The design of the resonant cavity radiator (RCR) is compared to that of the slotted waveguide array in terms of efficiency, weight, and structural integrity. It is shown that the RCR design has three significant potentials over the slotted waveguide array: (1) improvement in efficiency; (2) lighter weight; and (3) simpler structure which allows the RCR to be integrated with the RF tube to alleviate thermal interface problems

Carlise, R. L.

Schroeder, K. G.

Tomita, C. Y.

English

NASA Technical Reports Server

THE RESONANT CAVITY RADIATOR (RCR) 
K, G. S c h r o e d e r ,  R. L. C a r l i s e ,  C. Y .  T o m i t a ,  R o c k w e l l  I n t e r n a t i o n a l  
1. INTRODUCTION 
The fundamental theory  of MW antenna opera t ion  and bas ic  a r ray  technology 
development s ta tus  was used i n  the  design o f  t he  1-km diameter 5-Gw SPS 
microwave antenna. However, the  aper ture s i ze  and the h igh  e f f i c i e n c y  
requirements make the MW antenna extremely complex. Studies have shown 
t h a t  the  s l o t t e d  waveguide a r ray  i s  one o f  t he  most e f f i c i e n t  r ad ia to r s  
f o r  the  antenna. Subsequent analyses have shown t h a t  t he  temperature 
i n t e r f a c e  between waveguides and dc-RF conversion tubes can cause severe 
thermal design problems on the  array.  An a l t e r n a t e  design, the  Resonant 
Cavi ty Radiator,  i s described here. 
2. RADIATING ELEMENT DESIGN 
2.1 Basic RCR P r i n c i p l e  
Conventional waveguide designs such as the  TE mode waveguide s l o t t e d  . 
a r ray  make tube i n s t a l l a t i o n  f a i r l y  complex. 'Po solve the  r e s u l t a n t  
temperature i n t e r f a c e  problem and poss ib l y  increase the RF e f f i c i e n c y  o f  
t he  rad ia to r ,  Rockwell developed the  resonant c a v i t y  r a d i a t o r  (RCR). The 
RCR i s  a resonant c a v i t y  box e x c i t e d  w i t h  the  TE mode. Phys ica l l y ,  
the RCR i s  a conventional s tanding waveguide rad$%tor w i t h  the common 
wa l l s  removed. The RCR has th ree  s i g n i f i c a n t  po ten t i a l s .  They are: 
1. Improvement i n  e f f i c i e n c y .  
2. L i g h t e r  weight.  
3 .  Simpler s t r u c t u r e  which a l lows the  RCR t o  be 
i n teg ra ted  w i t h  the  RF tube t o  a l l e v i a t e  t he  
thermal i n t e r f a c e  problem. 
2.2 RCR Theore t i ca l  A t tenuat ion  Estimates 
The l oss  mechanisms o f  the  RCR can be bes t  expla ined by comparison t o  
conventional arrays. The t y p i c a l  f l a t  p l a t e  antenna a r ray  i s  formed by 
p lac ing  side-by-side several sect ions o f  rec tangu la r  waveguide as shown 
i n  F igure 1. 
F igure 1. Typica l  TEIO SWR Array 
2 35 
https://ntrs.nasa.gov/search.jsp?R=19810008038 2020-03-21T15:12:22+00:00Z
The mode that  propagates down each waveguide i s  the dominant TE . 
The mode designation simply describes a particular electric-mag@tic 
f i e l d  configuration that  s a t i s f i e s  Maxwell's equations. A portion 
of the top wall in waveguide No. 2 i n  Figure 1 i s  cut away t o  show 
the current flowing i n  the side wall. Not shown i s  the adjacent 
currents flowing in waveguide No. 1. These currents (waveguide No. 1)  
are flowing i n  the opposite direction and because the system i s  
symmetrical, they are of equal magnitude. If the side walls are 
removed as i n  the RCR,  these two equal and opposite currents cancel. 
Since conduction losses are simply I ~ R  losses, any reduction i n  sur- 
face currents will make the antenna array more e f f i c ien t .  
The closed-form analytical expression fo r  conduction losses for  a 
si lver-plated RCR supporting the TEm30 modes i s  given as: 
- 2.8738 x 
O1c - meter 
For an "a" dimension of 4.460 inches and a "b" dimension of 2.130 
inches (11.319 cm by 5.40 cm) the loss calculated from the above 
equation i s  tabulated i n  Table 1.  This shows tha t  fo r  a typical 
array length of 2.5 meters, a TEzo RCR has the potential of saving 
4.3 x 106 watts of power. Weigh savings i n  the MW antenna i s  
achieved by two design features:  (1)  the RCR i s  designed w i t h  
no s ide  walls with the exception of the cavity walls, and (2 )  i t  can 
be designed to be s t ructural ly  integrated w i t h  a magnetron or  klystron 
heat d iss ipator  because of the simp1 i c i  ty  of the structure.  
2.3 Typical Integration Between RCR and Tube 
Figure 2 shows a typical anode heat radiator integrated w i t h  the RCR 
bottom. The area required fo r  heat dissipation computed by Rockwell 
indicates that  the RCR has more than suf f ic ien t  area to dissipate 
the excess heat. In the aperture high-density area, only 0.76 per- 
cent of the to ta l  RCR area i s  required t o  replace a 48-cm magnetron 
anode. The RCR bottom wall can be constructed of pyrolytic graphic 
composite, or equivalent, and plated fo r  high RF conduction. The 
plating technique of pyrolytic graphite to operate a t  extremely high 
temperatures should be investigated i n  future studies,  The potenti a1 
weight savings of the RCR i s  then the removal of the side walls and 
the weight reduction achieved by incorporating heat dissipation in  the 
waveguide bottom wall . The integrated assembly a1 so provides techniques 
fo r  solving the high-temperature interface problem. I t  should be noted 
tha t  the RCR may of fe r  other advantages fo r  ease of maintenance and 
assembly. 
Table 1 : Theoret ical  Power Saving of  RCR Over Conventional 
Standing Wave TEIO S lo t t ed  Arrays 
I 'RCR PYROLYTIC 
6 CM GRAPHITE BOTTOM 
RCR TOP 
Figure 2.  Magnetron Modified Heat Sink (Input-Output Connections 
May be Dif fe ren t )  
2 3 7  
2.4 Measurement Results 
One of the primary uncertainties w i t h  the RCR i s  the suppression of 
higher order modes. One of the eas ies t  ways of detecting higher order 
mode existence i s  by observing radiation patterns. Higher order modes 
will collimate i n  off-boresight locations, causing nu1 1 f i l l i n g  and 
higher side1 obes . Rockwell developed special feed techniques which 
led to  the reduction of higher order modes. To prove the technique 
does suppress higher order modes, scaled t e s t s  were conducted. A 
TE70 RCR shown in Figure 3 was fabricated and tested with resul ts  
shown i n  Figures 4 and 5. The RCR was uniformly excited fo r  -13 dB 
peak sidelobe level.  Measured sidelobe levels in the E and H planes 
were -13 dB fo r  good correlation.  Off-axis patterns also were taken 
a t  predicted higher order mode locations. No existence of higher 
order mode propagation was found. These t e s t s  were performed on a 
limited scale;  however, i t  def ini te ly  proves tha t  the RCR has a 
potential fo r  a major breakthrough i n  array technology. Efficiency 
verification t e s t s  wi 11 be performed by Rockwell to  verify theoretical 
predictions. 
SUBARRAY DESIGN 
Rockwell ' s design of the MPTS transmit array consists of 6993. subarrays , 
each 10 meters square. The optimum s ize  of the subarray i s  a function 
of the electronic scanning range of the antenna. A small subarray allows 
more electronic scanning range: however, the to ta l  number of electronic 
scanning c i r cu i t s  increases w i t h  the increased number of subarrays. With 
a subarray larger than 10 meters square, the pointing requirements of the 
subarray i s  extremely t igh t ,  therefore undesirable. The base1 i ne subarray 
s ize  of 10m by 10m requires the subarray to  be pointed to  within 5 1 a rc  
minutes fo r  l ess  than 0.5-percent loss.  Typical power plots in dB and 
percent of the subarray i s  shown i n  Figures 6 and 7. A typical subarray 
may consist  of 20 to 50 RCR's, depending on the power density of the 
subarray. 
Figure 3. Experimental RCR 
2 39 
PATTERN NO. 2 DATE: 9/24/76 
PROJECT: RCR 
ENGINEERS: TOMITAIKONIECZNY 
REMARKS: 
RADIATING SLOT PLANE (H-PLANE) 
FREQ. 9.7 GHz 
SILVER EPOXY MODEL 
36 0 36 
ANGLE 
Figure 4. RCR H-Plane Pattern 
PASTERN NO. 1 DATE: 9/24/76 
PROJECT: RCR 
ENGINEERS: ToMITAIKONIECZNY 
REMARKS: 
FEEDLINE PLANE (E-PLANE) 
FREQ. 9 7  GHz 
SILVER EPOXY MODEL 
108 72 36 B 36 72 108 
ANGLE 
Figure 5. RCR E-Plane Pattern 
Figure 6. Far-Field Radiation Pattern' 
( 1  0-Meter Square ~ u b . a r r a y )  
Figure 7 ,  10-f leter  Square Eleme'nt Factpr  
4. TUBE SUBARRAY INSTALLATIONS 
One of the prime advantages of the RCR i s  i t s  adaptabil i ty &o numerous 
magnetron or klystron tube ins ta l  1 a t i  ons. Rockwell has studies various 
tube/RCR integrated and non-integrated concepts to determine potential 
solutions to the weight and h i  gh-temperature interface problem. Figures 
8 through 11 i l l u s t r a t e  various magnetron and klystron mounting techniques 
to  the RCR. Figure 8 which shows magnetron mounting, i l l u s t r a t e s  the 
configuration where the back face of the RCR i s  integral to the magnetron. 
I t  should be recognized that  these techniques are advanced and unproven; 
however, i t  offers  the MPTS antenna designer a1 ternative ins ta l  lat ion 
concepts. The simplicity of the RCR fo r  maintenance also i s  shown i n  
Figure 8. The RCR modes for  various ins ta l la t ion concepts will vary as 
a function of the power density or structural  in tegr i ty .  In the low 
density areas such as shown i n  Figure 9,  a TE RCR may be used. In 
the higher density areas of the array a TE30 6 @ ~  can be used. The 
interconnecting feed l ines  of the RCR as shown i n  Figures 9 through 11 
represent implementation of the old version of Rockwell's phased array 
retrodirective network. Separate p i lo t  and reference pick-up antennas 
are used in the new phase control system, similar to the one described 
in connection w i t h  the sol id-s ta te  concepts. 
Figure 8. RCR Element Maintenance 
\ 
IWPUT FEED WAVEGUIDE 
2460 MHz POWER OlVlDER 
Figure 9 .  Low-Densi ty  10-Meter-Square Subarray 
PHASE SHIFTER 
I 
2440 MHz BOWER COklBlDER 
INPUT FEED WAVEGUIDE 
2450 Y W  POWER DIVIDER 
Figure 10. High-Densi ty 10-Meter-Square Subarray 
2 4 3 
POWER DENSITY = 3 KW/M' / ,,, 
Figure 11 . Low-Densi ty  30-Meter-Square Layout Array 


The Resonant Cavity Radiator (RCR)

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