Should I use a balun for a G5RV?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE0CPH, Oct 14, 2018.

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  1. KK5JY

    KK5JY Premium Subscriber QRZ Page

    Don't judge a book by it's cover... or a website by its background. :cool:
     
  2. KE0CPH

    KE0CPH Ham Member QRZ Page

    KA2RRK likes this.
  3. KA2RRK

    KA2RRK Platinum Subscriber Platinum Subscriber QRZ Page

  4. G8ADD

    G8ADD Ham Member QRZ Page

    I think people tend to forget what a G5RV really is. A G5RV is just a simple doublet, nothing more, but the length of the doublet was selected to give four lobes on 20 metres, which happened to suit the originators QTH. If you read his original articles, the version that we are all familiar with - a matching section of window ribbon to coax - was just one option that he described, which happened to give a low enough SWR on a few bands for the tube rigs of the time to match to. Note, incidentally, that the design called for 75 ohm coax, not the 50 ohm that vendors usually recommend! He made it clear that the preferred design was to use open wire feeder to the shack, which is why I tried out extending my 300 ohm ribbon to my shack - the layout here required over 100 feet of coax which meant serious losses were unavoidable even with quality coax. As for baluns, my tuner is an MFJ-949E which has one built in. As a simple antenna with all-band capability (as I use it) it is as good as you can get.
     
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  5. KA2RRK

    KA2RRK Platinum Subscriber Platinum Subscriber QRZ Page

    Yeah, what he said!



    KA2RRK
     
  6. KK5JY

    KK5JY Premium Subscriber QRZ Page

    The nice thing about the G5RV is that it's a popular and basic antenna, that is reasonably easy to match across a few popular bands, and can handle QRO without complicated or expensive matching devices.

    As such, it is also handy as a learning tool for people interested in antennas. It helps them to start thinking outside the box with respect to matching and transmission line techniques. It has been widely studied, and several variants have been designed, all of which are good for comparing to each other, which further helps new people think about antenna design.

    The G5RV may not be an optimal design, but it's a good "teacher" antenna. :)
     
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  7. K2WH

    K2WH Premium Subscriber Volunteer Moderator QRZ Page

    Just put up a dipole, feed it all the way with ladder line to a 4:1 balun somewhere near the operating position, short piece of coax into a tuner.

    A G5RV is a poor antenna, hard to make work and limited on how many bands it works.

    Put up a wire dipole as I suggested and work all band even 6 meters (with the proper tuner) and you are done.

    A G5RV antenna was designed for those who didn't want or have a tuner.
     
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    If you choke I3, you force a balanced condition between I1 and I2. That's why common mode chokes are very commonly(and appropriately) called 1:1 current mode baluns when used at the junction of a unbalanced line to a nominally balanced load.

    [​IMG]
     
    WB2UAQ likes this.
  9. N2EY

    N2EY Premium Subscriber QRZ Page

    Actually, it gave a low enough SWR on ALL amateur HF bands of the time.

    The G5RV was put together back when the HF amateur bands were 80/75, 40, 20, 15 and 10 meters....and that's all. For hams in G-land back then, "80/75" was 3.5 to 3.8 MHz and 40 was 7.0 to 7.1 MHz. 160 could be worked by using the whole thing as a T against ground.

    The original design didn't use "window line", it used real open-wire line. The rest of the transmission line was either 75 ohm coax or 72 ohm Twin-Lead (yes, there actually was such a thing).

    Most of all, "low SWR" meant anything less than about 4 to 1 back then. All that really mattered was whether a tube rig of those days could match it.

    G5RV did not have fancy analyzers nor antenna modeling software, so some assumptions he made turned out to be rather optimistic...

    The antenna gained great popularity because of its size (only 102 feet, so it would fit in many places a full-size 80 metre dipole won't) and simplicity. The incredible conditions of Cycle 19 didn't hurt, either.

    Where hams today get into a fix is that they put one up and expect 1:1 SWR on every HF band from 3.5 to 29.7 MHz, and performance rivaling a beam. Doesn't work that way.

    73 de Jim, N2EY
     
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  10. G8ADD

    G8ADD Ham Member QRZ Page


    You appear to be one of the people I referred to above, who forget what a G5RV really is. The G5RV is just a dipole, nothing more, nothing less - though I prefer to call it a doublet to avoid confusion with half-wave dipoles. It was designed by Louis Varney as a 1.5 wave dipole for twenty metres, giving four main lobes and a little gain on that band, and although the most comprehensive description was in Radio Communication, July 1984 pp 572-575 it had been around for many years before that (IIRC the first mention of it that I heard was about 1960), and was designed in an era when tube PAs, commonly with pi-tank outputs, made tuners relatively unnecessary. It was not therefore expressly designed to avoid the use of a tuner and its ability to give a low(ish) SWR on a handful of bands when fed with coax was probably just a happy accident.

    The article goes into the function of the 34 foot matching section on each band but makes it clear that "by far the most efficient feeder is the open wire type" (page 575). In other words, the 34 foot matching section to 75 ohm coax was just one available option, the essense of the design was the length of the dipole section, and feeding with open wire all the way to the rig was the preferred option. I have found that feeding it this way with 300 ohm window line it will tune on all bands 160 metres to six metres, and though low in efficiency on 160 metres (and a cloud warmer) it will get me contacts at up to several hundred miles at night.

    Incidentally, Louis Varney's 1984 article was reprinted in ARRL's Wire Antenna Classics, page 2-3, and is well worth reading.
     
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