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Transmitters Towed Through Air Tests Antenna's Radiation Pattern
October 18, 1965 Electronics Magazine Article

October 18, 1965 Electronics

October 18, 1965 Electronics Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Electronics, published 1930 - 1988. All copyrights hereby acknowledged.

"Xeledop" is the Word of the Day for October 31; use it often. Xeledop is an acronym for "transmitting elementary dipole with optional polarity." Nope, I've never heard of it, either. The Xeledop (probably pronounced "zeh'-le-dop") is an air-towed transmitter that flies a pre-planned path around the ground-based antenna under test (AUT) whose radiation pattern is being measured. The circular power level plot at the bottom of the page shows the results of an actual test flight. In this application, a high frequency (HF, 3-30 MHz) transmitter is towed behind an airplane like target drone while it broadcasts signals at eight distinct frequencies toward the AUT, while the downstream receiver records power levels. The pilot flies on the surface of an imaginary hemisphere to maintain a constant radius from the antenna. Ground equipment tracks the aircraft azimuth and slant range is calculated using aircraft altimeter data and measured elevation angles. Both horizontal and vertical radiation patterns can be measured. A VHF model was also tested.

This is somewhat similar to the Drone-Based Field Measurement System™ (dB-FMS)™ that I proposed back in 2014. My scheme uses a remote controlled drone with onboard measurement receiver and GPS to fly a programmable profile for determining the radiation pattern of a ground-based transmitter - basically the opposite of the Xeledop system. Since antennas are reciprocal devices, the measured radiation pattern of a transmitter is equivalent to that of a receiver using the same antenna, and vice versa. I hope to achieve millionaire status some day when someone pays royalties for commercializing my idea ;-)

If you have a subscription to the IEEE Library, check out "Measurement and Modelling of HF Antenna Gain and Phase Patterns and the Effect on Array Performance," by R.W. Jenkins and L.E. Petrie. It is a modernized Xeledop measurement method that uses differential GPS for a precise tow aircraft position.

Transmitters Towed Through Air Tests Antenna's Radiation Pattern

Transmitters Towed Through Air Test Antenna's Radiation Pattern, October 18, 1965 Electronics Magazine - RF CafeSignals transmitted by airborne equipment are measured at the antenna to obtain accurate measurements at high and very-high frequencies

By Cecil Barnes Jr.,
Stanford Research Institute, Menlo Park, Calif.

In theory, the radiation pattern of extremely large antennas can be calculated. In practice however, it is not always possible because the pattern is affected by local topography, conductivity of the soil, and by reflections from other antennas, power lines, or metal structures in the area. Because of all these factors, methods of checking are needed to determine whether existing antennas meet directional requirements. One cannot complacently assume that the calculated values will give the correct radiation pattern of an antenna constructed in the field.

Modeling techniques are impractical because the conductivity of the antenna cannot be scaled and the ground constants are often unknown. The only sure way to determine the pattern of a large antenna is by direct measurement.

Towing the Transmitter

High-frequency Xeledop - RF Cafe

High-frequency Xeledop, equipped with fins for horizontal polarization, assembled and ready to be loaded on plane. Second Xeledop is kept in plane as spare. Only one unit can be flown at a time.

One way of getting direct measurements is to fly around the antenna as a signal is transmitted from the aircraft and to record the reception of the signal by the antenna. A plot of the voltage measured at the antenna terminals as a function of aircraft position will give the radiation pattern. The airborne technique covers angles above the horizon, and is considerably faster and more accurate than either walking or riding around an antenna while it is transmitting, and measuring the pattern with a field-strength meter.

Simple as the measurement from an airplane sounds, many complications arise. One, in particular, is that at high frequencies (3 to 30 Mc) some part of the airframe may be resonant and introduce an error in the results by reradiating additional signals. For this reason, when testing high-frequency antennas from the air, it is necessary to place the transmitter in an aerodynamically stable housing and tow it at a distance that makes re-radiation negligible.

The latest transmitter designed for towing by aircraft is called the Xeledop, an acronym for transmitting elementary dipole with optional polarity. Eight transmitters in the Xeledop can broadcast at eight different frequencies. These frequencies are selected to cover the antenna's complete bandwidth. One Xeledop can be used to test several antennas simultaneously. To measure the polarization characteristics of the receiving antenna, the Xeledop can be easily oriented in either a horizontal or vertical position.

The Xeledop system is an improvement over the pioneering techniques previously used [Electronics; Nov., 1955, pp. 134-136]. The advent of transistors has eliminated tube filaments and consequent battery drain, allowing operation of several transmitters over many hours. When oriented vertically, the Xeledop remains vertical regardless of air speed. Today, the pilot has a constant-distance indicator to assist him in maintaining a constant radius around the antenna being tested. And data processing is speeded with the aid of computer techniques.

Xeledop Design

Glass fiber fins on one radiator and an iron counterweight on the other keep the Xeledop horizontal - RF Cafe

Glass fiber fins on one radiator and an iron counterweight on the other keep the Xeledop horizontal when towed, as shown in the top diagram. For vertical polarization, the fins and counterweight are removed and a lead weight is substituted for one of the hollow balls. The bottom diagram shows how the drag cone is deployed to aid in keeping the Xeledop vertical as it is towed.

Two different Xeledops are used for high and very-high frequencies. The h-f Xeledop is a glass fiber sphere approximately 11 inches in diameter, from which extend two streamlined 44-inch radiators. Each radiator terminates in a 3-inch-diameter hollow metal ball. The h-f Xeledop is normally towed 300 feet behind a fixed-wing aircraft, traveling at speeds between 100 and 170 miles per hour, on a single 3/16-inch-diameter line of braided nylon with a breaking strength of 1,000 pounds. The line is attached to the Xeledop with a Dacron bridle and a non-conducting phenolic handle pivoting on an axle extending through the center of the sphere.

To make the Xeledop assume a fore-and-aft horizontal position and thus radiate horizontally polarized signals, glass fiber fins are attached to the end of one radiator and counterbalanced by a small weight inside the other.

To achieve vertical polarization, the fins and forward counterweight are removed and a 5 1/2-pound lead ball is substituted for one of the hollow balls. A drag cone is added to prevent pitching. In this way, the center of pressure (from the slip stream) lies on the axle; the center of gravity lies below it.

The third direction of polarization is horizontal with the dipole axis at right angles to the line of flight. For this, a helicopter is used, flying at slow speed with two separate tow ropes attached from the ends of the Xeledop dipole to two support points on the helicopter; in this case the drag cone is used to prevent yawing.

The length-to-width ratio of the radiators, the diameter of the small top-loading balls, and the shape of the main housing (spherical) are chosen so that the assembly will function electrically and at the same time be aerodynamically stable whether vertical or horizontal. The batteries and other components inside the sphere are placed so their center of gravity is at the center of the sphere. Flight tests with an airplane have shown that the Xeledop is stable in the first two configurations between 40 and 250 mph, it is not stable below 40 mph, and it has not been tested above 250 mph. The changes required to switch from one polarization to the other do not detune the antenna system or affect the radiated power because no conducting parts are affected.

The vhf Xeledop is similar to the h-f model except that the sphere is nine inches in diameter and the radiating elements are a half-wavelength long at the highest frequency. Glass fiber extension rods hold fins and weights, as needed, 16 inches away from radiating elements to avoid electrical interaction. When flying vertically, a lead weight is carried 16 inches below the bottom radiator and a balsa-wood ball of equal drag, is mounted 16 inches above the top radiator.

Dipole and Bandwidth

Minimum distance from the antenna to Xeledop - RF Cafe

Minimum distance from the antenna to Xeledop, based on the highest Xeledop frequency to be used, is plotted for different antenna apertures, largest antenna dimension or distance between nearby reflecting objects.

The Xeledops must be designed to simulate elementary (or short) dipoles so that their radiation pattern will be known. This means that the total length of the radiators cannot exceed one-half wavelength. However, short elements have low radiation resistance; if the elements are too short, it becomes difficult to match the transmitter output to the dipoles without excessive losses in the matching network. On the other hand, if the elements are too long, the dipole cannot be considered "elementary," and it will not have the desired radiation pattern. The dipole dimensions govern the bandwidth over which the Xeledop operates.

The configuration of the h-f Xeledop allows op­eration at frequencies as high as 50 Mc before the pattern is distorted. Operation at frequencies as low as 2 Mc is achieved by top loading each ele­ment with the 3-inch metal ball and by suitable choice of high-Q circuit components in the dipole matching networks. Each transmitter is matched to the dipole through a balanced ceramic-ferrite toroidal transformer using different materials for frequencies below and above 20 Me,

The h-f Xeledop can transmit pulses sequen­tially on eight crystal-controlled frequencies in the 2-to-50-Mc band. The pulse width depends on the cycling rate. This is set to give the desired sam­pling rate for one frequency, and is governed by the detail of the pattern to be studied and the speed of the airplane, normally about half a degree of azimuth per second.

Stepping Switch

High-frequency transmitter delivers one watt into dummy load - RF Cafe

High-frequency transmitter delivers one watt into dummy load. The actual power radiated is much less. Ring counter switches antenna from transmitter to transmitter. All transistors in this unit are type 2N3053.

An electronic stepping switch is used to key each transmitter sequentially and connect it to the dipole. The stepping switch, or keyer, consists of a free-running pulse generator which drives a 9-stage ring counter using silicon-controlled switches. The first ring counter stage generates a quiet pulse period for timing and other operations in an automatic recording system. Each subsequent stage drives an antenna switching relay and keying circuit for its associated transmitter unit. The frequency of the pulse generator is variable from 2.5 to 40 cycles per second; each pulse from the generator causes the ring counter to step ahead, keying a transmitter, releasing one relay, and closing the next so that one transmitter at a time is turned on and connected to the antenna. The ring counter may be adjusted to bypass one or more stages; thus the Xeledop can cycle at a steady pulse rate (5-millisecond interval between pulses) on any number of transmitters from two to eight, or it can transmit continuously on one frequency. Pulses transmitted are the same width and unmodulated.

The vhf Xeledop was designed to transmit on three frequencies. The keyer is similar to the h-f unit except a four-stage transistor ring counter is used, and the need for antenna switching relays is eliminated by a passive multi coupler.

Positioning the Xeledop

When he's ready to take the Xeledop aloft, the pilot takes along aerial photographs or topographic maps of the area with a flight-path circle drawn around the antenna. At low altitudes this circle is a guide enabling him to fly the airplane on an accurate track. At high altitudes or when flying over water or above clouds, the pilot is guided by a zero-center milliammeter mounted on the instrument panel. This meter, known as a deviation indicator, is driven by an interrogator, operating with a transponder at the antenna. The meter, similar to an instrument-landing system indicator, shows the pilot his deviation from the desired circular path and gives him right-left steering indications.

Lines on the face of the meter represent deviations of approximately one-tenth mile off the circular course. The deviation indicator is initially set with reference to the ground. When over a landmark on the flight circle at a specified altitude, the pilot sets the indicator to zero and locks the control. The pilot may deviate appreciably from the flight track without introducing serious error in the results. A 10% change in the distance between the airplane and the antennas causes less than 1 decibel change in the signal level. In any event, changes in range are allowed for in the data reduction.

Orbiting a Hemisphere

3 of the Xeledop's transmitters can be seen - RF Cafe

With the cover removed, three of the Xeledop's transmitters can be seen. One of the nickel-cadmium batteries is visible below the transmitters.

To conduct the pattern measurements at elevation angles below 45°, the pilot flies over the surface of an imaginary hemisphere, keeping his radius constant during each orbit by means of the deviation indicator. He covers the surface of the hemisphere in steps, usually from 3° above the horizon to 45°, maintaining a constant altitude during each orbit plus a 30° overlap for a validity check. Seven to ten orbits are usually required to get satisfactory coverage for an antenna pattern; one orbit is chosen to take the Xeledop through the antenna's estimated main beam. Calibration of the ground equipment is repeated while the pilot is changing altitude.

The radius at which the airplane is flown depends on the frequency of the test signal and on the size of the antenna to be measured. In some cases; it is limited by the airplane's ceiling. According to a rule of thumb commonly applied to pattern measurements, the aircraft should be far enough from the antenna to satisfy the equation:

R = (2D2)/λ

where R = slant range from antenna to aircraft, D = diameter of antenna aperture, λ = wavelength of highest Xeledop frequency.

The term D may represent simply the largest over-all dimension of the ground antenna includ­ing its ground plane, if any; or it may be the largest dimension of a complex installation of many antennas if reflections from other antennas and guy wires are to be considered. In most cases, a radius of 3 to 5 miles is satisfactory. The minimum slant range at which the airplane should fly for various frequencies and antenna apertures is indicated in the graph on page 98. Even so, bringing the airplane in to one-half this distance introduces an error of only a few percent in the measured antenna gain.

Circle or Grid

Rawin receiver (radar used to measure wind velocity and direction) - RF Cafe

Ground equipment necessary to test one antenna at six frequencies. Adjacent antennas can be measured simultaneously. A Rawin receiver (radar used to measure wind velocity and direction) tracks the airplane, in azimuth and elevation, and supplies the pulse to the recorder for coordinating received signals with plane position.

Experience has shown that a pilot can fly a fixed-wing aircraft above a cloud layer on a constant radius of 4 to 10 miles with the deviation indicator as his only guide at angles up to 45° above the horizon. At higher angles, the indicator becomes difficult to follow and accuracy drops.

For elevation angles above 45°, therefore, the airplane flies a rectangular grid pattern at a constant altitude over the antenna site, with the ground equipment tracking it on each pass. To fly this pattern, the pilot must have a clear view of the ground unless some rather exotic navigational equipment is on board. If the Xeledop is horizontally polarized, the pilot reports his heading, and consequently the direction of the dipole axis, on each pass. Because of cross winds, these headings may not coincide with the grid tracks; this conflicting data is later corrected by a computer.

The azimuth and elevation of the aircraft is determined by automatic ground radio tracking equipment. The slant distance to the aircraft is calculated from the measured elevation angle above the horizon and the altitude reported by the pilot.

On circular flights, the airplane can complete about three orbits per hour depending upon radius, including climbing and descending, or three hours to complete data-taking below 45° at one polarization. A grid pattern takes six hours.

Getting the Information

Typical chart recording of the signals received at the ground station - RF Cafe

Typical chart recording of the signals received at the ground station clearly show the lobe structure at four different frequencies. Each spike on the recording represents one pulse from the Xeledop. It is clear from the plot, second from bottom, that the reference antenna had two nulls, 180° apart, at 15.3 megacycles.

The recorder input signals are the automatic gain control supply voltages of the receivers. The gains are set so that 40 decibels occupy the width of one channel. Attenuators handle signal variations in excess of 40 db by adding or subtracting attenuation in 10-db steps. Analog-type recording has been used so far, with recorders designed so that the operator can continuously monitor the recorded information and immediately detect any malfunction. Since high-frequency response is not required, a multichannel paper-strip recorder is satisfactory, producing a clear, easily interpreted record. The figure on page 101 shows a sample record of four frequencies recorded as the aircraft made an orbit around a pair of antennas. The pilot flew more than 360° in azimuth to create an overlap, thus providing one method of checking the results. Each spike on the record represents a pulse transmitted from the Xeledop. The space between pulses shows the noise level on each channel, indicating the signal-to-noise ratio; in this example, however, the noise on all the channels is below the threshold set for the test. The height of the pulses represents signal strength. The lobe structure of the antennas being tested is clearly visible. A rectangular waveform made by the marker pen along the top edge of the chart provides a synchronizing signal for comparison with the separately recorded plane position.

The first step in the data reduction process, a screening process, consists of a visual inspection of the strip charts and field notes. During this inspection, the data for further analysis is selected and the azimuth synchronizing-pulse correlation numbers are written on the charts. Next, the analog information from the strip charts is transferred to punch cards along with data relating to the antenna, frequencies used, aircraft altitude, Xeledop polarization, nominal slant range, date and test number. On each card is entered the appropriate synch-pulse number and an amplitude for each channel of interest. Readings are taken for every 5° of azimuth and at all points of maximum or minimum recordings.

The azimuth-elevation information from the ground tracking equipment is punched into a second set of cards from a record, which is printed at six-second intervals, showing azimuth angle, elevation angle, and a synch-pulse number. This completes the manual processing of the data.

The two sets of punched cards are fed into a digital computer which combines the information on the input cards and incorporates corrections for the following: parallax due to the distance between the antenna and tracking equipment, change of slant range when flying a grid pattern or due to an eccentric or off-course orbit, Xeledop antenna pattern, distance of Xeledop below and behind airplane, and a shift of azimuth zero reference from true north to the nominal direction of the main beam. In addition, the computer remembers the largest signals recorded; this information is used later to normalize all signals of one frequency to zero decibels. This information is also used to compare the gain of one antenna with another. The punched card output of this computer is fed into another computer along with a program for drawing and labeling contours.

Drawing the Pattern

Contour lines, at 3·db intervals - RF Cafe

Contour lines, at 3·db intervals, are plotted by the computer from information it has received from punch cards. This pattern is of a horizontal dipole 2·feet·high and 78·feet·long. The signals were recorded at 10 Mc with the Xeledop vertically polarized.

The magnetic-tape from the second computer is fed into an automatic plotting machine which plots contour maps of the antenna patterns by drawing contour lines for 3-db intervals and writing the decibels below the maximum reading at suitable locations along the contour lines. The pen-recorder also makes several small registration marks near the edge of the paper; these are later used as guides for photographically superimposing a polar grid. In the process of computing the contour-line locations, the computer interpolates between measured values and thus is able to establish field-strength levels at locations between orbits where the airplane did not fly, It is because of this capability that the aircraft need not fly a perfect grid or a perfect orbit around every antenna.

A T-11 Beechcraft and a modified B-25 bomber have been used satisfactorily for pattern-measurement. The Xeledop is carried inside the airplane during takeoff and landing and lowered through a hatch in the floor for use. Helicopters have been used on special occasions where vertical descents or horizontal polarization at right angles to the line of flight were required.

Statistical distribution of burst durations as measured on the Seattle-Bozeman link. Curve is a plot of the measured probability of meteor bursts occurring whose duration is greater than an arbitrary duration.

Measured statistical distribution of period between bursts.



Posted October 31, 2018

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