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BE INFORMED No. 65 DIPOLE DIMENSIONS John B. Johnston W3BE A half-wave dipole is one of our most relied upon amateur station antennas. It consists of two metal conductors oriented in line with each other, with a small gap between them. It can be constructed of copper wire. A coaxial feed line from the transmitter - and/or receiver - is attached to the dipole's two conductors at its center.
 To maximize the transfer
of radio frequency energy to and from the dipole, it should be approximately one-half wavelength overall. Each conductor,
therefore, should be approximately one-quarter wavelength. The charts below show the approximate conductor length for
each frequency band. HF Phone Bands
HF CW Bands
Trimming Adjustments A dipole antenna usually
needs some adjustment after being installed. The lowest standing wave ratio should be at the frequency
desired. The amount of adjustment needed, therefore, is determined from measuring the frequency at which
the SWR is the lowest. If it is at a frequency lower than desired, the far ends of the antenna can each
be trimmed by an equal amount. If it is higher than desired, the ends can be lengthened. It
is usually easier to clip off than it is to add a bit of wire, so the above dimensions should allow for slight trimming.
The following charts indicate how much each conductor should be trimmed in order to effect the desired change. Shorten (clip wire) antenna to move its lowest SWR to a higher frequency. Lengthen (add wire) to move its lowest SWR to a lower frequency. Phone
CW
FAQ Q. There are so many variables with antennas. How accurate can those tables be? A. They were obtained from mathematic calculations, but they should provide a good starting point. Q. I realize the diagram is simplified down to the very basic; but what about lightning protection? A. The coaxial cable feed line should be disconnected from the transmitter/receiver and attached to a good ground whenever nearby lightning activity is eminent. Q. In the diagram, it should be noted that some of the RF energy from the transmitter will travel back down on the outside of the coax feed line. That will make the feed line a radiator close to you. A. One way to prevent that from happening is to wrap about 20 feet of the coax into a coil near to the antenna feed point. That should choke off the unwanted RF. Q. An antenna I used for several years back in the 50's and into the 60's was just what you describe and it worked fine. In 1954, as a novice using a single 6L6 crystal controlled oscillator as my transmitter, I had many favorable QSO's on 80 meters. So, even if the balanced antenna is fed with unbalanced coax line, it does radiate. When I upgraded to General and used a Heath AT-1 with an 813 Amplifier running about 250 watts input, I had RF in the shack. I suspect the condition was there all along but was never realized because of the low power. I then learned about a balanced transmission line and added a length of coax in parallel with the main feeder from the center of the antenna to a point 1/4 wavelength down the line where I bonded the 2nd coax outer shield to the main feeder coax shield. That solved the problem of RF in the shack although I never noted any difference in performance of the antenna. A. For simplicity and lowest cost, it is hard to beat a good old reliable half-wave dipole. See BE Informed No. 41 for a discussion of my similar 1954 Novice station. Q. Isn't it typical to work on your antenna when it is the worst weather possible? At least that is the way it seems to always turn out. I thought antenna work was to be done when it was windy and raining or snowing. From my experience antenna work done at these times always provided the best results. A. It seems that way. An antenna erected on a nice warm sunny day never seems to perform as desired. An antenna that stayed up last winter just wasn't high enough.
April 4, 2011 Supersedes all prior editions April 2, 2011 Supersedes all prior editions
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