NEC model of the quarter size G5RV

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 VSWR(50) 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 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 (0.005,13).

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 VSWR(50).

Screenshot - 2_01_2016 , 4_30_55 PM

Above is the VSWR(50) 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.