Dipole

(>-oniinal) Diameter

111 Inches

0.001 0,003

0.001

0.003 0.002

0.005

0.001

3Iaterial

Copper Copper

Platinum Platinum Platinum I'latinum

Bismuth

31.0 3.79

229. 53.5 23.7 8.56

2800.

Con1putcd Z<

i n 6 1 / m

lSY+jlSO 62.5+j59.i

measurement of back scattering cross section is esti- mated a t 5 4 per cent and the lengths of the wires used in the experiment were determined to within f 0.002 X. The limit of error i n cross section due to the values of resistivity used for the skin impedance i n the computa- tions is estimated to be an additional 4 2 per cent. The resistances of the wires u-ere measured on a General Radio Resistance Limit Bridge, and the permeabilities \\.ere all taken as ,uo=4aX lo-' henrys per mcter. From these the values of Zi were computed.i

('OKCLVSIOS

Resistance losses i n a t h i n cylinder modify the back scattering results i n the region of first resonance by re- ducing the cross section and increasing the bandwidth. The first resonant peak occurs virtuall!- at the same

I- 0.712 0.805

0.733 0.768

n -603 0.572 ' 0.725 0.690 i 0.760 ;:x; 0.788 i

-

I'er Cent Bandwidth

11.0 13.5 12.1 11.2

13.1 14.0 13.5 ' 11.4 1 3 . 8 1 5 . 3 15.6 16 .-I

cylinder length. 'The experimental results reported above show that the zero-order variational expressions for back scattering describe fairly accurately the be- havior when losses cause as much as a 75 per cent re- duction in the peak cross section. Furthermore, for COITI- putations in the region of first resonance, no additional accuracy is gained by the use of the next higher order of approximation of Tai.

-ACKSOKI,EDGXENT

The \\-ark described in this paper was supported by the Wright 4ir Development Center under Contract No. AAF 33(616)-68. The authors are indebted to J . AI. liinkowski for his many helpful suggestions and to Dr. J. hI . Kopper for review of the manuscript. F. G. 'I'rageser performed the experiments.

A Multipurpose Radar Target" J. W. CARRt

Summary-Consideration of methods of simulating a moving target by making a mechanically stationary target appear [to an MTI (Moving Target Indicator)-equipped radar] to be moving resulted in the use of crystals as switching elements in a low-voltage low-energy battery-powered device. Extending the use of these switching elements results in a target that is visible to any polariza- tion. By applying these concepts to composite waveguide structures, a dual-band simulated target head was developed and field tested.

* RIanuscript received by the PG-AP, October 9, 1958; revised manuscript received, ;\ugust 19, 1959. The work presented here \\-as done at Gilfillan Bros., Inc., Los ;\ngeles, Calif., where it \vas officially disclosed on June 6, 1'955.

Calif. t 1Iissile S>--stem Div., Lockheed Aircraft Corp., St~nnyx-ale,

IKTRODUCTION

N several radar systems, especially i n some particu- lar aircraft landing systems, two of the most \ d u - able adjuncts are 1) an XITI for detecting moving

targets in the presence of stationary ground clutter and 2) circularly polarized antennas for better target detec- tion in the presence of rain. Standard geographically- located stationary reference targets provide definite range and angle markers for normal radar use when 1) and 2) are not employed. Hou-ever, when the KIT1 is placed in operation, a standard passive target reflector is cancelled out along with the rest of the stationary

January

clutter; this leaves only synthetic geographical reference points, if such are provided, on the indicator. V'hen circular polarization is employed in the radar antennas, the plane reflector-type target is masked, since the re- flected wave is polarized in the opposite sense to the radar antenna. An active device is required in order to provide a visible target when I I T I is used, whereas the requirements for circular polarization alone can be achieved by a passive scheme.

After considering several schemes to simulate a mov- ing target, the nonlinear impedance characteristic of a crystal looked the most promising from the standpoint of low power requirement. Also, there mas a large range of commercially available silicon and germanium crystal diodes t o choose from.

PRINCIPLES OF OPERATIOK

Fig. 1 shows qualitatively the dc forward resistance characteristic of a crystal diode. For little or no applied forward voltage VI, the resistance is high in the vicinity of A. For some value 172 of applied potential difference, the resistance is some low value in the vicinity of C. If t he RF resistance has a similar characteristic, one sees the possibility of controlling the R F impedance by ex- ternally applied biasing voltages. Suppose a crystal and a transmission line are so chosen that the characteristic impedance Zo of the transmission line lies in the vicinity of B. Further, assume that the combined crystal and mount can be designed such that the impedance pre- sented to the line is mainly the crystal resistance. Let 2, be the predominantly real terminal impedance pre- sented to the line by the crystal and its mount. Let kef8 be the reflection coefficient.

For V = Vl, Zt l>Zo, and klej81 k l . For V = V 2 , Ztz <&, and knej82 E - k 2 .

Thus, if the applied bias is Vl on the 72th incident radar pulse and VZ on the (nf 1) radar pulse, it is noted that the phase of the reflected R F signal has been changed on the order of 180" from pulse to pulse. On a voltage basis the efficiency of the target head (transmission line- crystal network) may be expressed as

kleih - Rseion I per cent voltage efficiency = 100 - I

1 2 4

= 50 Ikl - kzei(cn - 01) . I When k l = k g = 1 and &-O1 =T, the efficiencl- is 100 per cent.

A voltage efficiency is defined since the most common MTI systems linearly detect the echo pattern from one pulse, delay it and compare it with the return from the next pulse, etc.

Fig. 2 shows some measured efficiencies at S band. X-band efficiencies were approximately the same. These 4 per cent bandwidth data do not represent band- width limitations. Tests were made only on commer-

Fig. 1-Forward bias.

I

6-S 6 a+&- FREQUENCY

Fig. 2.

cially available crystals. With switching characteristics considered as the main design parameter it may be pos- sible to develop and package a crystal, or family of crystals, which has much more efficient switching char- acteristics. In an attempt to apply the technique to an externally pulsed low-power T R device only about 10-db rejection was realized. No further investigation was made.

Fig. 3 shows schematically a primary feed horn with a crystal-terminated transmission line.

Fig. 4(a) shows an R F pulse chain and Fig. 4(b) shows an enveloping symmetrical waveform from a multivibrator with a frequency half of that of the radar PRF.

The multivibrator waveform is the bias waveform ex- ternally applied to the crystal. I t may be assumed, for the sake of presentation, that the multivibrator is syn- chronized with the radar PRF. However, this is not necessary in systems where the pulse-to-pulse signal differences due to a phase differential at the target are integrated over many pulses, i .e . , over the number of pulses per sweeping antenna beamwidth.

The above discussion has demonstrated how such a device can return a detectable signal to a linearly polarized radar set equipped with a pulse-to-pulse phase comparison MTI. If only an amplitude-sensitive radar detection system is used, it may be desirable to design

9

(b) Fig. 4-(ai Pulsed R F envelope. (bi Pulsed bias.

a high reflection coefficient for the nth pulse and a matched termination for the (n+l)th pulse. This mode of operation was accomplished.

DESIGN OF THE TARGET FOR CIRCULARLY POLARIZED RADARS

Consider Fig. 3 again, but now use a waveguide with orthogonal symmetry such that it can support tn-o or- thogonal modes with the same phase and impedance characteristics. A4dd a second crystal half-a-guide-wave- length towards the horn from the first one and a t right angles to it. Appl~r l,rl to one crystal and I;2 to the other. The arrangement is shown in Fig. 5.

For any R F pulse, if the polarization is circular or elliptical, the vertical component will be biased for a positixle reflection coefficient, while the horizontal com- ponent \vi11 be biased for a negative reflection coef- ficient. The Xg/2 spacing is necessary in the above scheme i n hollow waveguide to provide a round-trip 360" phase lag of kg with respect to kl, so that the phase difference between orthogonal components is 2 m r + ( B s - B 1 ) . (6,-Olj is the phase difference due to the difference of bias levels. This results in a target which is now detectable on a radar equipped Ivith both XITI and circularly polarized antennas. If k , = kz and B2 -61 =T, the sense of polarization of a reflected circularlJ- polar- ized wave is of the proper sense to be received most cfiectiuely by the circularly polarized radar antenna.

X DCAL BAND TARGET Some radar systems are equipped with a higher fre-

quency micro\vave radar for short-range precision data , and a lower frequency microwave radar for longer range search purposes. Fig. 6 shows a diagram of a combina- tion SS-band target head, which, v-hen used by itself or with an axially symmetrical reflector such as a pa- raboloid, n-ill provide a target return for the following types of operation on both Sand X band (pulse-to-pulse

Fig. 5.

X,'- - x,

x2

(b) Fig. 6--ia) Side half-section. ib) Front view.

circular polarization, horizontal polarization, vertical polarization, circular polarization with the 3ITI in operation, horizontal polarization 1%-ith the MTI i n operation, 1:ertical polarization with the MTI i n operation.

1:oltage efficiencies of the device on the order of 50 per cent were achieved over 10 per cent bandwidths for total battery current drain on the order of 20 ma. This is for the combined feed shown i n Fig. 6, where 6 crystals are driven by a transistorized free-running multivibrator powered by a 6-volt drk- cell (see Fig. 7 j .

Keither the frequent>- of the multivibrator nor the battery voltage (limiting the driving current for the crystals) is critical. Dropping the battery voltage from 6 to 3 volts resulted in only a small reduction in ef-

phase comparison AITI) : ficiency. This is due to the fact that the crystal current

10 IRE TRANSACTIOKS O N ARTTEATATAS AATD PROPAGATION Ja.nua,rry

I +6v

is still near its saturation value for the lower applied voltages.

As suspected, it was found that the efficiency of the crystal as a control element depends greatly not only on the type of crystal but on the crystal "package" and mount as well.

The S-band target head is a square coaxial arrange- ment in which 2 TEll orthogonal modes are excited. The terminal impedance for each of these modes is controlled by 2 crystals, one on each of opposite sides of the square center conductor. The cross talk between the modes is not great enough to cause any difficulty. Values of cross talk obtained were estimated as being about -20 db.

The X-band target head is a square quadruple ridge waveguide with a gradual transition into a square mave- guide, the open end of which serves as the horn aperture. I t supports two orthogonal ridge waveguide TElo modes each terminated by one crystal. This choice of mave- guide was made to obtain a working combination of waveguide impedance relative to the crystal character- istic (for orthogonal mode operation) and to minimize crystal lead inductance.

Because of the following factors, microwave crystals were not used in a working model. Microwave crystals have low burnout levels, a weakness which was en- hanced by the forward biasing. Also, the microwave crystals did not exhibit as large a change in impedance between low- and high-level biasing states for higher in- cident RF power levels. The amount of change varied much more with change of incident R F power level than was the case for the low-frequency point-contact or

junction-type diodes. On a sample germanium point- contact crystal the efficiency remained practically con- stant (dropping off a little a t higher power levels) for incident 0.5-psec pulses of peak incident RF power level variations over a 40-db range from one milliwatt to ten watts.

A compromise multivibrator frequency of about 0.7 of the S-band radar PRF and about 0.2 of the X-band radar PRF was used. I t was estimated that this gi~7es pulse-to-pulse bias state changes to about one-third of the pulses per beamwidth on the higher PRF X-band radar and about one-half of the pulses per beamwidth on the S-band radar. Field tests disclosed that the multi- vibrator frequency was not critical. The optimum bias frequency for a single fixed PRF radar would be in the vicinity of one-half the radar PRF so that practically 100 per cent of contiguous incident radar pulses are al- ternately biased and unbiased. Fig. 7 shows the transis- tor multivibrator circuit and shows how the crystals are driven. This particular circuit had very good frequency stability up to ambient temperatures of 140°F and for large changes in power supply voltage. As previously noted, the voltage level or frequency is not critical. The main concern is to sustain oscillations under large changes in ambient temperature levels.

COKCLUSION This type of target [meeting the requirements of 1)

and 2) in the Introduction] appears to offer the most versatility and most economical fabrication and main- tenance of the various types considered. For fewer func- tional requirements fewer crystals are required and therefore longer battery life is obtained. The low power requirements make its use more widely applicable than some device requiring a line power source. The ability to produce pulse-to-pulse amplitude changes as well as phase changes may make it applicable for monitoring radar systems equipped with pulse-to-pulse amplitude- sensitive MTI.

L4CKNORLEDGMENT

The author wishes to express his appreciation to those who contributed to the development of this work. H. Schauwecker designed the transistor circuit and con- ducted environmental tests and H. Kusunoki did much of the developmental fabrication, most of the labora- tory testing, and made several valuable contributory observations.