LF antennas by Dr. J.A. Pierce

[Andre' Kesteloot]

Walter Blanchard wrote:

> I've recently been editing the memoirs of Dr. J. A. Pierce, an American
> engineer who was responsible for much of the design work on the Omega and
> Loran-C navigation systems and I thought some of his remarks might strike a
> chord with this Group. See for yourselves!!!
> His full memoirs run to some 350 pages and will be published as soon as
> I've finished editing.
>
> Walter G3JKV.
>
>   ------------------------------------------------------------------------
> Excerpts from the memoirs of Dr. J. A. Pierce
>
> (He is talking about setting up Omega in 1956)
>
> Omega Antennas (Omega frequencies were 10.2 to 13.6 kHz).
>
> Transmitting antennas for the low frequency constituted a major problem. We designed and ordered
> for use in the Pacific three 625 foot lattice towers to be used in an "umbrella" configuration with
> heavy top-loading cables. These could not be produced quickly so we settled for balloon-supported
> wires for the experimental stations set up in the United States. The balloons were little "VLA" (very
> low altitude) models about 35 feet in length and with a diameter of 114 feet. They had stabi1ising
> fins so that the kite effect gave them some additional lift from the prevailing winds. Each balloon
> supported an antenna of about 1300 feet of copperweld wire with large insulators at the top and
> bottom. The Army Air Force provided the balloons and helium to fill them, and also assigned small
> crews to fly them for us. We had three stations at East Brewster, Massachusetts; Cape Fear, North
> Carolina; and Key Largo, Florida.
> The balloons flew more reliably than we had dared to hope but, of course, were less than perfect.
> They had a maximum load-carrying capacity (in still air) of 58 pounds while our antenna wire and
> insulators weighed 48 pounds. It was therefore essential to keep the balloons well inflated. It
> surprised me that, even with the balloons flying in three quite different weather patterns, all three of
> them were in the air for more than ninety percent of the time. This would not have been satisfactory
> for operational use, where absolute reliability of the signal is highly desirable, but it was adequate for
> the experimental work.
>
> The antenna at Jim Creek (a U.S. Navy LF communications site).
>
> The size of this station was a revelation to me. The antenna consisted of ten copperweld cables
> 8,000 feet long strung across a narrow valley between two ridges 3,000 feet high. The centers of
> these strands were connected to downleads that were brought together into a sort of transmission
> line that carried them back to the transmitter building. The antenna was actually separated into two
> halves, each excited by its own transmitter, so that in case of accident or the need for maintenance
> the station could operate at half-power for a time. The transmitter building was a concrete box a
> hundred feet or so square without windows and with access to the area of the transmitter itself only
> by elevator from below. As befitted a station with a transmitter whose component sections were
> mostly of the order of cubes ten feet on a side, the elevator was so big that we simply drove our
> truck into it for the ride up to the operating level,
> We spent two or three days setting up our equipment and erecting a whip antenna for receiving the
> signal from Criggion. As the transmitter building was the only possible site for our gear in the
> immediate vicinity, the whip was installed on the roof about fifty feet from the "lead-in" which
> carried about 700 amperes of radio-frequency current. It was in setting up this
> antenna that we discovered the falsity of the common statement that "r.f. doesn't shock; it simply
> produces surface burns". This may be the truth for small quantities as high-frequency currents tend
> to flow only on the surface of a conductor, but it fails by a wide margin to explain the behavior of
> large currents at such a low frequency as Jim Creek's. Our rough calibration of the field strength near
> the transmitter lead-in was as follows: a bit of metal up to five or six inches long (such as a
> screwdriver or a pair of pliers) stings like a nettle; rubber gloves are a necessity for handling metal
> objects a foot or two long; and touching a conductor five or six feet long can knock one down.
> The minor pain we encountered in setting up this antenna was wasted, as we never detected a signal
> from Criggion at that site. Two or three days passed while we searched for the signal. This was a
> slow process as the only indication of its presence would be the tracking behavior of our servos over
> a half-hour or more. The search was complicated by the fact that our oscillators had completely lost
> calibration in the trip across the country, while the signal from WWV which we had expected to use
> to find the correct frequency was received so poorly as to be essentially useless. At that date, the
> only real access to precise frequency was through the signals transmitted for the purpose by WWV
> from near Washington, D.C., and also from WWVH in Hawaii. Neither of these signals was received
> well enough for the very accurate calibration we had to make. It therefore was a painful and erratic
> search, moving our oscillator in small steps through what we hoped was an adequate frequency
> range, and watching the servo record for symptoms of proper tracking. Frequently random behavior
> would look real for a few minutes and lead us to erroneous corrections because our patience was
> under such strain.
> In the end we gave up trying to operate at the transmitter site. In the search for an alternate we
> found that there was a little "microwave hut" at the top of one of the mountains that supported the
> large antenna. This hut received signals, from Seattle I suppose, and relayed them down to the
> station in a telephone cable. The hut was not much more than a mile in a direct line from the
> transmitter, but was reached by seven miles of mountain road. The hut was near the
> southwestern-most "stub" tower that supported one of the strands of the big antenna. This tower
> was about 200 feet high and made an ideal point to which to tie a fairly long wire receiving antenna.
> At this site we set up our gear in an odd corner and, without too much difficulty, detected the
> Criggion signal. Without any proof, I still believe that our failure down below was due to the
> weakness of the Criggion signal at the bottom of the narrow valley - the signal was none too strong
> at the top of the mountain. We estimated the one microvolt per meter that I mentioned above from
> the degree of sluggishness of our servos. In other words, if the signal had been stronger the tracking
> would have been faster or better.
>
> The antenna at Haiku (Hawaii)
>
> The antenna at Haiku worked very well, partly because the site was in a crater with a bottom a mile
> or more wide, so that the outer ends of the antenna cables, where the voltage was at a maximum,
> did not hang too close to the conducting earth at the mountain top. The similar antenna at
> Jim Creek in the Cascades in the state of Washington had a disappointing
> efficiency. I have always thought that the difference was that this Jim Creek site was in a valley with
> a very narrow bottom. The large sag of the heavy cables brought much of their length too close to
> the slopes of the mountains on both sides. Jim Creek was indeed very useful as a large input to the
> antenna made it one of the more powerful stations in the world. The amount of power actually
> radiated, however, was only about a third of what its designers hoped.
> The first Omega-like transmissions from Haiku were made from the "small" TCG antenna, named for
> the type number of its transmitter. This was the antenna used for my slow-speed experiment.This
> antenna was a single strand of cable across the same crater but at a slightly lower height and with
> only a thousand-foot downlead. It was used for early tests by NEL, but later the work was
> transferred to the main antenna, which had not been in use for years. The Navy Electronics
> Laboratory crew working at Haiku were allowed to use the large antenna under a curious agreement.
> They could transmit what signals they pleased provided that they would rotate the armature of the
> Alexanderson alternator about 90 degrees once a week, to prevent it from developing curvature of
> the spine, or shaft.
> Even the large antenna at Haiku was somewhat inadequate at ten kilohertz. When it came time to
> promote the station from experiments to full Omega operation, I believe that the four strands of
> antenna were increased to six, thereby achieving ten kilowatts of radiated power. This was an early
> estimate of what was needed, but operation of Trinidad at low power had shown the desirability of
> such an increase.
> Apart from these "valley-span" antennas, the second type of antenna used for Omega is the "umbrella".
> This is a single central tower
> surrounded by radial cables extending from the top of the tower toward the ground at considerable
> distances. These many radials have insulators at such a distance from the tower that they hang at
> about half its height. The remainder of each radial is interrupted by several more insulators, so that
> the antenna part is well isolated from the ground system below. For use at the Omega frequency
> such a central tower should be about as high as any man-made structure, I believe that the one at
> Tsushima is at least 1500 feet in height. In that case, the central tower takes the form of a steel tube
> a dozen feet in diameter. The antenna at Monrovia is supported by a triangular lattice mast 1457 feet
> high. The whole umbrella antenna usually rests on massive porcelain insulators that are strong
> enough to support the whole weight plus serious strains from wind loading, and insulated for a
> couple of hundred thousand volts. This is probably the most economical type of antenna to use when
> "ready-made" mountain sites are not available, as seldom happens in convenient places.
> The base-insulated umbrella is the kind of antenna we designed for the LF Loran stations at the end
> of World War II. We surely did not originate the idea, but such antennas were certainly not used very
> often before then.
> I have only recently learned that a variant of the standard umbrella configuration is used at the
> Omega station at La Reunion and perhaps in Australia. This is the umbrella with the central tower
> grounded at the base and insulated at the top from the radial top-loading cables. A great advantage
> of this construction is that the tower is not electrically "hot", and it also permits some simplification
> in conducting to ground the strokes of lightning that invariably hit such a tall structure. Because the
> grounded tower offers a short capacitive path to ground from the high voltages on the radials
> (somewhat like the situation in a valley antenna with too small a height of parts of the radials) the
> efficiency is sure to be reduced in comparison with the same antenna with an insulated base. The
> large and expensive porcelain insulator, however, tends to be mechanically the weakest part of the
> structure and limits the weight that can be placed on it. The base-grounded antenna can therefore be
> extended to a greater height and thus permit radiation of as much power as a somewhat lower tower
> with an insulated base. The tower at La Reunion, for example, is no less than 1,600 feet tall.