OARC Transmission Line Project 2013 Session 5 Home TCA Index TCA Files Antennas Presentations Oarcproject  2013 Oarcproject 2015 Oarcproject 2017 Gallery Contact
Q3Q Transmission Line Balun Transformer
In session 4, we built and tested the classic 1/2 wavelength 4:1 transmission line transformer for the two metre band. Now we turn to the Quarter 3 Quarter transmission line transformer (Q3Q). It is made from one section of a quarter wavelength long line and one section 3/4 wavelengths long as shown above in its symbol. This type of transformer can have different transformation ratios. We will build it for a 1:1 ratio by connecting a 50 Ohm resistor to the load end (top). The input (left) is fed from a 50 Ohm source. If time permits, repeat the experiment using a 25 Ohm load and record the input impedance in the 2 metre band. Compare with TLDetails. This type of transformer injects a constant current into to load: Current does not depend on the load value because of the properties of a quarter wave transmission line. A one quarter wavelength transmission line has two important features: 1) Zin = Zo^2/Zload  In our case, Zo = 50 Ohms and Zload= 25 Ohms...Zin =100 Ohms     The 50 Ohm resistor is shared with the two lines. (25 Ohms each) 2) Load current = Vin/Zo independent of the load impedance.... a current source The  three quarter wavelength  line can be thought of a phase reversal one half wavelength line connected to a one quarter wavelength current source line.  
3/4  wavelength line
1/4 wavelength line
The Experiment:  2 metre band
1)  The dielectric length of the quarter wave length section is 407 mm. (from experiment) 2)  The dielectric length of the three quarter wave section is 3 times 407 = 1221 mm 3)  Extra cable length for connections (see photos below) is 10 + 10 + 5  =  25 mm. 3)  Hence the total cable length is 1653 mm. 4)  Now use the supplied RG8X wire stripper to strip off the cable ends. See the photos below. 5)  Tin the exposed center conductors. 6)  Now mark the cable at the short end 407 mm from the dielectric end. 7)  Use the wire stripper to cut the cable at that point. (I will demonstrate this procedure) 8)  Carefully remove the dielectric (see photo #1 before dielectric removed.) 9)  Fold the two ends together as shown. Solder the braid with a shorting wire. 10) The cable assembly must be connected to a piece of cable with a UHF connector.  Use the wire stripper to strip the cable ends. See Photo #3. 11)  Solder a 50 Ohm resistor to the load end. (not shown on Photo) Now connect to the AIM impedance meter and observe the results. (100 to 180 MHz) The SWR/Return Loss will not be perfect for the following reasons: 1) The 3Q3 design is always a bit off and there are discontinuities at the connections. 2) The reflection at the connector has a Return Loss between 25 and 30 dB. 3) The load resistor is slightly inductive. 4) The AIM calibration algorithm is quite simple (Not like an HP 12 term correction). Hence the noise floor of the measurement is around 25 dB Return Loss. Measure the center frequency by averaging the 20 dB Return loss frequencies.
RG-8X Cable Supplied
Sample Only Not to Scale
Tin the Center Conductor Do not tin the Braid
Wire Stripper
Carefully remove the dielectric with a cutter. Press the cutter blade down without dragging it. You don’t want to nick the center conductor.
Connect feed line after stripping the cable Solder center conductors Short braids with a U shaped wire.
 (1653 mm long +/- 2 mm)
Photo #1
Photo #2
Photo #3