Jun. 18, 2002
JR4MYL
The impedance of one lambda loop antenna without a balun is about 130 ohm. (SWR is 2.6) To match the antenna to 50 ohm feeding cable, I used 4:9 balun and it improved the SWR to 1.2. But the core used in that balun was too small to handle much power.
Instead of the balun, 1/4 lambda 75 ohm coaxial cable matching line could be used. The impedance at the end of matching line would be 112.5 ohm ( 50 * (75/50)^2 ). I can expect the matching line:
I bought normal 3C-FV (common 75 ohm cable in Japan) at my local store and checked the velocity factor of the cable.
The setting of experiment is shown above. The SWR will drop down at the frequency where the length of matching section is 1/4 wave length.
AEA SWR-121 HF showed the bottom SWR at 5920 [kHz].
Wave length of 5920 [kHz] in air is 300/5.92 = 50.7 [m]
1/4 wave length = 12.67 [m]
10 [m] of the cable is equivalent to 12.67 [m] in air
The velocity factor is : 10/12.67 = 0.79
Now the velocity factor is given. Assume to match the line to 21.20 [MHz], we need: 300/21.20 * 0.79 * 1/4 = 2.80 [m] of this cable.
To be shown soon.