Basic RF System Design

Basic RF System Design

RF distribution systems can be very complex or fairly simple, but they all follow the same rules.  The following basic system design won’t turn anyone in a master RF system designer, but it illustrates how signals are originated and attenuated. It’s also a very common small-system solution, where all RF feeds are home-run to the RF head end. This design assumes:

  • 33 dBmV or less RF level
  • Home-run RG6 coax
  • Relatively short runs to TVs, some in 300-500′ range
  • 2-28 QMOD channels

What we’re solving in this design is that there are several factors that will attenuate (reduce) the RF level to the TVs (note when I use dB it’s the same as dBmV – just RF shorthand):

  • Combiners – minus 3.5 to 20 dBmV if not amplified
  • Splitters – subtract 3.5 for 2-way, 7 for 4-way
  • Tap – subtract the amount rated for each tap (typical 20dB), but lose only about 1 dB passing through the tap to the next. Taps that are rated less than 20 dB lose 3.5 or more dB passing through
  • Distance – lose -3 dB per 100′ up to channel 13 on RG6 cable – that’s actually 28 channels because the actual frequencies of channels jump around a bit:
    • 2-6
    • 95-99
    • 14-22
    • 7-13
  •  If you’re using less channels, the loss per 100 feet will be much less

In this design exercise, I’ll use 4 types of components to distribute RF for up to 20 TVs. This design is scalable, as each 8-way tap only loses 1 dB.

• 3 QMOD HDTV Modulators, output at 29 dBmV
• 1 QCA9-33 Active Combiner, output at 33 dBmV
• 2 8-way Taps @20dB. Subtract 1 dB at the RF Out, 20 dB for every tap
• 1 4-way Splitter, subtract 7 dB at every output

I’m using the taps to send RF to most TVs within a relatively short cable distance, using the splitter at the end if I need a stronger RF level to TVs at a much longer distance.

Here’s how the RF attenuation works out in this design:

  1. 29 dB from QMODs
  2. 33 dB from QCA9-33 Active Combiner
  3. 13 dB from first 8-way 20 dB tap (33 – 20)
  4. 32 dB to second 8-way tap
  5. 12 dB from tap, net 6 dB through 200′ cable (3 dB per 100′)
  6. 12 dB from tap, net 11 dB through 50′ cable
  7. 31 dB to splitter, net 25, 24 dB at each output
  8. 24 dB to 600′ cable, 6 dB to TV (6*3=18 dB loss)

The great thing about digital channels, is that the target level to each TV is very wide, generally about 0 to 25 dB. Theoretically, HD tuners can still work at -dB levels, but you really don’t want that as a design goal.  It’s hard to measure exact cable lengths, so having a safe target is better in the real world.

Note I’m using the -3dB per 100′ as my rule of thumb. Generally, if you’re using a third less channels, the loss could be factored closer to -2 dB. When I say that, I mean the highest channel would be about 20. If your highest channel is 13, even if you only use 2 channels, you have to use the higher loss figure.

The goal of this session is to cover the basics of RF design, levels and losses. This layout has worked well for many small RF systems that distribute 10 – 30 TVs. Larger systems will be more complex, but the tools of RF level, combining, amplification, taps and splitters remain the same.

If you have more  demanding applications, there are resources that can assist with full-scale design and documentation. Contact a Contemporary Research RF Design Expert for more information.

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