View Full Version : Understanding Polarity

04-28-2011, 11:32 AM
Digital signals are transmitted from the satellites on either Vertical (V) or Horizontal (H) polarity for linear feeds, or on Right (R) and Left (L) polarity for circular feeds. Standard big dishes are most likely to have feed horn that can receive linear (H/V) polarity. Other system such as Dish Network and Direct TV use circular polarity.

If you have a big dish, most manufacturer of feed horns such as Chaparral and ADL provide what is called a "Teflon Slab" that can be added to the feed horn which would allow it to receive both linear and circular polarity signals. This will cause a loss of about 1 dB in signal level on the linear polarity signals. A Wideband Chaparral feed horn can receive both linear and circular (Ku band) polarity signals.

In order to receive the digital TV signal, you must have the feed horn set to the correct polarity. For LNBFs, the polarity is controlled automatically by a voltage transmitted form the receiver to the LNBF via the coax cable. The receiver will send the LNBF 18 volts for horizontal polarity, and 13 volts for vertical polarity. For standard LNBs, the polarity is controlled by a motorized motor. In this case, odd channels represent one polarity, and even channels represent the other polarity. Standard satellites have the even channels set the polarizer to horizontal polarity, and the odd channels set it to vertical polarity. If the Satellite Polarity is inverse, then the even channel set it to the vertical polarity, and the odd channels set it to the horizontal polarity. Furthermore, some LNBFs such as Dishnetwork's DP LNBFs control the polarity change by shifting the frequency rather than using 13/18 volts. In fact
the new shifted horizontal polarity is 25600 minus the original horizontal polarity.

If you are unsure of which channel represents which polarity, simply set the digital receiver to display the signal strength meter, then switching between odd and even channels on the analog receiver will generate a marked difference in signal strength.

Polarity can also play an important role in whether you are able to receive the digital signal at all. Digital signals are not as forgiving as analog signals, it is either you receive it or you do not. If the signal meter shows a high signal (over 85% on the Sat Cruiser), and you are still unable to receive the digital signal, then adjusting the SKEW will almost always help.

Most analog receivers have auto tune feature, it will automatically find the best dish position and the best polarity for the analog signal. Be forewarned that this is not always the best setting for digital signals. However, in most cases it does produce the best setting for the reception of digital signals.

If you are able to receive only the odd or even channels on a given satellite, then your polarizer is defective and need replacement. The polarizer is a motor located on your feedhorn which switches the unit to receive the vertical or the horizontal polarity. Since every time you switch channels the motor moves, eventually it will wear out and will need replacement. It is connected by three wires from the back of your analog receiver to the dish (the wires are normally (Red '5VDC', White 'pulse', and Black 'ground'). Before replacing the polarizer make sure to turn the receiver's power off. It would be prudent to remove the power cable from the electric socket since most receivers continue to provide power to the dish even when the unit is off.

04-28-2011, 11:55 AM
The LNB is actively involved in the tuning and signal selection process, under supervision
and control of the receiver itself. There are two adjacent frequency bands used, and two
signal orientations.

With terrestrial, you have Horizonally polarised signals and Vertically polarised signals,
depending on which transmitter you have your antenna pointed at. Most are horizontal in the
UK, but some are vertical. You can see which is which by looking at the aerial itself. If
the elements are laying flat, your local transmitter is transmitting the TV signal horizontally
polarised. If you twisted your aerial through 90 degrees so that the elements were all standing
up, you would get cr@ppy reception for sure!

Satellite works the same, but the signals on adjacent channels tend to alternate polarisation
so that they don't interfere with each other, and so that more can be squeezed into the
available spectrum.

How does your dish cope with both? The LNB does it for you, and the receiver tells it which
way to expect the signal based on the transponder definition it has stored in the firmware
(and which you can manually change) for a given frequency.

So, a quick review. The satellite signal can be in one of four 'states' if you like.

1) Low band, Horizontal polarisation
2) Low band, Vertical polarisation
3) High band, Horizontal polarisation
4) High band, Vertical polarisation

The LNB/receiver combination sorts this out based on which channel you're tuned to, and
dynamically switches when you do. Cool.

What the LNB also does is take that really flimsy, weak microwave signal, and knocks a
whole heap of frequency off of it.

Now, getting to the point at last ( it does this in a whole block of frequencies.
The LNB (low noise block down converter) down-converts a block of frequencies into what
is known as the IF (Intermediate Frequency) which is now at a much more useful wavelength,
amplifies it, and pokes it down your coax to the receiver.