THE SECRET TUBE
THAT CHANGED THE WAR
Today it's junk - a bargain - priced surplus special - but
it is also history, the WW II tube no one knew about
By WILLIAM I. ORR, W6SAI
THE YOUNG RADIO AMATEUR saw the dull glint of glass in the bottom of the
dusty box and immediately plunged his hand into the receptacle, searching
for the unknown object that caught his attention. Grasping something,
he slowly drew forth a curious, large misshapen radio tube. Holding
the dusty object up to the bare light bulb dangling from a faded sign that
read "UR CHOICE - 29c," he examined his find carefully. Puzzled,
he turned to the proprietor. "Hey, Sam! What do you know about this
tube? Can I use it on two meters?"
"Surplus Sam," owner of the radio junk shop, took the tube and examined it as if it were a fine jewel. He sighed. "Who knows? Buy it! I don't know what it is, but you can't go wrong for twenty-nine cents!"
| WHERE SHALL WE start the story of the curious tube?
On a June morning twenty years ago in Normandy? Or before that, at
the Panama Canal, or
years later on the slope of a numbered hill in Korea? It's a strange tale of a
unique tube, an Army major and American ingenuity-a true story whose ob-
solete residue was finally found by the inquisitive amateur in a surplus shop.
Panama, 1940: America is not yet at war, but it is obvious to some that we soon will be. The Panama canal is a tempting and vulnerable target from the air. Radar, the radio eye, had been invented a few years before, but the only available equipment worked on the relatively low frequency of 110 megacycles, and then not very well. The safety of the canal could not be trusted to this primitive, unsensitive gear which showed an almost complete blindness in detecting low-flying airplanes.
A decision is made to construct a small number of radically new and powerful radar sets capable of locating and detecting small planes, and to put these sets aboard picket ships located in the approaches to the canal. Laboratory ex-
|periments show that a good frequency for the new sets would be 600
megacycles, but no available tubes can produce the required power at what
was then regarded as an unusually high frequency.
By a stroke of fortune of the kind that often changes history, a radar tube is invented by young Major Harold Zahl of the Army Signal Corps that can produce the power required. A prototype of the vital search radar employing the major's radically new tube is to be secretly built and tested as fast as humanly possible.
On the M.S. Nordic off the New Jersey coast: The vessel is equipped with the new radar, and testing is going forward. Suddenly, a German submarine, intent on spying, surfaces close by. It does not go unnoticed, and as the sub's periscope turns, it sees a destroyer closing in together with a blimp overhead, both carrying depth charges. The sub crashdives as the depth charges drop. The new radar and those aboard the Nordicshaken up by the explosions-are safe. The tests continue. The search radar can detect a single bomber over one hundred
The powerful high-frequency Zahl tube was used in critical radar applications-to detect low-flying aircraft, and to trace the sources of deadly mortar barrages.
|miles away with the radar antenna mounted only fifteen feet above
the surface of the water!
THE SECRET, revolutionary canal radar
equipment was so successful that the Air Force asked the Signal Corps to
repackage the equipment into a lightassault type radar which could be airlifted
to a battle zone and then hand-carried to the front. A prototype of the
repackaged radar was built in February, 1943. To prove it was air-transportable,
the unit was loaded aboard a. bomber at the Newark (ICJ.) airport and flown
to Florida. It was up and in operation four hours after it arrived at Orlando.
The AN/TPS-3, known as "Tipsy
Three" is shown below installed in a tent. It was the first radar set to
operate at high power in 600-megacycle range.
The Zahl tube and its inventor, Dr. Harold A. Zahl, now director of the Army's Research and Development Laboratories, Ft. Monmouth, N.J. The radically new tube-four triodes in parallel with tuned plate and grid lines to make it an oscillatormarked a point of departure for modern tube designs containing resonant circuitry within the tube. Fortunately for the Allied cause during World War II, the Germans never obtained a Zahl tube intact, or guessed its secret. It was, without doubt, one of the factors that won the war and saved countless lives.
(Continued from page 59 )
The first twenty-five production units followed by many more-were built
by Zenith Radio Corporation, and went to England and then to the beaches
THE SUCCESS of the "Tipsy Three," as
it was known to its operators, was due to the secret tube invented by Major
Zahl. Essentially four triode tubes connected in parallel, the tube envelope
also contained tuned plate and grid lines which made it an oscillator.
As much as 250,000 watts peak power could be extracted' from the tube during
a radar pulse. Because of the plate dissipation and cathode emission required
to produce the 250-kilowatt pulse, the anode elements of the secret Zahl
tube ran red hot.
|VT-158, but the tube was soon given the unconditional Joint Army-Navy
(JAN) approval and placed on the "Preferred List."
Doctor Zahl, now the Director of Research at the Army's Electronics Research and Development Laboratories, Ft. Monmouth, N.J., wrote recently, "Within my recollection, this tube passed through its entire life cycle of usage without ever having been the subject of an unsatisfactory report from the field. Eitel-McCullough did a superb job in the production-design of this tube. Even now, I wonder how they did it."
THE TUBES, still unknown to the public
and the enemy, saw action in the Pacific Theatre as well as Europe. In
Doctor Zahl's article, "One Hundred Years of Research," published in the
October, 1960, IRE Transactions on Military Electronics, he said, "But
with all the assistance total mobilization brought (to the development
of new electronic systems) there were many problem areas where the most
learned hesitated to travel, lest the war be over before the problem could
be solved-if it could be solved at all. Riding high in this category was
the location of enemy mortars, the deadly devices which caused the majority
of our ground casualties.
|the problem. was solved. Under the personal` urging of 'General Stilwell
to hurry the equipment into emergency overseas freight, Captain Marchetti's
task force of twenty Signal Research scientists worked for an unbroken
stretch of ninety-six hours-to the verge of collapse on the , first prototype
radar unit. The deadly problem of enemy mortars had a solution-the Zahl
tube used in the AN/TPQ-3 mortar radar set.
During the Korean conflict, the Army again called on the aging Zahl tube and the semi-obsolete AN/TPQ-3 mortar radar-both resurrected from World War II.
THE ZAHL TUBE is no longer manufactured, but the concept has
not been forgotten. While the once-secret, revolutionary VT-158 may now
be found in dusty surplus bins, work is still being done on powerful new
ultra-high frequency radio tubes that contain the resonant circuitry within
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