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G5RV Install Questions

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE8CTH, Sep 16, 2018.

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  1. KE8CTH

    KE8CTH Ham Member QRZ Page

    Hello All,

    Recently moved into a house with no HOA so I want to get a good antenna up for a future ham shack. I been doing research and the G5RV peaked my interest. I was planning on getting the MFJ 1778 kit for this build. I did some quick layouts for everyone's review. I am planning on using a 5ft Rohn tripod on my rooftop with a 10ft pc. of schedule 80 pvc (.25in wall thickness). this is going to be a bit of a stretch as Rohn says not to go over 5ft with any mast. Got a couple questions regarding this antenna...

    1. If I truly do go with a 10ft pc of pvc, should this puppy be guy wired?

    2. Ladder line seems to be a critical part of how this antenna operates. I will not be able to get the entire 32.5ft (length given from the MFJ 1778 kit) length of the ladder line into the air. I have two different configurations attached below of how I can erect the G5RV. Which of the two configurations do you prefer?

    3. I am trying to get at least 20ft of ladder line exposed per the MFJ install instructions. If there is some remaining ladder line on the ground does that affect the performance of the antenna? Of the two configurations, I can choose to drape the ladder line approx. 25ft or angle the ladder line and secure it to a nearby fence to expose approx. 29ft. Should I take advantage of the additional 4ft gain shown in configuration 2? The angled ladder line would be essentially parallel with the antenna leg being attached to the fence post.

    4. Now the ultimate question...am I wasting my time with a G5RV in this location? The max height I am able to get is around 25ft. Should a chase a straight dipole in this situation?


    I operate mostly SSB. I am planning to get into CW quite heavily (learned the code, need to get the speed up). Right now I operate at QRP levels around 10watts. I eventually want a 100watt rig so I want to put the time and money into the antenna build now.

    Let me know your thoughts,

    Thanks
    Brant, KE8CTH




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    upload_2018-9-16_15-50-15.png
     
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    The ladder line needs to be away from everything. Do what ever that takes to keep it from touching anything except insulators.

    Good Luck.
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd just use a taller rooftop support. If you have no "restrictions," that should be pretty easy.

    A Glenn-Martin (or similar) 4-legged "roof tower" is similar to a tripod but much stronger and has four legs instead of three, so once installed it can be tilted over in either of two directions, not just one. The G-M RT832 can support a 20' mast, and the mast can be aluminum or galvanized steel (not PVC), which will work fine as long as the window line is hanging at least 4" or so away from the mast (easy to do); it needn't run "down the mast."

    With a setup like that, you'd have about 25' height above the roof, then another 15' to drop to the ground = 40 feet, plenty of room to let the ladder line hang mostly vertically and never reach the ground.

    The antenna will also work better at that height, since you'll be >1/2 WL above ground on 20m and >1/4 WL above ground on 40m.

    A G5RV can work well on 40-20-12 meters, and a "bit" of 80 meters; it does not do well on 30m/17/10m (enormous mismatch on those bands) but can work passably on 15m. Since right now, and for the next few years, the "hot bands" are 20/30/40/80m, it will cover three of those well enough to make a lot of contacts.

    You could modify it to become a ZS6BKW doublet, which is similar but covers bands differently.
     
    NH7RO, WB5YUZ and K2CAJ like this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Also, it is desirable to keep the ladder-line symmetrical with respect to the two wires. The laddel-line does not have to be vertical, but if the wires were in a N-S plane, then the ladder line should be in the E-W plane, i.e. orthogonal.

    I would use a real ferrite common-mode choke instead of a coil of coax at the coax-to-ladder-line transition. A coax coil can work for a single band; not so much on a multiband antenna.

    I prefer the much improved version of a G5RV called a ZS6BKW. It was further improved by Cecil (w5DXP) by adding a series capacitor ahead of the common-mode choke (aka 1:1 balun) to improve the match on 75m.
     
    Last edited: Sep 16, 2018
  5. KE8CTH

    KE8CTH Ham Member QRZ Page

    WB2WIK, thanks for the recommendation on the Glen-Martin towers. I have never heard of them. I like that idea of a sturdier roof top mount because I live down in Charleston, SC with all the hurricanes and living next to the coast it is always breezy.

    Those Glen-Martins are considerably more money than the Rohn towers but I think it is in the right direction.
     
  6. KE8CTH

    KE8CTH Ham Member QRZ Page

    WA7ARK, I see what you are saying. Configuration 1 might be the better of the two. To prevent the ladder line from hanging on the ground, I could come up with a small support maybe 10 ft from the house to allow the ladder line to sit on. This of course would still be 90 degrees to the two antenna elements.
     
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    +1 on the Glenn Martin towers but these are basically the same design and a bit more affordable: http://www.w8io.com/rooftower.htm
     
    NH7RO likes this.
  8. KD6RF

    KD6RF XML Subscriber QRZ Page

  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I agree, the W8IO products seem to be nearly carbon copies of the G-M ones and do cost less.

    But 4 legs is a lot stronger than three legs and these designs are substantial compared with almost any kind of tripod; plus, the advantage of tilting it over in either of two opposing directions can be very helpful.
     
    NH7RO likes this.
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Follow-on to the excellent article about the G5/RV and ZS6BKW antennas by G3TXQ, I offer the following:

    Assume an apex height of 30ft. Assume that the Ladder-Line (LL) is Wireman #552 (JSC#1315). The ends of the wires are fixed at 10ft agl.

    I vary three parameters of an AutoEz/EzNec simulation to minimize SWR simultaneously at three specific frequencies: 3.8, 7.15, and 14.225 MHz. The variables are the length of the wires, the length of the LL, and the value of the series capacitor placed between the coax center conductor and the input to a 1:1 balun (Common-mode Choke) per Cecil W5DXP. As the length of the wire leg is changed, since apex height and end height are fixed, that effects the apex angle.

    The first pass of the optimization found a capacitor value close to 500pF, so I fixed the capacitance value at 500pF, and allowed the AutoEz optimizer to play with only LL length and wire length, which converged on 47.6ft and 40.25ft, respectively.

    I then asked AutoEz to plot SWR vs freq for the 75, 40, and 20meter bands:
    (click to enlarge)

    forKE8CTH80.gif forKE8CTH FieldsPolar80.gif
    forKE8CTH40.gif forKE8CTH FieldsPolar40.gif
    forKE8CTH20.gif forKE8CTH FieldsPolar20.gif

    You notice that the SWRs for these three bands are quite good, well within the range of built-into-rigs automatic tuners. Because of the low height of the antenna, the elevation patterns are not great, especially on 20m.

    Note that this antenna also can be used on four additional bands:

    forKE8CTHt.png
    but, the elevation patterns will be quite bizzare.
     
    Last edited: Sep 17, 2018

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