The Problem With Microwave – Alignment

May 2, 2016
CastleRockMicrowave

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.

Mounting Hardware
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
* Fatigue
* Laziness

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.

Weather
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.

Alignment Equipment
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.

Alignment Procedures
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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12 Comments. Leave new

Mike Veselka
May 2, 2016 7:53 pm

If your using waveguide or coaxial cable, I want full system and feedlines sweeps before starting alignment. For 6 Ghz paths level the pipe mount, level the antenna feedhorn, coarse align with compass. I use the radio transmitter for alignment and a power meter at the bottom of the receiver filter stack. Of course lock the transmitter on high power. On ACM radios you will probably have to lock the radio in its lowest modulation mode but the power levels should be plainly stated in the equipment specs. After the expected receiver signal is found disconnect the power meter and check against the radio software readings. I really like using a power meter because I get instant feedback. Some radio software RSL readings are delayed or a laptop can be sluggish and slow you down.

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CastleRockMicrowave
May 2, 2016 8:16 pm

Great points Mike, thanks! The power meter idea is a great one. It would be important to ensure that it’s measuring all of the power in the channel and not just the peak power. We’ve even seen where the signal level indicated in the radio isn’t very accurate due to poor calibration at the factory. That’s a difficult problem to troubleshoot without taking the radios down and doing a back to back test, which we’ve had to do. Thanks for the excellent feedback!

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Bob Moldashel
May 3, 2016 12:52 am

A few comments…

Level mast pipe is very important. It is extremely critical for E band systems. I have seen and repaired my fair share of 60, 70 and 80 Ghz systems that were installed on 1 1/2″ fence pipe or cheap non-pen mounts and these links were never installed correctly and subsequently cannot be aligned to target RSL.

Initial alignment should always be done at the lowest possible modulation rate.

ATPC should be off. :-). Stop laughing….

I 100% agree on sweeping waveguide prior to turn-up and alignment. The big issue is installers without the proper equipment to perform sweeps and installation people that have little to no experience with waveguide. Waveguide is not heliax….we know that. Unfortunately many tower dogs are not “gentle” when it comes to installation practice.

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CastleRockMicrowave
May 3, 2016 1:58 am

Thanks Bob! Great points regarding ATCP and ACM. That’s the second time it’s come up today.

One issue we’ve run into recently with people doing sweeps is that they can do the sweep, but aren’t qualified to interpret the results. That obviously makes for frustrating situations.

I wasn’t sure if I’d ever need the ability to sweep up to 18GHz, but was glad to be able to last week while we did some back to back testing. It’s really handy for characterizing your test cables and attenuators.

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Palmer L. Greene
May 3, 2016 10:55 am

Brett,
Two comments, Patience is a virtue when aligning MW dishes, go slow you will find that RSL.
Secondly pause at each adjustment, some radio displays take a moment to react to a change in received level, you can pass it up and not know it.
Your first point under Mounting Hardware about educating the crew (all of them) is far and away the quickest route to successful alignment. Understanding the systems you are dealing with helps!

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Jim Johnston
May 4, 2016 2:32 am

Great post, Brett, and good comments from others.

Squaring the mount and squaring the dish are step one, everyone mentions it, but crews get confident and skip this step and when they do, it can take hours instead of minutes to align a dish. If someone tells you they eye-balled the mount, you’ve go the wrong guy, particularly on lattice towers where there’s no such thing as a straight angle.

Having someone drive a truck or put a cone or marker on the azimuth out in front of the tower helps the dish installer quickly get close to the azimuth. This helps when it’s one crew and they put of the first dish without any signal to use for alignment. There’s usually enough signal at the far end that the second dish can come up and be aligned on one trip.

I’ve found if the installer understands the antenna pattern and how much the signal drops on side lobes can save time. After a couple weeks of throwing up 3′ and 4′ dishes, I’ve seen crews struggle when trying to align an 8′ dish with a .5 degree main lobe. I encourage sweeping side to side slowly just to watch the RSL rise and fall before beginning fine align.

Taking time to make sure the dish is mounted so that it can be swung to left and right without hitting the tower can save time having to drop the dish and change legs or flip the mount.

Please explain to crews to not mount on the climbing leg if at all possible. What good is a safe climb if you have to unhook and climb around an obstruction. Also, consider other crews that come after you when mounting hardware or attaching cables. That clamp or cable you installed becomes a lever or a foot brace for the next guy.

Making sure the installers know the calculated RSL before beginning alignment helps them get off the tower sooner. They know when to quit.

Tall guyed towers without torque arms and with big dishes, the weight and wind on the installer on the tower can affect alignment. I’ve seen paths come out of alignment when the installer climbs down the tower.

Correct tools can save time also. Make sure the climber has the correct size wrenches. How many times have we gone to align a link installed by someone else with nuts and bolt heads rounded off by someone using a crescent wrench. Or, when we went to cast mounting brackets from steel clamps and had contract installers tighten the bolts until the casting breaks, letting you find out weeks later when the dish goes out of alignment and they are long gone.

Make sure the power settings at both ends is the same. I’ve seen crews spend hours looking for a discrepancy in RSL when the issue was the power setting at one end.

Another time suck is trying to align a dish in a multi-path null. We’ve all seen cases were some engineer thought the “higher the better” and the link never came in. I would rather deal with losing a few dB dragging the Fresnel on the ground than deal with multi-path. The crew needs to know when to step back and review what they are doing and be prepared to move the dish up or down.

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CastleRockMicrowave
May 4, 2016 12:29 pm

All great points Jim! Thanks for taking the time. I almost mentioned that there should really never be a crescent wrench on the tower.

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Brett,

This is a great blog post and just the tip of the iceberg. Other antenna alignment topics for the future:
– Diversity antenna alignment
– Cross-polarization alignment (especially with XPIC)
– Sector antenna alignment-understanding downtilt
– nLOS/NLOS antenna alignment tips
– E-Band/V-Band antenna alignment considerations
– Passive repeater antenna alignment tips

It amazes me how the importance of antenna alignment is overlooked. Transmission engineering is an exact science. A good Transmission Engineer designs a link to deliver an expected capacity/throughput with predicted up-time only to have it all compromised by a misaligned antenna system.

If only I weren’t afraid of heights…

-Mike

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CastleRockMicrowave
May 4, 2016 1:56 pm

Great stuff Mike! Perhaps you’re offering to be our guest blogger??

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Ted Langmack
May 4, 2016 4:25 pm

All good procedures from many very experienced technicians. But, I haven’t seen any comments addressing two items that I teach to my students. I provide Consulting Services and Training for Microwave radio equipment and antenna systems. I have always used the same method for path alignment as recommended by a major antenna system manufacturer. This is the best procedure I have found. I’ve been in the business for over 45 years and have aligned a lot of paths. I’ve corrected more paths that others aligned Back when I was a young technician we struggled with finding that “peak” and soon found out about the character of electromagnetic waves. As the electromagnetic wave transmits through the atmosphere many things happen to it. One being the buffeting to the main lobe can leave the alignment technician endlessly looking for the peak value. When buffeted by transmission we might see a flat area where the peak should be and a rippling effect that varies over time. This lets the technician peak on something and when coming back to it; it is gone. Thus, endless amounts of time are used trying to find that elusive peak. The manufacturer of one particular line of antennas created his alignment procedure way back in 1973 or so and it is still their standard. The procedure teaches you how to find the center of the beam…..not the instantaneous peak value at any specific time. This procedure is simple to understand and is best administered by an experienced microwave technician. So the second point of this comment is to advise that the equipment technician who is skilled at path alignment does in fact direct the efforts of the tower crew. When a tower crew makes peak alignments with a path alignment box then his peak may be happening out near the edge of the beam width instead of at its center. Today, may tower crews are over confident and are unwilling to work with the equipment personnel. The two parts must be able to work together and the equipment team must be experienced enough to properly guide the tower team. This procedure will provide very accurate alignment and save money for both teams. If that alignment team misses the center of the beam and aligns the signal near the edge of the beam width then the path can suffer catastrophic “wind fades” when the tower twists past that point. Note: On short paths the beam may exhibit a nice sharp peak since the path is short enough not to have a lot of flattening or buffeting but even with nice peaks the procedure of the manufacturer is true. And one last note; I train personnel from both technologies and there are about 25 equipment technicians to 1 tower technician in my classes. I would thing that ratio would be reversed.

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CastleRockMicrowave
May 4, 2016 4:49 pm

Thank you Ted. I respect and appreciate feedback from someone with the experience that you have.

I have often associated this “buffetting” with the depolarizing effects of reflections. In fact, I almost shared a story in the post about a 14mi path I had over corn fields of Iowa, but felt the post was already too long. On this path, the RSL was down 15 to 20dB and varied over time. I wasn’t involved in the initial alignment, but came out to help fix it. My first thought was polarization, so I rotated the feed horn 90deg expecting something to happen and the RSL practically stayed the same. This lead me to believe that propagation anomalies and reflections likely impact polarization. At that time I hadn’t had any training yet to indicate that.

I’m glad you mentioned communications between the tower tech and the radio tech on the ground. There’s no question of the importance of cohesiveness, but to your point this is often more of a social and political issue than say a technical one. It’s definitely something to be considered.

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I would agree with you with all the factors that causes a microwave problem as you have mentioned above, thank you for sharing you interesting information. It enlightens me. Thanks

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