Combining RF Signals and the Home Run
We’ve already determined that contemporary MATV installations must do much more than distribute the signal from a single antenna to multiple locations. They system must seamlessly allow for the integration of bi-directional communication for cable modem and interactive programming features, allow for distribution of satellite IF (intermediate frequency, the output of the satellite’s LNB), allow for the integration of locally modulated sources such as a cable or satellite tuner, DVR, VCR or DVD player, and even relay remote control commands. How do we inject these signals into the antenna system? Through the use of a diplexer.
A diplexer is a device that combines radio frequency inputs from two or more radio transmitters into a single output, or, when used in the opposite direction, divides a single RF input into two or more outputs based on frequency. Depending on how it is used, a diplexer may be called a combiner or splitter. In fact, a common 2-port splitter is a diplexer and can be used to combine like spectrum signals. As an example, if you modulate the output of your DVR onto an unused UHF or cable frequency by using an active modulator, you can combine the output of the modulator and the output of the antenna system by taking both signals to the “out” ports of a splitter and running the “in” port to the distribution amplifier or distribution block. In essence you’ll be installing a splitter “backwards.” No harm is done with this application because a passive 2-port splitter is electrically identical to a 2-port passive combiner. This is only true when both signals are of a like frequency, however.
In the residential installation a diplexer is typically used at the head-end to inject signals of different frequencies into the same distribution infrastructure, and then another diplexer is used at the drop location to separate those components to their basic constituents again. Using direct broadcast satellite such as DirecTV or DISH as an example, a diplexers allows the DBS signals from the dish to the receiver to piggyback on one regular coaxial cable, along with lower-frequency signals from the outdoor terrestrial TV antenna for local channels. (Note: The cables must be 3MHz tested RG-6, as RG-59 will not pass the high intermediate frequency (usually 950 to 1450 MHz) output from the LNB).
In this case, one diplexer joins the two signals together. Another diplexer then separates the signals to the receiver of the TV set, and the IRD of the DBS set-top box at each drop location. More complex systems may have a distribution amplifier, which allows each IRD to access multiple LNBs with different antenna polarizations.
Selecting the right diplexer for a particular application requires careful consideration. It is vital that each component in the distribution chain possess the correct bandwidth and expected signal gain or loss. Some satellite systems may require a DC Passive (passes DC signals) device to allow for power at the LNB. Using the wrong component can keep the system from functioning and may even cause damage.
System gain should also be considered in the design of the multiplex matrix. That is to say that signals should be balanced before and after each split or combination. As an example, if the output of an RF modulator used to inject the signal of a DVR into the antenna distribution system is higher than the signal arriving from the roof-mounted antenna, the antenna signal should be amplified (or the modulator output padded) so that the combined output has a flat power bandwidth. This will keep the injected signal from overloading (or being overly noisy due to inadequate power) at the input of downstream tuners.
The Home Run System
Most residential installations are best served by the Home Run installation method. As previously mentioned, this required that each drop have a dedicated coaxial cable that runs directly to the demarcation or distribution point in the equipment closet. At the distribution point all these cables are connected to a distribution block or splitter. There are power splitters with a different number of output ports. However, all power splitters are built upon the basic one-to-two power splitter component. A one-to-four power splitter/combiner is made by cascading three one-to-two power splitters together. Inside a one-to-four power splitter, output ports of the first one-to-two splitter are connected to input ports of two other one-to-two splitters resulting in a total of four external output ports. Of course this means a 4-port splitter has twice the insertion loss of a 2-port splitter. In a similar manner, an 8-port splitter will exhibit at least three times the loss of a 2-port splitter. Beyond 8-ports splitters can seriously compromise signal levels without proper amplification. Each splitter component drops the signal at least -3.5dB.
In a typical residential installation there will likely be full use of 8-ports. From the antenna there will be one drop to the theater TV tuner, and perhaps a second to an FM tuner and a third to a DVR. Assuming three bedrooms we have used 6 of the available 8-ports. Add a television in the kitchen and, perhaps another in the master bath, den or home office and we’ve reached our allotment of connections. One problem that frequently arises is the issue of multiple drop placements in a single room or zone. Do those all need to be home-run? The answer is no; we can use a hybridized system of taps and home run for such a situation. See below for details on trunk and hybrid designs.
At this point we have a single coaxial cable input that has our antenna and satellite (or locally modulated sources) in one hand and eight cables running to various rooms in the other. How do we connect the ends? The 8-port splitter is the obvious answer, but signal attenuation caused by insertion of the devices themselves, as well as cable-induced roll-off, must be taken into account.
