The Problem With Microwave – Alignment
A recent estimation from an industry test equipment manufacturer indicated that as many as 40% of all microwave paths are not aligned optimally. I believe it’s more, perhaps as high as 60%, or higher. It’s a difficult assessment to make. Our experience in troubleshooting deployment issues and observing thousands of microwave system deployments leads me to my estimation.
Optimal antenna alignment ensures the following:
1. Compliance with regulatory and frequency coordination rules
2. Maximizes system performance
3. Provides predictable system availability
There are several factors that impact proper antenna alignment; appropriate antenna construction and installation, proper mounting hardware and mounting hardware installation, the installer, weather, and the procedures and equipment employed to perform alignment.
Antenna Construction and Installation
Anecdotal experience has taught that the fewer parts required to perform field installation, the better. Occasionally we have to install antennas that we don’t like. Under those circumstances we do our best to minimize the likelihood of issues by building the dish in a controlled environment (rather than on site) and transporting it to the site. This is particularly helpful when a dish comes with a lot of small parts.
Often, the source of the alignment problem stems from the mounting hardware. It’s either not appropriately sized for the dish, or the pipe mast is not perfectly plumb (I mean PERFECTLY). Adjusting a dish through anything other than a single plane will result in completely unpredictable results. Installers will forego perfect leveling of the pipe for a lot of reasons, but here are the most likely:
* Not properly educated on the importance of a perfectly level pipe
* Improper mechanical hardware to compensate for a tapered tower or uneven mounting surface
* The wrong tools and no level
It goes without saying that mating the appropriate sized pipe clamps to the appropriate sized pipe is necessary. Unfortunately, we see corners cut in the interest of time and money.
The best way to ensure that the dish mounting mechanism is installed properly is to require enough pictures to remove any doubt that it was done incorrectly. Most important are pictures of a level placed on both axes of the pipe mast to prove that it is indeed perfectly level. This will help rule out any issues with the mounting hardware. A photo of the level on the back of the dish, showing it’s level, is necessary too.
Installer Fatigue and Frustration
Often, when proper alignment becomes difficult to achieve, and all other possibilities have been eliminated, it’s time to consider the installer. It can be physically and mentally grueling to spend hours on a tower performing the same procedure over and over, and not improving the situation. Nine times out of ten the installer and their support people on the ground resign to the frustration and lack of discipline of proper alignment and just settle for low RSL.
I have witnessed proper alignment take multiple attempts by multiple inexperienced people, until the “right person” arrives. Of course, the more experience someone has, the better they get at it. In fact, some installers can pre-align a dish with a high degree of success using simple navigation tools like a compass and nearby landmarks.
A favorite saying I’ve heard is that, “Plan B is not Plan A, with enthusiasm”. Sometimes it’s necessary to give the project a rest and come back to it, or put a new set of eyes and hands on it.
It’s very difficult to install relatively large dishes in the wind. Likewise, dishes that sustain exposure to high winds can move over time. It’s critical that appropriate tie-back hardware is used on larger antennas. Some customers require that 3′ dishes and larger have tie-back hardware. Often 6′ dishes have two tie-backs and 8′ dishes and larger have three tie-backs. The use of tie-back hardware (sometimes called “struts”) requires forethought and planning, to ensure that all of the correct hardware is purchased in advance.
In areas where propagation is more susceptible to inversions and weather-related phenomena (varying k-factor), it is very important to perform alignment during periods of stable weather. Generally speaking, ‘good’ propagation is accompanied by wind and sun, cold fronts can be good – once they pass, warm & stationary fronts are ‘bad’. I credit my friend and colleague, Tom Hendricks for this information.
Anticipate challenges in antenna alignment when shooting over or through reflective surfaces (ie large bodies of water or between many tall buildings or a narrow canyon). With a good design the results are predictable.
We often use the microwave radio to provide fine alignment of the system. Most radios provide an indication of Received Signal Level (RSL), sometimes called RSSI. One should always be aligning to a planned RSL (dBm) value, which is derived from path planning tools or software. In addition to providing an RSL value, some radios also give audible feedback with a buzz or beep, or visual feedback with LEDs. Regardless, we generally only use these aids for course alignment and rely on an RSL measurement for fine alignment.
On occasion, the antennas are being installed before the radios are available, so the radios cannot be used for fine antenna alignment. There are some popular products available for antenna alignment under these circumstances, like the Spectracom Path alignR.
Similarly, we recently witnessed a demonstration from a company called Sunsight Instruments and their alignment tool called the AAT-08. We found it to be a highly innovative tool and seemingly valuable in terms of expediting alignment, versatility and reporting capabilities. We’re anxious to get feedback from the field about how they work.
It’s already been stated that the right tools needs to be used. This is typically ratcheting-style wrenches and sometimes socket sets, of the correct size. Also, a small torpedo level to ensure everything is perfectly plumb prior to alignment. We see a lot of folks carrying digital levels, which seem to work well. The installer should also carry a marker so they can put an indicator on the threaded rod to remind them how to get back to a certain signal level.
Prior to getting on the tower or building, a course alignment can be achieved using a simple magnetic compass, adjusted for proper angle of declination. This of course assumes that the path azimuth is known. Mapping tools like Google Earth can be helpful in identifying landmarks nearby and in the distance that the installer can use to aim the dish.
We always align the antennas with no uptilt or downtilt, unless the path is extremely short. This is the easiest way to begin alignment if the expected tilt adjustment is less than a couple of degrees.
It should be stated that only one side of the path should be adjusted at a time. Also, everything below assumes that both dishes are on the same polarization.
Once everything is level, azimuth should be swept. Folks often see one or more side lobes of the antenna, indicated by the RSL measurement, before they see the main lobe. The mail lobe is often distinguishable as having approximately 20dB more signal than the nearest side lobe. Likewise, it tends to rise and fall off very sharply, where side lobes can persist longer through a sweep cycle. Also, the main lobe is accompanied by the presence of at least two side lobes on either side of it.
So, if one was to start a sweep on the far left side of the radiation pattern of a dish, sweeping the dish to the left (pulling the left side of the dish towards them), they should see some amount of signal gradually increase and then decrease (one side lobe), next they will see the signal increase more than it did with the prior lobe and then decrease (another side lobe), and then the signal should increase significantly (~20dB) and quickly fall off. This is the main lobe. After this, the reverse of what was just described should be witnessed as the dish continues to be swept in the same direction, until the signal completely goes away.
Commonly, installers will only see a single lobe in the entire sweep. This is seldom the main lobe and evidenced by a signal level well below (10 – 20dB) the planned RSL. A lot of time is often spent aligning both sides of the link to keep landing on the lower-than-expected RSL. This is often due to one of the issues described above; mounting isn’t level, there’s tilt in the dish, adjustments are being made on both side simultaneously, etc
Another common symptom of poor alignment is that two rises and falls of signal will be seen, but again well below the expected amount. This is just more evidence of either being above or below the main lobe, or not sweeping far enough through the entire radiation pattern.
Once the main lobes of both dishes are found, then elevation should be adjusted to optimize the link. Once both azimuth and elevation are fine tuned, the the RSL should meet the planned signal level dictated by the path design.
There are obviously other things that can go wrong with microwave radio links, but more often than not, the problems stem from a system that isn’t optimized and the most neglected step in optimization is proper alignment.
We’d love to hear your thoughts on the matter. Please take a moment and let us know how you deal with an installer or tower crew that swears the path is “on the main lobe”, when all of the evidence points to it being on a side lobe.
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