Antenna for terrestrial based over-the-air signals
m (minor edit in introduction; add link)
m (moved DVB-T antenna to Antenna for terrestrial based over-the-air signals: name)
Latest revision as of 16:10, 25 April 2010
A good antenna can make a lot of difference in improving reception of terrestrial based over-the-air (OTA) digital TV signals, especially for indoor reception. This is because a proper choice of antenna can amplify your signal more.
Antennae are widely available in stores in the EUR 10-50 price range. When buying one, try to match the wavelengths or the frequencies to be received, as well as the impedance of the antenna system. Often, the antenna you use(d) for analog television reception will do nicely if digital terrestrial TV in your area took over from analog TV on the same or similar frequencies, at the same transmitters (often the case).
Build your own antenna
It's more fun, and may even be better, to build your own antenna, following one of the following tried and tested designs.
The simplest form is the half-wavelength dipole antenna. An amazingly easy (and cheap!) way to make one is as follows. It really does work (possibly better for digital terrestrial TV than analogue TV), and is probably all you need.
- take a coaxial cable (i.e a TV or satellite cable) with no connector on one end (and the standard connector on the other)
- strip the outer sleeving off the last 12.5cm (actually one quarter-wavelength see below) of the end with no connector
- pull the exposed metal braid shielding (and/or metal foil shielding) down and back over and around (or on) the remaining intact cable, extending it also to 12.5cm, and tape it in place
That's it! You now have a dipole antenna: the geometry is the same as the classic design by Heinrich Hertz (1886). The two parts are the (1) the last 12.5cm of coaxial core connector inside the dielectric and (2) the shielding you just taped down behind it, with a (shielded) central connection cable starting at the point where (1) and (2) meet. Together the two parts form a 2*12.5=25 cm (half wavelength) antenna, and is connected to the receiver (at your TV or PC) with the connector on the other end. Find a way (tape on window/table, thin non-metallic tubing) to keep the antenna straight (not coiled) and oriented in the same direction as the signal polarization (i.e. horizontal for H polarization, and vertical for V polarization). For horizontal polarization, experiment with rotating it around the vertical axis until you get the best signal, keeping it horizontal. (Or if you have a clear line of sight, find out where the transmitter antenna is and orient it transverse to that direction). If possible place it out of reach of people and away from sources of electrical interference.
The resonance frequency of the 2*12.5cm antenna lies around 600 MHz, but it will catch frequencies around this as well, although not a very broad band. Calculate the exact quarter-wavelength for the signal you want to receive in metres = 0.25 * (speed of light in metres per second / frequency in Hz) = 0.25 * (300 / frequency in MHz). The UHF TV frequency bands are listed here http://en.wikipedia.org/wiki/Uhf, or you can get the exact frequency from the broadcaster.
Laptop users will appreciate that this is just as portable as the mini antenna which comes with USB DVB-T sticks, and probably better. Tape/blu-tack it to a table or window to hold it down if necessary.
A German description is available. It is not necessary to remove the dielectric as suggested there -- leave it on for greater mechanical support of the core. It is exposed to radio waves because you have removed the shielding.
A more complex design, but also more sensitive, is the log-periodic antenna design. This one must be pointed to the source (because greater gain means narrower directionality), with the side bars either horizontally or vertically to match the transmitter's polarisation. This type of antenna can pickup a broad frequency range with the lowest frequency about half of the highest frequency.
Another interesting antenna design is the double quad. That design is less prone to interference and has a higher gain than a simple dipole. There is a very good German article on how to build such an antenna from scrap (or for less than 5 Euro if you don't have any of the material at home).
The Google translation is horrific but should be enough to give you an idea. Be careful with inch/cm though sometimes they translate the unit without correcting the numbers.