 100 MHz transmitters in the Netherlands. Sporadically, some radio amateurs broadcasted in the 144-146 MHz frequency band. That is why an own VHF transmitter, the PE1PH, was built. It operated on 144.00 MHz and had an ERP (Effective Radio Power) of approximately 4 kW. A rotatable one-sided beaming and horizontally polarised Koomans beam antenna was used. It had five phase-fed full dipoles mounted above each other at a distance of half a lambda in front of a reflective screen of horizontal wires. The gain in the forward direction was 15 dB (32-fold); the background radiation suppression was approximately 28 dB. Measurements were conducted with this transmitter in close collaboration with the radio amateurs active in this frequency band.
The former PTT became interested in the behaviour of the VHF-UHF waves in connection with the development of FM for broadcasting and TV. Three 146 MHz measuring transmitters with an ERP of 400 W were installed in Hoogezand, Hengelo and Hulsberg. Relative to Waalsdorp at distances of 204, 172 and 172 km.
The Netherlands is small. Therefore, trials with connections over distances larger than 200 km require cooperation abroad. It was not possible to find interested parties through official channels. Foreign radio amateurs were called in for help, for communicating over distances of 250 to 500 kilometres.
Also, an experimental 70 cm (432 MHz) UHF transmitter was built to operate in the amateur band. This was linked to the 8.5-meter diameter Würzburg-Riese antenna of the laboratory. Parabolic a diameter of ~8.5 m, a gain of ~28 dB (700 *), and an ERP of ~100 kW.
Hard work was put in designing and building low-noise receivers (in fact complex tube voltmeters) given the very weak received signal. More information about the 145 MHz (2 metres) and 435 MHz (70 cm) receivers can be found in an article [in Dutch] by S. Gratama.
Some highlights of the research include:
- Long-duration measurement series.
During periods of three years and even sometimes longer, a large number of propagation measurements have been made over distances of 20 to 500 km. Two-way communication was tried one or more per days with transmitter stations in the Netherlands, Belgium, England, Germany, and France. The furthest stations were Paris (405 km), Würzburg (472 km), and Preston and Oswestry in the UK (510 km). From time to time, communication was made over even larger distances with stations in Switzerland. Based on the results of many measurements, a (formula for a) transmission damping curve could be drawn up.
- Experimental VHF connection with Norway.
With Norwegian assistance, a broadcasting station was built on the island of Tromöy, near the Norwegian town of Arendal in 1956. The antenna had 120 dipoles in phase with the reflector (28 dB) behind it (see the old photo below). The three masts were placed 8 meters apart; each was 25 metres high. The frequency was 150.00 MHz. The output of the FM transmitter was 3.5 kW. The carrier was “Frequency Shift Keying” (FSK) modulated with only two positions that differed 85 Hz from each other. The data rate was only 75 characters/s (75 Baud). The received and demodulated signal went directly to a Teleprinter running at this speed. At that time, the abbreviation FM RTTY (Frequency Modulation Radio Teletype) was used for this type of setup. A 30 metres long NKF coaxial power cable (34 mm aluminium/16 mm copper) was used. The distance covered by the transmitter was approximately 800 km. The large antenna aimed at the Netherlands provided an effective radiated power (ERP) of approximately 2.3 MW. The receiver was at the Physisch Laboratory at The Hague Waalsdorp, Netherlands. The connection was established in 1956 and was closed down in mid-1959.
- Troposcatter installation for the Dutch Armed Forces
In 1959, two complete troposcatter communication posts were built for use by the armed forces. Frequency diversity was used, whereby two frequencies were transmitted simultaneously and received at the ‘other side’. The so-called “fading” of the troposphere is different at different frequencies.
In the frequency range from 80 MHz to well above 10 GHz, there appear to be more paths in the troposphere, as a result of the dispersion (partly due to turbulence) of the dielectric constant of the moist air, a property that slows down the propagation speed relative to the speed of light. Due to the difference in delay of two such paths, extinction can occur on one frequency and at the same time addition at another. Interesting in Gratema’s story (see reference below) is the figure on page 147. A sharp directional antenna does not help because the waves come towards the antenna thanks to those paths with a larger opening angle.
By now connecting each of the two receivers to its own loudspeaker (or headphones), it is more likely that one of them will produce the desired sound. A technical challenge was building the “crossover filters” to connect two transmitters or receivers to a single antenna.
In 1959 two complete troposcatter communication posts were set up for use by the armed forces. Frequency-diversity was used. Each station had two transmitters and two receivers, all operating with a single, rotatable directional antenna. Initially, this equipment was intended for the Royal Netherlands Navy, which was interested in this technology. In 1963, the Navy’s interest in a troposcatter system diminished because of all kinds of tactical objections. The so-called trapo-installation was then made available to the Royal Netherlands Army. During the years 1964 to 1967, it has been extensively tested over various routes. It was most recently in use on the Seedorf route at the 1st Army Corps, West Germany and the Physics Laboratory (distance 360 km), whereby telex was used in addition to telephony. A high-quality telephone connection was possible for over 90% of the time. In mid-1967, this test connection was decommissioned and the installation was converted for use in the tactical satellite communication project.
Tactical Satellite Communication (TacSatCom)
On January 4, 1967, the Dutch Defence Council decided that the Netherlands would participate in the NATO Tactical Satellite Communications (TacSatCom) project as presented by the United States in November 1966. The Netherlands signed the “Memorandum of Understanding” with the United States on November 10, 1967. The project was a Research and Development project that aimed to investigate the extent to which satellite communication could be used for radio connections with and between smaller tactical units such as eg. vehicles, submarines, aeroplanes, and surface vessels.
The scientific program included:
- Propagation effects such as absorption, scintillation, “Faraday”-rotation, etc.
- The multiple access to the satellite problem
- Electronic Countermeasures (ECM). This included deliberate disturbing a satellite that is of high importance from a military point of view.
The launch of the relevant geostationary communications satellite, the LES-5 (Lincoln Experimental Satellite) took place on July 1, 1967.
The operating frequency, as well as that of the later satellite (the LES-6), was in the 240 MHz band. Already on July 3, the beacon signal, as well as the test signals from some American stations, were received at the Laboratory. On July 27, 1967, the Physics Laboratory could transmit with a power of 3 Watts. This setup established a good telegraph connection via the LES-5 with the British station G4P of the Royal Aircraft Establishment at Farnborough. Shortly afterwards, a connection with the United States was also established. On August 4, the full transmission power of 600 Watt was available. Many connections have since been made and countless experiments conducted, often in collaboration with other nations such as the United States, Canada, the United Kingdom, West-Germany, Belgium, and Italy.
Initially, the permanent observation station PAG-27 was used equipped with our Würzburg Riese antenna as a parabolic reflector. This station had a large effective radiated power (120 kW) and gained international fame. The PAU-21 station was completed at the beginning of 1967, housed in a 3-ton military truck. In 1970, a military jeep was added, equipped with a small fully transistorised transceiver. This transceiver was developed and built by the Laboratory, just like a demountable helical antenna. This third station had the call letters PAP-22. The dimensions of the transceiver were 42 x 40 x 10 cm, the weight was 12 kg, the transmission power was 80 Watts. Capacity: one very good quality telephone channel; one or more telex channels.
In the period 1968 · 1973 numerous TacSatCom demonstrations were given in national and international contexts with the three transceiver stations.
In 1971, the PAP-22 transceiver station was placed aboard the HNLMS Evertsen (F815). A rotator kept the antenna aimed at the satellite irrespective of the ship’s course. The ground station was stationed in Noordwijk, The Netherlands. The connection from the ship with Noordwijk and other TacSatCom stations went so well that they continued to be used by the Royal Netherlands Navy on several ships until 1975, the year in which the satellite LES-6 was decommissioned.
In 1971, the National Aerospace Laboratory (NLR) developed an electronically adjustable antenna as part of the TacSatCom project. the antenna that was installed and tested on a Fokker F27 ‘troopship’. The TNO Physics Laboratory cooperated in this project. The project officially ended in 1975 after extensive reporting about the project results to NATO. Despite the attractive possibilities offered to small stations by a satellite in the UHF band, in 1977 TacSatCom had not (yet) reached operational applications. One reason is the limited number of channels offered by the UHF band.
S. Gratama (1957), Troposferische voortplanting van VHF- en UHF-radiogolven ver voorbij de horizon en enkele praktische toepassingen, NRG, mei 1958, no 3, p117-185. <pdf [Dutch]>
Physisch Laboratorium 1927 – 1977 <pdf [Dutch]>