Background: How the DirecTv signal gets to you.

What is an LNB, and why do I need one?

Can I just split the cable from my single LNB for 2 receivers?

What is a dual LNB and why would I need that?

What if I want to connect more than 2 receivers? (multiswitch)

Is there any way to run multiple receivers with just one cable?

 First, you need a general understanding of how the signal gets to your receiver...

The DirecTv satellite position is at 101 degrees west longitude directly over the equator. This is where most subscribers get all their signal from. The satellite is geostationary, which means that it does not move relative to the earth. At roughly 22,500 miles above the ground, the satellite is actually orbiting the earth at the same rate and direction that the earth is rotating. From our perspective on the ground, the satellite stays perfectly still.

There are 32 transponders on the satellite. A transponder is essentially a radio transmitter. Each of these 32 transponders is transmitting a signal on a specific frequency, and each of these frequencies carries a set of DirecTv channels.

Key Concept #1: The frequencies that the satellite transmits are in what is known as the Ku band. They range from 12.224 GHz on Transponder 1, to 12.676 GHz on Transponder 32. Just for reference, consider that your FM car radio receives commercial broadcasts in the range of 87 to 107 MHz.

The Catch: The Ku band frequencies are fine for reaching the earth through the air, but will not travel down your RG6 coax. Household coax will choke off signal at a much lower frequency than that. In addition, your DSS receiver is not designed to deal with frequencies this high. It can only see frequencies in the range of 950-1450 MHz.

Key Concept #2: Each transponder broadcasts its signal with a particular polarization. Technically, polarization has to do with the physical orientation of the electric and magnetic field components of the broadcast signal, but that is too complicated. Just remember this...Half of the transponders broadcast their signal with "Right Hand Circular Polarization." These are the odd numbered transponders. The other half, the even numbered transponders use "Left Hand Circular Polarization." I believe that the reason for this is to eliminate interference between adjacent frequencies. That is why they alternate polarization for every other frequency.

The Catch: To efficiently receive a radio signal, an antenna must be polarized in the same direction as the transmitted signal. Therefore, if you want to pick up the signal from Transponder 1, you need your receive antenna to be Right Hand Circular Polarized. (RHCP) If you want to receive a channel on Transponder 22, you need your antenna to be LHCP. (I'll abbreviate that from this point on.)

Summary: The upshot of all this is that you need an antenna system that can switch between LHCP and RHCP depending on which transponder you need to receive. Additionally, it needs to provide your DSS receiver with a useable signal in the 950-1450MHz range. How do we do this? Why, that's the job of the LNB...

The LNB is a "Low Noise Amplifier - Block Downconverter." That is where the name comes from. There is an LNB (or 2 of them with a dual LNB setup) at the end of the arm on your dish. The dish itself focuses the signal from the satellite directly at the LNB. Your RG-6 coaxial cable is connected to the LNB output.

Following is a diagram which shows what the LNB actually does. This first example is the Single-LNB dish situation:

Summary: What you should understand after this is that an LNB must be told which polarization to select. At any given moment, only half the available frequencies are being received, converted, and fed to the receiver. Because of this, we come to Key Point #3: If you want to supply signal to 2 or more receivers, you CANNOT simply split the coax with a regular splitter and connect one line to each. If one receiver wants to tune in a channel that is coming over an even numbered transponder, and the other wants one from an odd numbered transponder, you are S.O.L. Forget it....not gonna happen.

Well, this is actually pretty simple. If you put 2 independent LNBs in one case at the focal point of the dish, then you can connect each to its own receiver.

This is the proper configuration for two receivers. Note that each receiver can request (via voltage) to see either the even or odd numbered transponders. They are completely independent. The picture above shows receivers tuned to opposite polarities.

Well that seems reasonable, but what if I have more than 2 receivers? The answer to this dilemma is the miraculous multiswitch...

By now you understand what the limitations of the LNB are. Each one only supplies signal from half the transponders at any given moment. But what if you have 3 DSS receivers? Or what if you have a regular receiver and a DirecTv recevier with Tivo? The D-Tivo actually has 2 receivers or tuners built in, so now you actually have 3 receivers. (...counting the days 'til the second tuner gets activated....) I know, I know... there is Ultimate TV from Micro$oft too, and the same applies. Anyway, there is a solution, and it's called the Multiswitch.

The multiswitch is a special kind of splitter. It has 2 dedicated inputs that are to be connected to the outputs of each LNB on your dual LNB dish. It doesn't matter which one goes to which LNB. The outputs of the multiswitch are then connected to the individual receivers. The multiswitch applies the request voltage for even transponders to one LNB and the voltage for odd transponders to the other.

When an individual receiver tunes to a channel, and selects the polarization it needs with a request voltage, the multiswitch connects that port to the appropriate LNB with a relay. The end result is, both polarities are available to all receivers at all times. In the picture above, Rcvr 1 is selecting one polarity, and the other 3 are selecting the other. The multiswitch shown is a 2x4 model. This means that there are 2 inputs and 4 outputs. There are other configurations available, such as 2x8 if you have such a need. Some models add a third input that you can connect a regular over the air (OTA) antenna to. That will combine your OTA signals onto the receiver lines. You would need to separate out the OTA signal with a special diplexer at your TV. These are sometimes referred to as "3x_" multiswitches.

Some multiswitches are powered only by the current they "borrow" from the receivers. You don't have to plug them into a power outlet. These are sometimes referred to as "unpowered" or "passive" multiswitches. Other models do require an external power source. These are sometimes called "powered" or "active" multiswitches. I don't have experience with these, but I have heard that some unpowered models can have a momentary disruption of signal to other receivers when one receiver switches polarities.

Sample Product Links:

2x4 Multiswitch

3x4 Multiswitch with OTA input

3x4 with built in signal amplifier

DSS/OTA Diplexer

 Can I run both LNBs on a single cable?

I recently became aware of a solution mentioned by some users on the AVS Forums Tivo message board. What if you only have one cable routed to a dual receiver unit such as the D-Tivo, and there is just no way to fish a second cable through for the second tuner? There actually is a way to do this. You can use an up-converter and down-converter. Here's how it works.

 I haven't tried this, but it is a pretty neat idea if you need to do it. The converters are available from Holland Electronics.