Sector Antennas 101: Patch arrays

[Tom]

 

 

Patch array (PA) antennas are widely used sector antennas in unlicensed 5 GHz networks. In this webinar we talk about all the technical details of this antenna technology. We'll look into:

- Why are these antennas called 'Patch array'

- What are PA built from

- What are the advantages and disadvantages of PA when used in unlicensed 5 GHz networks

Why 'patch array'

The antenna is composed of patch antennas. A patch antenna is a very common antenna type used in many applications. It is a metal patch of various shapes etched on a printed circuit board (PCB) with a ground plane (metal layer on the bottom side). A single patch antenna has a low gain and point source-like radiation pattern that is good for GPS and other applications, but definitely not for sector coverage in unlicensed 5 GHz networks. Stacking more patches on top of each other, a patch array is formed. The array has higher gain thanks to the array effect, narrower beam width (BW), and side lobes (SLs) due to the interference of wave radiated from each patch antenna.

The higher gain is favorable, but the narrowing beam width and SLs are not. Narrowing beam width puts a definite limit on the maximum gain that is usable in practice.

At the limit, the beam width becomes so narrow that it becomes hard to aim the antenna such that it effectively covers the intended area. Moreover, the narrower the BW is, the harder it becomes to use an antenna to cover valleys of mountainous regions.

SLs of PA antennas are undesirable in general but are especially harmful in unlicensed 5 GHz networks. Here, the no. 1 problem of interference is largely caused by the SLs of antennas, through which the signal is transmitted and received from unintended directions. While the azimuthal SLs can be dealt with, the SLs caused by the array effect are unavoidable by and large. It is technically possible to suppress them, but the price for such antenna would be higher than most of WISPs are willing to pay for an antenna.

Beam Efficiency (BE) quantifies the amount of SLs an antenna has. It can have values from 0 to 100 %, where the 100 % is the best possible case. Here an antenna has literally zero SLs. The smaller the BE is, the more SLs an antenna has. Typical BE of PA antennas is on the order of 65 %, which means that 65 % of the energy the antenna radiates is contained in the main lobe and the remaining 35 % is in the SLs. This is rather low BE for a sector antenna in 5 GHz network.

The manufacturing cost and price of PA antennas is typically low, which makes these antennas very attractive - the entry barrier is rather low. Scaling for higher gain is also relatively easy (considering the two limiting factors mentioned before).

Compared to horn antennas - another frequently used antenna type for sector coverage in 5 GHz networks, PAs definitely come out as having very narrow span of application cases. The PAs are useful when:

1. The landscape is rather flat (because of the narrow radiation pattern in the elevation plane).

2. High gain is needed.

3. There are very few to no interference sources in the area - which rules out most of the urban / sub-urban areas and nowadays even rural communities.

4. Customer density is rather low - the radiation pattern is wide in the azimuth plane covering wide areas.

Comparing Horn sectors and PA sectors, we can conclude:

1. Horn sectors have high BE while PAs have low BE

2. Horn sectors have high frequency stability radiation pattern while PAs have very unstable radiation pattern causing coverage instability.

3. Horn sectors are easily scalable for various gain/beam width requirements and offer a versatile tool set (https://rfelements.com/products) for any coverage scenario. PAs usually have very limited gain/beam width options.

4. Horn sectors have wide bandwidth covering the whole useful spectrum the 5 GHz radios cover. PAs typically have varying properties within this spectrum which makes the network performance unstable as well.

5. While the manufacturing cost of PAs is typically lower than Horn sectors, by optimizing the manufacturing process, it is possible to obtain a product of comparable price and very high quality even with Horn antenna sectors - the RF elements antennas are a proof of this - check them out: https://rfelements.com/products

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