Introduction |
Loading coils can be used to shorten a dipole. Another method is called Linear Loading. With Linear Loading, the dipole elements are effectively folded back on themselves, 2 or 3 times, as shown in the drawing on the right. That drawing was in a published in the ARRL Antenna Handbook, but originally came from an article by John Stanford, NN0F, that was in Volume 5 of the ARRL Antenna Compendium.
The drawing shows the relationship between f, the resonant frequency of the antenna, verses f0, the resonany frequency of a standard dipole. For the antenna at B, the two wire loaded dipole, the resonant frequency is reduced to about 0.67 to 0.70 that of the reference dipole of the same length. The antenna at C, the three wire loaded dipole, the resonant frequency is reduced to about 0.55 to 0.60 that of the reference dipole of the same length.
What isn't obvious from the drawing is that, the element wires in A, B, and C are all the same lengths. So the antenna at A is 1/2 λ, end point to end point. But the antenna at B is only 1/4 λ, end point to end point. And, the antenna at C is only 1/6 λ, end point to end point.
9V1KG Linear Loaded Dipole for 21 MHz |
This idea came from Dr. Klaus D. Goepel, 9V1KG. Klaus did not have very much room, but wanted a antenna for 21.0 MHz. Normally, a 1/2 Wavelength dipole would require 468 / 21.0 MHz = 22.3' (6.797 m) of space. However, he only had about 12' (3.65 m) of space available. So he used the Linear Loaded Dipole approach to fit in the small space.
In the drawing, the rope is installed first and used as a carrier for the wire. The spacing between each of the ropes is about 4" (0.1 m). Then, cable ties are used to secure the wire to the rope. This keeps the wire flexible so that it can be trimmed for the right frequency, without taking down everything.
Initially, at the feed point, a 1/4 λ transmission line transformer was used to convert the 50 Ω coax to 12.5 Ω at the feed point. The transformer consists of two parallel lengths of 50 Ohm coaxial cable (RG-58), connected as shown. Each length is 7' 9-11/16" (2.38 m), (299.792458 / 21.0 (MHz)) × 0.66 (VF) / 4 for RG-58.
This type of impedance matching is not very common, as it is only good for one one band. While information was scarce, and often contradictory, I did find out that other combinations are possible. This paralleling of coax lines transforms the resistance of the radiator to the 50 Ω of the feeding line. In all cases, the coax is 1/4 λ × VF (Velocity Factor).
- Using 2×75 Ω cables, you can transform a 50 Ω feed line to 28 Ω.
- Using 2×50 Ω cables, you can transform a 50 Ω feed line to 12.5 Ω.
- Using 3×93 Ω cables, you can transform a 50 Ω feed line to 18 Ω