Friday 3 July 2009

Are coax-fed G5RV's lossy?

These were some tests I did with a G5RV mounted as an inverted V at about 25 feet and the ends at about six feet.

I first measured the coax losses of 30ft of RG58 with an MFJ 259 analyser, which were as follows:

3.6 MHz: (0.6db)

7.1 MHz (0.7db)

14 MHz (1.3db)

21.2 MHz (1.4db)

28.5MHz (1.8db)

I then took SWR readings on five bands - below are the SWR figures as measured at the bottom of the G5RV's ladder line with an MFJ 259 analyser. The figures in brackets are the SWR readings as measured in the shack after a run of 30ft of RG58.

Using the calculator at: the final figure is therefore the calculated total loss taking the measured loss and SWR loss into account.

3.5MHz: 2.0 (1.7) - 0.646dB

3.6MHz: 3.0 (2.2) – 0.724dB

3.8MHz: 4.2 (3.6) – 0.823dB

7.0MHz: 2.2(2.2) - 0.784dB

7.1MHz: 2.4 (2.3) – 0.806dB

14.150MHz: 4.5 (5.7) – 1.954dB

21MHz: 3.9 (9.6) – 2.83dB

28MHz: 3.6 (12.4) – 3.862dB

29MHz: 3.2 (9.5) – 3.397dB

As you can see the antenna was only really a good match at the bottom end of 80m and on 40m. The total loss gets worse as you go up in frequency. At 28MHz the combined loss was 3.8dB - you are losing half your power.

But the worst case scenario of 25ft+30ft of generic window line at an SWR of 12.4 on 28MHz is only 0.405dB.

So the conclusion is, a G5RV fed with RG58 coax is fine for 80m and 40m, but losses mount as you go higher in frequency. If you want to work the higher bands think about using open wire feeder all the way to the tuner. You then have a doublet with much lower losses.


  1. Hi Steve,
    great blog you have here, I have added
    a link onto my blog if that is ok?

    With regards to the g5rv with it being set up in an inverted V configuration, wouldnt that tend to lower the impedence at the feed point so it no longer matched to the 300 Ohm twin feeder, or am I barking up the wrong tree?
    P.S. I am no expert.
    Cheers Mark

  2. Hi,

    I don't think the G5RV is ever "matched" to the 300 Ohm ribbon - it tends to work as a whole with the ribbon acting as some sort of impedance transformer. But you're right - see my other post on what happens to a G5RV's pattern when you make it into an inverted V.

    Steve G0KYA

  3. Hi Steve,
    Excellent information on the G5RV. I also found your posting on the inverted V very informative. I would be interested to see the results for a half size G5RV (51ft) on its design frequencies of 10m,20m and 40m?
    Best Regards
    Fred EI4GMB