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Understand Your Dish to Keep Your Audio Online

Satellite receivers present some unique challenges; here’s how to address them

Fig. 1: LNA with two 90 degree elbows

When I was the contract engineer for three local radio groups some years back, an interesting event occurred. It was a summer day and a call came from a station that had just lost its audio feed from a Ku Band (12 GHz) satellite dish. Before I could even go out the door, another station called with the same problem.

The sky was black with clouds and street lights were coming on. There was no rain, however; moisture stayed in the cloud cover.

A look at one satellite receiver confirmed the signal had gone away. Then the C Band (4 GHz) satellite receivers went silent. Satellite programming on all of the radio stations in town was unavailable.

The event lasted for about an hour until the clouds dissipated. C Band satellite receivers came alive first, followed by Ku Band.

Today, relatively small consumer television satellite dishes often lose signal when it rains. Fortunately, the broadcast industry uses larger dishes to avoid most annoying outages.

[Next-Gen PRSS System Is on the Way]

In RW’s Oct. 24, 2018 issue, the article “A Sampler of Common Sense Helpers” featured an anecdote about a problem in which some, but not all, transponders were working on a C Band satellite dish with an LNA (low-noise amplifier).

Fig. 1 shows the configuration where two 90 degree N elbow adapters were combined into an innovative and convenient U arrangement. As it turned out, the distance between two sharp right angles resulted in notching two transponders from the received signal. All other transponders were fine. Replacing the adapters with a short flexible cable solved the problem. The wavelength of a 4 GHz C Band signal is only 7.5 cm (2.95 inches) and about 2 cm (0.79 inches) for Ku Band. Significant portions of a wavelength and right angles combined to create the situation.

While we don’t see many LNAs anymore, the moral is that unexpected things can occur when handling microwave RF signals.

WIND

A major troublemaker for satellite dishes, as you know, is wind. You’ve seen it where a weather event caused misalignment of a satellite dish.

Fig. 2: A storm-damaged satellite dish

I’ve gone so far as to fabricate steel struts that attach to the sides of dishes with the other ends fastened to the dish pole or even to screwed-in ground anchors. However, those supports need to be detached before any realignment can be done.

I made it a practice to mark the locations of adjustments so the dish can be put back to where it started before things went wrong. Those spots are on the fixed pole mount to its rotatable portion. A permanent marker works well for that. I also marked the elevation adjustment and feed horn polarization. It is easy to do and saves lots of time in the future.

Then, of course, there are tornadoes and straight-line winds that can demolish a dish. Fig. 2 shows such a dish at a station in northern Minnesota. Even the feed horn was swept away, never to be seen again. This was on level ground — imagine trying to keep a dish aligned on a tall building!

COLD WEATHER

Years ago, when I still had brown hair, I was a contractor for all of the local radio stations. A call came at midnight from a station where the satellite audio feed had failed. It was a cold winter night, –54 degrees to be exact.

I drove out of the garage and down the street. Soon, the steering on my car became a bit stiff and difficult to handle. I made it across town to the station and took the time to pull out the satellite receiver book while warming my hands. It fell open to the specifications page. There it was — the receiver down converter was rated for service to –50 degrees F. That was the clue.

Soon, I had a floor rug wrapped around the downconverter at the cold satellite dish. I hung my trusty 100 watt trouble light under it. That was back in the days when lights were incandescent and produced a lot of heat when running. Back inside the studio, my hands were just recovering from sub-freezing temperatures when an announcer yelled, “It’s working again!”

The next spring found me digging in a coaxial cable so the down converter could be mounted indoors where the temperature was an even 72 degrees, summer and winter. Problem solved.

Yes, we engineers are tasked to be problem-solvers. It is in our DNA to examine a situation and come up with an answer. Rarely did I go to a transmitter site without taking away a list of things that needed to be purchased to keep the building and equipment in good repair. Often that involved improving something that failed, rather than replacing it for another failure down the road.

TERMINOLOGY

Many people don’t understand the vocabulary of satellite technology.

How many times have you heard that a satellite audio outage was caused by “sun spots”? You might politely correct people by saying the event is similar to an eclipse where the sun is aligned with the satellite and the dish. The meager satellite transmitter is drowned out by billions of watts of broadband RF noise from the sun. No wonder a satellite receiver can’t decode a signal under those conditions.

You can also tell the station staff that they can confirm such an event by looking for the shadow of the low-noise block downconverter. If it is centered, or nearly centered, you know the dish is looking at the sun. That applies to most dishes, but not the ones where the LNB is designed to be off-center intentionally.

JUST THE RIGHT CURVE

The physics behind a satellite dish are simple but can be mechanically difficult. Dishes are typically RF reflecting parabolas that must be exactly shaped so a satellite signal will arrive at the LNB from all parts of the dish.

Fig. 3: The physics of a satellite dish

Think of a dish as an optical mirror where light focuses on a single point. Radio waves follow the same principles as light, except you can’t see them. A satellite signal heads toward Earth as a wave front. The system works correctly when a wave striking the satellite dish then bounces off to arrive at the focal point (LNB) as a combined signal.

The dish shape is a double-edged sword. Signals arriving straight in (on-axis) reflect and add at the focal point, while off-axis signals are reflected to a different point, or are just scattered away from the LNB. As they say, the angle of incidence equals the angle of reflection.

A parabolic dish is good at focusing received energy at one point. That is why a dish looks at just one satellite, not others nearby in orbit. A warped dish might reflect signal to the wrong point, causing low dish gain. Ouch!

OUT IN FRONT

Placement of the feed horn and LNB is critical, too. Misalignment at the focal point, even by just a few centimeters (fractions of an inch), could mean the difference between a reliable dish and one that is a marginal performer. That usually happens when a dish is warped.

You’ve seen this one, too. The satellite signal goes away, and you find a bee or hornet hive in a feed horn. Those critters love a location like that. It is away from animals and somewhat sheltered from rain.

Any electrically transparent material can be used to cover the horn. A little plastic sheet, or even plastic food wrap, will do. It’s a maintenance item.

Be sure to mark the EbNo, a signal-to-noise performance number, on the front of every satellite receiver. It is easy and quick to check on regular inspections and will tell you when trouble is brewing.

Hey entrepreneurs, how about developing a curved broom that will help clear snow and debris from satellite dishes?

Knowing the facts helps you be a better engineer. It makes perfect sense.

Mark Persons, WØMH, is an SBE Certified Professional Broadcast Engineer and SBE Engineer of the Year for 2018. Mark recently retired after more than 40 years in business. His website is www.mwpersons.com.

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