Tuning ½ And ¼ Transmission Lines And Stubs - AEA CIA-HF Operating Manual

TUNING ½ AND ¼ TRANSMISSION LINES AND STUBS
Example: Using a standard Wilkenson power divider to stack two 15-meter yagi antennas with
two ¾ wave sections of RG-11, 75-ohm coax lines. The ¾ wavelength cable is used in this
example because two ¼ wavelength cables would not be long enough to allow optimum stacking
distance between the two antennas.
Process:
1. Use the following equation to determine the length of coax cable needed for ¼ wave phasing
lines. (For solid polyethylene coax, assume a velocity factor of 0.6.)
For ¾ wave phasing lines, plug 6.61 feet into this equation:
2. Cut two, 22 foot-long lengths of coax cable (the lengths of coax cable are cut approximately
10% longer than the computed length to allow for fine tuning).
3. Solder a coax "PL-259" connector to one end of each cable.
4. Attach the "PL-259" connector to a "T" connector already attached to the antenna connector.
Attach a 50-ohm load to the other end of the "T" connector (Figure 2).
5. Turn the Analyzer on. At this point, follow the steps for either the ½ Wave Cutting Method or
the ¼ Wave Shorting Method
½ Wave Cutting Method
1. When the S screen appears, press the WIDTHp key twice to select an initial width of 500
kHz. Enter 4 2 5 0 0 ENTER to set the center frequency to 42.500 MHz (the frequency at
which the tuned phasing lines will be 1.5 wavelengths). This center frequency value is twice
the 21.250 frequency value used in the above equation.
2. A "V" or "U"-shaped SWR curve will appear.
3. If the calculations for determining the length of the coax cable were correct, the SWR curve
should dip at a frequency of approximately 38.250 MHz. You are now ready to start tuning
the phasing line.
11
234 x Vf = 234 x 0.6 = 6.61 feet
F 21.250
6.61 x3 = 19.8 = 19 feet 9 5/8 inches
FIGURE 2. T Connector