Today we discuss the SHF band. Wavelengths are between 1 and 10cm. This band is mainly used for radar, microwave point-to-point links and satellite traffic (both telecommunication and television).
Last week we saw the first two microwave bands (L and S bands), this part of the spectrum has many more of them. These band names are often used when referring to radar and satellite frequencies.
Apart from short range communications (such as Wifi), antennas in this part of the spectrum are highly directional. Conventional coax cables and conventional dipole antennas are seldom used in this band. At the upper end of the SHF band a half-wave dipole is only 5 millimeters long and a full-size Yagi antenna would be the size of a fingernail. In reality this type of antennas is not used in this part of the spectrum. We use waveguides (metal pipes) instead of coax cables to move a radio signal between the transmitter (or receiver) and the antenna and the antenna itself is a feedhorn. Large horns can provide high directionality. They are mainly used by radar installations.
More antenna gain is provided by parabolic reflectors. A parabolic reflector concentrates all energy from a single direction into a single focal point where the antenna itself is located. Conversely, all energy radiated from that single focal point is reflected by the parabolic reflector into a highly directional beam, not unlike the reflector found in a flashlight.
An antenna is said to have gain if it concentrates most of the radiated energy in the desired direction. No antenna can really amplify the signal and no antenna can radiate more total power than it gets from the transmitter. But an antenna can concentrate nearly all energy into a very narrow beam radiated in a single direction. Therefore the intensity of the radiated energy in that direction is much higher than without gain. A high gain antenna can also concentrate more of the received energy from one direction into the receiver. Large parabolic dishes have a higher gain than smaller ones. They can radiate the transmitted energy into a narrower beam and they can collect more energy into the focal point where the receiver can pick it up.
The ability to create such high gain antennas makes SHF very suitable for long distance communication into a single direction, but only if most of the distance travelled by the radio wave is not through the Earth's atmosphere. This means satellite communication. SHF (and EHF even more so) is absorbed by the atmosphere, especially by rain. This limits the distance through the atmosphere to a few tens of kilometers and it absolutely has to be line of sight. A few tens of kilometers through the atmosphere highly attenuates the signal, but thanks to the high gain antennas, communication is still possible. On the other hand, terrestrial broadcasting would not be possible.
There are several bands for terrestrial microwave links. These microwave links are point-to-point links, for instance between several offices of a single business. Also the backbone of the GSM network (that connects the cell transmitters to each other) consists largely of terrestrial microwave links. Each end of the link has a parabolic dish and these parabolic dishes point straight at each other.
In The Netherlands, several bands are available for microwave links, from 6GHz to 38GHz. The longer the distance, the lower the frequency band, as lower frequencies are attenuated less by the atmosphere. Distances of up to 45km are possible, but such a long distance requires really tall antenna towers (height 80m).
The 5.8GHz band (wave lengths just over 5cm) is another ISM band. As in the 2.45GHz band, low power users are also permitted. This band is now used for Wifi (wireless LAN) as the 2.45GHz band has become very crowded.
The SHF spectrum contains four amateur bands. The higher the band, the fewer amateurs make use of it. Next week's EHF amateur bands are even more exotic and used by fewer radio amateurs still. The 3cm band is extremely wide (0.5GHz), so it can be used for wide band experiments. No commercially made amateur equipment is available for any of these bands, so radio amateurs have to homebrew their equipment or to repurpose microwave equipment that was designed for other purposes.
Most radars work in the SHF bands. though there are some radar bands below 3GHz. There are radar systems form many different purposes, such are shipping, aviation, weather analysis and highway speed traps. There are long range radar systems to track ships or airplanes in a range of tens (or even hundreds) of kilometers and there are shorter range systems. A radar aboard a ship usually has a shorter range than the radar found on the shore. Different radar systems use different frequency bands. Usually the longer range radar system use lower frequency bands.
Radar can be used to measure the distance of an object by measuring the time interval between the transmitted pulse and the received echo. Early radar systems were analog. For each transmitted pulse they wrote a scanline onto a cathode ray tube from the inside to the outside of the screen in the direction the (rotating) antenna was pointing at that moment. The cathode ray tube had a very long persistence time, so the echoes would be visible for many seconds, until they ware refreshed by the next rotation of the antenna. Modern radar systems are fully digital and use conventional computer displays. They show additional information such as maps of the terrain and transponder results from aircraft.
The radar echo reflected by a moving object has a slightly different frequency from the transmitted pulse. If the object is moving towards the radar, the echo has a slightly higher frequency, if it is moving away, the echo has a slightly lower frequency. This is called the Doppler effect. By accurately measuring the frequency of the echo it is possible to measure the velocity of an object. This is used around the world by traffic cops and speeding cameras. The most widely used frequency for this type of radar is 24.1GHz (K band), but other bands are also used.
Radar detectors are illegal to sell and to have installed in vehicles in The Netherlands, like it is in most countries around the world. Receivers for radar frequencies are only illegal if their apparent purpose is to warn for speed radars. Other receivers that could possibly pick up those frequencies are legal. As radar detectors contain microwave oscillators that are not well shielded, they can be detected from a certain distance by another receiver, The Dutch police use such receivers (radar detector detectors) to catch motorists who use a radar detector illegally.
In the mid 1960s the first communication satellites were launched. Communication satellites are located in the geostationary orbit at 36000km height, right above the equator. Satellites in this orbit move at the same speed as the earth rotates, so they appear to stand still when viewed from the earth. The parabolic dish antenna of a ground station has to be pointed exactly at that satellite, but once it is pointed, it does not have to be adjusted all the time.
Intercontinental telephone cables already existed at that time, but their capacity was rather low. Satellites added to the available capacity considerably. Islands without submarine cables could now be well connected to the telephone system. They had to rely on shortwave radio links before. Satellites made intercontinental television links possible. The telephone cables of that era could not carry video signals.
In the 1990s it was technically possible (but not legal in The Netherlands) to receive signals from analog telecommunication satellites and hear international telephone conversations. This was one of the very few radio signals that you were not allowed to listen to in The Netherlands.
The importance of telecommunication satellites has decreased somewhat due to the availability of high capacity fiber optic cables. The signal path from earth to satellite and back to earth again adds about 0.25 seconds of delay, which is just starting to get annoying for telephone calls. A transatlantic fiber-optic cable only adds a delay of about 20 milliseconds.
In the early 1970s the first television programs were transmitted by satellite in the USA. These were not intended to be received by consumers, but rather by cable companies, so they could further distribute them to subscribers. These satellites operated in the C-band around 4GHz. Reception required large parabolic dishes (around 3m diameter). Some hobbyists did build their own satellite receiving stations though.
Nowadays TV satellites operate around 12GHz (downlink). In Europe the official satellite TV broadcast band runs from 11.7 to 12.2GHz, but frequencies between 10.7 and 12.2GHz are in use. Transmitters in satellites are more powerful and receivers are more sensitive than in the 1970s, so a dish size of around 60cm is sufficient to pick up satellite signals for which you are in the target area. Satellite signals for which you are not in the target area (for instance those aimed at the Middle East when you are in Europe) require large dishes, say around 1m diameter. Satellites contain transponders with both horizontal and vertical polarization, so they can transmit two signals on each channel. Satellite reception is degraded by heavy rain.
The first stage of the receiver (that contains a preamplifier and a down converter) is called the LNB and it is mounted inside the satellite dish. The signal from the dish to the satellite receiver is downconverted to the range 1-2GHz. Therefore such a satellite receiver can pick up signals from the 23cm amateur band without additional converters.
Broadcast satellites (like those operated by SES Astra) were introduced in Europe in the late 1980s. at first the signals were analog. The video signals used ultra wide band FM (27MHz channel width). Few countries still broadcast analog satellite signals.
In 1989, commercial broadcasting was still prohibited in The Netherlands. RTL in Luxembourg introduced the station "Veronique", later called RTL-4 with Dutch programming. As this station originated from a legitimate foreign broadcaster, cable companies in The Netherlands were allowed to distribute this program to their subscribers. Some other satellite stations were successfully banned from Dutch cable networks. When commercial television could no longer be kept out of Dutch homes, commercial television stations eventually legalized in The Netherlands.
A satellite television signal can be picked up throughout Europe, but television stations would have to pay a fortune to the rights holders when they transmit third party programs. Therefore most broadcasters want to make sure that their programs can be viewed only by a limited number of viewers (only those in the target countries), so they use encryption. Encryption of analog signals is rather weak. The need for encryption was one reason why digital video was adopted early in satellite TV, increased capacity was the other one.
In The Netherlands all public and commercial television stations and most radio stations are available on satellite, but nearly all television stations are encrypted. Only the station BVN (Beste Van Nederland, Best of The Netherlands) is free to air. The German and British stations on the other hand, are free to air. Subscriptions to the Dutch satellite stations used to be available to residents of The Netherlands for a small fee to cover administration costs, but this fee was raised considerably over the years. It is still cheaper than cable television, but far from nearly free.
Most television satellites contain (at least it was that way in the 1990s) linear transponders. They retransmit whatever they receive on the uplink frequency on the downlink frequency. When a satellite receives a signal from a pirate transmitter it happily retransmits that signal as well. With modest transmitter power and antennas it was possible to transmit a narrow bandwidth signal just at the edge of a transponder channel. The legitimate payload signal would still be relayed and the pirate signal would not noticeably interfere with it. The pirate transmission would be hard to detect and it would be almost impossible to find out from which location it was uplinked. Some businesses used this method of bandwidth stealing for private satellite links.
See you next time when we discuss the EHF bands.
I added a band diagram.