YOUR ANTENNA

    Last month we studied the quarter wave vertical that most of use on our VHF and UHF rigs. As you progress in your ham capabilities you will want to explore the HF band (the frequencies from 3 to 30 MHz.)  The most common antenna will be found to be the half wave dipole.  This month we will take a look at how to make this antenna.

    In addition to wire for the antenna, three insulators will be needed. These can be made from PVC pipe. Three six inch pieces of one inch pipe will be plenty.  Drill 1/4 inch holes through the tubes about an inch from each end that will be used to fasten the ends of the wire.

   Using one of PVC tubes, fasten one piece of wire to each end of the tube. Then fasten the other two tubes to the remaining ends of the wires.  Make certain that the two lengths between the fastening spots are the same and make a careful measurement of those lengths.  Write it down!  Since you will probably be using coax to connect the antenna to your radio, one of the ways that you can fasten the coax to the antenna is by peeling back a few inches of that coax and separate the braid and the center conductor. The braid is fastened to one side of the center insulator and the center conductor is fastened to the other side.  There are several other ways that the coax and the antenna can be a attached. Use your imagination!

    After you've put the center of the antenna up as high you can get it, you'll fasten the ends some place (maybe a tree) with some rope and check it out. The antenna made as was just described will be too low in frequency on purpose. We will fix that with one adjustment!

    Suppose that when we measured our lengths they came out to be 61 feet long and when we try to find our best SWR we find it to occur at 3.6 MHz.

    Remember from your first studies that Frequency times wave length equals the speed of light? 

         The equation: F x W = C

    Thus:  F1 x W1 = F2 x W2 or F1/F2 =W2/W1
    Let F1 = 3.6 and W1 = 61 and F2 = 3.8

    Stuffing these numbers into the equation:

    3.6/3.8 = W2/61  or W2 = 57.8

    This means that if 61 feet was too long then 57.8 feet will be just right. We pull the ends down and chop off 3.2 feet and when the antenna goes back up, our magic SWR of 1:1 will be there!  Hot diggity dog!

    After you have chosen the band that you plan on working you need to calculate the length of wire needed.  The same formula that we used before for the vertical antenna will be used for our half wave dipole. Just remember that it is a half wave, not a quarter wave.  One more consideration needs to be made and that is that we will twist some of the wire at the ends around insulators and need to provide for that, an extra foot at each end will do the job.  As an example, if we choose to operate on 3800 KHz, 468/3.8 will be 123.16 feet. Adding 2 feet to each end will make about 125 feet. Here's a little idea that will help get that SWR thing close to 1:1. Add another foot on each end. we'll come back to that in just a bit.  Cut this wire into two equal pieces.