The formulas for common antennas are:

Table 1 shows lengths for some common frequencies.

The recommended procedure when erecting an antenna is to make it 2% to 5% longer than the calculated value.., it’s easier to delete wire than splice it on. The calculated values are seldom exact in practice due to nearby objects, ground effects, etc. After erecting the antenna, use your meter to find the frequency where the SWR is lowest. If this frequency is too low you need to shorten the antenna; if too high, you need to lengthen it.

The procedure for changing the antenna length can be illustrated with an example. Say you erect a 40M (7.1 MHz) dipole and cut it a little long at 70 feet (35 feet per side.) You raise the antenna and measure the antenna as in Fig. 2. The lowest SWR occurs at 6.521 MHz. So, you antenna is too long. The correct length should be:

Freq.

For the example:

Desired Length =70 ft. x 6.521 I 7.1 = 64.29 ft.

(This is shorter than the formula, which is not unusual.) So you must remove 70-64.29 = 5.71 ft, or 2 ft 10 inches from each side. This is a big adjustment, so you might want to remove only 2 feet and repeat the above procedure to zero-in on the correct length.

MAKING 1/4 and 1/2 WAVE LINES

The lengths are often used for phased arrays, stubs, and have other uses. Formulas are:

Using a loose length of cable (not connected to your antenna), connect the meter to the cable (Fig. 3.) You can either short the other end of the cable or leave it open. Now, measure the Z of your cable vs. frequency. You’ll get a curve like Fig. 4.

Fig. 3. Measunng Transmission Lines

- - Note: Disconnect any antenna . . _____

Fig. 4. Transmission Line

Impedance vs. Frequency

Since maximum Z is probably off scale, we recommend looking for the minimum Z. So, to measure:

1/4 Wave Line: Open the line

1/2 Wave Line: Short the Line

In either case, look for the lowest-frequency minimum Z.

As an example, lets say we have 50 ft. of line. We short the far end and measure Z starting at 0.5 MHz. We see Z rising as we increase frequency, then it peaks and falls again to a broad minimum around 6.48 MHz, probably as low as a few ohms. This is the First Null Frequency. The line is exactly 112 wave at this frequency. By manipulating eqn. (4), the velocity factor of the cable is:

So, in the example,

Now that we know Vt, we can calculate the appropriate length using eqn. 4. (Or Eqn. 5 if we want a 1/4 wave- line.) Using eqn. 4, a 1/2 wavelength of this line would be:

If we cut the cable to 22.8 ft. and short the far end, we should see the minimum Z at 14.2 MHz now, confirming that we have 1/2 wave of line.

If we had cut a 1/4 wave line at 14.2 MHz (11.4 ft for this cable), we would OPEN the line and confirm that we had a minimum Z at 14.2 MHz and, thus, a 1/4 wave line.

These measurements are remarkably accurate with only a

slight discrepancy between maximum and minimum Z

frequencies due to second-order effects.

FREQUENCY WHERE LINE IS 1/4 WAVE (F-1/4)

This frequency is needed to use the Rs ant and Xs ant functions of the VA1, and can also be used to determine line impedance. (see below.) To find F-1/4 for a random length of

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