Understanding RF Signal Levels and Trunk Runs

Signals in cable systems are measured in dB relative to 1 mV(millivolt) across the 75 Ω characteristic cable impedance. Expressed in dBmV (decibel-millivolts), the minimum room-temperature noise (noise-floor) in a perfect cable system is -59dBmV. Ideally we need to deliver at least 0 dBmV of signal, but no more than +10dBmV, to the terminal on the television receiver. Lower numbers produce snowy pictures and higher numbers may overload the television receiver’s tuner, resulting in cross modulation of the channels.

There are two types of signal attenuation that must be considered in a successful RF installation; flat power loss and roll-off. Flat power loss occurs when a device is inserted in the signal path and power is lost at all frequencies more-or-less equally. Roll-off is a function of cable length and frequency. Looking at the latter first, 100 feet of quality RG-6u will exhibit about 8.5dB of signal loss at 800MHz (in comparison, RG-59u will suffer nearly 50% greater signal loss at this frequency and length). A 100 foot straight run in a residential installation isn’t that unusual, so let’s use that as a starting point for our calculations. To have the higher numbered UHF stations display the same picture quality as the lower frequency VHF signals (keep in mind OTA VHF broadcasts are slated to be curtailed by the FCC by 2009, leaving all OTA television broadcast within the UHF spectrum) we need to add 8.5dB of tilt compensation just to prevent losses from the wiring used.

Now let’s look at flat power loss. If we are using a 1-in-8-out splitter we can expect at least -10.5dB per port signal loss. Assuming we have a good antenna installation in a strong signal area, and the combined signal after the diplexer is +3dBmV, we’ll need minimum of 7.5dB to a maximum of 17.5dB in additional gain (amplification) to deliver a signal in the specified range of 0 to +10dBmV at the drop location. Additionally we’ll need at least 5.5dB in tilt compensation for the cable loss. For this hypothetical installation we might select a flat 15dB VHF/UHF amplifier that features a tilt compensation control to peak the highest frequencies for the best reception. Clearly it is important to be able to measure our starting signal and balance the power at each stage of signal manipulation. There is no way to design and install and effective RF distribution system without knowing the real amplitude of our signal components!

There are additional considerations, which are beyond the scope of this particular article, which must be taken into account to maximize performance. It is vital to determine maximum and minimum signal input levels for distribution amplifiers based on amplifier gain, amplifier noise figures, and CTB (composite-triple-beat) performance parameters. Additionally, the potential for signal splitting to multiple devices at the terminal location of the network should be anticipated. Finally it is vital that any good RF distribution design take into account eminent signal introduction. Digital ATSC signals broadcast on the UHF band will soon become the standard. Systems designed only for maximum VHF or cable band performance will likely suffer when the user attempts to distribute these new signals. A complete accounting of all system variables and a written table summarizing the starting and ending actual and theoretical signal levels is invaluable documentation of system performance that will aid in future system expansion.

The Trunk-Run System

A trunk-based distribution system is what a cable plant creates to distribute signals to subscriber homes. We can emulate the same system within the context of a residential installation. In this type of installation a main trunk line is created and RF taps are used to provide each individual drop. The advantages include minimizing installation labor and cable use as only a single coaxial cable is capable of serving a full zone of drops. As an example, let’s assume a residential installation in a home with three floors. We can run one coax to each floor and then run taps off this trunk to provide drops to each access location. So if there are three bedrooms and two bathrooms on the upper floor, and we want to provide TV access in each bedroom plus the master bath, we can run one coax from the equipment closet to the first floor, then from the first floor to the upper floor.

In a trunk system, splitters are not used to divide the signal. Instead a directional coupler is used. A directional coupler has much less insertion loss than a splitter, on the order of 1.5dB. But a directional coupler (tap) has much higher loss to the drop location, as much as 9dB or more. This allows us to create a single master trunk line using a coaxial cable like an RG-8u that exhibits much less loss per foot than either RG-6u or RG-59u. Lower frequency-based attenuation means fewer issues with tilt and a flatter overall loss. This is much easier to control in a large system.

Let’s examine what such a system would look like if we had the same starting conditions as the example above. Signal levels are +3dB from the multiplexer and we have 8 drops on three floors. The longest distance to the top floor is 100 feet. RG-8u exhibits about 6.5dB of loss over 100 feet, but for such a short run we’ll stick with the RG-6u also used in the example above.

Now instead of having 8 wires coming into the equipment closet from the drop zones we only have one. Coming out of the amplifier we’ll run to the first tap, which will have four outputs at -12dB. From the output side of this tap we’ll run our trunk to the first floor of the home where we might have an additional 4 drops. We’ll then continue the same cable to the top floor for an additional 4 drops. The final tap will be closed with a 75 Ω termination to hold the characteristic impedance of the system. Also, it is very important to use a 75 Ω termination on each unused tap location to maintain characteristic system impedance at 75 Ω.

Our losses in this system look like this: (+3dB initial signal level) + (-8.5dB attenuation) + (-12dB flat loss per drop) = -17.5dB system loss. We can easily add a 20dB amplifier to compensate for these losses and provide a +6dBmV signal to each drop location. If the longest run is 100 feet, but the average run for all drops was 60 feet, we may have just saved 300+ feet of RG-6u plus the time needed to install it. The drawback? Each drop will always have the same signal content. There is no way to isolate a single drop to change the system configuration in the future. You’d better get it right the first time if you’re designing a trunk-based system!