The polarity of the signal refers to the whether the transmitter is designed to transmit a Vertical signal or a Horizontal signal. This means that the carrier wave is transmitted either horizontally or vertically and the antenna MUST match the polarity of the transmitter if it is to receive the signal correctly. In Australia about 40% of our transmitters transmit their signal vertically and these are always low powered transmitters intended for coverage of a smaller area. The high powered signals are transmitted horizontally. With analogue signals the signal loss for an antenna mounted with the wrong polarity was around 9db (remember that an extra 3 db is a doubling of signal strength so a loss of 9db is significant). This would result in a noisy analogue picture where the signal power was reasonably strong down to a complete loss of picture as the strength of the original signal at the antenna gets weaker. Analogue signal is all about the signal strength.
Digital signal however is very different. You must match the polarity, it does not matter how strong the signal is; it will not work with the wrong polarity. With digital signal it is all about the equipment reading the zeros and ones and when you have the wrong polarity it cannot read them and you get a ‘No Signal’ reading on the TV. If you could put a meter on the signal you may find that there is plenty of signal power, yet there is no picture. This is easy to see for yourself. If you can get a simple antenna or just a set of rabbit ears that can be laid out horizontally just like an ordinary antenna and connect it to your digital tuner. Now with the picture operating and the antenna held horizontally turn the antenna vertical and you will see that the picture disappears. Turn it back to match the transmitter and bang the picture will start up again.
So use your transmitter guide book and check for the correct polarity of the transmitter. Or you can check out the antennas on the local houses to identify on what polarity the local transmitter is transmitting on