|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|
It's obvious that this antenna should not be fed with coax alone except on 20m. Losses in the coax due to VSWR would be prohibitive on all other bands. However, with a twinlead or ladder-line series section, some of these antenna feedpoint impedances are transformed to an impedance much more acceptable to the coax, some are not.
The following table shows how 27.7 ft. of 300 ohm twinlead (velocity factor = 0.8) transforms
the EZNEC antenna impedance at the twinlead/coax junction in a
typical G5RV. The figures were obtained from EZNEC using the transmission line feature.
|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.