G5RV SR SWR problem

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N0RTR, Jul 13, 2011.

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  1. N0RTR

    N0RTR Ham Member QRZ Page

    Greetings,
    I have a G5RV SR which I do like very much and have made alot of great contacts with it. The problem I am having is on certain bands such as 20m my tunner which is a LDG AT 200pro tunes it to a 1:1. I then start talking and the more I talk the higher my SWR goes until it reaches over on the tuner and the lit display on my IC-718 begins to dim with my audio. I started out using hi power but my tunner and swr would not get over 20 watts so I decreased the rf power to 50watts. I have a pic of my shack and my antenna system here on QRZ. I would love to get as much performance as I can out of this. also with the ladder line being 68 feet long I have looped it to save space and keep the better half happy. I dont get on 160m that offten, can I cut down the ladder line to about 40 feet and leave the legs the same length they are and get better performance with the swr and power output. When you look at the pic notice the gutter that the ladder line passes infront of. I am going to bring that section of ladder line at least 18" away from the gutter, could that gutter be some of my problem or the looping of the remainder of the ladder line. I greatly appreciate all comments and suggestion.

    73
    N0RTR
     
  2. W5DXP

    W5DXP Ham Member QRZ Page

    The 204' G5RV SR will not work well on 20m, just as the standard G5RV doesn't work well on 10m. The SWR on the coax is sky high, in the neighborhood of 50:1. I doubt that the LDG AT 200pro tuner is spec'ed to handle an SWR of 50:1.

    The G5RV SR works well on 160m, 80m, 40m, and the WARC bands. It does NOT work well on 20m. If you want a 20m antenna, I would suggest a 33 foot coax-fed dipole. (P.S. I have a rotatable 33' dipole fed with ~64' of 300 ohm ladder-line that I use on all bands from 20m to 10m.)
     
  3. N0RTR

    N0RTR Ham Member QRZ Page

    I have made alot of contacts on 20m with this G5RV SR. Do you think by getting the ladder line away from the gutter would help my performance? And what about cutting down the ladder line or is it ok to loop it like is shown in the pic? The apex is up approximately 45' and the end of the legs are up off the ground approx 10'. I just dont want the high swr to damage my 718, or will it?
     
  4. KB1CKT

    KB1CKT Ham Member QRZ Page

    The 718 should have SWR foldover, which will limit transmitting into a too-high SWR. [Or it ought to--look in the manual, and see if it has any sort of SWR protection.]

    You can try altering the feedline length. A different length may cause your tuner to see a more-friendly SWR. A few years ago I was able to model my G5RV (standard model) on EZNEC, the freebie demo version. IIRC the demo version did not do feedlines (maybe it does now), so I took the feedpoint impedance and went someplace else on the web to find out what would happen after x feet of feedline. [Or perhaps I just used a Smith chart--don't remember now.] You could try to do the same, and see if you can pull in a bad SWR to something better. I'm not familar with the SR version but I'm not sure about altering the twinlead section--I'd probably play with the coax section instead. Surely the twinlead length was chosen to get SWR "better" on the primary bands; altering that will alter everthing.
     
  5. N0RTR

    N0RTR Ham Member QRZ Page

    Yes thd 718 does have a SWR potection built in, so what would cause my display to dim and brighten as I talk longer on 20m primarily and noticing my SWR increasing as well the longer I talk continuously?
     
  6. KB1CKT

    KB1CKT Ham Member QRZ Page

    Get a DMM and put it onto your supply. I wonder if RF is getting into that. I *think* the new LCD backlit displays are not supposed to change in brightness with respect to power supply level--but who knows. Might as well verify that at this time.

    Is the radio running hot?

    I don't feel like going and figuring out the SWR presented to the radio/tuner, but it sounds like (from W5DXP) that 20m performance is likely to be bad, period. Perhaps you could run a second dipole, as a fan or otherwise, below this? Heck, if this is a homebrew antenna perhaps you can add a second dipole under it, same feedline, and run as some kind of fan dipole. Not sure how that would work out, but I'm going to guess that a 32' dipole under your 200'+ dipole will at least tune easier, even with the twinlead/coax conspiring to give an odd impedance to the tuner.
     
  7. G3TXQ

    G3TXQ Ham Member QRZ Page

    Components in your tuner heating up, or possibly even the coax. Cecil already explained that you will have a very high VSWR on the coax on 20m - perhaps as high as 50:1; that means when you are running 100W the coax is experiencing heating equivalent to running 5kW at "hot spot" current maxima. It also means the tuner may be seeing an extreme load impedance - worst case 1 Ohm!!! Worst case, that's 10A flowing in the tuner components or in the coax at a current maximum.

    Changing the ladderline length may improve things but will likely make the VSWR worse on bands where it is currently reasonable. Changing the length of the coax could improve things if the problem is in the tuner rather than the coax. As Cecil said, the G5RV SR is not well suited to 20m operation.

    Steve G3TXQ
     
    Last edited: Jul 13, 2011
  8. N0RTR

    N0RTR Ham Member QRZ Page

    The radio is not running hot at all. I will check the supply and see if rf is getting into that. Thank you all very much for the opinions. Now what about looping the ladder line that is left over at the base of the tower, is that ok?
     
  9. EI4GMB

    EI4GMB Ham Member QRZ Page

    The G5RV works fine on its design frequencies. The problem is that advertisers market it as an all band antenna which it is not!

    73,

    Fred EI4GMB
     
  10. KB1CKT

    KB1CKT Ham Member QRZ Page

    I'd be careful of looping the twinlead too close to itself or to anything metallic. But in the end I'd get something else for 20m. Anything. This 204' is going to have many many lobes (and nulls) and may not give satisfactory general coverage (IOW working in multiple directions).
     
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