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Large size antenna alignment (Part 3 of 3)

20 2017 June

This blog post explains large (6ft/1.8m and larger) size antenna alignment. As mentioned in the previous blog post, the main difficulty in aligning these antennas lies in finding the initial signal and doing the final fine tuning.

In order to understand why it is difficult to find the initial signal, we have to look at the radiation pattern of large size antennas. Additionally, we have to remember that, typically, larger antennas are used for links that are at least 40 kilometers/25 miles long.

As an example, one of the longest links with SAF Tehnika radios is 148 kilometers /92 miles. There are definitely quite a few links that are longer than that.

For our example, I have chosen the Commscope 8ft/2.4m UHP8-59W-D3A antenna. Its beamwidth is just 1.6 degrees and the gain is 41dBi. From the radiation pattern (pictured below) we can see that this antenna, similarly to any other large size high performance antenna, has no prominent side lobes. At a 10 degree angle from the main lobe, this antenna has just a +5dBi amplification, and it drops to -4dBi at a 15 degree angle. With that, it is clear that this antenna requires a very precise and smooth alignment.

UHP8-59W-D3A radiation pattern envelope

UHP8-59W-D3A radiation pattern envelope

Imagine that two of these antennas are installed at least 50 kilometres / 31 miles from each other, and need to be aligned with 0.5 degree precision using a wrench mechanism to turn the antenna based on the RSSI readings on a voltmeter. This could potentially result in a very long alignment process and/or incorrect alignment. Here are the two main issues to keep in mind:

  1. You have two antennas with a very narrow beamwidth, so the first, and most challenging, issue will be to precisely point each of them to the tower on the opposite side of the link.

  2. RSSI readings are limited by the radio sensitivity and management refresh time. This means that, most probably, you will only see a change in the voltmeter reading if the antenna is aligned on the main lobe. It is important to note that if you sweep the antenna too quickly or with steps that are too large, you will skip over the main beam and will not be able to align the antennas at all.

How would a spectrum analyzer with an additional antenna help?

The key element here is the sensitivity and the reading speed of the spectrum analyzer. In my previous blog post, we covered the fact that spectrum analyzers have a better sensitivity than radios and, therefore, finding a side lobe (in case of small antennas) or the initial signal (for large antennas) is much easier. In other words, during a sweep of the antenna, you will be able to see the signal in a larger sweep region than you would with an RSSI measurement.

Comparison of antenna sweep area using different alignment methods

Comparison of antenna sweep area using different alignment methods

Now, let us get back to the previously mentioned 50 kilometer / 31 mile link.

We now have a spectrum analyzer with a sensitivity that is better than that of the radio. This will help us spot the signal much quicker and, due to the faster scanning speed, it will be easier to understand how the antenna sweep affects the received signal level. This is already a step up from what we had with the radios, but while it does solve the second issue and somewhat remedies the first, it will still take some time to properly align the antennas.

From our perspective, the best solution to the first issue is to use a small size directional wide antenna. For this purpose, SAF uses a 20dBi 14 degree wide antenna. Since the spectrum analyzer has a significantly better sensitivity than a radio, it is still possible to detect the signal from the far side despite using a smaller antenna. It is much easier to align the far side antenna to this small wide beamwidth antenna, which allows you to align it to the correct position on the tower. Once that is done, you just connect the spectrum analyzer to the large antenna and fine tune it as was described in the previous blog post. This approach will be much easier and less time consuming than the traditional alignment with RSSI readings.

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In case you missed our previous posts on the topic, here is a first article about Point-to-point antenna alignment

and the second one about Point-to-point antenna alignment using spectrum analyzer.

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Andrejs Bogdanovs

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By Daniele Zedda   •   18 February

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