RF Electronics Chapter4: Transmission Line Transformers and Hybrids Page 107
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
These couplers behave exactly like the broadside couplers shown in figures 4.52 and 4.53,
When the line length is one quarter of a wavelength long, figure 4.73 shows that these
couplers typically have a 3 dB coupling to both the direct port (S21) and coupled port
(S31) and more than 20 dB isolation (S41) and return loss (S11). This is called a Narrow
wireline coupler. By increasing the coupling slightly, the bandwidth over which a -3
0.5 dB direct output and coupling outputs can be maintained is more than one octave.
That is called an Octave wireline coupler. Figure 4.74 shows that the phase difference
between the direct and the coupled output is close to 90 degrees over an octave bandwidth.
Couplers with -10 dB or -20 dB, coupled outputs, like the one shown in figures 4.44 to
4.47 can easily be obtained by using much shorter than a quarter wavelength long coupler.
Having a capacitor between the coupler output to ground, like C2 and C3 in figure 4.47,
results in a reasonably flat frequency response over a wide frequency range.
Figure 4.74 Wireline phase response.
By varying the spacing between the coupled conductors, the coupling ratio for a quarter
wavelength long cable can be varied. Wirepac cables with -10 dB coupling at a quarter
wavelength are available. Ham radio operators use this principle to make detectors for
antenna matching by sliding an insulated wire underneath the shield of a short length of
standard coaxial cable. Then connecting a diode detector and a meter, results in a circuit
similar to the Bird Wattmeter shown in figure 4.54, and with suitable calibration allows
forward and reverse power of their transmitter and antenna to be measured.
References
1. W. C. Johnson, Transmission Lines and Networks, Mc Graw Hill June 1950.
2. Wikipedia, Transmission lines: http://en.wikipedia.org/wiki/Transmission_line
3. D. M. Pozar, "Microwave Engineering", 3rd Ed, Wiley, 2005.
4. E Wilkinson, "An N way Hybrid Power Divider," IRE Trans. Trans Microwave
Theory and Techniques, vol. MTT-8, pp. 116-118, Jan. 1960.
5. Rogers R04000 Series High Frequency Circuit materials. https://rogerscorp.com/
advanced-electronics-solutions/ro4000-series-laminates
6. Bird RF https://birdrf.com/Products/Test%20and%20Measurement/
RF Electronics: Design and Simulation
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