One of the challenges of getting on the High Frequency (HF) bands is putting up an effective antenna. Assuming you’ve got the cash available, buying an HF radio might be the easy part of assembling a station. Putting up the antenna may look like a daunting task the first time around. This is where the classic half-wave dipole antenna comes into play, since it is relatively easy to construct and make work.
The basic construction of the dipole is two elements each 1/4 wavelength long, fed in the center by a transmission line (as shown in the figure below).
The total length of the dipole is given by:
Length (feet) = 468 ÷ Frequency (MHz)
For example, for the 10 Meter band, we might cut the dipole for the frequency of 28.4 MHz (right in the middle of the Technician phone band). Using the formula, we can calculate the total length of the dipole (a half wavelength).
Length (feet) = 468 ÷ 28.4 = 16.48 feet
You can use just about any old wire that is 16 gauge or larger but it needs to be a good conductor, with copper being the most popular choice. Stranded wire is generally better since solid wire can stretch under tension. As shown in the figure, you’ll need some kind of insulator at the center and at both ends of the antenna. You can buy insulators designed for this purpose or you can just fabricate something on your own.
As shown in the figure above, the coaxial transmission line may be attached right to the antenna wires without a connector. The coaxial center conductor attaches to one of the ¼-wavelength elements and the ground side shield or braid attaches to the other segment. You may also purchase special center insulators that have a coaxial connector integrated into them so the transmission line can be easily attached. And of course, there are many commercially available dipole antenna products ready for a simple coaxial cable connection and minor trimming or “tuning” to proper length.
You will probably hang the antenna by attaching rope or cord to the end insulators. This may be your biggest design challenge…to figure out where to string the antenna. A pair of tall trees can work well, if they are spaced at a convenient distance. Or you may have to connect one end to your house and the other end to a pole. The main thing to keep in mind is that the antenna should not be in close proximity of large metal objects. For example, if you mount the antenna close to a metal rain gutter, it will severely detune the antenna and it will not work well. You’ll want to get the antenna up into the air about ¼ to ½ of a wavelength above ground, but you may have to settle for less than that.
Dipoles are commonly erected in one of three configurations: 1) flattop, 2) inverted V, or 3) sloper. The inverted V configuration has the advantage of requiring less horizontal space and only a single high support in the center, although the ends should be kept high enough to be out of normal human reach. This configuration also provides a bit better signal propagation off the ends of the antenna than a flattop arrangement. The flattop configuration tends to provide slightly better gain, or signal strength in the broadside directions (right angles to the wires), but performance out the ends of the wires is poorest. The sloper also uses a single tall support and has a more omnidirectional azimuthal (horizontal) radiation pattern, much like the inverted V. In each case, route the transmission line away from the elements at a right angle or directly between the elements for the inverted V, as depicted in the figure below, to avoid detuning the antenna. Every antenna is a compromise, so decide which dipole configuration will work best for your situation.
To get the antenna trimmed to the proper length, start out by cutting the antenna a little long…you can always trim it shorter. Use an SWR meter or antenna analyzer to adjust the antenna for best (lowest) SWR at the desired operating frequency. Remember, make the antenna longer to lower its operating frequency and make it shorter to increase the operating frequency. See Stu's excellent article, Trimming Your Dipole Antenna for a detailed description of tweaking your dipole for good performance.
Since this is your first dipole, I’d encourage you to just try and get something working. It does not have to be a work of art; it just needs to radiate your signal. You may end up installing a more complicated antenna later but the main idea here is to get on the air.
This is just a short article to get you started down the path of Your First Dipole Antenna. For a more detailed look at this type of antenna, see the QST article, “Antenna Here Is A Dipole” and Stu's article mentioned above.
Bob K0NR