G5RV Antenna

This article reports the results of searches performed on the G5RV antenna. As stated in  several articles concerning the G5RV antenna an specially in old French books written by R.A. Raffin, F3AV and R. Piat, F3YX and more recently. Unfortunately years have passed and I only kept in memory that at the bottom of the line the impedance has a small reactive part and a 75ohms resistive part then any length of 75 ohms coaxial line can feed it.

In practical
Today , as regards what I heard on air it is always used with an ATU, mainly on 80, 40, 30 and 20m. I think it's partly due to the fact that the solid state PAs don't allow any impedance adaptation when in the past tube PAs had an output circuit tolerating more or less a load with a reactive part. On 20m and higher it seems that commercial multiband antennas needing no settings have superseded the G5RV antenna.

The G5RV on internet

It provides us with several KE2DI articles about the G5RV antenna. They are the most interesting I was able to find because they provide original information and also today consideration. As a sum up, the features are:

q The antenna length is 31.10m (102ft), so it's a 3/2-wave on 20 m and installed horizontally at 12m (39ft), the resonant frequency is 14.150MHz and the resistance is about 80ohms.
q On 20m and up the gain is better than a dipole because of the presence along the wire of several 1/2-waves, which give a collinear effect.
q The 10.36 (34ft) stub line length is designed to be a 1/2-wave on 14.150MHz and on the other bands it acts as a useful impedance transformer.
q The aim was to get a multiband antenna without traps and for limited area

Remark
Applying a 0.97 velocity coefficient (air ladder line) to the 10.36 m (34ft) of the stub line give exactly an electrical 1/2-wave on 14.150Mhz. Therefore the stub is a 1:1 transformer on 20m, so on 14.150MHz there is at the bottom of the line the same impedance as at the antenna centre.

Variants

F8CI
Today in France a lot of hams use this variant. The difference lies in using a 1/4 current balun at the bottom of the 10.36m (34ft) stub line where it ensures the transition symmetrical to asymmetrical. This balun configuration is named remote balun.



Flat-top installation
Theoretically to be perfectly symmetric the two antenna wires must be aligned in the same horizontal plane and not threw out of balance by environment masses. If it is not the case there will be unbalanced currents in the conductor line, it will radiate and will influence the antenna. Also the collinear effect will be decrease

Inverted-V installation
Theoretically to be perfectly symmetric the two antenna wires must be straight in the same vertical plane and not threw out of balance by environment masses.  If it is not the case there will be unbalanced currents in the line conductors, it will radiate and will influence the antenna. Also the collinear effect will be decrease. If the apex angle becomes lower than 120�, the benefit of the collinear gain on the highest bands will be progressively lost. It means that if the apex angle is 12m (39ft) high the wire ends must not be lower than 4.25m (14ft).

 

Conclusions 

I haven't yet built up a G5RV antenna in order to check the above features, which are obviously theoretical. However I have carried out some tests with a 2x10m center-fed antenna fed by a 450 ohms twin-lead and a 1/1 balun connected to the asymmetrical out put of my ATU, see remote balun, so:

q An ATU is required to match the various impedance got at the end of the stub
q The ATU can meet its matching goal on some band but possibly not on some other
q The ATU cannot suppress the SWR in the coaxial line and consequently the losses as well
q If a balun is used there will be more losses on some band
q Although the original article tell that it is conceivable to use a balun, to day on the web it is recommended not to do so, of course it is understandable that the wonderful balun  doesn't exist.

 

Solution 

q If like me you hate to waste power in a loss-making system of antenna feeding, the solution to run the multiband properties of a 3/2-wave-20m is to use an ATU with symmetrical  output. The symmetric line length required is no longer 10.36m (34 feet), but it must be optimised as regards the ATU possibilities. Even if it is right that the line work with a very high SWR, due to its conception a parallel conductor line generates tiny losses in standing wave rating. But it's no longer definitively a G5RV antenna.