AIN SHAMS UNIVERSITY
Microwaves Faculty of Engineering
Laboratory
𝝏𝝂 𝑭𝝁𝝂 = 𝝁𝟎 𝑱𝝁
ECE 433 – Spring 2023 Microwave Circuits and Dr. Hussein Kotb
Systems
Microwave Circuits and Systems
Exp. 5: Characterization of Directional Coupler
1. Objective:
a. To characterize a directional coupler by measuring its coupling and directivity.
b. To measure the return loss of a test device.
2. Theory:
A directional coupler is a device consisting of two transmission lines. The main arm and the
auxiliary arm, which are electro-magnetically coupled to each other, so that the power entering
port 1 in the main arm divides between port 2 and port 3, and almost nothing comes out in port 4.
Power entering port 2 is divided between port 1 and port 4.
To define the coupling of the coupler we use the setup shown below. Port 4 of the coupler is
terminated with a built-in termination.
𝑷
𝐂𝐨𝐮𝐩𝐥𝐢𝐧𝐠 𝐂 = 𝟏𝟎 𝒍𝒐𝒈 ( 𝑰 ) 𝐢𝐧 𝐝𝐁.
𝑷 𝟑𝑭
To define the directivity of the coupler we use the setup show below. Port 4 of the coupler is
terminated with a built-in termination.
𝑷
𝐃𝐢𝐫𝐞𝐜𝐭𝐢𝐯𝐢𝐭𝐲 𝐃 = 𝟏𝟎 𝒍𝒐𝒈 ( 𝟑𝑭 ) 𝐢𝐧 𝐝𝐁.
𝑷 𝟑𝑹
We can use a high directivity waveguide coupler for measuring the reflection from a test device.
The signal is incident at port 2, the test device is connected to port 1 and the reflected signal is at
port 3. See the setup shown below:
First, the incident power at port 2 gets reflected partially at the test device, and appears at port 3
𝑃
with power 𝑃3 = 𝐶𝑟 . If we replace the test device with a short, all power is reflected and appears
𝑃1
at port 3 with power 𝑃3 = . The ratio between both is called the return loss:
𝐶
𝑷
𝑹𝑳 = 𝟏𝟎 𝒍𝒐𝒈 ( 𝒓 ) = |𝚪|𝟐 in dB
𝑷 𝟏
3. Equipment:
• Gunn oscillator • Slide screw tuner
• Rotary vane attenuator • Termination
• Detector • Waveguide carrier(s)
• Ferrite Isolator • Shorting plate
• Directional coupler • DC meter
4. Procedure:
a. Coupling C:
i. Set up the equipment as shown below and set the rotary vane attenuator to 0
dB.
ii. Achieve a reference reading on the meter. Record in your lab report.
iii. Disconnect the directional coupler from the attenuator and connect the detector
instead of it.
iv. Increase the attenuation until the reference reading is achieved. The Coupling
is now the attenuation value. Record in your lab report.
b. Directivity D:
i. Set up the equipment as shown below and set the rotary vane attenuator to 0 dB
ii. Achieve a reference reading on the meter. Record in your lab report.
iii. Disconnect the directional coupler from the attenuator and connect the detector
instead of it.
iv. Increase the attenuation until the reference reading is achieved. The attenuation
value now is C + D in dB. Record in your lab report.
c. Return Loss of a Test Device:
i. Set up the equipment as shown below and set the rotary vane attenuator to 0
dB.
ii. Achieve a reference reading on the meter. Record in your lab report.
iii. Replace the test device with a short.
iv. Increase the attenuation until the reference reading is achieved. The attenuation
value now is the return loss in dB. Record in your lab report.
v. Calculate the magnitude of the Reflection Coefficient.
AIN SHAMS UNIVERSITY
Microwaves Faculty of Engineering
Laboratory
𝝏𝝂 𝑭𝝁𝝂 = 𝝁𝟎 𝑱𝝁
ECE 433 – Spring 2023 Microwave Circuits and Dr. Hussein Kotb
Systems
Microwave Circuits and Systems
Exp. 5: Characterization of Directional Couplers - Lab Report
Date:
Section Number:
1.
2.
Group Names:
3.
4.
Directional Couplers
Coupling
Reference Reading on the meter
Attenuation Value
Coupling C
Directivity
Reference Reading on the meter
Attenuation Value
Directivity D
Return Loss
Reference Reading on the meter
Attenuation Value
Return Loss
|Γ|
Suppose that the directional coupler has a coupling of 20dB and directivity of 40dB.
How many dB below p2 is the unwanted signal at port 3?
How much power related to p2 is dissipated in the load at port 4?
