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About RF Cafe
Copyright: 1996 - 2024 BSEE - KB3UON RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while typing up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived... All trademarks, copyrights, patents, and other rights of ownership to images
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Carl & Jerry: Elementary Induction |
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While a bit far-fetched, this Carl & Jerry saga has the two amateur radio hobbyists cum detectives applying their knowledge of standing waves and an invention called SNARE (Signal Net for Actuating Radio-sensitive Explosives, by Irwin Ehlmann) to thwart an assassination attempt on a visiting foreign dignitary. The choice by author John T. Frye of a halo antenna on their mobile shortwave rig was probably no coincidence given the guardian angel role it played in the adventure. Carl & Jerry: Elementary Induction By John T. Frye W9EGVA Carl and Jerry Adventure in Electronics
" ... see if you don't make out a pair of wires going down over the edge of the bluff ... " The dew-washed June morning found Carl and Jerry testing out their super-duper Field Day radio station atop a high limestone bluff overlooking the river. Radio amateurs, on their annual Field Day, familiarize themselves with the operation of emergency radio equipment by competing to see which station can make the most contacts during a 24-hour period. The score made is multiplied substantially if the station is set up out in the "field" with no connection to commercial power lines. Determined to run up a high score, the boys had two five-hundred-watt transmitters in the back of a station wagon borrowed from Carl's parents. One was a band-switching job that could be operated on all amateur bands from 75 through 10 meters. The other transmitter was for six meters only. An all-band trap antenna had been swung between two trees for use with the band-switching rig. On Field Day, there would be a rotatable beam for the six-meter station, but today the boys intended to test the station out with a halo-type antenna mounted on the rear bumper of the station wagon. A two-wheel trailer containing a husky 10-kw. gasoline-powered a.c. generator was hitched to the rear bumper of the station wagon. It was puttering away while Jerry hunkered down inside the station wagon connecting antennas, mikes, switches, relays, etc. Carl was outside playing around with a powerful, tripod-mounted, prism-type spotting telescope he had recently acquired. Carl looked back along the steep, narrow, twisting dirt path up which the station wagon had clawed its way from the paved road running alongside the river down below. Then he raised the front of the telescope and looked across the river at the road running along the top of the bluff on that side. "Hey, wonder why the law is so busy across the river," he called to Jerry. "I can see the sheriff's car and two state police cars cruising along over there." "Didn't you look at last night's paper?" Jerry asked. "The president of a South American country is going to inspect a typical Midwest corn farm about two miles west of here on the other side of the river at ten this morning. It's feared political enemies from his country may try to kill him and so create an international incident. Naturally, our government intends to do everything possible to prevent anything of this nature. That's undoubtedly why the police are checking the route so carefully." "So that's it," Carl muttered, swinging the telescope to the right and peering through a thin screen of bushes at the top of another limestone ridge a half-mile away on his side of the river. "Boy, this 'scope is really a honey," he remarked. "There are a couple of guys sitting over there on that other ridge, and I can see the buttons on their shirts. Wonder what they're doing. They seem to be just sitting there with a kind of funny-looking gasoline can between them. One of them keeps watching the road across the river through a pair of binoculars."
Jerry slid out the back of the station wagon and came over for a look. "That's no gasoline can," he exclaimed; "it's a blasting-cap detonator! Look closely and see if you don't make out a pair of wires going from it down over the edge of the bluff to the road below and then across the road through some treetops and on down toward the river." "Yeah," Carl agreed, "and those wires keep right on going across the river and up the bluff to that little culvert under the road over there. So help me: those jokers must be planning on blowing up the president when he crosses the culvert! We better go tell the police." "There's no time," Jerry objected, looking at his watch. "The president's car should be along any minute now, and there isn't a farmhouse either way for two miles. By the time we got to a telephone it would be too late. I'm going to try to raise someone on seventy-five meters and have them call the police!" The boys scrambled into the station wagon and fired up the bandswitching transmitter. Jerry pushed the button on the mike while he watched the meters on the front of the transmitter. A worried frown creased his forehead as he quickly rechecked the knob settings. "Something's wrong," Jerry said. "We're getting no drive to the final." Gingerly he raised the lid of the exciter and peered inside. "The driver tube is stone cold," he announced, "and we don't have a spare. The filament must have been jarred in two by the rough ride up here." "Try the six-meter rig," Carl suggested, glancing nervously across at the empty highway on the other side of the river. Jerry quickly put the high-frequency transmitter into operation and desperately called "QRRR," the amateur emergency distress signal. No sign of an answer was heard in the receiver. Twice more he put out the distress call, with absolutely no results. "It's no use," he said as he snapped off the receiver and started unscrewing the coax cable antenna lead from the other transmitter. "No one monitors six meters around here this time of day. Our only chance is to get down there to where the wires cross the road and try to break them before the president comes along. "You drive," he suggested hurriedly as he threw the end of the cable out the rear of the car, "and don't spare the horses." "Hadn't we better uncouple the generator?" Carl asked as he climbed into the driver's seat. "No time for that; get going!" Jerry urged. The drive up that steep zigzagging path had been spine-tingling, but it was nothing compared to the ride down. Carl sent the station wagon hurtling along the narrow, twisting path while behind, the heavy generator, still humming away, bumped and jolted and careened first over on one wheel and then on the other, threatening to overturn at any instant. Somehow, though, they finally reached the black-topped road that paralleled the river. Just as Carl made a tire-screeching right turn onto the road, Jerry glanced up at the top of the bluff directly across the river and uttered an exclamation. "There's the motorcade now," he shouted. "See if you can gain a good lead on them before we reach the wires." Carl tried. The accelerator was clear to the floor as the station wagon and the heavy trailer raced along the winding black-topped road, but the cavalcade across the river was also traveling at a fast clip on a stretch of highway that ran perfectly straight. As the station wagon neared the point where the wires should run across the road, it was obvious to both boys the scant 200-yard lead they had over the state police car leading the motorcade was not enough to give them time to locate the detonating wires, stop the car, get to a point where they could reach the wires, and cut them. Carl kept taking quick upward glances as he let the car slow down. "There they are!" he said, pointing to where two inconspicuous wires crossed the road from one treetop to another. A glance was all that was needed to convince the boys that the wires were far too high to be reached even if they stood on top of the station wagon. "Stop the car with the back bumper directly under the wires," Jerry called to Carl. Carl slammed on the brakes, and the wagon came to a lurching stop with the halo antenna squarely underneath the twin strands of wire. Jerry threw full power into the six-meter transmitter, and he and Carl stared in sickening fascination at the rapidly closing gap between the motorcade and the mined culvert across the river. Suddenly there was a muffled roar, and a fountain of dust and broken pieces of concrete erupted from the point where the highway crossed the culvert. As the rocks and chunks of concrete rained down into the river, causing splashes that reached almost halfway across, the boys saw the entire cavalcade brake to a halt a scant hundred yards from where the explosive had torn a gaping trench clear across the highway. The boys leaped from their car and looked upward at the point where the wires disappeared over the top of the bluff above. Two dark-skinned faces silhouetted against the blue sky stared down at the boys and at a car parked at the side of the road not far away from the station wagon. After a few frozen seconds, the faces disappeared.
"That must be their getaway car," Carl exclaimed. "They probably will be afraid to try to get to it now with us here, but just to make sure we'd better take the rotor out of their distributor. They must really feel pretty sick about getting impatient and blowing up the culvert too soon." "They didn't blow up the culvert; we did," Jerry corrected him. "We what? You must have had your little gray cells jarred by that explosion. How could we have set off the dynamite, or whatever it was ?" "We did it with r.f. from our six-meter transmitter. Radio frequency currents from the halo antenna induced similar currents in the wires leading to the dynamite caps. Standing waves on these wires produced heating in the caps and detonated them." "So that's why you wanted "me to stop with the back of the car directly under the wires! What ever made you think of detonating the caps with r.f.?" "Well, we both know it's dangerous to use transmitting equipment in an area where blasting is going on. We've seen highway signs warning against that sort of thing. And then I was reading a story recently about a patent that had been taken out to eliminate explosions on airplanes caused by bombs concealed in baggage. The apparatus is called SNARE, or Signal Net for Actuating Radio-sensitive Explosives. A man by the name of Irwin Ehlmann of Hatboro, Pennsylvania, is the inventor. "The device consists of a 30-foot bombproof chamber with a conveyor belt running through it. Baggage on this belt is exposed to strong radio frequency waves of different wavelengths. These radio frequency waves will detonate any caps concealed in the baggage and so set off the explosive. "When I realized we couldn't reach the wires in time, all this flashed through my mind. There wasn't anything else we could do; so it seemed worth a try-" Jerry was interrupted by a wailing siren, and in a few seconds a state police car came to a stop behind the generator which was still sitting in the middle of the road. The police across the river had seen the wires leading across the water, and they had dispatched a cruiser to investigate. The officer, "Doc" Watson, was known to the boys, and they quickly explained to him what had happened. He relayed the information over his radio, and a net was quickly thrown up around the area. "Those fellows will be picked up shortly," officer Watson prophesied. "They don't stand a ghost of a chance of getting away on foot. "We all certainly owe you two a big debt of gratitude," he said then, "but I must admit I still can't understand how you set off that dynamite clear across the river without even touching the wires." "Elementary induction, my dear Dr. Watson, elementary induction!" Carl replied with a teasing grin.
Posted May 7, 2014 |
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