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Optimising a typical G5RV

Introduction

Since the G5RV was described by Lois Varney (G5RV) decades ago, it has been a popular antenna with amateur radio operators.

Though Loius Varney described a number of configurations of the G5RV, most amateurs erect a G5RV that has either a flat-top or inverted-v dipole, fed with a length of parallel wire transmission line and then an indeterminate length of coaxial transmission line to an ATU / tx with or without a balun at the coax to parallel line transition. 

This article explains why a G5RV should be "fine tuned" and a method that mostly uses the basic equipment that is required to operate the antenna.

Fig 1: Optimised typical  G5RV antenna system efficiency.

Fig 1 shows the efficiency of a optimised typical G5RV. Note that only the peaks do not necessarily coincide with amateur bands.

Be aware that other antennas in close proximity might prevent optimisation of the G5RV. When many antennas covering similar frequencies are in close proximity, they may behave more like a single antenna with multiple feedpoints.

Configuration

This section describes the G5RV configurations that are covered by this setup procedure. The configuration here is not represented as an ideal configuration, rather a typical configuration.

Dipole

The dipole may be either flat top or inverted-v, and is electrically three half waves in length at 14.15MHz. The physical length of the dipole will depend on its height above ground, the nature of the ground, inverted-v angle, proximity of nearby structures, trees etc.

Parallel wire transmission line

The dipole is centre fed with a section of parallel wire transmission line that is electrically one half wave in length at 14.15MHz. The physical length of this line will depend on the Velocity Factor of the line. Velocity Factor is the ratio of the velocity of a wave on the transmission line to the velocity of a wave in free space. Velocity Factor may vary from around 0.65 to 1.00 for parallel wire transmission lines, depending on their construction.

Balun

Varney originally described the G5RV with a balun at the coax to parallel line transition, and changed his mind in a later article due to uncertainty about the balun design. More has been learnt of baluns and antennas in the meantime, and there is no doubt that inclusion of an effective choke balun at that transition will assist in minimising feedline contribution to radiation, and conversely, feedline pickup.

Coaxial transmission line

An indeterminate length of coaxial line connects the parallel line /balun to the ATU near the transmitter. The coaxial transmission line is usually the single greatest contribution to poor efficiency. Fig 2 shows the contribution to loss of the three main loss components for a G5RV using nominal 450Ω ladder line and 15m of RG58C/U to the ATU.

Fig 2: Typical G5RV antenna system losses.

Optimising a G5RV is about adjustments to minimise feedline losses, and mostly the coaxial line loss.

ATU

Most modern transceivers are designed for a load impedance close to 50Ω, the tolerance is often specified as a maximum allowable VSWR (in 50Ω line). For example a radio's specifications might say that load impedance is 50Ω, VSWR must be less than 2.

Fig 3: VSWR at both ends of the RG58C/U coax

Fig 3 shows the expected VSWR on the coaxial feedline, both at the load end and the source end.

Note that there is only one amateur band in this model where the VSWR is less than 2, it is at 14.15Mhz.

Fig 3 also shows that the source end VSWR can be much lower than the load end VSWR, a result of line loss, and a clear warning that source end VSWR measurements may not on the face of it reveal the true extent of problems.

Since coax is relatively lossy, high VSWR is a problem because it usually exacerbates line loss in a typical configuration. Note the correlation between coax loss in Fig 2 and VSWR in Fig 3.

Setup procedure

Step 1 - tune the parallel wire transmission line

The electrical length of the parallel wire transmission line must be one half wave at 14.15MHz. 

This is a very important step because subsequent tuning of the dipole will depend on the accuracy of this step.

If you know the velocity factor (VF), the length of the line is 300/14.15/2*VF m or 10.60*VF m. Velocity factor is often given in line specifications, but the figures for parallel wire transmission lines in specifications are quite unreliable. It is better to not depend on published specifications for parallel wire transmission lines.

There are a number of ways to determine velocity factor for a transmission line, and these use a variety of test instruments. The method set out below is designed to use equipment needed to use the G5RV and a 50Ω dummy load.

  1. pre-cut the line to 10.6m (~35') in length, unroll it and suspend it in the clear away from other materials;
  2. connect a 50 ohm resistive load of sufficient power rating to one end of the line (making sure it is clear of other materials, especially if it is an unbalance load);
  3. connect the other end of the line to a choke balun which is then connected to a VSWR meter using 50Ω coax, which is then connected to a transmitter;
  4. set the transmitter for just enough power on 20m to measure VSWR, transmit for short periods only to avoid overheating the dummy load if it is of low power rating;
  5. measure the VSWR at different frequencies in the 20m band, initially VSWR should be quite high, and higher at the high end of the band;
  6. shorten the line a little at a time and repeat from 5 until the VSWR minimum (should be close to 1) occurs at 14.15MHz.

At the successful completion of this procedure you have a parallel wire transmission line section that is an electrical half wave in length at 14.15MHz.

If you are using stock Wireman 551 ladder line, the section should be 9.56m long (300/14.15/2*0.90m).

Step 2 - tune the dipole

The dipole must be electrically three half waves in length at 14.15MHz.

This step depends on accurate completion of the previous step.

When the dipole is electrically three half waves in length at 14.15MHz it will present a purely resistive load of typically 80Ω to 90Ω at the centre feed terminals.

  1. pre-cut the dipole a little long at 32m (~105') overall and attach the parallel wire transmission line;
  2. connect the other end of the line to a choke balun which is then connected to a VSWR meter using 50Ω coax, which is then connected to a transmitter;
  3. erect the dipole and feedline in final position making sure to route the parallel wire transmission line clear of other materials, especially conductors;
  4. set the transmitter for just enough power on 20m to measure VSWR, transmit for short periods only to avoid interference to others;
  5. measure the VSWR at different frequencies in the 20m band, initially VSWR should be lowest at the low end of the band;
  6. shorten the radiator equally at each end a little at a time and repeat from 5 until the VSWR minimum (should be about 2) occurs at 14.15MHz.

Step 3 - check 80m

If the G5RV is optimised at 14.15MHz as described above, a VSWR minimum should occur in the 80m band. Although the minimum VSWR is quite high, the dip is important to minimisation of feedline loss on 80m.

  1. set the transmitter for just enough power on 80m to measure VSWR, transmit for short periods only to avoid interference to others;
  2. measure the VSWR at different frequencies in the 80m band and find the frequency with minimum VSWR.

 The exact location of this minimum will depend on the specific implementation, but if that minimum is outside the band, efficiency is compromised on 80m.

Step 4 - ATU checkout

The G5RV will need an ATU on all frequencies except possibly 14MHz.

Try tuning up with the ATU in line on various bands.

It may be possible to "load up" using the ATU on some bands even though system efficiency may be very low. The G5RV is not an efficient all-band antenna.

Why does it work

The theory behind it

The setup procedure is designed to use mainly the components that are required for operation of the G5RV antenna.

Step 1 is a procedure to adjust the length of the parallel wire transmission line to an electrical half wave at 14.15MHz.

It depends on the fact that the impedance presented by a length of low loss transmission line of Zo<>50Ω that is terminated in a 50Ω load is also 50Ω when the electrical length of the line is exactly an integral multiple half waves.

This condition is detected using a 50Ω VSWR meter in conditions where its accuracy is typically best (where VSWR=1), the 50Ω line between the VSWR meter and the balun / parallel wire transmission line transition and a practical choke balun itself has little effect on the accuracy of location of the line length where the indicated VSWR is minimum.

In addition to the antenna system's VSWR meter, 50Ω coax, and choke balun, the procedure requires a small 50 ohm load (balanced, or close to it). A suitable load can be constructed from 20 x 1000Ω ohm metal film 2W resistors soldered in parallel in a smallish bunch for a cost of ~$2. A physically small coaxial load will give reasonable results so long as it is suspended clear of other objects, especially conductors.

Step 2 is a procedure to tune the dipole to resonance at 14.15MHz. The dipole will have a feedpoint impedance in the region of 90Ω resistance and no reactance at resonance. If you were to place a 50Ω VSWR meter at the feedpoint, you would observe a minimum in VSWR at the frequency of resonance, and the indicated VSWR would be ~1.8.

It is inconvenient to measure the VSWR directly at the feedpoint of the antenna since it is in the air. Having "calibrated" the parallel wire transmission line length to faithfully reproduce the dipole feedpoint load exactly at 14.15MHz (and ONLY at that frequency), we can measure VSWR remotely from the dipole feedpoint at that frequency to detect the condition where the dipole is resonant at three electrical half waves length. If the VSWR is measured at the transmitter end of the coax, the indicated VSWR will be less than at the antenna end of the coax, but for practical lines / line lengths and expected VSWR=~2, the difference is small and does not affect the accuracy of the procedure for find the length of dipole where the VSWR is minimum.

The practice behind it

A 9.6m length of Wireman 551 terminated in 50.0Ω will have an input impedance of 50.3Ω (VSWR=1.006). The loss of the line does not mask the VSWR minimum or the frequency at which it occurs.

Taking a dipole feedpoint impedance of 90Ω for example, a 9.6m length of Wireman 551 terminated in 90Ω will have an input impedance of 93Ω. A 15m length of RG58C/U terminated in 93Ω will have an input VSWR=1.7. Again the loss of the lines does not mask the VSWR minimum or the frequency at which it occurs.

The procedure works, I have applied it many times.

Links

Feeding a G5RV

Changes

Version Date Description
1.01 20/05/2006 Initial.
1.02    
1.03    
     

Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.


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