Directional Couplers

Directional Couplers also known as Taps, Line Taps or just DC, are used to split off a portion of a signal from a main distribution line. Unlike splitters which divide signals evenly to all ports, direction couplers allow a signal to be removed from the main line without lowering the signal level of the main line as much as a signal splitter. The signal level measured at the tap port(s) is "X" dB less than the signal level measured at the input. Typical "X" values for a tap port(s) are 8dB, 12dB, 17dB, 20dB, 23dB, and 26dB.

 

The signal level at the out port, called through-loss, varies depending on the value of the tap port. A typical 26dB tap through loss value is .3 to 1.2dB ranging from 30MHz to 1000MHz, whereas an 8dB tap may have a through loss of 3.5 to 5dB for the same frequency range. The lower the tap value, the higher the through loss. A DC-4 has no through port and effectively terminates the distribution line.

 

2-way splitter

The first graphic shows a typical television signal going into a 2-way splitter. The signal is divided between the two outputs. Each output is 50% of the signal level entering the splitter. Television signal levels are measured using a logarithm scale measured in decibels or db's. A 50% reduction of a television signal level, measured in db's, is a -3db reduction. Every 3db increase, doubles the signal strength and every 3db reduction cuts the signal level in half.

 

Directional Coupler

 

The next graphic, shows a 2-port directional coupler. The amount of the signal that is tapped of the main through-signal, is marked on the tap. This image shows a 20dB tap. The signal level measured at the tap port will be -20dB less than the input signal level. A DC-8 has a tap value of 8dB lower than the input signal and a typical through loss of around 4dB.

Applications

Directional Couplers have their greatest advantage when used in television distribution networks. The image below illustrates a Hotel Cable TV distribution system if 2-way splitters where used at each TV location. On the left we see a +30dB signal entering the first room. The signal splits evenly, dropping the signal level by 3dB. The first problem with this scenario is the TV is provided a VERY strong level. TV's are designed to operate with 0dB of signal. Anywhere from -3 to +10dB usually provides sufficient signal level for a television to produce a clear image, and not overload the tuner with too strong of a signal. At the first TV location, we would need to reduce the signal level using an attenuate to avoid damaging the TV. At each TV location, we reduce the signal level to 50% of the input. After 5 TV's the amount of signal available for distribution has dropped by 15dB. After 5 more TV's the signal would be 0.

 

 

Distribution System using 2-way splitters

 

 

The image below demonstrates the same scenario above, but this time we use 2-port Directional Couplers. The first port is the through port and the second is the tap port.. The first signal tap uses a DC-26. The output is 26dB less than the input. The through loss of a DC of this value is typically 1dB. The second DC is that same value, but the third DC has a lower tap value because the input signal level is lower. For this system we need at least 3dB of signal level, at each TV, to assure a clear picture. To maintain the 3dB minimum the tap value is lowered to increase the tap output signal level. As we move down the distribution line, the tap value of the DC is reduced as the signal level reduces. This allows us to maintain a minimum signal level while not wasting signal power. The result in this example is a power savings of 9dB and the proper signal strength for each TV.

 

 

Distribution System using Taps

 

The examples above did not factor in signal loss from cable or connectors. Engineering a Cable TV Distribution system for a hotel, apartment or town is far more complex then this simple scenario, but it should give you a basic understanding of Directional Couplers.

 

For residential applications Directional Couplers are most often used in a loop through distribution system. As you can see, even a small number of TV's can benefit from using line taps over splitters.

 

Standard 2-PORT Directional Coupler Specifications

 
26dB
23dB
20dB
17dB
14dB
12dB
8dB
Frequency
Typical Through Loss in dB
10-30 MHz
.3
.5
1.2
1.4
2.1
2.2
3.4
30-150 MHz
.4
.6
1.1
1.3
2.0
2.3
3.3
150-300 MHz
.5
.7
1.2
1.4
2.1
2.4
3.4
300-450 MHz
.5
.7
1.3
1.6
2.2
2.5
3.4
400 - 600 MHz
.6
.8
1.4
1.6
2.4
2.6
3.6
600 - 750 MHz
.7
1.0
1.5
1.7
2.4
2.8
3.7
750 - 860 MHz
.9
1.0
1.5
1.7
2.5
2.9
4.0
860 - 1000 MHz
1.2
1.1
1.9
2.2
3.0
3.6
4.8

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