Microwaves

Microwave antenna on tripod in field of long grassIn amateur radio, the bands above 1GHz are in the microwave and millimetre-wave domain.

They are the home of long range weak-signal work, home construction, precision propagation beacons, data links and ATV. Activity is not just confined to terrestrial modes – the microwave bands are also home to a high proportion of EME activity and high data rate satellites.

The microwave spectrum or GHz bands, in amateur radio terms extending from 1GHz to above 250GHz, is much more than a resource of short-range, broadband communication links. The idea conveyed by some, that microwaves are strictly for line-of-sight communication is simply not the case. Indeed, radio amateurs are the only group of microwave users to use this part of the electromagnetic spectrum for intercontinental communications using a naturally occurring propagation mechanism.

As with all aspects of the amateur radio hobby, ‘microwaving’ can be what you make it. It can involve ‘hill-topping’ with very simple equipment, the operation of data networks, voice and video repeaters, both digital and analogue, satellite communications, and DXing using CW, SSB and digital modes. This can lead to interests beyond normal amateur radio activity into related areas such as amateur radio astronomy and deep-space monitoring.

Background to the Microwave (GHz) bands

All UK amateur radio licence classes provide access to parts of the spectrum above 1GHz.

The performance of the GHz bands differs from that of the lower frequencies; this is reflected in the operating techniques employed.  While repeater operation is much the same, the characteristics of microwave propagation and the equipment used, requires different approaches to conventional station-to-station communication on the lower frequency bands.

Very simple equipment – which nowadays probably involves re-purposed Wi-Fi modules rather than the Gunn oscillator technology of thirty years ago – is still used for line-of-sight and very local communication, nowadays mainly on the 5.7GHz (6cm) band. By tuning the modules to a frequency for which we are licensed within that band, and making small modifications to support wideband FM operation, it is possible to make contacts of up to around 100km between suitable hilltops.

Another use for WiFi modules is to construct data networks. These usually operate in our 2.4GHz allocation, and can provide reliable communication. Networks are typically initially provided for high-speed emergency communication, but can also usually be accessed by any licensed radio amateur. An example of one of these is Feednet, which operates around the Firth of Forth.  The firmware and software which allows these networks to run is Open Source.

Amateur television repeaters operate within most of our bands below 24GHz. Many of these now use modern DATV technology. There is also a network of voice repeaters, mainly on 1.3GHz, using both analogue and digital modulation schemes. Spacecraft-borne amateur repeaters use the microwave spectrum, and the planned launch of satellites carrying microwave transponders into geostationary orbits – and even into orbits of the Moon and Mars – will make operation on the microwave bands even more exciting.

For a significant proportion of microwave operators, as with many people operating on the lower frequency bands, working DX using natural propagation mechanisms is a primary interest. This still centres around SSB/CW operation, but the use of weak signal ‘digital’ techniques is rapidly becoming more common.

 

Long Distance Propagation

  • Tropospheric Scattering. Most microwave station-to-station contacts beyond the line-of-sight take place using a very reliable propagation phenomenon known as ‘ tropospheric scatter’, or more informally, ‘troposcatter’. This involves scattering of the microwave signal from natural discontinuities in the upper part of the lowest layer of the atmosphere, the troposphere. SSB/CW stations on sites without major local obstructions, running 1W on 10GHz to a 60cm diameter TV dish, can reliably communicate over about 250km. This performance is obtained because the dish concentrates the majority of the transmitted energy into a narrow cone, typically around 2° wide in both the vertical and horizontal directions for a small TV dish at 10GHz. The problem in finding contacts is that, unless they are very local, the station will only hear, and be heard by, other stations within that beam. Some simple maths shows that, even ignoring the potential for errors in vertical alignment, the chance of two stations directing their beams at each other is small: about 1 in 32400! So, the practice of ‘talkback’ has developed.
  • Rainscatter: This literally the scattering of energy from rainfall or other forms of precipitation is common and can result in contacts over several hundred km. More about this can be found at – www.mike-willis.com/Tutorial/rainscatter.htm
  • Tropospheric Ducting:  In various forms this can vary from brief lifts in a specific direction to widespread openings covering much of Europe. See here for forecasts and maps – www.dxinfocentre.com/tropo_nwe.html
  • Aircraft Scatter: Scattering from aircraft is a regular occurrence. With feedback from aviation websites, and the use of suitable software, this can be predicted surprisingly accurately. Contacts of up to about 800km can be made by this method. http://www.airscout.eu/

Moonbounce (EME):  This provides the opportunity to make intercontinental contacts on the microwave bands. UK stations are active using moon reflection on all bands to 24GHz. Microwave ‘moonbounce’ does not require enormous antennas or transmitter power required on the lower frequencies – although both can help!. It is possible to make marginal EME QSOs with some of the larger stations using just a 1m TV dish and 10W on 10GHz, while an optimised 2.4m dish with 50W on the same band can provide SSB echoes. On the ‘low-bands’ at 1.3 and 2.3GHz, a 1.8m mesh dish with ~200W will provide many QSOs using CW or digital modulation schemes.

Satellites: There are numerous Amateur satellites. A number carry GHz band transponders and beacons.

 

And there is more …

The microwave bands don’t finish at 24GHz, although above 30GHz, it’s more technically correct to talk of millimetre waves. The UK Intermediate & Full Licences allow access to many bands in the frequency range to 275GHz. Contacts have been made on all our allocations at 47, 76, 122, 134GHz and 241GHz. Our mm-wave bands are usually located in regions of the spectrum where absorption of transmitted energy by atmospheric components such as water vapour and oxygen are significant, adding attenuation, ‘gas losses’, to the usual path losses. Despite the difficulties of generating significant transmitter power, making sensitive receivers and pointing antennas accurately, progress is being made and as this is being written, the UK distance record on 76GHz now exceeds 120km.

Minimising the number of failed contacts – or maximising success!

With narrow beamwidth comes the need for good control of antenna direction: ideally this should allow pointing of the antenna to within its 1dB beamwidth, or less. It is now easy to determine the direction of another station from its QTH locator by means of readily available software tools. Being completely in control of your antenna’s pointing direction in both azimuth and elevation is very important, as it removes one of the common reasons for a failed microwave contact.

Another reason for failed contacts is inadequate frequency accuracy. At 10GHz a simple crystal oscillator with ±10ppm frequency stability can lead to a frequency error of more than ±100kHz!  While this needn’t be a big handicap if you can calibrate the error by reference to an accurately maintained local beacon, increasingly microwave operators are locking their local oscillator to an accurate frequency reference, such as a GPS steered oven-controlled crystal oscillator. With this technique, frequency accuracies of a few tens of Hz can easily be obtained.

A third major reason for failed microwave contacts is timing. QSO attempts should always include an agreement between the operators as to who transmits – and who listens – at specific times. Often this takes the form of agreeing to transmit in 30 second or one-minute periods.

Talkback

In the past, talkback often took place via the VHF/UHF bands, but as technology has developed, it has become possible to work further under normal conditions on the microwave bands than on, say, 144MHz. Also, the microwave bands are more frequently subject to propagation enhancements than VHF; it is quite possible to experience an exciting ‘opening’ with big signals from 8 – 900km on, say, 10GHz while 2m is dead!   VHF is still used by some, but most talkback is now carried via the internet, particularly by the ON4KST chat room.

The use of talkback isn’t, in any sense, a substitute for making contacts. Passing QSO information via a talkback link will invalidate any ‘contact’ where this happens. Doing so on a link such as that provided by ON4KST will almost certainly provoke strong comment!

Access to ‘KST’ can be via its web interface. A very convenient way to use ‘KST’ is to do so via the Telnet client written by OZ2M – www.rudius.net/oz2m/software/kst2me.  The use of Telnet has the great advantage of minimising data usage. This is very helpful when using ‘KST’ from portable locations.

Beacons

There are a surprising number of propagation indicator beacons covering much of the UK, operating on all microwave bands from 1.3GHz to 134GHz. Most UK beacons are now frequency locked to GPS derived frequency standards, so their frequency can be relied-on to a few tens of Hz.

A key resource is www.beaconspot.eu, provided by a member of the UK Microwave Group, listing the callsigns, frequencies, and operational status of VHF/UHF/microwave beacons throughout Europe, as well as worldwide 6m beacons.