Proximity Fuze Jamming - W.W. Salisbury
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Proximity Fuse Jamming 
By Winfield W. Salisbury (c) SMEC

An interesting side of electronic warfare came to my attention about ten days after I joined Dr. Terman at the Harvard Radio Research Laboratory. Fred asked me to come to his office to discuss a new project. When I came in he showed me some small pieces of odd looking equipment and remarked that these parts were especially secret and should be kept in the safe when not in use.

I asked what application was intended and Fred explained: "These parts are radio activated fuses for artillery shells. Vannevar Bush and Alfred Loomis have suggested you for this job so I guess Iíd better let them explain," He picked up the phone and I heard him say Iím sure Win can come right over.

When I arrived in Vannevarís office at MIT, Alfred Loomis was there. Alfred said, "We have a special secrecy problem. Dr. Merle Tuve at Carnagie Institution of Magnetism is developing a proximity fuse for artillery shells, He is very tight on secrecy, but he is telling the appropriate military people that his fuses cannot be interfered with. We are somewhat skeptical about this, but we have to be careful, because if the military users thought that these fuses could be jammed they would never get used. Vannevar and I believe that, contrary to Dr. Tuve, any electronics device can be interfered with. The only real question is, what the is cost in terms of knowledge and equipment? We want you to analyze Merle's fuses and make and test a jammer, if you can find a way. Merle Tuve is rather touchy about this so do not contend with him. We will insist he allow you to make a test but you are to report all results to us, and keep your work completely secret from any military or naval personnel. We want the fuse to be used but we believe that if they have any weakness we should know it before any enemy does."

I agreed to take on the project and returned to my lab to analyze the samples. A description of how the fuses were intended to work depended on the Doppler shift of reflected radio waves from the target causing a low frequency beat with the transmitted frequency.

I devised a variable frequency transmitter which could be adjusted to the necessary frequency range and which, by means of a motor driven variable condenser, emitted a wave that varied at the approximate beat frequency, which depended on the velocity of the shellís approach to the target.

Dr. Tuve reluctantly agreed to a test at a fort in North Carolina, where he was testing fuses by firing them more or less vertically and observing the burst with a smoke puff rather than the usual explosive charge as they approached the ground. I came to Tuve's test site on the appointed day and he demonstrated by firing them while we were protected by a heavy roof of Palmetto logs. The gun used was available because it was considered too worn for military service. On this account the shells tumbled on the way down and took an amazingly long time for the trip up and back down. The fuses worked well and the test shells puffed smoke at about 30 to 40 feet above ground as they returned. After several demonstration shots, I observed the shellís radio signals on a special radio receiver which I had provided. I suggested that on the next few firings I would use my jammer. Tuve agreed, and I tuned my jammer to the observed frequency.

The next few shots all puffed smoke at about 4000 feet altitude, much to Dr. Tuveís dismay. He immediately suggested that we had hit a bad batch of improperly manufactured fuses. However when we tried several shots with my jammer turned off, the shells again puffed smoke at about 30 feet above the ground.

Dr. Tuve was still unwilling to admit that his fuses could be jammed and said he would report that he had encountered a few defective samples, I replied that, of course he was free to report the tests as he saw them but I would make my report independently. Merle was very angry with me, but he later forgave me and we became good friends.

I turned in my report and my jammer and nothing more came up until the Germans captured 20,000 proximity fused shells during the Battle of the Bulge. We then had a marvelous flap to build 200 of my jammers.

However, the Germans had been told by their engineers that proximity fuse shells were impossible because of the forces exerted during acceleration in the gun, so they never recognized what they had captured.
 
 
 

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