Let’s look at some of the characteristics of a horizontal 102 ft. center-fed dipole strung between two 37 ft. poles. The antenna runs North/South for the purpose of displaying radiation patterns. For now, we won’t worry about what kind of feedline conducts the RF power from the transmitter to the antenna. “Impedance” in the following chart indicates the antenna feedpoint impedance as predicted by EZNEC(R), an antenna modeling program. We will include one frequency per HF band with the knowlege that the antenna feedpoint impedance changes with any change in frequency. We will also list the predicted VSWR for 50 ohm coax, 300 ohm twinlead, and 450 ohm ladder-line. Radiation patterns can be observed by clicking on the band of operation.
Radiation Patterns MHz Ant. Impedance SWR, 50 ohms SWR, 300 ohms SWR, 450 ohms
80m 3.8 31.9 - j326.5 69 20.6 21.5
40m 7.2 558 + 1215 64 11.1 7.8
30m 10.125 1882 - j2504 100 17.5 11.7
20m 14.2 103.3 - j48.6 2.6 3.0 4.4
17m 18.14 2089 + j1964 79 13.2 8.9
15m 21.3 298 - j1009 74 13.3 9.7
12m 24.95 188 + j327 15 3.9 3.8
10m 28.4 3113 + j491 64 10.6 7.1
MHz, Twinlead/Coax Junction Impedance, 50 ohm Coax SWR
3.8 14.6 + j2.9 3.44
7.2 27.9 - j54.8 4.3
10.125 31.6 + j274 50
14.2 103 - j49 2.6
18.14 44.3 - j290 40
21.3 24.2 + j80.7 7.8
24.95 79.3 + j41.5 2.2
28.4 3049 + j655 64
Because a wavelength is so long on 75 meters (about 260 feet) we can change the length of the antenna and 300 ohm twinlead slightly without having much of an effect. But on 10 meters, where a wavelength is only about 35 feet, slight changes in the lengths of elements can have a large effect, about seven times the effect that it has on 75 meters. This wavelength effect makes accurate preditions of how an antenna will respond more difficult as we go higher in frequency. If our G5RV is really 104 feet long instead of 102 feet long, we won’t detect much of a difference on 75 meters but we will definitely see a large difference on 10 meters. The standard G5RV has trouble on 30, 17, and 10 meters. On all the bands where it is a good antenna, it requires an antenna tuner.
EZNEC is a registered trademark of Roy Lewallen, W7EL
Article by K2HQ originally available here https://www.qsl.net/k2hq/g5rv.htm