What Is The Difference Between Co-Polarization and Cross-Polarization?
What is Antenna Polarization?
Antenna polarization is the orientation of its dipole or dipoles. This orientation (or polarization) affects, in turn, the orientation of the electromagnetic field and the radio waves that the antenna transmits through space.
Confused? Try thinking of it this way:
Inside the housing of any antenna is a metal rod or two. The first rod ‘sends’, or transmits, radio waves out. The second rod receives radio waves. (An antenna with a single rod will switch between roles).
The polarisation of an antenna refers to the orientation of these rods to the ground.
What Does Cross-Polarized Mean?
Cross-polarization, then, is when an antenna’s dipoles (the metal ‘rods’ we’ve been referring to) form a plus (+) sign or a cross (x) shape. IE, they’re oriented at right angles (90º) to each other.
What Does Co-Polarized Mean?
Co-polarization is when an antenna’s two dipoles are oriented in the same direction. For example, a linear, horizontally polarized parabolic antenna is co-polarized.
In practice, you’ll hear about co-polarized antennas less often. Cross-polarized antennas carry several advantages, which we’ll get to later. These advantages are why the term ‘cross-polarized’ is an often-used term by manufacturers and salespeople marketing their antennas to customers.
Types of Antenna Polarisation
We mentioned linear, horizontally polarized antennas just above. But what are those?
There are three principal types of polarization. We’ll cover linear polarization first.
1) Linear Polarization
Linearly polarized antennas can be vertically or horizontally polarized. (Technically, they can be polarized at a slanted angle as well).
Poynting’s MIMO, OMNI and PUCK ranges are all linear, vertically polarized antennas.
2) Elliptical Polarization
Elliptically polarized antennas are seen less often. Cloverleaf shapes are sometimes employed in antenna design to achieve this type of polarization.
The elliptical polarization introduces phasing to the antenna beam. The breadth of the antenna beam will vary with this phasing in a process known as beamforming.
Beamforming controls an antenna’s directionality and can improve signal quality.
Other types of antennas can employ beamforming strategies too. Parabolic antennas are linearly polarized but use the parabolic reflector for beamforming.
3) Circular Polarization
Circularly polarized antennas are commonly used in instances where a high degree of directionality and range are necessary. That’s why antennas incorporating this kind of polarization are often used in industrial mining and tunnelling operations.
Helical antennas, as they’re known, incorporate an element that spirals forwards and the radio waves propagate through space in the same orientation as the spiral.
Circular polarization is therefore referred to as either RHCP (right-hand circular polarization) or LHCP (left-hand circular polarization), depending on whether the antenna spiral is clockwise or counterclockwise.
What are the Benefits of Cross Polarization?
As mentioned, cross-polarization is the orientation of two dipoles at right angles to each other. This allows for plus-sign and cross-shaped orientations between an antenna’s transmitter and receiver.
A 90º orientation allows for something called signal decorrelation, which helps to reduce interference for received signals.
No matter how important or unimportant this sounds, the ramifications for performance are huge. Signal decorrelation is one of the biggest advantages of cross-polarized antennas.
Most antennas are deployed in less than ideal conditions, but most antenna manufacturers test their products under ideal conditions. For example, anechoic chambers are used for testing antenna performance with clear line of sight paths.
In practice, an antenna is likely to receive a fractured, reflected signal since it will be operating under non-ideal conditions. What’s more, radio waves arriving at different angles will interact and phase, affecting each other and sometimes cancelling each other out.
Capturing a signal adequately under imperfect conditions is the challenge for antenna engineers and cross-polarization is an effective solution. De-correlating signals which arrive at the antenna simultaneously results in increased performance.
Handling High Signal Density
Spatial diversity is something that can be achieved with multiple antennas, spaced a certain distance apart and oriented at 90º to each other, but in circumstances where space is at a premium, cross-polarization produces similar results with a single antenna.
Urban areas have strict aesthetic requirements and restrictions on space. What’s more, line of sight is almost impossible to achieve in developed towns and cities because of the number of buildings that act as obstacles.
Radio waves bounce and reflect from these obstacles, making predicting the direction of any incoming signal impossible.
Signal density (the number of radio waves being sent through the air) is higher in urban areas too. But strong, clean signals are no less important.
Achieving a clean signal, therefore, entails a bit more creativity in antenna design.
Cross-polarisation is that extra creativity.
How Cross-Polarisation Helps
Omnidirectional antennas, with spherical radiation patterns, receive and transmit signals in all directions. Cross-polarising these antennas further improves performance in urban areas by de-correlating problematic radio waves.
As a result of the extra work that goes into the R&D of cross-polarized antennas, as well as the multiple radios and components required to manufacture them, they do tend to be a bit more expensive than co-polarized antennas.
In exchange, you’ll get improved performance via signal decorrelation in a single package.
Examples of Cross- and Co-Polarized Antennas
Examples of Cross-Polarized Antennas
As you might guess, the ‘XPOL’ in the name here stands for ‘cross-polarization’.
A cross-polarized, 5G-ready, omnidirectional cellular antenna, Poynting’s XPOL-1-5G is well-suited to urban applications where line of sight with your local cell mast cannot be established.
Various mounting options and an IP65 rating make this antenna suitable for mounting either indoors or outdoors.
Poynting’s XPOL-2 series is the directional sibling to the omnidirectional XPOL-1.
This directional cellular antenna is also cross-polarized, 5G-ready and compatible with older generations like 4G and 3G.
Since it’s directional, the XPOL-2-5G is best deployed in suburban and rural areas where you can establish a fairly clean line of sight to your local cell mast.
As a directional antenna, mounting indoors is out of the question as orientation is still too important. Elevating the antenna can help to clear obstacles like trees, while tools like Cellmapper can help to locate your nearest mast and orient the antenna accurately.
That being said, there’s more room for error when orienting this antenna since it’s cross-polarized. This means you’ll be able to replicate the stats listed on its datasheet more readily under less-than-ideal conditions.
Examples of Co-Polarized Antennas
The best examples of co-polarized antennas are those with a dish design. Dish antennas are also known as parabolic antennas, thanks to the parabolic reflector they use to focus those RF waves.
Parabolic antennas mount a transmitter and receiver in parallel and in very close proximity with each other in the centre of the dish. Since they’re oriented in exactly the same direction, the antenna is co-polarized.
The dish acts as a reflector (think mirror) on either side of the transmitter/receiver pair, focussing transmitted radio waves into a narrow beam (that beamforming process we mentioned earlier). The parabolic reflector also focuses the radio waves being received onto the focal point at the receiving dipole.
Ubiquiti’s point-to-point antennas utilise this strategy for effective directionality. Their Powerbeam line of antennas, for example, is a range of dual-linear polarized dish antennas.
The PowerBeam M2
The PowerBeam M2 directs RF energy in a narrow beam, blocking and spatially filtering out noise.
In this way, the co-polarized antenna’s dish achieves a similar end result to the signal de-correlation that’s achieved with cross-polarisation.
The downside of this kind of antenna is that orientation has to be incredibly, incredibly precise. If one antenna in the communicating pair is buffeted a little too strongly in a storm, performance will plummet until it’s reoriented.
That’s down to the whole ‘ideal conditions’ thing we discussed earlier. Cross-polarized antennas are more resilient in the face of less-than-ideal conditions, making them more suitable for deployments where great orientation accuracy cannot be achieved. If you don’t have specialised equipment to orient two antennas within fractions of a degree with each other, cross-polarized antennas are likely to be the best choice for you.
NetXL is a leading UK distributor of antennas and network hardware for both businesses and consumers. If you’re looking to upgrade your network or implement a fixed wireless access solution, browse our range of products or get in touch with our team today.