The quarter sized G5RV with hybrid feed

(Varney 1958) described his G5RV antenna in two forms, one with tuned feeders and the more popular form with hybrid feed consisting of a matching section of open wire line and then an arbitrary length of lower Zo coax or twin to the transmitter.

(Duffy 2005) showed that the hybrid feed is susceptible to high losses in the low Zo line as it is often longish, is relatively high loss line and operates with standing waves.

Lets look at measurement of a real antenna, broadly typical of the G5RV. The antenna measured is a G5RV rigged in Inverted V form, 11m height at the apex and around 8m at the ends. The feed line is 2mm diameter copper spaced 50mm with occasional plastic insulators.

To some extent, the measurements are dependent on the environment, and whilst there will be variation from one implementation to another, the measurements provide a basis for exposing the configuration challenge.

Screenshot - 01_05_16 , 19_33_56

Above is a plot of VSWR50 essentially at the lower end of the matching section and low Zo line. The measurement is made looking into 0.5m of RG142 and a Guanella current balun that uses about 1m of 110Ω pair, it is essentially the load end VSWR of a hybrid feed were it used.

The plot shows that VSWR dips close to four pre-WARC HF amateur bands.

The optimisation challenge is to steer these minima into the desired bands, especially the lower ones where the dips are narrowest, the low Zo line can easily account for most of the loss.

Fractional G5RV antennas seem very popular in the US market, and they appeal to hams wanting multi band performance in small space.

One of the offerings is the quarter size G5RV, commonly marketed as the G5RV Mini.

The original concept set out by G5RV was a combination of a centre fed dipole and open wire transformation section to successfully deliver a lowish VSWR50 on several of the pre-WARC bands. This enabled arbitrary length low Z feed extension to the transmitter, and allowed direct attachment to transmitters of the common design of the day (1950s).

The quarter size G5RV (G5RVMini)

The quarter size G5RV is as the name suggests one quarter length of both the dipole and the transformation section. Radio Oasis shows dimensions of a 25.5′ dipole and 8′ transformation section.

An NEC model was constructed assuming Wireman 551 nominal 450Ω tranformation section, though ignoring loss. The dipole is made of 2mm diameter copper and is 10m above ‘average' ground (σ=0.005, εr=13).

Screenshot - 2_01_2016 , 4_30_29 PM

Above is a plot of impedance Z looking into the transformation section, swept from 10-35MHz. As expected, there is a high impedance resonance around 21MHz, and low Z resonances around 13.5 and 31MHz. It is the latter that offers the best prospect of low VSWR50.

Screenshot - 2_01_2016 , 4_30_55 PM

Above is the VSWR50 plot with minima at 13.9 (VSWR=3) and 31.5MHz (VSWR=1.04).

The combination could be shortened a little to move the 13.9MHz VSWR minimum up to say 14.2MHz, and of course that will push the 31.5MHz resonance higher.

Screenshot - 2_01_2016 , 4_47_50 PM

Above, the dipole shortened a little moves the minimum VSWR up to 14.2MHz, and the next minimum even further out of the 10m band. VSWR at 28.5MHz is 8.4, and higher in all the bands between that and 14MHz.

This configuration does not deliver G5RV's designed lowish VSWR on multiple bands, and the VSWR=3.3 on 20m is achieved by slight tuning and is barely lowish. It perhaps could be described as a quite poor monoband antenna.