A High Directivity Microstrip Coupler Technique - IEEE Xplore
A High Directivity Microstrip Coupler Technique - IEEE Xplore
A High Directivity Microstrip Coupler Technique - IEEE Xplore
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
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