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Thread: 80m monoband dipole or G5RV for 80, 40m?

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  1. #1

    Default 80m monoband dipole or G5RV for 80, 40m?

    I have 140ft of solid copper wire and a length of 300 ohm twin lead feed line. I am primarily interested in the 80m and 40m bands for now and want to build a dipole that will suit my needs. I will probably use it for both CW and phone. I would like to gather some opinions on whether I should cut the wire for an 80m half wavelength or if I should follow the G5RV spec and cut the wire even shorter.

    I have an MFJ-949e antenna tuner, so I have a decent range for tuning as well as the opportunity to bring in a balanced or unbalanced feed line.

    I don't have any coax or an so-239 so I am planning on using the 300 ohm twin lead. Unfortunately, the twin lead is the cheap flimsy kind until I can get the heavier duty kind.

    Let's assume the best configuration due to space will be an inverted V at probably 20ft apex (I know, that's low for a dipole). I don't know what kind of ground soil I have.

    Let's pick the following frequencies (just a middle spot on the bands):
    3.750
    7.150

    Will I be better off cutting for 80m monoband or going with the G5RV? Depending on which, what would be a ballpark length for the 80m inverted V monoband? (62'6")?

    How about for the G5RV? (51')?

    And feedline length? (32')?

    I also have thought about trying out a fan dipole and just getting another length of wire, so perhaps both a 102ft wire and a 125ft wire....
    Rigs: HW-8, HW-100, Swan 600 Twin, Kenwood TS-570d(g)
    Keys: Bencher BY-1, Heathkit HD-1410, J-38
    Dipoles: 160-30m inverted V, Buckmaster OCF
    Beams: Homebrew 6-band Spiderbeam (30-10m), K4KIO Hexbeam (20-6m)
    Verticals: ZeroFive 43-foot Vertical

  2. #2
    Join Date
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    I would do a dipole for each band.

    Dipoles are the simpliest antenna there is and they work pretty well. The height, 20', will limit your performance on 80 or 40 antenna but you will still work stations.
    Leroy
    Be sure to listen for my beacon on 28.278.8 MHz

  3. #3
    Join Date
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    9812 Banway Drive, Greenwell Springs, LA.
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    Default

    Well, I assume you are talking about one leg of the dipole when you mention the inverted V monoband at 62.6 feet, and the G5RV at 51 feet, because the actual dipole length will be double that for both, of course.

    There are some calculators online for optimizing a balance fed doublet antenna. One of the problems that you'll run into is that an 80 meter resonant dipole is going to be a double zepp on 40 meters. This means whereas on 80 meters you'll be feeding the dipole at the 50 ohm current maxima, on 40 meters you'll be feeding it at the high impedance points, which can be several thousand ohms.

    On 80 meters this will be fine if you use a 1/2 wave length of feedline (that would be approximately 120-130 feet give or take) because a 1/2 wavelength of BALANCED feedline will repeat the feedpoint impedance and you'll have approximately 50 ohms at the tuner. However, on 40 meters your 1/2 wavelength feedline will be nearly a full wavelength, which is two full 1/2 wavelengths, and you'll also be bringing the same HIGH impedance point to the tuner. Your tuner may do fine with it, but it might have some difficulty too.

    There are various "tuned feeder doublet" designs out there that utilize antenna length and feedline matching to bring a reasonable SWR to the transceiver on multiple bands. Basically, you'll be looking for the best compromise that brings the lowest impedance on your bands of interest to the transmatch unit.

    I would start by googling "tuned feeder" or "tuned feeder doublet" to find the design that looks best to you. You'll likely have the added benefit of having a decent gain antenna on some of the high bands, albeit that the lobe patterns may be very narrow with deep nulls.

    At any rate, I'd recommend this sort of design over a G5RV if 20 meter DX is not your interest. The G5RV was really, by Varney's own words, intended to be a 4 lobed 20 meter antenna that happened to work well on many other bands as well. Since 80 and 40 meters were only "afterthoughts", I'd be surprised if you couldn't make a tuned feeder doublet work a little better for you on those bands.
    73, Heath/KE5FRF
    CWOps#776/SKCC#1940/NAQCC#1712/WAS#52445
    EchoLink Node#268023
    W5YI-VE
    My favorite mode? Morse, of course.

  4. #4

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    I would go with the full size 80 meter length. It is slightly more efficient on 80 meters. The fan dipole will also work well.

    The MFJ-949e will do fine with either configuration. I don't think the balun in it is very good, so you might look at getting an external one sometime in the future. DX Engineering, Array Solutions, and Balun Designs all have good reputations for baluns.

    For feed line length recommendations, look at the "Choosing the Correct Balun" article on the DX Engineering site.

    73,
    Jim AD4J

  5. #5

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    Quote Originally Posted by AD4J View Post
    I would go with the full size 80 meter length. It is slightly more efficient on 80 meters. The fan dipole will also work well.

    The MFJ-949e will do fine with either configuration. I don't think the balun in it is very good, so you might look at getting an external one sometime in the future. DX Engineering, Array Solutions, and Balun Designs all have good reputations for baluns.

    For feed line length recommendations, look at the "Choosing the Correct Balun" article on the DX Engineering site.

    73,
    Jim AD4J
    I agree with this. Generally people use something like G5RV dimensions for better matching. Since you won't have any trouble matching either with this setup, go for the 80 meter dipole.

    20 feet is pretty lousy. No antenna for 40, and even worse for 80, is going to perform well 20 feet off the ground. You'll have good local contacts, pretty good regional contacts under 1000 miles, and the rest will be rare and sheer luck. This is from experience using a dipole at 20 feet of 40 and 80 meters.

    You really need height to make a dipole perform well. 20 feet isn't even a quarter wave high on 40. The antenna pattern will be putting most of your radiated energy straight up... hence the term cloud burner...

    Focus on getting the antenna high and in the clear. That is going to be much more critical to the performance than whether you cut it as a G5RV or 80 meter dipole. Height has a huge impact. Even another 20 feet would probably make a big difference. A half wavelength high (70 feet on 40 meters) would be much better.
    Charley K1DNR

  6. #6
    Join Date
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    Quote Originally Posted by WA4OTD View Post
    I would do a dipole for each band.

    Dipoles are the simpliest antenna there is and they work pretty well. The height, 20', will limit your performance on 80 or 40 antenna but you will still work stations.
    That would be my choice too...if you have the real estate and the copper. Next choice for me would be a TRAP dipole for 80 / 40

    Eric
    "The more you know, the less you don't know."

  7. #7

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    If he doesn't have coax and he is using twin lead and a manual tuner, what advantage does cutting two antennas give him vs tuning one 80 meter dipole?

    Especially at 20 feet.

    Get them up to 100 feet, with two resonant antennas and proper coax and it might make a difference.

    Even still, twin lead let alone ladder line is pretty low losses matched or unmatched at 3.5-7MHz

    I'm not sure I understand the advice.
    Charley K1DNR

  8. #8
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    Quote Originally Posted by AE2CS View Post
    If he doesn't have coax and he is using twin lead and a manual tuner, what advantage does cutting two antennas give him vs tuning one 80 meter dipole?

    Especially at 20 feet.

    Get them up to 100 feet, with two resonant antennas and proper coax and it might make a difference.

    Even still, twin lead let alone ladder line is pretty low losses matched or unmatched at 3.5-7MHz

    I'm not sure I understand the advice.
    A more predictable pattern. An 80 meter dipole on 40 meters has some pretty sharp nulls. This may or may not be a problem...but if they happen to lie in the wrong direction it could be pretty obnoxious!

    Eric
    "The more you know, the less you don't know."

  9. #9

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    I started to post the antenna wouldn't be very directive on 40 meters at a height of 20 feet. Fortunately, I modeled it before posting that misinformation. The 135' dipole will have nulls 30 dB deep off the ends (at 15 degrees elevation) on 40 meters. With a 67' dipole the nulls are only 5 dB. So, a separate dipole for 40 meters would give wider coverage on that band.

    A fan dipole may give similar coverage and has the advantage of a single feed line, but I haven't modeled that.

    Jim

  10. #10

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    You mentioned a fan dipole with 125' and 102' in the original post for this thread. If you erect fan dipoles, I think you would be happier a half wavelength for each band - about 135' for 80 meters and 67' for 40 meters.

    Jim

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