Taps, Cable – Contemporary Research QMOD-SDI Integration Guide User Manual
Page 16
Contemporary Research
16
QMOD-HD Integration
Taps
Taps are used at the end of the wire in a system, feeding from one to eight TVs. A Tap has an In
and an Out port – 18-28 dB taps lose only about 1 dB passing through. The Tap Outs feed each TV
subtracting from 18 -28 dB in the process. For example, if the net signal arriving at a 20 dB tap at
25 dBmV, each TV will receive a net +5 dB of signal, fine for digital. When you factor dB levels, you
don’t count the loss of -1 on the tap that connects to the TV – that only applies to the output that
connects to other taps.
Cable
You lose power over the coax over each 100 feet. How much loss is determined by how many
channels you’re planning for and what kind of cable you are using. Get the free Broadband
Reference Guide from Blonder-Tongue for full charts and other handy information.
Here’s a short summary:
Cable Attenuation – dB per 100 feet
MHz
Channels
RG59
RG6
RG11
55
2
2.5
2.0
1.5
211
13
3.59
2.87
1.81
250
28
3.89
3.12
1.98
500
70
5.5
4.51
2.85
750
117
6.96
5.62
3.58
870
137
7.54
6.09
3.90
Cable channels don’t step evenly through frequencies. There are actually 24 channels from 2 -13.
Channels 2-6 (55.25 – 83.25 MHz)
Channels 95-99 (91.25 – 115.25 MHz) *Some designers only use these channels as a last
resort, they are in the FM band and could experience interference
Channels 14-20 (109.25 – 157.25 MHz)
Channels 7-13 (175.25 – 211.25 MHz)
Channels 23-135 (217.25 and up, in 6 MHz steps)
Note that the frequency of a channel can often have a greater impact on attenuation than splitters
and taps. This is why, when you create a full in-house system where you’re not carrying the burden
of all those cable channels, keep your channels down in the lower range.
For practical design, use of RG11 should be limited to trunk feeds to branches where distance is a
factor and you need conserve loss. RG11 has the least loss, but is harder and stiffer to work with,
and most tools and connectors are made for RG6 cable.