Theory and use of channel amplifiers
Channel amplifiers - basics. A common problem that is encountered in terrestrial TV antenna installations is unbalance of the level of received channels, often about 10-20 dB. In case of small installations (5-20 outlets) such a difference (not recommended) usually doesn't make any trouble yet. It is the real problem for bigger installations, where the difference of levels at the input is increased by inconstant (being a function of frequency) attenuation of cables and may easily exceed (in an outlet) the accepted limit (12 dB) all over the band.
Therefore, for bigger installations (more than 20 outlets) it is recommended to use channel amplifiers. In the example we want to receive the channels: 25, 30, 40, 51, and 60. Analyzing frequency spectrum we see that reception of channels 30, 51, 60 won't cause any problems even if we use a broadband amplifier. However, the channels 40 and 25 may be distorted or received with considerable noise.
For small and medium installations the problem of low level of the channels 25 and 40 could be solved by adding separate antennas with preamplifiers, connected to separate inputs of multi-input amplifier like CF-512, and adding filter FR-401 (channel stop filter) into the path of the 40 channel. In the case of a bigger installation, especially when it uses extended cabling, the levels should be even better balanced, due to large number of connected receivers that generate additional distortions on different frequencies.
The example of combination of received channels and their levels
Channel amplifiers - selective amplifiers. The role of channel amplifier is to selectively amplify a single (or in the case of digital channels - a few close digital broadcasts) television channel. It means that the most amplified channel is the desired one, the rest is attenuated. Basic parameters of channel amplifiers are: working channel, gain, maximal output level, selectivity. Channel amplifiers, in opposite to other types of amplifiers are characterized by high gain, usually 40-54 dB and very high output level, above 120 dB.
Illustration of selective properties of channel amplifiers
Channel amplifiers - gain. The most "spectacular" feature of channel amplifiers is their gain, achieving values not common among other types of antenna amplifiers. For example, the ZG series of Alcad amplifies the working channel by 53 dB. So high gain (in connection with high output level above 120 dBuV) makes possible to use set of the amplifiers as a backbone for big installations, having even above 200 outlets, without necessity of using additional building amplifiers.
The high amplification factor has its disadvantages, too. It requires to use well shielded cables and high-class connectors to achieve good stability of the whole system (to avoid the risk of parasitic coupling). The good examples can be screened connectors IEC by Alcad company, MC-095 (R90800) and HC-095 (R90801), and TRISET-113 E1015_500, TRISET-113 HF E1019_100 cables.
For the same reason, it is not advised to install preamplifiers directly on channel amplifier's input. If needed, it is better to install them in antenna boxes.
Channel amplifiers - output level determines number of outlets. Maximal output level of channel amplifiers, similarly as in the case of other amplifiers, decides on the number of outlets that can be used in an installation, i.e. the size of the whole installation. Usually channel amplifiers have maximum output level equal 120 dBuV and more (for example ZG series has permissible level 123.5 dB).
It is worth to remind that the level relates to a single channel and does not depend on the number of channels (as each of them is supported by individual amplifier). It can be estimated that output level 100 dBuV allows to build installation with 20-40 outlets, 104 dBuV with 40-50 outlets, 110 dBuV with 60-80 outlets and 120 dBuV allows to build installation with 140-180 outlets. Obviously, the start point for selection of amplifiers is the design for a specific installation.
Channel amplifiers - distortions coming from neighbor channels. There is an erroneous assumption that channel amplifiers can ensure perfect quality in all conditions. Obviously it is not like that, and channel amplifiers have some limits, too. The receiving problem occurs when neighbor channel (unwanted) has higher or close level to the received (desired) channel.
In this case the reception is simply impossible and no channel amplifier will secure effective attenuation of the neighbor channel. Audio subcarrier of the channel lying below and video carrier of the channel lying above are critical disturbances for the received channel. To get right reception, not disturbed by propagation fluctuation, the carriers should be attenuated at least by 15 dB in comparison to the values of the useful signal. It is impossible to realize, because the spacing between them is only 1.5 MHz.
At frequencies reaching 900 MHz the 1.5 MHz spacing is a very small value, not allowing to build effective passive or active filters. So the reception is only possible when neighbor channel has its level not higher than the level of desired channel and when special channel amplifiers, e.g. ZG-421 are used.
Channel amplifiers - inputs and outputs. What are two inputs and two outputs in channel amplifiers used for and how to terminate them?
Input connectors have the same priority, and they are used for connection of several channel amplifiers to a single source i.e. antenna. Also the outputs are identical and used for combining signals from all channel amplifiers in a set. The peripheral outputs of amplifiers can be used as trunk lines, giving signals to separate line sections (e.g. in staircases).
General rule is that, if a few (at least two) amplifiers are connected to single antenna, the last free input in series is terminated with 75 ohm resistor. When single amplifier is connected to an antenna, unused second input should be closed with original hole plug, included in the set. It is important not to mix it up with the other ones coming with amplifiers working in UHF band.
An example of installation with properly closed inputs and outputs
Broadband amplifiers versus channel amplifiers. The choice isn't simple. The first criterion is the number of outlets. A little generalizing, if the number of outlets exceeds 50, it is recommended to use channel amplifiers. Below 10 outlets broadband amplifier will be enough for sure, unless certain specific conditions are met. The problem occurs when we have 10-50 outlets. The main point is to secure for the subscribers output levels at their outlets not less than 60dBuV and not higher than 80 dBuV.
When choosing the solution there should be considered concrete receiving conditions, especially differences in levels of channels in the spot. Antenna installation diversely amplifies different frequencies, and the differences are additionally increased in subscriber outlets. The calculation should take it into account, in order one does not have to perform empirical verification at the outlets. To avoid problems, it is better to employ channel amplifiers in any case when the provisionally determined margins are narrow.
Channel amplifiers versus broadband amplifiers. In previous point we have said that the main issue is to secure signal levels in subscriber outlets not less than 60 dBuV and not higher than 80 dBuV, i.e. the difference should not exceed 20 dB. Most of devices in antenna installations have flat characteristics, independent from frequency. Practically, the only exception is coaxial cable.
In the case of channels 6-12 there is no problem, as usually only one program is transmitted in the band and, in addition, the whole bandwidth is quite narrow - ca. 56 MHz (174-230 MHz). More complicated situation is in the range of 21-69 channels, where the bandwidth is equal 392 MHz (470-862 MHz). In this range almost all programs are transmitted, in addition - with different levels and often from different locations. If we have assumed that the difference of levels at an outlet can't exceed 20 dB, the difference of levels at installation input can't be higher than:
dT = 20 - a
where: a - the difference of cable attenuation in the range 470-862 MHz (of the length to the furthest outlet)dT - maximal difference of levels on installation input.
Assuming that for smaller installations the length won't exceed 40m, if we use cable like YWD 75-1.0/4.8 we get attenuation difference equal 2 dB, and for the cable YWD 75-0.59/3.7 - 4 dB. Therefore a broadband amplifier may be used for input level difference not higher than 18 dB (20 - 2) in the first instance, and 16 dB (20 - 4) in the second.
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