ADVANCED APPLICATIONS

BASIC PRINCIPLES

There are some basic principles you should have in mind:

1. Changing the length of a feedline does not change SWR... to a first approximation.

Changing the length of a feedtine can drastically change Rs and Xs if the SWR is high, and this might make your transmitter or tuner happier, but the SWR is virtually unchanged, except for the effects of transmission line losses.

2. The SWR is always a little lower at the transmitter end of the feedline than at the antenna ; due to line loss.

3. An antenna tuner does not change SWR on the line, It only makes your transmitter happier since it sees the lower SWR. However, all the line losses caused by SWR (See Fig. 6) are still there. In addition, the tuner introduces additional loss, which can be significant. The popular “T” configuration tends to be the lossiest. If you can, put an efficient matching network right at the antenna and avoid all these losses.

4. If Xs=O,SWRis Rs/Zo orZo/Rs,whichever is >1

For example, 25 and 100 ohms both yield an SWR of 2 on a

50 ohm line. Any Xs always increases the SWR. (Eqn. A6)

Many antenna myths are humorously discussed in a series called Aerials, by Kurt N. Sterba, available from Worldradio, Box 189490, Sacramento, Ca. 95818. (916) 457-3655.

USING As ant and Xs ant

If your antenna is 100 ft in the air, you can’t usually measure Rs and Xs at the antenna. In addition, the feedline often becomes part of the antenna and changes Rs and Xs, at least slightly. Rs ant and Xs ant allow you to measure at the transmitter end of the feedline. There are a series of equations that the VA1 solves to do this. First, you must enter:

1. The Feedline Impedance.

This is 50 ohms at turnon, but can be changed by holding down the SWR button as described on Page 2.(lf your feedline is 50 ohms, nothing need be entered.)

2. The Feedline Electrical Length via F-i 1/4

This is entered by determining the frequency where the line is

1/4.wavelength long ,and entering this frequency into F-i /4.

Once these values are entered, the VA1 compensates for the length of the feedline and calculates the RS ant and XS ant at the antenna, even though you might be measuring at the other end of 100 feet of feedline.

The best way to determine F-1/4 is to measure it. See Page 6. The value is somewhat critical, especially near frequencies where the line is 1/4 wavelength long or a multiple. If the line is many multiples of 1/4 wave you may want to measure, say, the 3/4 wave frequency (See Fig. 4) and divide by 3 to get F-1/4. A less accurate way is to calculate F-1/4 using:

Where Vt is the velocity factor of the cable. (Also see Page 6.)

For example, if you have 50 ft of RG8, RG213, or AG58 cable,
Vt is approximately 0.66. So:

F-1/4 = 246 x 0.66/50 = 3.247 MHz

ENTERING F-1/4

The VA1 has no numeric keypad, but F-1/4 is easily entered by tuning the unit to F-i /4.

To enter F-1/4, first tune the VA1 to the approximate frequency desired. Then select Function 4 and press the F-1/4 button. You will see “----“, meaning that F-i /4 has not been entered. Press the button again, and you will see a display exactly like the display in Frequency mode. Now, adjust the frequency display so it reads the desired F-1/4, without changing bands. When you have the desired F-1/4 on the display, press any other mode button, such as Rs ant. Pressing the other button saves F-1/4 until the unit is turned off.

To verify that you have saved F-i/4, go to the frequency mode and change frequency. Then tap the F-i /4 button once. You will see the frequency saved above. Then, again, tap any other mode button to keep this value of F-i/4.

Just as with Line Impedance and Function, the last value you see is the one saved and used in the calculations.

To change to a different F-1/4, tap the F-1/4 button twice. After the first tap you see the last value saved. After the second tap, tune to the new desired F-1/4 without changing bands and save by tapping any other mode button as before.

For Vt = .66, and a frequency range of 0.5 to 30 Mhz, lines between 5.4 feet and 324 ft. can be entered.

INTERPRETING Rs ant and Xs ant

See the drawing on page 2. With this measurement, it is as if you were standing where the coax is connected to your antenna. The readings will seem to bear no resemblance to Rs and Xs, except at frequencies where the feedline is 1/2 wave or a multiple, where they should be equal (except for some possible roundoff in the computer.) Xs can be highly reactive (large Xs), yet Xs ant will can be zero. Or vice-versa.

Rs can be very low, yet Rs ant is very high, or vice versa.

These measurements can be used to determine what matching network to try at the antenna. Please note that the VA1 calculations do not take line loss into account. If your line loss is small there is little effect. Line loss usually acts to bring Rs closer to the feedline impedance. For example, Rsant might read 110 ohms at resonance, but, because of line loss, the actual value is 125 ohms. Usually the error is small and resonance is not affected.

We recommend a program called “TLA” which comes with the ARRL Antenna book. ARRL, 225 Main St., Newington, Ct, 06111 . (888-277-5289)

With this program you can enter Rs and Xs and it calculates RS ant and Xs ant, including losses. But each frequency needs a separate entry, so the VA1 is much more convenient.

VA1 Page 8