Good 40M DX antenna?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KD0KZE, Jan 6, 2016.

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  1. KD0KZE

    KD0KZE Ham Member QRZ Page

    Currently I'm using a G5RV (w/TS-590) and achieve reasonable DX on 20m when conditions are good. But 40m behaves asymmetrically. When I leave my rig on for RX at night with WSJT-X JT65, it can hear a lot of stations -- dozens in Europe, South America, Asia, Africa, obscure islands, etc. However, I can barely TX 40m outside of my hemisphere. The G5RV at about 28' causes 40m behavior to feel more like NVIS. It'll work all of North America, down to Central America, and only very sporadically to Japan or Europe.

    I am able to receive much better than I can transmit DX, based on what I see on I'm thinking that my elevation angle is too high, and the signal passes right out into space.

    So what's a good 40m antenna? How does a dedicated 40m dipole compare with a beam or vertical? I've poked around a bit here, and searched some reviews over at eham, but I don't see a clear winner yet. Most of what I see are either "compromise" verticals, multi-band dipoles, or expensive tower-mounted antennas.

    73, KD0KZE / Paul
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Higher is better for a G5RV or any kind of horizontal dipole, inverted vee, loop, etc.

    28' is too low for "much" DX on 40m. The same antenna, but up at 60' above ground, will improve your DXing performance dramatically without really changing anything other than its height. "Dramatically" only has to be 3-6 dB in many cases, especially on CW or digi modes where a signal 3 dB over the noise is pretty easy copy.

    Also, of course if you're hearing them but they're not hearing you, an amplifier will help. Only time one doesn't help is when you're also not hearing them.
    WB5YUZ likes this.
  3. KD6RF

    KD6RF Premium Subscriber QRZ Page

    As said above, 26 ft is not quite high enough for decent DX on 40 M.

    That's why many use the good ol' vertical or inverted L on the lower bands where a properly radialed system will typically outperform the low dipole for DX. That's not a "compromise" when compared to dipole-like antennas of the same height.
    W9IS and WB5YUZ like this.
  4. W1VT

    W1VT Ham Member QRZ Page

    I've been thinking of that myself--I may be able to squeeze a 40M 4 square with elevated radials close to the house--and use another set of receive antennas for diversity reception.

    There are some good articles on the YCCC web site.

    Zack W1VT
  5. K8JD

    K8JD Ham Member QRZ Page

    The 40M DX Antenna Choice
    first off the G5RV is not a 40M antenna, it just "kind of works on 40M" .
    The best SIMPLE DX antenna would be a quarterwave vertical mounted high on a roof with s few radials,
    second best is ground mounted with a LOT of radials.
    Best is a HIGH Yagi but that involves a lot of money and work to get it high and keep it there in stormy weather !
    I have worked 40M with Dipoles and Verticals, and even went to a horizontal Fullwave loop as an ANTI DX antenna, when Radio Free Europe, Radio Moscow and the BBC were waging the cold war on 40M with multi hundred Kilowatt Broadcast stations. I wanted to work US novice stations and the low flat loop brought down the European QRM and brought the US novices up out of the noise!
    Last edited: Jan 6, 2016
    W8ZNX, WA8FOZ and K5VV like this.
  6. AK5B

    AK5B Ham Member QRZ Page

    Look into building a small transmitting loop for 40M; while a properly built one won't be inexpensive you'll get both low-angle and NVIS radiation from a 4-6-foot loop only a few feet above ground. If done right with a continuous single radiating element (around 3/4" or more diameter copper for low I2R losses), it'll be only 6db down from a 3-element Yagi 65-feet above ground, too---imagine that!

    K8NDS just finished building his latest experiement; an 8.5-foot helically loaded fractional wave loop and he's already getting fantastic results on 160, 80 and 40 meters with it. (check out his Helically Loaded Magnetic Loop Group on Yahoo for more details including bandwidth on the various bands if interested)

    Btw, my comments apply to ones vertically polarized---but if you have a tower they also perform best half a wavelength above ground or more---snd they'll be omnidirectional with no nulls to take advantage of. Given such decent performance constructed and placed as a vertical with the benefit of some directionality (maybe 20-22db according to Rich's findings) the choice of orientation is a no-brainer. Before the usual naysayers pipe up about inefficiency and so on---his efficiency on 40 meters is close to 90% with his 8.5-footer made with 3" copper flashing wrapped over a 3.5" PVC form.

    My next antenna project is going to one similar to this (two-turn copper pipe or Heliax loop about 5-6 feet in diameter) for 40/80 meters and I'll start a thread here once I get started with the actual build.

    Food for thought in the meantime.

    73, Jeff
    Last edited: Jan 6, 2016
  7. K5VV

    K5VV Ham Member QRZ Page

    Key for DX is high gain at low takeoff angle. I agree with K8JD. Vertical is better if height above ground is less than 1/2 wavelength (~65').

    WB5YUZ likes this.
  8. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    I think the OP will find there is more consensus on this than there is on most topics.

    Below: unless I've made an error, here are the radiation patterns of a simple 40m dipole at 33, 45, 54, and 66 feet over "good" ground. Note that the greatest increment of improvement in low-angle performace comes in the last 12 feet. Also note that, if this model is accurate, there will not be much improvement between 33 and 45 feet, but significant improvement between 33 and 54 feet; in other words, if OP's existing antenna can be raised to 54 feet, he should see a significant improvement in performance, but raising the antenna to only 35, 40, or 45 feet is unlikely to bring much change.

    Last edited: Jan 6, 2016
    W4MMC likes this.
  9. KD0KZE

    KD0KZE Ham Member QRZ Page

    This is basically what I figured. Did you model this based on working 40m on a G5RV, or a "dedicated" specially-cut 40m dipole?

    Here are my parameters:
    I have about 30' height and 100' linear distance to work with.
    Could probably set up a ~20-30' vertical this spring.
    Tower is currently out of the question (time, space & $$$).

    So the consensus I'm seeing is that there's probably no point in taking down my G5RV for a couple months to work 40m with a dedicated (measured & cut) 40m dipole at the same height. Height is the real bottleneck here, not the declined efficiency due to the G5RV's compromise length -- though this doesn't help either.

    I've had good luck with my Comet GP-3 for basic VHF/UHF work, and see they make an HF vertical (CHA250B). Anyone have experience with those on 40M, or might they recommend another brand for a smallish vertical?

    My time is also rather limited (rat race), so I may not be able to tackle a DIY/build until I quit. retire or get outsourced. So is there an out-of-box mag loop that works reasonably well for 40m DX? I've seen some reviews of the MFJ-1786, but hadn't looked at it too carefully in the past since I'm not in a covenant/restricted area -- and that seems to be their main demographic. But would it provide lower-elevation at 40m for those of us who can't get a dipole up to proper 40m DX height?

    73, KD0KZE / Paul

  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If the model is accurate, at 30 degrees elevation angle the difference between the high dipole and the low one is 3 dB; but at 15 degrees, where a lot of the "DX" actually is, the difference is about 6 dB.

    That's a lot of difference.

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