Polarisation & TV Antenna/ Sat & Telecoms Signals
You may already be aware of the term but may not necessarily know the importance of polarisation with regards to TV aerial satellite, wireless telecommunications transmission and why this must get this correct when aligning antennas. In this article, we discuss the types of polarisations and how to adjust your antennas accordingly.
Sine Wave/ AC Basics
To understand why polarisation is used and why it’s important in the transmission of wireless signals, a basic understanding of Alternating Current (AC) and the sine wave is important. Alternating Current differs to Direct Current in the sense that constant charge is not present, but rather a current that constantly switches between a positive and negative charge. The sinusoidal wave (Sine Wave) with respect to signal propagation, would be an almost S shaped curve. The sine wave begins at a zero charge point, progressing to a positive voltage charge until it reaches its maximum amplitude before progressing back to the zero charge position, and then repeating this journey to maximum negative amplitude and back to the zero charge point, creating the S shape. The amount of times this is completed is referred to as the oscillation rate, represented as a frequency, total oscillations per second. With this considered, and with regards to TV aerial, satellite, wireless communications transmission would be the orientation of the electric field that is being transmitted/ received.
Vertical polarisation means that the carrier wave is transmitted with the electric field within the vertical plane, a vertical polarity. This will be at a right angle to the earth. To receive a vertical polarised signal the receiving antenna will need to be mounted with the receiving elements/dipole pointing up to the Sky, 90 degrees from earth. Most relay TV transmitters in the UK broadcast in the vertical polarisation.
Horizontal polarisation means that the carrier wave is transmitted with the electrical field within the horizontal polarisation, a horizontal polarity. They will run parallel to the earth. To receive a horizontal polarised signal the receiving antenna will need to be mounted with the elements/ dipole pointing so that they are parallel to the earth. Most main TV transmitters in the UK broadcast in the horizontal polarisation.
Circular/ Elliptical Polarisations
There are a couple of other polarisations that you should know about that are sometimes for satellite systems and wireless telecommunications these are the circular polarisations. Instead of propagating in a horizontal or vertical path, the signal follows a helical or screw like path. There are even some, polarisations that follow an elliptical path as opposed to a circular one but the principle are the same. There are two types of circular polarisations, these are LHCP and RHCP.
Left Hand Circular Polarisation (LHCP) is a polarisation where the electric field vector travels/ rotates in an anti-clockwise direction.
Right Hand Circular Polarisation (RHCP) is a polarisations where the electric field vector travels/ rotates in a clockwise direction.
Why Mounting Antenna For Correct Polarisation Is So Important
You can see from the descriptions above, that the charge in the electric field and maximum amplitude point on the sine wave (where the signal is strongest) which will propagate in either a horizontal or vertical polarity, is where the charge moves away from the central neutral point. This means that the antenna needs to be mounted and adjusted accordingly to receive this. You can see that to mount the aerial incorrect would effectively “miss” most of the transmitted signal, which will result in a dramatically weaker signal. Mounting an antenna for the correct polarity is easy, you just need to know what the polarity is that you’re trying to receive and mount this aerial that way. This is usually just changing how the clamp fixes to the mast/ antenna itself. Watch this video for more information.
Why Do TV Transmitters Broadcast Different Polarisations?
By broadcasting in different polarisations, helps maximise the available bandwidth for TV services. Obviously, this is achieved also by using different broadcasting frequencies within the UHF but when you install an TV aerial you do not want to (usually) want to receive signals from more than one transmitter. As previously explained, an antenna mounted to receive in one polarisation will only receive signals in the opposing polarisation at a much weaker level. This helps to protect against interference amongst other things. Typically, a main high power TV transmitter for an area will broadcast in a horizontal polarisation. Where this transmitter cannot be received in a specific area for whatever reason, usually a land mass blocks the signal, a relay transmitter is installed which receives the original transmission from the main transmitter and re-broadcasts it on different frequencies to an area that could not receive the signal from the main transmitter. Often the relay will only broadcast areduced number of services, this in the UK is known as Freeview Lite. A main transmitter will typically feed many relay transmitters. Some relay transmitters will even go on to feed further relay transmitters.
A working example of this is, in my area Eastbourne we have a relay transmitter mounted on top of the South Cliff Tower in Meads. This is a relay transmitter fed from the Heathfield main transmitter. It receives the Heathfield transmissions which are horizontally polarised and within the Group B frequency band, and re-transmits these in a vertical polarisation in the Group A frequency band. The Eastbourne transmitter (known locally as South Cliff locally) then feeds a further transmitter in the Eastbourne Old Town, which re-broadcasts approx. half of the Freeview service, Public Service Broadcast only (Freeview Lite) in the vertical polarisation and in the Group K frequency band to areas that cannot receive a reliable signal from neither the Heathfield or the Eastbourne transmitter. The Eastbourne Old Town transmitter now only serves very few properties as coverage has improved since the digital switchover. I for instance have only aligned an aerial to it for an installation once in sixteen years.
Even with the changes in frequencies and polarisations, you can still run into problems. If you follow the seafront northward, you will find a transmitter in Hastings that is also a relay transmitter from Heathfield. Like the Eastbourne (South Cliff Tower) transmitter this also broadcasts Group A vertical polarised signals which clash both clash on three frequencies and can cancel each other out creating an unreliable poor TV reception. This effect is called co-channel interference and is common in many of the surrounding areas. This can usually however be overcome by aligning the antenna to a different TV transmitter.
Exceptions To The Rule
Not all mains TV transmitters broadcast horizontal polarised signals, and relay transmitters vertical. A notable exception to this would be the Rowridge TV transmitter in the Isle of Wight which broadcasts in both. This means that when aligning an antenna, you would ideally have an aerial that have both horizontal and vertical elements protruding from it, but this is not common for UHF terrestrial TV reception. Typically you would just accept that some services will be received as a stronger signal that others. This isn’t a problem so long as the weak services are above the minimum signal level and stronger services not above the maximum.
Polarities of Satellite Signals
Satellite dishes are designed to receive both horizontal/vertical polarisations at same time. In theory it would be possible to install a Yagi type antenna with very small elements, pointing at the satellites in space, this would be impractical. Both polarisations are reflected towards the LNB feedhorn from the main reflector (the big round/ elliptical part) and are separated at the LNB itself. Satellite receivers that use a universal style LNB will then switch between these by varying the DC supply fed to the LNB by the satellite receiver. Wideband LNB’s like that used for Sky Q, have separate vertical and horizontal polarised signal outputs, both of these need to be connected to the satellite TV receiver to receive all to TV services.
Satellite ARC/ Skew Adjustment
Because the earth is round and the satellites themselves are situated in a semi-permanent fixed orbital position in geo-stationary orbit, depending how far off due south (in northern hemisphere) or due north (in southern hemisphere) that the satellite(s) itself is located will effect how much of an offset angle that the vertical/ horizontal will be. For example, if the satellite dish was mounted at an orbital position of 0 degrees, the vertical/ horizontal signals would be perfectly straight. But, if the satellite orbital position was east or west or this position, like nearly all of them are, the signal would effectively be transmitting the signal at an angle., the further off due south (or north) that you descend, the greater this effect will be.
This is important as an adjustment at the LNB needs to be mount to compensate for this, this is called the skew adjustment and it’s important so that the LNB can effectively separate horizontal and vertically polarised signals. This is why when you look at most correctly installed satellite dishes the LNB will look like it’s not sitting straight, it’s not supposed to!
Tips For Getting Polarisation Correct
When mounting your antenna, there are tools which can be used to make polarisation adjustments easy. For instance, I would use my TV spectrum analyser to adjust the skew correctly on a satellite LNB installation. But the following tips can also be helpful.
-The mast/ pole or fixings should be as straight level as possible
-Fix your mounting brackets to the wall using a level or plumb bob.
-Check that it’s straight using a level.
-Many dishes have a built-in level, pay attention to this when mounting/ adjusting the antenna.
Signal Polarity Questions
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Further reading - if you would like to read more on this subject. I recommend this article: wikipedia.org/wiki/Polarization_(physics)
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