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A High Directivity Microstrip Coupler Technique - IEEE Xplore

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A HIGH DIRECTIVITY MICROSTRIP<br />

Alan Podell<br />

Anzac Electron.<br />

COUPLER<br />

Cs<br />

TECHNIQUE<br />

Typically loosely coupled backward wave microstrip<br />

couplers have di.recti.vit.ies which decrease with i-ncreasing<br />

frequency. <strong>High</strong> di,recisivi,ty becomes more difficult to obtain<br />

as the coupling is loosened. The problem appears to be that<br />

the propagating velocities of the odd and even modes are not<br />

equal. This paper deals with a technique which equalizes<br />

these velocities up to an arbitrarily high frequency.<br />

<strong>Technique</strong>s such as capacitive tabs and lumps at the fnds of<br />

the coupler may be used to increase its directivity.<br />

Generally<br />

the techniques which increase the directivity seem to<br />

decrease the overall bandwidth of the coupler. The overlay<br />

coupler has equal even and odd mode velocities, and may be<br />

used in broadband couplers. The technique described herein<br />

is also a fundamental velocity equalizer, but without any<br />

additional processing beyond the normal printing and etching<br />

process of a microstrip coupler.<br />

Figure 1 shows a typical wiggly line coupler. Wiggling the<br />

adjacent edges of the lines in the manner pictured slows the<br />

odd mode wave without much affecting the even mode. If the<br />

outsides of the lines were wiggled in unison with the insides<br />

then the even mode would be slowed down, aud the wiggling<br />

would have to be more severe before velocity equalization<br />

would occur.<br />

Figure II shows a typical coupling and directivity versus<br />

frequency plot for a wiggly coupler. Note that the direc–<br />

tivity does not slope severely upward with frequency, demonstrating<br />

the fundamental velocity equalization.<br />

Wiggly couplers have been built with couplings ranging from<br />

7.8 to 27 DB on glass teflon, glass epoxy, and alumina.<br />

Achieving high directivity seems mainly a result of patience<br />

in determining the necessary wiggle depth, and care in fabri–<br />

eating the photographic mask.<br />

Five tandem pair 3DB wiggly couplers were built as shown in<br />

Figure III to check their reproducibility. Their performance<br />

is shown in Table I. A welded strip, a bond wire or<br />

a beam lead may be used to connect the lines at the crossover.<br />

In our case, the jumper was made with a pair of 1 mil<br />

bond wires. For just 3DB, one would guess that the single<br />

section interdigitated coupler would be as easy to construct<br />

33


and be less 10SSY.2 Above 7DB coupling, however, there<br />

is no need for the cross-over; a single coupler can be<br />

etched with the necessary tolerances for reproducibility.<br />

In order to exploit the broadband possibilities of a<br />

wiggly coupler, a 54 to 290 MHz 3 section 18DB coupler was<br />

built. The 3 Sections were individually built and tested<br />

before being patched together. Figure IV shows the coupling<br />

and directivity performance of this coupler, built on glass<br />

epoxy circuit board.<br />

The side effects of wiggling seem to be to increase the<br />

coupling for a given line spacing, and to slightly increase<br />

the loss. When the wiggle spacing becomes an appreciable<br />

fraction of a wavelength, the coupler will no longer appear<br />

continuous, but this does not appear to be a serious limi–<br />

tation until well above X band.<br />

What has been described here is a technique for the broadband<br />

equalization of the even and odd mode velocities of a<br />

microstrip coupler. This technique makes possible simple,<br />

multi-octave microstrip couplers of more than 20DB directivity.<br />

1 H.E. Brenner “Perturbations of the critical parameters<br />

of quarter-wave directional couplers” <strong>IEEE</strong> MTT and<br />

J.E. Morris, <strong>IEEE</strong> TMTT, July 1968. Overlay <strong>Coupler</strong><br />

2 Julius Lange, <strong>IEEE</strong> TMTT, December 1969<br />

34


g<br />

z—<br />

z<br />

0<br />

!-<br />

5<br />

15<br />

FIGUF@ 1 - TYPICAL WIGGLY LINE COUPLER<br />

COUPLING<br />

1<br />

25<br />

\<br />

/<br />

35<br />

150LA<br />

TION<br />

45<br />

.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0<br />

FREQ<br />

GHz<br />

FIGURE 2 - COUPLING AND DIRXCTIVITY<br />

FIGURI 3 - 3 DB COUPLER<br />

35


TABLE 1<br />

FREQUENCY I-2 GLiz<br />

COUPLING 3dB NOMINAL<br />

INSERTION LOSS .35d B MAX<br />

UNBALANCE 1.0 dB MAX<br />

ISOLATION 27 dB MIN<br />

5UMMARY1 5 UNIT5 VfOfi ST CASE<br />

(JI<br />

D<br />

10<br />

2<br />

20 —<br />

COUPLIN<br />

G<br />

so<br />

K<br />

o<br />

40<br />

\ 150LA TION<br />

/’<br />

5 0<br />

\__--’<br />

/’<br />

* 0<br />

50 100 150 200 ,50 300<br />

FREQ<br />

MHz<br />

FIGURE<br />

4 - COUPLING AND DIRECTIVITy OF 54 TO 290 NHZ COUmER<br />

TRANSCO PRODUCTS, INC.<br />

.&l Glencoe Avenue<br />

Venice, Cahf 90291<br />

~+ ,13,3,1_;2,,<br />

Coaxial & Waveguide switches, Airborne Antennas><br />

Power Dividers, Filtersj DiPlexers~ and ‘ther<br />

Microwave Components<br />

36

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