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Basic Antennas - Seeking clarification

Feb 8th, 05:04

KJ4ZCV

Joined: Nov 15th 2010, 10:55
Total Topics: 0
Total Posts: 0
Hello - I’m reading ARRL’s Basic Antennas book, and on page 5-4, it says:
The load impedance, resistive or complex repeats every half wavelength, for example. The impedance goes to the opposite extreme in odd multiples of a quarter wavelength. For example, our 25 Ohm load would get transformed to 100 Ohm in quarter or three quarter wavelength transmission line sections and vice versa.

- What is meant here by “opposite extreme”? Opposite to what? What extreme?
-How is the 100 Ohm value determined?
- What is meant by “vice versa” here? Zero Ohms?

Is there anyone who can refer to this section of the book able to throw light on this?

Thank you!
Feb 8th, 07:31

W9IQ

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The "extreme" is a multiplier of the characteristic impedance of the transmission line.

In the example you cited, the load is 25 ohms and the characteristic impedance of the transmission line is 50 ohms. The ratio 50/25 or 2 so the input impedance of the 1/4 wavelength line becomes 50 *2 or 100 ohms.

The vice versa refers to changing the load impedance to 100 ohms. Now the ratio is 50/100 or 0.5 so the input impedance of the quarter wavelength line becomes 50 * .5 or 25 ohms.

Note that these examples are assuming a purely resistive (no reactance) load in order to avoid the complex math.

- Glenn W9IQ

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