Chasing the Interference
Wesley Sizemore’s job at NRAO is to safeguard the NRQZ (National Radio Quiet Zone). As such, he is made aware of any RFI (Radio Frequency Interference) to the work being done at NRAO, whenever it occurs.
Following are stories about chasing the RFI, which are not only interesting in and of themselves, but provide very good data about how sensitive NRAO’s telescopes truly are and why it is imperative that the NRQZ is properly maintained.
RFI may occur at night, but Mr. Sizemore will still only start chasing it in the mornings. He prefers not to go out chasing signals at night, because most of the folks in the surrounding area are hunters, they have loaded weapons inside their houses and know how to use them. “You don’t go knocking on doors in the middle of the night unless you have a good reason, and you definitely don’t go prowling around people’s property.”
“You get in, you find that you have a source of interference, and that becomes your priority for the day.” As a result, he is never able to successfully plan his day. Every day was different. He’d come in expecting to do something, then the phone would ring and his schedule disintegrated.
For example, one day he got a call in the morning when he’d already planned his day that the 140 ft. telescope was receiving interference and after investigating the problem, they found it to be airborne radar. There was nothing to be done about it. An hour later, it was gone. He then started monitoring those bands just to see if and when it’ll return.
During most of his career at NRAO, he was the only person taking care of interference. That included administration of the NRQZ and responding to interference complaints from the astronomers. Now there are three, soon to be four people doing it. He’s got help on the administrative side and he can devote his time to interference problems.
The Grade School Thermostat
Thermostats arc when they close. They emit a small spark of electricity as two points of contact come together. They do so when the desired temperature in the room is reached. The thermostat measures the room temperature and can also be set to a certain temperature. Each temperature level has a metallic point of contact. When you rotate a thermostat lower or higher than the room temperature, you are in effect putting a little bit of distance between the two metallic points of contact. As the air conditioner or convector then works to increase or decrease the temperature in accordance with your desired setting, the point of contact assigned to the room temperature comes ever closer to the point of contact that corresponds with the setting you’ve chosen. When they get close enough, they’re supposed to make contact, creating a short that activates a circuit which then shuts off your convector.
As thermostats get older, the points of contact get corroded; the room temperature is also measured inaccurately, etc. As the points of contact get closer, they start to chatter before they close. Instead of making contact, they sit there, close enough so that sparks of electricity are exchanged between them. These sparks are tiny enough that you don’t see or hear them, but they’re big enough to bother the radio telescopes at NRAO.
In this case, Mr. Sizemore came in one morning and found that RFI was occurring in the proximity of the site, and it really affected the work being performed on one of the telescopes. He started to track it.
It took him 2 days to track the signal for ½ mile. The signal was only there for a few seconds every 15-20 minutes. He would have to take the antenna and swing it around, find out which direction the signal was strongest at, and move toward it. You may be able to go about 100 yards, but then you have to sit and wait until the signal re-occurs. When it occurs again for 3-4 seconds, you spin the antenna like mad, and determine the direction again. You slowly triangulate onto the signal. When you finally beat it down to a building, you can finally say that it’s in the building. Now it has to be found inside the building. You take portable equipment inside, and you begin to listen to the signal. When it occurs, you say, “It’s loudest over here!” Eventually you reach it.
It was one of the heaters at the local school. It would cycle on depending on temperature changes in the room, whether or not kids were going out of the classroom, if windows were opened, etc.
He got to the heater, unplugged it, and waited 30 minutes to see if the noise would reoccur. It didn’t. What he did was to take the heater off the wall, took it to the lab and replaced the switch onto the thermostat. Then he took it back down to the school, mounted it, and things worked fine afterwards.
The reason this thermostat was such a problem was that the grade school borders on the NRAO property. It was in a direct line of sight to the telescopes. The signals fall off just like optical signals, inversely proportional to the square of the distance.
Anything that generates an electrical arc will generate radio signals. A lot of time is spent chasing down just those things.
A lot of time is spent chasing powerline noise. A powerline can generate radio signals. You’ve got a power pole, a powerline coming in, and a powerline going out. What you don’t want to happen is to have the powerline touching the pole, because that shorts it to ground and creates problems. So what is normally done is that the line is jumpered around the pole. It gets wrapped around a Bell insulator with a little hook. The line then goes around the pole and wraps around another Bell insulator. These insulators are bolted to the pole. There’s a metal to metal contact there. If that contact, or hook, gets corroded, you build up a difference of potential between the two metal pieces. The potential will build up and then arc over in order to neutralize itself. This will be an ongoing cycle. Potential builds to a limit, then arcs, only to build up and arc again and again.
For example, if you tune your AM radio between stations and drive around, eventually you will hear the noise from powerline arcs. It’ll make a recurring “bzzzzt” noise. Those are the arcs of powerlines. This normally doesn’t bother most people, but amateur radio operators have problems with it. Most of it is spectral content, its radio energy is at lower frequencies. Because NRAO dishes are so sensitive, the relatively little arcs will cause problems for them. In the past, NRAO’s 300 ft. dish did a lot of pulsar observation, and that particular band is very susceptible to power line noise. Mr. Sizemore spends a lot of time tracking down those noises.
Once the noise is located on his monitoring station, Mr. Sizemore locates it by driving in its general location and using a CB radio as his guide. That can get him as close as a ½ mile of the source. Then he takes a handheld device that will localize it to a particular pole. He swings it around. Since the antenna has a very narrow beam, he can get pretty good direction. He then uses an ultrasound machine – any electrical arc has an ultrasound component to it – to localize it to an area the size of a half dollar. He then contacts the power company and asks for their help in fixing a particular insulator-wire assembly on a particular pole. Understandably, he prefers to let the power company take care of the poles, because of the inherent danger that comes from working with such high voltages.
The Dead-blow Hammer
In the middle of a night, on weekends, you can’t call the power company. They’re not going to respond to an interference complaint from the observatory. So what do you do if an interference develops during non-business hours?
One of the techniques used is a dead-blow hammer. This is sort of a like a sledgehammer, but it’s loaded with shot. When you strike something with it, like a power pole, the hammer hits the pole, then the shot impacts the head of the hammer and vibrates the pole without leaving big marks on the power pole. If you use a regular hammer, all of the energy goes into the pole immediately, not necessarily vibrating it, and you also make a big mark on it, which the power company won’t like. With a dead-blow hammer, which has a fairly soft head, you simply make a much less forceful contact with the pole, which in turn causes the shot to impact the hammer head while it is still in contact with the pole, inducing vibrations. These vibrations hopefully remove some of the corrosion at the contact point between the wire and the hook, thus eliminating – or drastically reducing – the arc interference for a matter of hours, or sometimes even days, until it corrodes again. Eventually, of course, the power company will need to come out and replace the worn-out components, tighten the line and make sure things are working well.
Another thing that can be done is to shake the guide-wire of the pole. This wire is what secures the pole to the ground. Think of it as a line coming down from the maypole, or as a line you would tie around a young tree to help keep it pointed straight toward the sky. If you shake or pull on the guide-wire, you can get the top of the pole moving a bit, and thus clear out some of the corrosion and cut down on the interference. One has to be very careful when doing this, because if the top is set into motion, the lines will start to swing and may touch each other, shorting and causing a very large arc to occur. They may even snap or break and fall to the earth, or worse, on you – and that would be the end of you!
At one time, there was some very strong power line noise, fairly close to the observatory site. Mr. Sizemore got a direction on it, got into the mobile unit, found the general area, looked out across the field to the power pole, and he could actually see the wires arcing. The cable from the transformer to the hot clamp, which clamps onto the power cable, was broken. It was laying there, beside it, metal-to-metal contact, visibly arcing. That was as close as he got to it, for safety’s sake.
You also run into situations where tree limbs get into the power lines. They will arc and spitter and sputter until the wires burn the limb in two, at which point they fall off. This is why you may see your power company out and about, trimming the branches of all the trees near power lines, once or twice each year.
Once an arc was traced down to a knot in an electric fence that people were using around their garden. They used a monofilament line that had threads of aluminum wire in it. It wasn’t a solid wire, so it bent very easily, making it easy to manipulate. A loosely tied knot in the monofilament line was causing interference for NRAO! The solution to that was simple. Mr. Sizemore got the owner to turn off his fence, then he took out his Leatherman tool and tightened the knot. Problem solved! But it took half a day to trace down that particular knot.
The “Damn Dog”
There was an elderly couple that had retired from the observatory – they had been employees there – and NRAO started receiving low-frequency interference. Mr. Sizemore jumped into the interference truck and started looking around the site. Normally power line interference is localized to within a line of sight, or a mile or two from the observatory. Because they’re in mountainous terrain, once there’s an obstacle – a hill, a wall of stone – between the observatory and an earth-bound signal source, they’re attenuated enough that they’re not a problem. Thankfully for Mr. Sizemore, he doesn’t have to patrol a big area because of that.
After chasing the signal for several hours, he localized it. There were several houses there. As usual, he got out of his truck and used his portable equipment to pinpoint the source. It so happened that it was in the couple’s backyard. There they had a very nice tent for their dog. Mr. Sizemore loves dogs, has dogs of his own, doesn’t hunt, but his characterization of the animal was stark: “it was an elderly dog, an ugly, nasty dog,” he said.
Being the lovely couple that they were, they wanted to make their dog as comfortable as possible. The lady had taken her heating pad and placed it outside, in the doghouse. The dog’s pen was off to the side of the building and the dog’s house was built right against the side of the house and raised off the ground. There was an outlet next to it, rated for outdoor use, and the pad was plugged directly into it. Still, you don’t lay on a heating pad. It’s written very clearly in the instructions for these products. Also written in the instructions are clear warnings about not using the pad in wet environments. The couple was breaking both rules.
Mr. Sizemore traced the interference signal to the heating pad, which was cycling on and off frequently, because it was outside, in a cold environment. Every time it would turn on, it would generate interference. It was an old pad, worn out, perhaps by the dog’s chewing or playing with it, and wires inside were broken. Luckily, for whatever reason, it didn’t shock the dog, which was good.
The problem with interference is that NRAO is operating equipment (their radio telescopes) that cost several thousand dollars an hour to operate, and they’re picking up garbage due to interference. The solution is to replace defective equipment, within reason.
Mr. Sizemore knew that they made heating pads specifically for dogs. What he had to do for the couple was to take their old heating pad, destroy it and purchase them a new pad after researching the market to see if one was rated for outside-use. A company called RC Steele sold it. He ended up on their mailing list for more than 10 years afterwards and couldn’t get off it, just because he bought that one heating pad from them.