O anténách TV-SAT
How to select antenna for digital TV (DVB-T)
The implementation of digital terrestrial television (DVB-T) in Poland and some other European countries is at the turning point. Alongside with the new digital transmitters and successive multiplexes, there are still available analog channels. Although in most cases the reception of DVB-T does not require replacement of the antenna array used to date for analog TV, the practice shows that in many situations the modernization of the array is necessary. This is mainly linked with a lower power of some digital transmitters in the transitional period, due to the simultaneous operation of analog broadcasting stations. And, of course, every day new homes in various locations must be equipped with suitable antenna systems.
So we try to answer the question: what antenna for DVB-T?
Of course, this question cannot be answered unequivocally. A full success in each case should be guaranteed by professional installer, who, using his own experience and a suitable measuring instruments, will be able to choose the optimum solution for the location. People buying antennas on their own should a least look at the roofs of buildings in the vicinity and learn from the experiences of residents who already receive digital terrestrial television.
The parameters of the antenna necessary for proper reception of digital broadcasts depend on several key factors:
- the distance(s) from the transmitter(s),
- broadcasting power of the transmitter(s),
- the height of transmitting antenna(s,)
- directivity of the transmitting antenna(s),
- terrain profile (natural and man-made obstacles on the line between the transmitting antenna(s) and the receiving antenna array),
- environment (open space, urban area etc).
The first three factors ca be checked with the use of dedicated services on the Internet. In the example, we will look for a suitable antenna for reception of DVB-T broadcasts available in the location of our headquarters in Krakow.
Checking the distance from the transmitter and its transmission power
This operation can be done in several ways. The fastest and relatively simple method is offered by sat-charts.eu service with its locator (it supports only Polish DVB-T network).
By clicking on the appropriate location on the map, we select the area of interest:
Using the slider on the left we may reduce the scale of the map and set down the position of the receiving point:
After selecting the location, we press the lower button:
We'll see the list of transmitters with multiplexes in order from the nearest to the farthest, relative to the specified location. In addition, the individual items on the list are marked with one of three colors:
- green - high probability of stable reception;
- orange - conditional reception, stability not guaranteed;
- red color - reception is virtually impossible.
The service also informs the user about the fact that the transmitter power and distance are not the only determinants of good reception (translation):
Stable reception depends on many factors, not only on the distance from the transmitter and its power. We must also take into account the terrain profile, the height the transmitting antenna is placed at, the gain of the receiving antenna, sensitivity of the tuner/receiver.
For the indicated location, i.e. the DIPOL headquarters in Krakow, Cieplownicza 40 Street, we get the following list of transmitters:
Based on the above list, we can find two multiplexes that should be received without problems, MUX3 (ch. 48, 2 kW) and MUX2 (ch. 64, 63 kW), broadcast from Choragwica transmitting tower located only about 10 km away.
We may also consider reception from Rabka, Katowice, Tarnow, Wisla, Zakopane, but only the Wisla-Skrzyczne transmitter provides MUX1 with a different set of programs.
The tables below help find optimal antenna based on information obtained from such sites. They are provided for guidance purposes only - as the reception of radio signals is the result of many factors, we cannot give anyone a full guarantee that the antenna selection based on them will ensure trouble-free reception in each location within the distances indicated.
On the basis of the information, in our location we can use some of the following antennas:
- for Choragwica (11 km, 2 kW and 63 kW) the best choice is the A1911 antenna,
- if we try to receive MUX1 from Wisla-Skrzyczne transmitter (82 km, 10 kW), we should rather use one of the A2670, A3710, A3380 antennas. It is worth mentioning the basic difference between the A2670 and A3710 antennas. The gain of the both antennas is similar, however ATX antennas (A3710) are more directional. If we hope to receive signals from several transmitters using a sigle antenna, the better choice will be the A2670 antenna with a wider beam.
Antenna with or without amplifier?
The vast majority of the antennas listed in the tables above can work with LNA preamplifiers mounted in their boxes. Some of them are sold with high-gain LNA-177 B4009 preamplifiers: A1615, A2665, A2675. Other antennas can be equipped with one of the following preamplifiers (to be bought separately): LNA-101 B4007 (15 dB), LNA-169 B4008 (24 dB), LNA-177 B4009 (30 dB). The first two types can be also used with the A1615, A2665, A2675 antennas, to replace the built-in high-gain B4009 in the case of problems caused by excessive signal level.
Generally, it is better to avoid using preamplifiers for reception of DVB-T signals, if they are not necessary. It mainly depends on the transmitter power and distance. In the case of the distance longer than 40 km and a low-power transmitter, an preamplifier may be indispensable. The environment also has a strong impact on the signal level - dense urban housing, forests etc. Sometimes the antenna receives a reflected signal (no line-of-sight with the transmitter) - in this situation an preamplifier may also be necessary.
What about broadband antennas with mesh reflectors?
Popular opinion says that broadband mesh antennas are not suitable for receiving digital signals. However, the practice shows that these antennas can be successfully used for that purpose and possible problems usually do not arise from their wide beam, but from low-quality preamplifiers which were used by some suppliers (no shielding, high noise figure).
So, before purchasing a new directional antenna, it is worth to verify the possibility of receiving DVB-T signals. There may be cases that the change of the antenna is a must, but on the other hand, these antennas may perform well in dense urban areas where, due to multiple reflected signals, it will be difficult to make use of very directional antennas.
Directivity of the transmitting antenna and coverage
High power and a relatively small distance from the transmitter do not guarantee correct reception of DVB-T signals. The decisive factor may be the directivity of the transmitting antenna. An example - one multiplex is broadcast with the use of directional antenna towards an urban area, whereas another one is broadcast with omnidirectional antenna to cover the entire area around the transmitter. This is the reason of frequently occurring problems with the reception of a multiplex by residents of a particular area.
The service that can help users in the final selection of suitable antennas is dvbtmap.eu providing means for:
- locating majority of DVB-T transmitters in Europe, together with their contents (multiplexes)
- displaying their coverage areas
- displaying the profile of the terrain towards the transmitters
View of the homepage of the dvbtmap.eu service. The site has several language versions (Croatian, Czech, English, German, Hungarian, Polish). The "Coverage Map" option visualizes theoretical signal level for a selected transmitter from the list on the right side of the page.
An example of coverage maps for transmitters with omnidirectional antenna (MUX2) and directional antenna (MUX3). It should be noted that in the latter case many places remaining in the immediate vicinity of the transmitter are not covered by the signal. In these locations it is necessary to verify the possibility of receiving MUX3 from another transmitter or even with the use a reflected signal.
Distance and terrain profile
The last thing that can be checked before purchasing an antenna is the profile of the terrain between the transmitter and the place of reception. It is especially important when we want to receive a signal from a distant transmitter. Even if the transmitter provides high power, some obstacles intersecting the straight line between the transmitting and receiving antennas can cut off the signal. In this case it's no use to buy even a best antenna - we must look for other options.
Choosing an appropriate transmitter from the list and selecting the reception location on the map, we get the distance and terrain profile between the points. The calculation takes into account the height of the placement of the transmitting antenna on the tower.
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