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Tuning the feedline for my G5RV

Discussion in 'Amateur Radio Antennas' started by AE5RD, Apr 8, 2010.

  1. AE5RD

    AE5RD Reformed CB'er

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    I've about had it. I am very frustrated. I read this website from the internet about tuning my feedline to resonance as a first step to building a G5RV. I cut some radioshack 300 ohm twinlead to 35 ft and hooked one end to a dummyload and the other to my swr meter and then to the radio. I can not for the life of me get the this feedline tuned to a low swr. I am down to 27 ft and the best i can seem to get is 3:1. Has anybody used radioshack twinlead and how many ft is the feedline supposed to be using this junk.


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  2. Beetle

    Beetle Well-Known Member

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    A "G5RV" antenna, by definition, is 102 feet long overall, fed in the center with 34 feet of 600 ohm open line. If you're using regular 300 ohm line, the length should be closer to 28 feet. Then any length of 50-75 ohm coax to the tuner. Any substantial changes to these dimensions yields something, but not a true G5RV.

    As a general rule, the only band on which a G5RV will "work well" without a tuner is 20M. That's what Varney, the original G5RV, was looking for. He found that a doublet with those dimensions worked fairly well on the other ham bands at that point in history, but WITH a tuner.

    I've never heard of "tuning feedline for resonance" -- and if you go through this pointless drill, a given length of feedline will only be "resonant" at ONE frequency.

    What band(s) do you plan to operate with this antenna?
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  3. HiDef

    HiDef Active Member

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    Can you post a linky to the place you found that?

    Dunno why a resonant feedline would be used to feed that antenna.

    Resonant feeders were popular before the use of coaxial cable. The feeders themselves were not resonant but their length was manipulated to present a non-reactive load to the transmitter. The combination of doublet and feeder length is trimmed to resonance. Take the feeder length X 1/vf and add one half of the doublet length to figure how the feedpoint behaves. Maybe this is what the site is trying to explain.

    BTW, Resonance does not mean 50 ohms load. All it means is the sum total of the reactances is zero. The impedance will act purely resistive. The IEEE definition of resonance INCLUDES maximum current flow which would dictate relatively low impedance feed. This definition differs from previous resonance definitions. Example: a 40 meter resonant doublet will also be resonant on 20 meters because here will be a frequency where the sum total of the reactances is zero. The feedpoint impedance will be in the thousands of ohms in this case so coax feed is not going to work. Some call this situation anti-resonant.

    Feedline is also made resonant when it is a different impedance than the source and load. Cheap cable TV 1/2" foamflex makes a great way to get low loss on VHF but it must me trimmed to length for low SWR.

    In any case you should have been able to find a low SWR somewhere with what you were doing. Is the dummy load physically large? Can you substitute a 50 ohm non-inductive resistor? A large load like a cantenna or big Bird type might be a problem due to it being unbalanced. Are you testing with an MFJ where you can try a bunch of different frequencies. Is the feedline in a straight line away from large metal objects?
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  4. AE5RD

    AE5RD Reformed CB'er

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    Here is the link. Optimising a typical G5RV I am using a small dummy load in a 2"x3" metal can. It is very primitive. The feedline is running out of an aluminum window in a straight line and hooked to the wooden fence on the other side of the yard. There is the minimum amount of objects in proximity of the feedline. I recognize that i need to purchase a MFJ 259b or equivalent. At the time I have limited resources. I took the day off from work today to try to get everything in order. I usually stay on 80, 40, 20 and sometimes 10 but i have the maco for ten meters.
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  5. Beetle

    Beetle Well-Known Member

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    Connecting a presumed 50-ohm dummy load to a length (any length) of 300-ohm feedline represents, on its face, a 6:1 SWR if the meter is normalized at 50 ohms, as most of them are.

    How are you determining "resonance"? As HiDef pointed out, the term simply means that the circuit has zero net reactance, and is purely resistive. Sometimes resonance occurs at an SWR minimum, but often it doesn't.

    I see you just posted a link, which I hadn't seen before starting this post. I'll check it as time allows.
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  6. SR385

    SR385 Active Member

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    A G5RV is three half waves on 20m is why. The balanced feeder is an active electrical part of that design. It is part of the antenna. This is why it is important to have that element hanging straight and free of the ground or metal objects. Ideally a G5RV should be flat topped as well due to the interaction of the balanced feeder and the top two radiating elements.

    The VK1OD site is very good for his explanation of constructing a G5RV.

    The part that may be fouling up the try here is using the 300Ohm twin-lead and what exactly the velocity factor of it is. If the OP has extra, start with it longer than indicated and see if you get better results. You may be too short where you are now.

    Also, don't overlook the requirement of the choke balun at the point where the coax feeder meets the balanced line. It's noted in the instructions, but it's important for a G5RV to operate properly.
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  7. W5LZ

    W5LZ Crotchety Old Bastard

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    That parallel feed line is acting an an impedance transformer, converting whatever the impedance of that 102 foot doublet is to something close to 50 ohms that your transmitter likes to see. It isn't 'tuned' in the way 'tuning' is thought of. That's all it does.
    If that 'linky' is for determining the original purpose of a 'G5RV', there are probably several around because Varney made it plain that it was a 20 meter antenna, and only -incidentally- could be made to work on other bands. All of the misconceptions with a G5RV antenna are a result of unsubstantiated 'BS' with advertising. You may have a parrot that 'barks', but I don't think I'd try putting a flea-collar on it...
    - 'Doc
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  8. hookedon6

    hookedon6 W9WDX Amateur Radio Club Member

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    BINGO,.............. we have a winnar(y)

    the feed line in a G5RV does radiate


    yikes,..... i just reread what i wrote. i must have 'dain bramage, the balanced feed line has equal and opposite currents on it and does NOT radiate
    Last edited: Apr 8, 2010
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  9. Happy_Hamer

    Happy_Hamer Administrator Staff Member

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    My G5RV has 31 foot of 300 ohm and 70 foot of coax. I bought it many many many years ago, it is a Van Gorden which I do not think they make anymore?

    It also has a transformer between the 300 ohm and the RG-8X

    I took the transformer apart when my original G5RV was tore down by a tornado. Inside the transformer is, I THINK 12 to 15? ferrite cores. RF Parts Company ~ Chokes that run over the coax where it ties into the 300 ohm.

    At one time, when I had my original G5RV in the air I cut the coax so I would not have to much coming into the house, the G5RV wouldn't tune very well at all. I called a friend and he said that I need a MINIMUM of 70 foot of coax for it to work properly because it helps in impedance matching? http://www.abc-comm.com/images/Antennas/VGORDON/G5RV.htm

    After I "spliced" the coax back together it worked just fine. Remember, do not coil excess coax.

    I do not know if any of this will help, just giving you my experience
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  10. hookedon6

    hookedon6 W9WDX Amateur Radio Club Member

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    70+ feet of coax is required BEFORE the balun for impedance matching:confused:
    without a tuner:
    When a G5RV is used with a short coax the only band resulting in a low SWR is 20m.

    However, when using the G5RV as an all-band antenna, (which it is NOT) the only time you will get a low SWR is when using a long, lossy coax, where the loss in the coax reduces the indicated SWR found at the antenna end to a much lower indicated SWR.

    i think your power was being lost due to attenuation in the coax.


    as soon as i saw this on that site, i "suspected" the author was really just trying to sell G5RV's:

    "The G5RV is an excellent all-band antenna (3.5 – 30 MHz) 102ft. dipole. The 102ft. dipole with 31ft. feeder of 450 ohm transmission line achieves a resonance on all bands from 80 to 10 meters ..."


    yada yada yada,......(sigh)

    just put up a 125 - 135 foot doublet, fed with ladder line and you are good to go
    Last edited: Apr 8, 2010
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  11. W5LZ

    W5LZ Crotchety Old Bastard

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    I'm afraid that if you consider a G5RV to be an 'excellent' antenna on more than one band, 20 meters, then you have very low expectations for 'excellence'.
    Resonant on all bands between 80 and 10 meters? Not hardly.
    Not bad advertising though, and it's sold a lot of G5RV antennas.
    - 'Doc


    And about that coiling up of feed line being a 'never do it' thingy, that's just not true for most coaxial cable. In the majority of cases, it does absolutely no harm. In a -very- few particular cases, it might. And naturally you don't want to wind it in coils that are too small for the size of the coax. That's one instance where it may not be a good idea. Winding '9913', or 'hard-line', into a coil is probably not such a good idea either.
    The important stuff happens on the inside of that coax, not the outside. The 'inside' of that coax has no idea of what 'shape' it's in and couldn't care less.
    Last edited: Apr 8, 2010
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  12. hookedon6

    hookedon6 W9WDX Amateur Radio Club Member

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    @ hf, if i have any coax left over and for one reason or another, i don't want to cut it off, i just wind it in a coil about 10 - 12 inches in diameter next to the feed point (aka:an RF choke). i think it helps to "uncouple" the feedline from the antenna.

    don't wind it too tight or make too sharp of a bend. it can pull the center conductor thru the pe insulator and if it contacts the braid inside the coax.:oops:
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  13. Captain Kilowatt

    Captain Kilowatt Professional Amateur Staff Member

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    Coiling up extra coax is fine except, as said if, you coil it up too tightly as to strain the centre conductor or when the feedline is actually part of the radiating portion of the antenna. In the latter case it will really throw a wrench into the tuning and matching.
    #13
  14. SR385

    SR385 Active Member

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    (y)(y)(y)

    My first wire was a hamfest G5RV in a bag special. Never liked it, though it met the 'spec' for what it should do.

    I neutered it. Clipped the choke off the window line and spliced more window line in and fed it from the tuner directly as a 102' doublet. The performance was immediately improved.

    I've continued to extend and raise my doublets and have had better results (I'm primarily a low band op) each time.

    The worst mistake I've made was making it the wrong length over 300' where it was resonant around 1.4MHz, which gave me headaches on 160m for a match. A storm 'fixed' that one, so this time around I'm cutting it for 160m.

    The only other change I've made was to get a DX Engineering 1:1 current balun rated for 10kW with a nice static drain on it. I feed coax out from the tuner to that balun and window line from there to the feed point. I've compared direct window line to this and found no significant difference. This setup does have the advantage of bleeding off static buildup on the wire though and unless I put two chokes on the window line to ground, the benefits of the quieter receive are very nice.

    I found that window line directly into the tuner, though the tuner is grounded, gives a lot of static buildup and discharge noise.
    #14
  15. W5LZ

    W5LZ Crotchety Old Bastard

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    As long as you can keep that coax to the outside balun short, and of 'good' quality (meaning low loss, high power capacity) I seriously doubt if there will be any significant problems, or differences from ladder line straight to the tuner. Sure, it's going to make -some- differences, but nothing that can't be handled by a decent tuner.
    - 'Doc

    (I don't 'like' baluns and don't use them if at all possible. This sort of thing is one of the few exception I'd make to that opinion.)
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