To eliminate lead inductance, measure L with test leads shorted to get the inductance of the test leads. Then connect the test leads to the coil, read L, and subtract the test lead inductance to get the true L of the coil.

When measuring a lumped capacitor, keep leads short, especially with capacitors over 100 pF or so. If leads are long, the positive reactance of the leads will cancel some of the negative reactance of the capacitor and make the capacitor appear larger.

The above discussion applies to lumped L or C. When measuring an antenna, the VA1 also converts Xs to its equivalent L or C. (See equations AiD and A13 in the appendix.) For example, you read

Rs =70

Xs =50

F =7 MHz

Because Xs is positive, the series reactance is inductive, not capacitive. Switching over to the L mode, you read

L=1.l4uH

So, your load looks like a 70 ohm resistor in series with a 1.14 uH inductor at 7 MHz. Switching over to the C mode at the same frequency you read

C-455pF.

There is a minus sign in front of the C. This is the capacitor value which has Xs = -50 ohms. This tells you that, at 7 MHz, 455 pF has the same reactance as t.14 uH, but opposite sign. In other words, this L/C ôombination has a resonant frequencyof 7 MHz.

Similarly, if Xs is negative, C will read positive since the series reactance is capacitive, but L will read negative and yield the series value of L required to cancel C.These applications are discussed in more detail below (matching.)

Rp&Xp

Just as any load can be described by Rs and Xs, there is an equivalent parallel combination of resistance and reactance, called Rp and Xp which is just as valid and occasionally more convenient.

See the MEASUREMENT SUMMARY on page 3. Rp and Xp are calculated from As and Xs using equation A8 and A9 in the Appendix. Some uses for Rp and Xp are discussed in more detail below (matching.)

F-1/4, Rsant, Xsant

A feedline can drastically change Rs and Xs unless it is 1/2 wavelength long or a multiple. These functions yield As and Xs at the antenna for ANY LENGTH feedline, so you can measure at the transmitter end of the feedline. Briefly, F-1/4 is the frequency where the feedline is 1/4 wavelength long.These modes are discussed in detail below. (Advanced Applicatk,ns)

4

BASIC APPLICATIONS

As shown in Fig. 1 and 2, the VA1 can be used to measure at the antenna, or at the far end of a feedline. For SWR, measuring at the end of the feedline may be preferred since it is often more convenient and the feedline may affect the antenna resonance. However, initial adjustments are often made at the antenna it possible.

Note: Disconnect teedline when measuring at antenna.

Fig. 1. Measuring at Antenna For SWR & Accurate Rs and Xs.

Table 1. Some Common Lengths

Fig. 2. Measuring at End of Feedline.

For SWR. Use Rs ant and Xs ant for R and X.

VA1 Page 4