American Embassy Amman | Chasing Third Order Harmonics

Mid-November 1977

I would never get used to the constant chatter of the HW-28 Teletype machines in the Communications & Records Units on the top floors of American embassies. How the CRU personnel managed to live with the persistent interference of thousands of mechanical parts endlessly clanging in their ears I would never understand.

Interference to the ear was one thing but trying to track down a silent and invisible RF interferer required skill, patience, and a bit of luck. Intermodulation or IM attacked radio signals without mercy. Sometimes manmade, sometimes a result of nature, RF interference didn’t pick its target and was often an accident caused by two unrelated phenomena.

As I sat in the corner of the Communications & Records Unit I closed CRO Messenger’s logbook. During my training last year at the G.E. Radio facility at Springfield, Virginia the instructors had dwelled on RF interference that could cause critical outages to a radio system. The instructors had stressed two main points: (1) proper grounding of the base station equipment and especially the antenna was essential. (2) to properly isolate RF interference the tech required a spectrum analyzer test set, which I didn’t have available.

Radio interference reared its ugly head in different ways. RFI radio interference could be the result of another radio transmitting on or near the user radio frequency. Intermodulation occurred when a mixing of difference frequencies occurred in the user receiver, causing distortion. P.I.M. or Passive Intermodulation was the result of interference caused by conditions such as the junction of dissimilar metals or dirty interconnects, which results in third order harmonics that distort carrier frequencies. An example would be a rusted antenna connection.

3rd-order harmonic
Intermodulation harmonics graphic by midians.com

Without a spectrum analyzer I’d have to rely on gray matter and a bit of luck. If I couldn’t locate the origin of the interference then my only other option would be to retune all the radio equipment at post on new frequencies (a daunting task in itself).

I had been searching for patterns in the information derived from Messenger’s logbook. Although the days of the interference were random, the times were not. Messenger and his crew had reported interference within a window of from one p.m. to four p.m.

After lunch I verified that the base station had been properly installed. Then I had Messenger escort me up to the roof to take a look at the embassy VHF base station antenna. We brought handheld radios to monitor for the RF interference disturbance. The two-way handheld radios were noisy—I had unsquelched them. We detected no interference, though.

VHF-omni-ant
VHF Omnidirectional Antenna by theantennafarm.com

The six-foot fiberglass VHF omnidirectional antenna stood about five feet above the roof. I gave it a brief inspection. It had been installed properly with adequate grounding. The RF coaxial connector had been sealed well with black tape. It showed no signs of weathering. I came to the conclusion that the antenna and the associated mounting hardware and coax cable were probably not the cause of the interference.

At this time I saw a maintenance worker walk by on the other side of the roof.

“Who is that?” I asked Messenger.

“We’ve been having problems with the building water supply. I think it has something to do with the pump that fills the water tank.”

When did the water problem start?”

He scratched his head. “Oh, I don’t know, about two weeks ago?”

That’s about when the interference was first reported in Messenger’s logbook.

He pointed at the large cylinder on stilts. “The guy has to come up here every time the water gets low and manually refill the tank. They had to order a new pump that’s due in next week.”

I said, “Can you ask the maintenance guy when and how often he comes up here?”

“Sure.”

When we got to the tank I could hear water begin to flow into the tank. The maintenance guy had fitted a portable pump to the tank intake. He stopped what he was doing when we approached.

“Habib,” Messenger said, “How many days do you have to come up here and refill the water tank.

He shook his head and said, “Two, three times a week, it depends how much water we use.”

I said, “What time do you come up here to fill the tank?”

“After I have my lunch.”

He gazed at us like what was the big mystery. I looked at Messenger and said, “Maybe its better we stay up here a few minutes while he does a refill.”

He nodded and said, “Thanks a lot, Habib.”

The man smiled and got back to work. I noticed the portable pump caused a fine vibration of the cylinder. The steel support structure of the cylinder showed a lot of oxidation (rust). Telltale signs.

A surprised Messenger said to me, “What do you think?”

I noticed that a small rusted antenna hung off the side of the tank. “It’s possible this cylinder may be somehow related to the radio interference.”

He looked at me incredulously and muttered, “What?”

“Do you know who uses that antenna hanging off the cylinder?”

As I pointed at the antenna our handhelds began to squeal.

“That’s it,” Messenger said, his voice full of excitement. “That’s the interference.”

“Ask the maintenance guy to turn off the pump.”

Seconds after the pump silenced so did the interference. I told the maintenance guy to turn the pump back on. The interference returned.

“Let me know if the interference stops,” I said to Messenger.

I reached up to the old antenna hanging off the cylinder. Someone had done a half-assed job installing it. I easily moved the rusty antenna base.

“It’s gone,” Messenger said.

“I’m pretty sure the combination of the water tank and that antenna is our culprit,” I said to Messenger. “When he turns on the portable pump it causes the cylinder to vibrate, which affects that rusted antenna, which provides a path for the interference through our antenna and into the base station receiver.”

Messenger tried to digest what I had said. He called downstairs and found out the rusty antenna belonged to U.S.A.I.D.

U.S.A.I.D.’s radio net operated on a frequency very close to the embassy frequency. I said, “I’m going to have to install a new temporary antenna for U.S.A.I.D.”

I would use a mobile type antenna as a temporary fix until my next visit. I pointed to a pipe on the other end of the roof. “On that pipe over there.”

Messenger called down and got the okay from U.S.A.I.D.

An hour later I had reinstalled and tested the U.S.A.I.D. antenna. Messenger had the maintenance guy start up the water pump. We didn’t see any interference the rest of the afternoon.

“I’ll be damned,” Messenger said. “I would never have thought that a water pump could have caused our radio interference.”

“Sometimes radio is smoke and mirrors,” I said. “Other times it’s just a big mystery.”

Messenger wore an “Ah shit,” expression that wouldn’t go away.

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