Dipoles & Inverted Vees

The term dipole usually implies a half wavelength, resonant dipole. Resonant implies zero reactance (i.e., the impedance is completely Real). An electrically resonant antenna is NOT physically ½ wavelength from tip-to-tip. Self capacity between the two wires means the antenna appears longer than it physically is. In addition, there is what is known as “end-effects”, additional capacity due to the loops at the end of the wires where they are tied to the end insulators.

All of these effects are also influenced by the diameter of the wire (in wavelengths), so the effects are typically smaller at HF then at UHF. A half wavelength in free space is:
                492/frequency in MHz (in feet).

But a good rule of thumb for wire antennas below 30 MHz is:
                468/frequency in MHz (in feet).

The impedance of a dipole in free-space is 72 W. The impedance of a dipole changes with the height above ground, and to some extent with the type of ground. On the left is a plot of the impedance as the height is varied. Once the antenna becomes far enough above the ground it cyclically approaches the free-space impedance of 72 W. However, it can be as high as 90 W (at about 3/8 l and very low when the dipole is close to ideal ground. Luckily, over typical grounds it presents a fairly good impedance match to either 50 W coaxial cable for HF antennas installed at typical heights (<.5 l ) for residential property.

For the inverted Vee the issues are more complex because the angle q also effects the antenna parameters. As the ends come down toward the ground there is an increased capacitive loading that can lower the resonant frequency, but only if the antenna ends are dangerously close to the ground (1-2 ft). This tendency is overcome by the reduction in effective inductance due to field cancellation as the antenna legs get closer together. So in general, the smaller the Vee angle, the lower the impedance and the higher the resonant frequency. Angles less than about 90° do not produce efficient antennas (impedance is too low). Changing the angle allows one to “tune” the antenna at ground level.

There are a number of commercially available center insulators and end insulators, but in a pinch almost anything will do. I threw together a quick 80-meter dipole made out of heavy gauge magnet wire that used a piece of plexiglass scrap for the center insulator, and folded pieces of plastic window blind for end insulators. An even simpler portable dipole for 30 meters was made from a length of Radio Shack speaker wire that was split down the center until the two individual pieces were the right length, and then a knot was tied to keep things from unraveling at the feed point, and the remainder of the cable used as a parallel conductor feedline. Knots tied at each end of the dipole connected to pieces of nylon fishing line for support.