Popular Electronics May 1958, V8 N5           Previous     Index     Next

JERRY was so busy looking at something in the small tub of water on the floor in front of him that he did not know his chum, Carl, had come into the basement laboratory until the latter suddenly blurted right in his ear: "What are you doing?"

Jerry took his hand from beneath the little aluminum capsule he had been gently supporting in the water and watched it sink slowly to the bottom.

"I'm adjusting the weight of this little sonic tag," he explained, as he mischievously flipped the water from his fingers onto the glasses of his friend.

"Sonic tag," Carl repeated, wiping the water from his horn-rimmed spectacles; "just what is a sonic tag?"

"Remember that story back in the February, 1958, issue of Popular Electronics about the little supersonic oscillators the Fish and Wildlife Service fastens to salmon to keep track of the movements of the fish? Well, that story stuck in my mind, and I wanted to know more about it; so I wrote to the Bureau of Commercial Fisheries. Mr. Parker S. Trefethen, the research biologist mentioned in the P.E. story, sent me back a whole mess of material that answered all my questions."

"Such as-"

"On what frequency does the supersonic tag operate? How long will it continue to oscillate? At what distance can you detect a fish wearing it? Exactly how is the tracking and ranging managed?"

"What did you find out?"

"The transistor oscillator inside the capsule drives a transducer-which is a device to convert electrical currents into sound waves and vice versa-at a frequency of 132 kc. This oscillation is interrupted at a frequency rate that can be adjusted but usually is in the vicinity of 2000 cps. In other words, we have a 'supersonic carrier' on 132 kc. modulated by a 2000-cycle audible signal.

"This 132-kc. signal is picked up by a cluster of four transducers, operating in pairs-one pair for right-and-left and an- other pair for up-and-down. The signal picked up by each transducer of a pair passes through an amplifier, is detected, and then is combined with the signal from the other unit in a differential amplifier . The output of this amplifier goes through a servo amplifier and drives a servo motor that positions the transducer cluster."

"I get it," Carl broke in. "As long as the fish is right in the center of the transducer cluster beam, the signals are balanced and the servo motors don't operate; but when a fish swims up or down or right or left, the signals picked up by a pair of transducers become unbalanced, and the proper servo motor operates to bring the beam back on the fish."

"Exactly right! And a sonar echo-ranging system shoots a pulsed signal right along the center of the tracking beam that strikes the fish and is returned. This device, by measuring electronically the length of time it takes the pulse to go to the fish and come back, tells how far away the tag-bearing fish is. The battery inside the tag will keep the oscillator going for about seven hours The presence of a fish can be detected at distances up to 2000 feet, but the ranging system is only reliable up to about 800 feet."

"All very interesting, but what's it got to do with us?"

"AFTER I got the information from Mr Trefethen, I wrote to my uncle who is in sonar work for the Navy; and he sent me a barium titanate transducer so that could make my own sonic tag. I've just been adjusting its weight so that it will barely sink in the water."

"But what are you going to do with it? I know you're not going to try to build up that elaborate tracking and ranging gear the Fish and Wildlife people use."

"No, but from the information they sent me, I learned that they use a small portable receiver with a hydrophone pickup to detect the presence of a tag-bearing fish. When my uncle sent me the transducer, he also sent along a sensitivity hydrophone, which is merely a special microphone for detecting sound waves in water.

"I padded the oscillator and r.f. circuits of that surplus long-wave command receiver over there so it would tune down to the frequency of my sonic tag, and then I revamped the antenna circuit a little so I could use the output of the hydrophone in place of an antenna. Listen while I twist together these two wires that start the tag oscillating and barely dunk the hydrophone in the tub."

As Jerry did this, a loud musical tone came from the small speaker plugged into the output of the compact little low-frequency receiver.

"Holy Toledo!" Carl gasped, his eyes opening wide behind his horn-rimmed glasses; "that thing has possibilities. How far do you think we could hear a fish wearing that tag?"

"I don't know, but we're going to find out this afternoon. Mom's going to take us and Dad's electric outboard up to Crystal Lake. The battery will furnish power for the receiver. First we'll check and see how far we can hear the tag. If everything works out as I hope, we can give it a real try. You get your rod and some of those night-crawlers out of the box buried back of the garage and be ready to go right after lunch."

CARL NEVER NEEDED a second invitation to go fishing, and he was sitting in the station wagon when Jerry and his mother came out of the house. As Crystal Lake was only a 45-minute drive, within the hour the boys were in their boat heading away from the pier. The electronic tag was suspended from the pier with a piece of line so that it was about five feet under water. As Carl steered the silent little outboard, Jerry listened carefully to the signal he was picking up on a small hearing-aid type of earphone connected to the receiver. The hydrophone trailed over the bow.

Finally he said: "I can still hear it, but that propeller makes quite a racket. Stop the motor and let the boat coast. Ah, now I can hear it fine, although the signal is getting pretty weak."

"No wonder," Carl exclaimed, looking back at the pier. "We're almost a half mile away from the noisy little cuss. Let's go back and get our little jewel before something happens to it."

The boys picked up the little aluminum capsule and headed up the lake to where they saw several other boats fishing for the huge bluegills for which Crystal Lake was famous. The standard method of fishing for these pan fish was to drift with the wind until a school of them was found. Usually two or three fish would be taken on one pass through the school. When the biting stopped, the motor would be started as quietly as possible, the boat moved four or five hundred yards into the wind, and an attempt made to drift back through the school. Sometimes this was successful; more often it was not. Starting the motor was likely to frighten the fish away, and once a school was gone it was hard to find.

Crystal Lake was a real fisherman's lake, and the people who fished it were true-blue disciples of Izaak Walton. They viewed the arrival of the two boys with attitudes that ranged from mild contempt to crusty hostility. Carl and Jerry joined the little flotilla as quietly and courteously as possible.

Luck was with them, and before Carl could get his tackle ready, Jerry had hauled in a bluegill that looked as though it might go to a pound and a quarter. Carefully dipping his hands in water before touching the fish, Jerry quickly fastened the clip of the supersonic tag firmly to its dorsal fin. He sat up in the boat, held the fish up critically for a moment, and then spoke loudly to Carl: "I don't like the scale arrangement on this fish. What say we throw it back?"

"Go ahead," Carl said with a merry twinkle in his blue eyes.

Without another word Jerry nonchalantly slid the fish into the water and released it.

"Hey, Buster," a hard-bitten man in an adjoining boat remarked; "I knew you modern kids were loopy, but I didn't know you were that crazy. That's the best fish I've seen caught in three days."

"Oh, we'll catch plenty more," Jerry answered brightly.

The man laughed a loud, scornful laugh that was quickly taken up by the fishermen in nearby boats.

Jerry, in the meantime, was listening carefully to the inconspicuous earphone he was wearing. As the signal became weaker, he suddenly stood up again and sniffed the air noisily.

"Carl," he announced, "I can't smell fish here any longer. Let's move, and see if we can find them again."

Obediently Carl started the motor, and their boat slid silently away from the others. Jerry kneeled in the prow sniffing this way and that like a coon dog that has lost the trail. Every now and then he would give a motion of his hand, and Carl would turn the craft in the direction indicated. Actually Jerry was listening to the signal in his earphone. Suddenly it became very strong, and he motioned for the motor to be stopped.

The boys dropped their baits over the side, and almost at the same instant each hooked a fish. They hauled these in quickly, and caught two more on the same baits. Carl took still another bluegill before the fish stopped biting.

The boys wasted no time. They started the motor and went through the elaborate routine of "smelling out" the fish. With practice, it became easier to locate the school again with the signal picked up by the hydrophone, and the boys really caught bluegills. In no time at all they were approaching their limit; so from then on they refused to keep any small fish unless it had swallowed the hook and would not live if returned to the water.

THE MEN who scoffed when the boys started to fish were now watching in amazement as they quartered back and forth across the lake, jerking in fish every time they shut off the motor. These men refused to believe the bait consisted of ordinary night-crawlers. One man tossed over a couple of the huge worms in a tobacco can and challenged: "Let's see if you can catch any fish with those."

Obligingly the boys stripped off their own baits and strung on the worms. They kept right on catching fish, and the man who had given them the worms just shook his head in bewilderment.

The boys were quite a little way from the other boats when Carl suddenly heard Jerry exclaim: "Well, what do you know?"

Carl looked at the fish hanging on Jerry's line and then broke into a laugh. Dangling from its dorsal fin was the little aluminum capsule of the electronic tag which Jerry had attached to that same fish such a short time before.

"Man, that fish must really be hungry!" Carl exclaimed.

"Yeah, and I guess this is a sign we'd better call it a day," Jerry said, as he gently pried open the jaws of the clamp that fastened the tag to the fin of the fish. "Both of us have caught our limits, and now we've got back our electronic tag. Let's return this Judas-goat of a fish to the water and head for home. I'm pretty, sure we've made fishing history on Crystal Lake today, and I can just imagine how this story is going to grow and grow and grow."

"You can say that again. If those fellows ever found out we were pulling their legs with that business of smelling fish, they'd use us for cut bait. Fishing is a deadly serious business with them, and it's plain to see that they don't want any foolishness mixed in with it."

Gently Jerry slid the big bluegill over the side of the boat and released him. For a second or so the fish did nothing, then he gave a flip of his tail and disappeared down in the blue water of the lake.

CARL AND JERRY turned the prow of the boat toward the pier, and they both felt that deep-down contentment which comes to a man starting home from a highly successful fishing trip. In this case, though, the primitive satisfaction of a full creel was augmented by the knowledge of an electronic experiment that really worked. The boys were so happy they could hardly stand it! -30-

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