Radio

Infra Red

Visible Light

Ultraviolet

X–rays

Gamma Rays

 (Long Wavelength = Low Frequency <-------> Short Wavelength = High Frequency)

The long wavelength limit is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length, although in principle the spectrum is infinite and continuous. 

Visible Light

There is no sharp division between sections of the spectrum.

Each colour is a single wavelength or Frequency

400 Terahertz / 750 nanometres to 789 Terahertz / 380 nanometres

Colours in rainbow or sunlight spread by a Prism

Trivia

Notice the Magenta type colours don’t exist. Our eyes work by overlapping sensitivity to the Red, Green and Blue part of Spectrum.  Deep yellow  we see can be really sodium lighting (Mostly yellow) or a flower that absorbs every colour except Yellow. But also a suitable mix of pure Red and Green light will appear to be yellow. This is why Colour TV works with only three Colours. But what if there is a mix of Red and Blue Lights without Green? We will know this is not Green as we can “see” Green. Instead we see colours that don’t exist on the Electromagnetic Spectrum as they are always a mix of two or more Wavelengths (frequencies).

Magenta Family of Colours

These are all mixes of more than one colour of light.

A CRT, LCD, Plasma, LCOS, DLP, LED, OLED or Laser screen or projector only has 3 (or occasionally 4) colours of light. If you use it as illumination a really orange, yellow or cyan object will be dark. It only appears to create the spectrums above as a kind of "illusion" that works because of how we perceive colour. Thus a Red, Green andf Blue LED shining on a screen can appear to be any colour or white, if viewed directly but if used as illuminaton most "real world" objects will not have the same colour as illuminated by true white light (which contains millions of colours rather than three.

2: Radio Waves

Radio waves are usable from 10kHz to 400GHz, that’s Wavelengths of  30km down to 0.00075m (below 1mm).

From 150KHz to 25GHz frequencies are  commonly used for communication and Broadcast.

Long | Medium | Short (HF) | VHF | UHF | SHF / Microwave

(Long Wavelength = Low Frequency <-------> Short Wavelength = High Frequency)

Approximate Bands

VLF                   10kHz ... 60kHz   (Whistlers, Submarine)

LF                     60kHz ... 150kHz (Beacons)

Long                  150kHz ... 300kHz  (Europe, North Africa Broadcast, USA Beacons)

Medium              500kHz  ... 1.6MHz  (A.K.A. "Broadcast")

Short or HF        1.6MHz ... 30MHz

VHF                  30MHz ... 300MHz

UHF                  300MHz ... 1GHz

Microwave         1GHz ... 40 GHz

EHF                  40GHz ... 400GHz

Just like “Red” Light gradually appears “Orange” then “Yellow” as frequency increases the “bands” are conventions, there is no sudden change, only a drift of  characteristics.

Some common bands Important to TV & Satellite.

• VHF Band II (FM Radio) is 87MHz ... 108MHz, (only 104MHz in some countries). Japan uses about 76MHz ... 90MHz.
• VHF Band III  approximately 175MHz ... 280MHz, also used for DAB.
• UHF TV is 470MHz ... 864MHz
• Satellite TV C band approximately  3.5GHz ... 4.2GHz.
• Satellite Ku Band 10.7GHz ... 12.7GHz
• VSAT Ku Uplinks 14GHz ... 18GHz
• Satellite Ka Band 18GHz ... 20GHz (usually a lot less than 2GHz is used)
• VSAT Ku Uplinks 20GHz ... 23GHz (approximately)
• Satellite IF Nominally within 950MHz to 2100MHz except for USA DirecTv

3: Reception and Transmission

All Radio reception and transmission uses aerials. Sometimes these are called Antenna. Even a satellite dish is simply a mirror to concentrate the radio signals into a very small aerial a few mm long inside the LNB (Low Noise Block) on the arm facing the dish.

Frequency, Analogue and Digital

There is no such aerial as a DTT, MPEG4 or Digital Aerial. The only difference between aerials is that the size for same number of elements or a dipole is directly proportional to frequency. A 50MHz aerial design will work at 500MHz or 5GHz if every dimension (including rod diameter) is scaled. In practice there is a small range of sizes desirable for width of plate elements or diameter of equivalent pure rod elements. Also the current only really flows on the surface so larger rods can be hollow with no change in performance. Radio waves are no different for Analogue and Digital. Only the method of Modulation is different. Even Analogue can use AM, FM, PM/NBFM, SSB or spread spectrum. Analogue can use single or multiple carriers.

Digital values are encoded as analogue changes in the Radio signal, since that allows more information to be transmitted. Only Morse code (also known as CW = Continuous Wave) and simple TV IR remotes carried over 433MHz use “real” digital RF (i.e. the RF carrier is turned on or off. This is also called OOK = On Off Keying and simple ASK = Amplitude Shift Keying with only two levels). Real digital transmissions use a minimum of 4 analogue values on satellite QPSK (DVB-S, Quad Phase Shift Keying). Cable TV Digital may use a carrier with 256 different possible combinations of amplitude and phase (256 QAM = Quadrature Amplitude Modulation).  The DVB-t version of Terrestrial TV uses about 1,700 (2K mode) or 6,700 (8K mode) carriers instead of the single carrier of Cable TV. Each carrier can then be modulated 1,700 to 6,700 times slower. Each carrier on DVB-T is either QPSK,  16QAM or 64QAM. The speed of modulation of a digital signal is the symbol rate and symbol rate x number of carriers is the absolute minimum bandwidth used.  256 QAM sends 8 bits of data per symbol and QPSK sends only 2 bits of data per symbol. This is covered in detail in Digital Modulation Theory. Even so-called “Digital Tuners” are mostly still analogue at the aerial end.

4: Propagation

While UHF and especially VHF does not need the quality of Line Of Sight (LOS) of Microwave, the losses from obstructions can make the signal unusable. Examples are large nearby trees, tall buildings, hills and mountains. Sometimes a large object in the middle of the distance can help (diffraction) or hinder (Interference due to Fresnel Zone).

Line-of-sight is the direct propagation of radio waves between antennas that are visible to each other. This is probably the most common of the radio propagation modes at VHF and higher frequencies. Because radio signals can travel through many non-metallic objects, radio can be picked up through many objects. Attenuation is then much higher than the normal inverse square law (double distance gives ½ the signal) Group C/D UHF is attenuated by objects and more strictly LOS than Group A UHF. Band II FM VHF is best and Band III DAB is between.

Ground plane reflection effects are important factor in VHF and UHF propagation. The interference between the direct beam line-of-sight and the ground reflected beam often leads to an effective inverse-fourth-power, i.e. the signal can be reduced to 1/16^th for double frequency or distance. Reflections from hills, trees and buildings cause “Ghosting” on Analogue TV. On Digital if the Ghosting is within the Guard band timing limits the effect is the same as multiple transmitters in a SFN (Single Frequency Network). On digital it may be possible to alleviate breakup in picture due to Wind Farms or large trees moving by combining two aerials at opposite ends of the house. On analogue this would increase the signal by 3dB but would usually create a very bad ghost effect.

It’s thus can be  important to have the aerial about 1m above the chimney because if it’s below the top of the chimney stack the nearby roofs will be seriously affecting the signal. An aerial in the attic can be 6dB to 12dB worse, with potentially too much ghosting even for Digital TV.  Unless you can actually see the transmitter mast from the window and indoor aerial could have up to 20dB loss and very severe multi-path compare to a roof aerial.  However it’s possible a better signal by 12dB to 18dB is on a wall only 2m above ground compared to a poor multipath signal on chimney if there are mature deciduous trees as they are very tall (20m+) and the lowest boughs can be well above ground exposing the TV mast site to a lower wall mounted aerial 

Path loss with frequency. (1GHz/1000MHz to 100GHz)

Received Noise and sources vs. Frequency

Path loss and background noise can be reduced by either more transmitter power, more directional (higher gain) transmitter aerial,  more directional (higher gain) receiving aerial or moving closer. More height helps. As a rule of thumb the “starting” height for good reception is about 45’ (14m), or just above chimney height. For a Line Of Sight (LOS) transmitter additional height has little effect. If the transmitter is not perfect LOS or near horizon (optical or radio), then you need to roughly double the height to get 3dB more gain or significant extra range. Hence most Mobile masts are about 12m to 15m (30’ to 50’) and most Mobile Radio or Tetra masts are about 25m to 30m (about 90’). This is also why TV masts are not only on hill tops but even “fill in” sites are usually at least 25m tall.

The type of Modulation for Analogue and Digital systems and the bandwidth affects the Signal to Noise Ratio for the same aerial, distance and power. Thus speech on SSB (2.5kHz minimum) and speech on FM (about 12kHz to 100kHz) at same power and frequency have different range. The narrow band SSB carries much further. The FM is a little like Digital. Up to a certain SNR (distance) it sounds fine, then it “falls of a cliff” and quickly gets very noisy then un-intelligible before vanishing into the background noise. Digital simply has a sharper cliff. You don’t either receive perfect or not. There is a narrow band of SNR where the sound or image degrades with increasing “bubbling”, “breakup”, “pixelation” or periodic “freezing”. It’s important to know the real signal levels with digital as the sound or picture gives no indication you are near the edge of the “cliff”. Rain, Fog, aging of cable or connectors, slightly reduced power, ice on aerial and even hot clear weather can all reduce the signal. Solar noise or background interference can increase. Thus digital systems need a hidden margin of signal  the user can’t see compared to analogue reception which degrades more gradually and gracefully. 

Effect of Modulation on SNR required.

Note there is a -1.6dB “Shannon Limit” that no Analogue or Digital system can beat no matter how narrow band or how slowly information is transferred. There is also an immutable Shannon Capacity Boundary. DVB-t and 3G/HSPA are close to this and DVB-T2 is as close as is possible. Due to this physical & mathematical limit no amount of clever modulation will get more range or more speed. You need either more bandwidth (wider channel) or more signal (better gain aerial or more transmitter power). A mast head pre-amp only ever makes the SNR worse. It only compensates for loss after the aerial (such as connectors, splitters, combiners and cable all of which attenuates the signal, solar noise, aerial thermal noise, cosmic noise and interference all equally).

Noise Levels

When we talk RF, the NF (Noise Figure) is the dB of excess noise compared to an theoretically perfect receiver.

Both at same temperature, bandwidth and in perfect faraday cage with perfect input matching at same temperature.

i.e. the NF is noise generated in the real electronics.

It's got nothing to do with threshold of hearing.

Noise Levels of sound are not noise levels, but Sound pressure levels. It need not be "noise" in sense of random. It's the power level above a reference of 20 µPa RMS, regarded as threshold of hearing.
90dB is heavy traffic, 0.001 W
130dB is threshold of pain, about 10W
A live trumpet (jazz horn) is about 3W.
Good "speakers" are only about 1% efficient, so 3W of live horn needs a 300W amplifier / Speaker! Bose are the least efficient and need much more power. Efficiency says nothing about reproduction quality.

With Digital and Analogue TV it can be more useful to know the Signal to Noise (Eb/No or quality or bit error rate).

You also need to know the absolute level too as if it's too low, then you can't split the signal or drive any coax at all without amplification with a low noise amplifier. If it's too high it may need attenuated or receivers or amplifiers may overload.

dBmW in 50 Ohms, 75 Ohms, or 300 Ohms is one reference.

Field strength is usually reference to 1uV/m (microvolts per meter).
60dBu is thus 1mV per meter.

Coverage plot contours are usually dBu (reference microvolt per meter).

On practical systems dBm (reference 1mW in 50, 75, 300, 600 Ohms, whatever the cable & equipment impedance is).

A dBu reading only makes sense on a calibrated aerial with exact known gain.

for example if the minimum spec is 35dBu for Digital and the signal strength is 20dBu, then an aerial with a gain of 15dB is needed. An amplifier of 15dB will not work as that amplifies the noise and interference too.

If the cable loss is 18dB from aerial to equipment, then can't put much more than 18dB of mast head gain. If the signal is 20dB more than needed, then no preamp is needed.

So it's more useful to know the dBm needed and the minimum SNR for analogue and minimum Eb/No (digital SNR) or maximum Bit Error Rate.

Different transponders or multiplex from same source with same power may need different SNR as the FEC and APSK/QPSK or QAM/PSK modulation levels may be different.

Module 701 covers Interference in detail,  Module 504 is Modulation theory with some SNR and Module 508 Advanced SNR theory for designing distribution systems covering SNR budgets and  noise contribution of each distribution component. Distribution systems usually need larger aerials and dishes.

Tropospheric scattering

At VHF and higher frequencies, small variations (turbulence) in the density of the atmosphere at a height of around 6 miles (10 km) can scatter some of the normally line-of-sight beam of radio frequency energy back toward the ground, allowing over-the-horizon communication between stations as far as 500 miles (800 km) apart. This is much more common on VHF than UHF and creates  reception issues for normal channels on FM band II, sometimes DAB band III and occasionally on UHF.

Tropospheric ducting

Changes  in the atmosphere's vertical moisture content and temperature profiles can on  occasions make VHF, UHF and even Microwave  signals propagate hundreds of kilometres up to about 2,000 kilometres (beyond the normal radio-horizon. The inversion layer is mostly observed over high pressure regions, especially if there is no wind to disturb the layering of the atmosphere and possibly a very clear sky at night allowing very cold ground and hot air in the morning.  The duration of the events are typically from several hours up to several days. Higher frequencies experience the most dramatic increase of signal strengths.  Propagation path attenuation may be below free-space loss.  There can be dramatic reduction of a medium distance station (say Maghera or Mullaghanish in Limerick City environs) and Reception of Spanish stations on a whip aerial in Limerick. If there is a co-channel DAB or DTT station when these events occur the effect can be much worse than Analogue. Total loss of signal is possible.

Sporadic-E propagation

Sporadic E (Es) propagation can be observed on HF and VHF bands, it’s very rare on UHF. It must not be confused with ordinary HF E-layer propagation. Sporadic-E at mid-latitudes occurs mostly during summer season, from May to August in the northern hemisphere and from November to February in the southern hemisphere. There is no single cause for this mysterious propagation mode. The reflection takes place in a thin sheet of ionisation around 90 km height. The ionisation patches drift westwards at speeds of few hundred km per hour. There is a weak periodicity noted during the season and typically Es is observed on 1 to 3 successive days and remains absent for a few days to reoccur again. Es do not occur during small hours; the events usually begin at dawn, and there is a peak in the afternoon and a second peak in the evening. Es propagation is usually gone by local midnight. The signals are not strong, so co-channel interference is not usually a problem. If a TV or set-box is on “autoscan” it will periodically have extra channels.

Terrestrial Digital receivers should not be left in an Autoscan/Autotune mode.

Aerials, Baluns and Polarisation

It's easiest to make, test and understand aerials and Radio at UHF and Microwave (450MHz to 2GHz).  VHF and even Shortwave aerials can be tested using scale models at 2GHz.

Dipole Aerial

A dipole antenna or aerial  is a radio antenna that can be made of a simple wire, with a centre-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other (in line with each other), with a small space between them. The radio frequency voltage is applied to the antenna at the centre, between the two conductors. These antennas are the simplest practical antennas from a theoretical point of view. They are used alone as antennas, notably in traditional "rabbit ears" television aerials, and as the driven element in many other types of antennas, such as the Yagi-Uda, commonly called Yagi. Dipole antennas were invented by German physicist Heinrich Hertz around 1886 in his pioneering experiments with radio waves.  A Wave or Wavelength in meters is 300/f  in MHz as the wavelength is speed of wave / frequency  (velocity/changes per second) and speed of light about 300 Million meters per second.

Wire ends are supported by insulating cords

A dipole has some “gain” as it doesn’t radiate or receive signal on its axis. This is about 2.15dB more than an impossible but hypothetical true spherical omni-directional aerial (isotropic antenna). So measuring this reference is called dBi. A fairer measure is comparing how much better an aerial is than a perfect Dipole. This is thus dBd. So 0dBd is about 2.15 dBi

The Balun

The sketch for the dipole is misleading as the voltage is symmetrical. This means the aerial should really be connected with balanced twin feed cable (The 300 Ohms version used to be common on older FM radios using a pair of screw terminals and some TVs with loop aerials. The consequences of connecting unbalanced coaxial cable to a balanced aerial connection  are increased pickup of interference, reduction of performance of aerial and risk of “feedback” on any mast-head preamp.  Some aerials are advertised as “Digital” because they have a balun (BALanced  to UNbalanced  converter). All dipoles, Grid/Bow-tie, loop, “rabbits ears” and Yagi aerials are “balanced”.  All types of single core coax is unbalanced. Even since the 1950s poor quality TV and Radio aerials have had no balun and good quality aerials have had a balun. A balun can made by tracks on a printed circuit board, a parallel centre tapped transformer, a series “common mode” choke or transformer, ferrite clamp on the cable or coil of the coaxial cable. A “transformer” type balun can even be a folded exact length of coaxial cable in the connection box.