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If an alternating current is applied to a metal rod or wire, electromagnetic radiation (composed of oscillating electrostatic and magnetic fields, perpendicular to each other, and to the direction of energy propagation) is generated. This is how a television transmitter works. Conversely, if electromagnetic radiation is intercepted by say a copper wire (such as that found in a portable TV aerial, for example) it causes electrons to move, “inducing” an alternating current in the wire, and a voltage between its ends, or the same frequency and magnitude as the original.
A portable TV aerial is used to capture and possibly amplify as much of the incoming signal as possible, although this must be done selectively. An aerial must be able to differentiate between different signals from the same transmitter, so that interference from unwanted signals received at the side or the back of the aerial, is minimised. Portable TV aerials are typically characterised by their mechanical construction and their performance electromagnetically, but the importance of choosing a high quality aerial should not be underestimated. An aerial is responsible for receiving television signals in the first place, and no matter what you attempt to do to the signal afterwards, a poor signal means poor picture quality.
Types of Portable TV Aerial
The simplest type of portable TV aerial is a simple stick or rod aerial, typically 8″ or so in height, which may allow adequate reception of DVB-T (“Digital Video Broadcasting – Terrestrial”) channels, such as those available on the “Freeview” service, but is likely to require a strong signal and direct line of sight to a transmitter. Bear in mind that a vertical stick or rod aerial cannot receive signals from transmitters that are horizontally polarised; that is, transmitters that generate electromagnetic waves with the electric field in the horizontal plane, which most main transmitters are.
As a general rule, it is advisable to fit a portable TV aerial that is designed specifically for the type of transmitter from which you receive your television signals; in other words horizontal for a main transmitter, and vertical for a relay transmitter. A circular, loop aerial, for example, functions perfectly well with either type of transmitter. Indeed, it may include monopole or dipole antennae to increase bandwidth coverage, and requires no specific orientation, horizontally or vertically.
A more sophisticated for of portable TV aerial is a directional aerial, modelled on a traditional outdoor aerial and known similarly as a “Yagi” or “Yagi-Uda” aerial, after its Japanese inventors Hidetsugu Yagi and Shintaro Uda. This type of aerial is composed of a rod, a reflector and a number of director elements; at least 5 or 6 is recommended for best results, and can be pointed directly towards a transmitter and adjusted horizontally or vertically with ease. This type of aerial may allow better reception than the types of portable TV aerial already discussed, but may also require correct placement and even then 64-QAM (“Quadrature Amplitude Modification”) channels may not be received, if the signal strength is poor. The more director elements a Yagi aerial has, the more directional it becomes; this means less interference, but also that the aerial needs to be pointed accurately towards a transmitter.
The use of a signal booster, or aerial amplifier (which can typically boost a signal by anything up to 40dB) may increase the likelihood of picking up weak signals in “fringe” DVB-T areas, and in some cases can allow stable, uninterrupted reception on all channels. Do bear in mind however, that a signal booster should only be used in areas where digital television reception is poor in the first place. Quality and strength of signal are required for satisfactory reception, but a signal booster increases only the strength; this means that if a signal booster is used in an area where the signal is already strong, any “noise” in the signal will also be amplified, and if anything, the overall reception will be worse, not better.
Portable TV Aerial Characteristics & Considerations
Digital television is broadcast in groups known as “multiplexes”, each of which contains perhaps a dozen or so individual channels. In addition, whereas traditionally, a weak analogue signal meant that television pictures were grainy and plagued by interference (but nevertheless, viewable) a weak digital signal often means no picture at all. These factors in combination mean that in an area where digital television signals are weak, an incorrect choice of aerial may deprive you of not one, but many, channels.
Aerial manufacturers are obviously aware of this fact, and many portable TV aerials designed for the reception of digital TV (there is no such thing as a “digital” aerial, per se) are manufactured slightly differently from their analogue counterparts, and include a device known as a “balun”. A balun; the term itself is a contraction of the words “balanced” and “unbalanced”, is in effect a type of transformer, converting an unbalanced signal to a balanced one, or vice versa. A balun affords certain benefits, not least improved rejection of interference.
Another important characteristic of a portable TV aerial is its “gain”, or its ability to receive signals at specific frequencies in comparison to a reference aerial. Aerial gain is typically quoted in dBi, if compared to a perfect, zero gain, isotropic aerial or dBd, if compared to a dipole aerial, and the higher the gain the better. A high gain aerial can provide solutions to problems such as poor signal strength, interference and ghosting, but by its nature, is highly directional, and should be pointed directly at a target transmitter. Indeed, the intensity of gain may vary greatly through just small angles.
Aside from the characteristics of a portable TV aerial itself, the characteristics of the cables, and connectors used to connect it to TV can have a major impact on the quality of reception. For the best reception, you may need to position an aerial some distance from a TV, so it is important to have sufficient, high quality cable to allow a single run, with no joints. High quality cabling and connectors experience less “attenuation” (loss of signal due to resistance) and cabling is shielded, so that it is less susceptible to the effects of external magnetic fields. In fact, you may well find that double screened, low loss, FT100 coaxial cable, for example, allows better reception of Freeview channels than the addition of a signal booster to conventional coaxial cable.
Portable TV Aerial Tips
Portable TV aerials are susceptible to interference from domestic electrical appliances, such as computers and microwave ovens, so are often best positioned close to a window, just above head height, and away from any such appliances or intervening walls or other obstructions.
A powered aerial is likely to provide better results than one without power, and in either case, an aerial should be directed towards your local television transmitter, and oriented (horizontally or vertically) to suit the polarisation of that transmitter. If you are unsure as to whether you receive television signals from a main or relay transmitter, the orientation of rooftop aerials in your neighbourhood are a good indication.
Using a portable TV aerial to tune a set-top, Freeview box, or similar equipment, may prove problematic in weak signal areas even if a signal booster is used to amplify incoming signals, because a signal of insufficient strength often means just a blank screen. A solution to this problem may be to use a rooftop aerial; either your own, or that belonging to a neighbour, to complete the tuning process, before plugging in the portable TV aerial and adjusting its position for optimal reception.
Do bear in mind that unless you live within 20 miles or so of a main television transmitter, and the path to that transmitter is unobstructed by tall buildings, hills, trees, etc., it is unlikely that you will receive digital or analogue television signals perfectly, via a portable TV aerial. It may be worth your while, before you choose a portable TV aerial, to speak to your neighbours to ascertain whether they can receive all Freeview channels without “freezing”, or loss of signal or analogue channels without noise, or interference, using a portable TV aerial. If they cannot, a high gain rooftop aerial may ultimately be necessary.