Finding the Correct Dipole Length

We have discussed the standard formula for cutting a dipole to length [L = 468/f(MHz)]. First, you may wonder why 468 is used instead of the free-space factor of 492 for a half wavelength. This is a good question: If our dipole was in free space and with no feed line, the length would be greater than that obtained from the 468 factor. The end effects of the insulators and guy lines, plus antenna proximity to trees, the earth and other influences, causes the antenna to have stray-capacitance effects that tune it lower in frequency. 

You may think of the dipole as an inductance, and stray capacit- ance appears in parallel with it, just like in other tuned circuits. These effects are compensated for by changing the length factor from 492 to 468.

Let's assume that you have cut your dipole wires in accordance with 468/f, as discussed earlier. How can you be certain that this is the correct length (antenna installed) for your chosen frequency of operation, say, 3.7 MHz?

 First, you need to know that the lowest SWR occurs at the resonant frequency of your antenna. It may not be the ideal 1:1 ratio we seek. Rather, it may be 1.5:1 or even 2.3:1. If the SWR is high at the reson- ant frequency of the dipole, you will know that the feed line is not matched to the antenna feed point. The SWR will rise either side of the resonant frequency as you change the operatYou will need an SWR indicator or bridge, as they are called, in order to check your antenna resonance and SWR. 

The SWR indicator must be designed for the impedance of your feed line, such as a 50-ohm bridge for 50-ohm coaxial line. The SWR instrument is installed between your transmitter and the feed line. 

Next, check across the amateur band for which your dipole is cut, observing the SWR reading every 25 kHz or so. Note the frequency at which you obtain the lowest SWR reading (minimum reflected power). 

This will occur at the resonant frequency of the antenna. If the lowest SWR occurs lower in the band than your design calls for, remove small amounts of wire from the ends of the dipole until the lowest SWR is noted at the preferred antenna resonance point. If the lowest SWR reading is observed higher than your chosen frequency, you must add a small amount of wire to the ends of the dipole. Continue this process until the lowest SWR occurs at the chosen operating frequency.

You may use a dip meter for checking the resonance of your antenna. You may do this by connecting a three- or four-turn small coil between the shield braid and inner conductor of your coaxial cable (antenna erected, and feed line attached). A solid-state or tube type of dipper may be used. Insert the dip-meter coil into the small coupling coil on the feeder. Adjust the dipper through its range and locate the frequency at which a deep dip is noted on the meter. Listen to the dip- meter operating frequency by means of your receiver, and note the frequency. Adjust the dipole length until the dip meter indicates resonance at your preferred operating frequency.

We will discuss methods for mismatch correction, later in in this notebook. A minor mismatch (1.5:1 SWR or less) is not of concern to us for routine operation, provided the trans- mitter is designed to deliver normal output power at low values of SWR.

source : Novice Antenna Notebook by DeMaw, Doug