Connecting Multiple Directv Receivers

The issue of connecting multiple Directv receivers keeps coming up from time to time. This article will try to shed some light on how to connect multiple receivers to a single dish and the issues that frequently come up with such installations.

Usually the question pops up, “can I use common CATV signal splitters” to connect dual tuners?” With the HD Tivo and other HD DVRs becoming more and more prevalent, people find that they only have a single cable coming from their dish to the receiver. With the dual tuners there is a need to connect the second tuner to the dish somehow. The short answer is you can not use signal splitters to make that connection.

Why Can’t I Use A Splitter?
To understand why a signal splitter can not be used it is necessary to know how the satellite dish systems work. In order to pack the channels closer together frequency wise, satellite transmission uses a polarized signal. To try to keep it into layman terms the signal is sent from the satellite transmitter in either a horizontal beam or a vertical beam. The LNBs at the dish have a corresponding horizontal or vertical “filter” to match. On the C-band satellite systems the signal would alternate between even numbered channels and odd numbered channels and there is a mechanical mirror like device to switch between the horizontal and vertical filters. This is very similar to the polarized glasses where you are dealing with light rather than RF signals. In any case by employing this system the channels can be placed closer together and thus allowing more channels on a satellite without cross-talk, so to speak.

On the C-band system there was a separate wire for the polarization of the LNBs and it was included in the cable that also ran the servo for pointing the dish to different satellites. The small dish systems, Directv, Dish and Voom, the polarization signal is sent by superimposing a DC voltage on the coax cable from the receiver. Although the proper terms are Left Hand Circular Polarization and Right Hand Circular Polarization, many satellite devices will still label them Horizontal and Vertical. For LHCP the receiver will place an 18-volt DC voltage on the coax and for RHCP the receiver will place a 12-volt DC voltage on the coax. This voltage is not only used to select polarization, but is also used to power the tiny amplifiers in the LNBs.

So what would happen if you tried to use a signal splitter like used by cable or antenna systems? The tuner that was outputting the highest voltage would set the polarization for that cable and eventually the LNB. This means that if one tuner was tuned to a LHCP channel (18v to LNB) then both tuners would only be able to receive the LHCP channels. Only if both tuners were to have a RHCP channel (12v to LNB)selected would a RHCP channel be able to be received. This erratic behavior is why when some people try a splitter thinking it will work end up confused and drive themselves nuts looking for other reasons why they can’t get all of the channels. It needs to be stressed that with satellite connections, unlike cable or over the air antenna hook-ups, there are two components occurring in the lead in cable. A DC voltage to set the LNB polarity and the RF signal from the satellite. Splitters will allow the RF to get through to the receiver, but can not differentiate between the LHCP and RHCP voltages.

So what Is Required?

This is where the multiswitch comes in. If we take a look at a 2×4 standard multiswitch there will be 2 inputs to the switch and 4 outputs. INPUTS

OUTPUTS


These inputs and outputs are from the RF signal perspective. As far as the DC voltage is concerned their roles are reversed. The 2 cables that are connected the dish LNBs their input connections will be assigned by the multiswitch to be LHCP and RHCP respectively. Then the proper DC voltage will be applied so as to polarize one of the inputs to be LHCP and the other to be RHCP. The RF signal is sent by the multiswitch to whichever input is desired by the receiver. If a receiver requests a LHCP (18v) channel the multiswitch will switch the connection for that receiver to the LHCP RF input to the multiswitch.

HD elliptical dishes have two dual LNBs, one for sat-101 and one for sat-119, and a single LNB for sat-110. This means there are 5 total connections at the LNBs. Most HD dishes are furnished with a built in (intergral) 5×4 multiswitch, providing service for up to 4 receivers or tuners without the need for additional equipment. Sometimes it could be desirable to use individual dishes instead of the single elliptical HD dish and that is a separate subject covered here: Using Multiple Dishes for Directv HD

So what to do when a HD DVR is dropped into the system?
Say you had a HD receiver in your family room and some SD receivers in the bedrooms. Then you decide to move up to a HD DVR in the family room. You will quickly find out that from a wiring perspective the DVR is considered to be 2 tuners and will require two coax feeds. Here is a typical installation diagram:
HD hookup
If you have only two other rooms with SD receivers, then you can run the 4th cable to the DVR to provide service to the second tuner. It is not possible to run two tuners over a common cable when we are talking about satellite signals for the reasons discussed above. You have to have a feed for each tuner as this diagram shows:
HD hookup 1

Now if you have four or five rooms hooked up to the HD dish, you will need to add a multiswitch of some sort to accommodate the extra tuner. If the other three rooms have SD satellite receivers then it is possible to use a standard 2×4 multiswitch. What you would do is to run 2 cables from the dish 5×4 multiswitch to the DVR thus providing individual feeds for each tuner. The other 2 cables from the dish 5×4 multiswitch are then run to the input of the new 2×4 multiswitch. From this multiswitch you can connect up to 4 rooms with SD receivers. per this diagram:
HD hookup Basic

What About Over The Air (OTA) Antenna Wiring?
Now in the interest of completeness, the other issue that comes up is the hook up of the over the air (OTA) antenna. Since the satellite receivers do not get all of the networks in HD, most HD satellite receivers have an OTA tuner built in. That’s great, but it also requires a separate antenna system. The best way to hook up the OTA antenna is through separate runs of coax with splitters or distribution amplifiers. Alternately, some multiswitches have a connection for the OTA antenna connection so that the antenna RF (which is at a different frequency than the satellite) can be sent through the satellite cables that feed the receivers. The device that splits out the OTA RF from the coax is called a diplexer. Diplexers will cause some signal loss, so it is generally considered better to use a separate coax system for the OTA. Sometimes due to physical constraints it is not possible to run separate coax for the OTA and that is where diplexers can save your bacon. The use of amplifiers at the antenna or integrated in the multiswitch (some Spaun units have an integrated amp) will normally make up for the loss at the diplexers. Diplexers are hooked up right at the receiver. There is an input that hooks to the coax from the dish or multiswitch, and the an output for the satellite signal and another output for the OTA RF signal. Diplexers will pass through the DC voltage sent out by the receiver. Here is a diagram showing a typical HD hookup with OTA included:
HD hookup 2
Note when using a HD multiswitch there are the four cables from the HD dish. All four cables are required for HD operation because with HD in addition to the polarization voltages, there is a High and Low band select required. The band is selected by the receiver sending a 22khz pulse up to the LNBs to tell the LNB to change bands. Therefore to cover all of the possibilities, 4 cables are required:

LNB INPUT 1 INPUT 2 INPUT 3 INPUT 4
12V 12V 12V 12V 12V
18V 12V 12V 18V 18V
22kHz 12V 12V+22kHz 18V 18V+22kHz

So another question comes up. “I have a SD TiVo DVR and it has two coax cables already connected to it. Can I use a multiswitch on these two coax cables to connect a HD DVR?” The answer is no. Usually the person asking this question has a multiswitch installed to connect a SD receiver for use with a VCR or to connect to the TV for PIP operation. As long as you are talking about SD receivers the addition of a multiswitch will work, but with a HD DVR you will need four inputs to the multiswitch, so you may as well run another feed and forget the multiswitch.

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Update: The information in this article is applicable to the 3LNB Directv systems. The new AT9 dish with 5LNBs and the mpeg4 local HD service has eliminated the OTA diplexing ability. If you have that system you will need to run separate coax from your OTA antenna to your HDTVs or HD DVRs because the added channels are carried in a band that conflicts with the OTA band.

For additional information see: Eric Oshlo Asks a reply from a reader that asked about the AT9 dish system.

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