Help me visualize tree mounted dipole at new QTH

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KN4CQB, Jan 13, 2018.

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  1. KN4CQB

    KN4CQB XML Subscriber QRZ Page

    I am using an end fed antenna currently but about to move to a new QTH in March where I will have an opportunity to install a 20/40M dipole. I have searched every nook and cranny of the web but can't find what I need to understand. Please correct me where I am wrong or if there is a site on the web I have missed I would appreciate a link
    • Shack will be on the 2nd floor - so coax will come down to ground level
    • I want to bury the cable for about a 100 foot run to the woods behind the house as there are no other supports available until the tree line. I assume some type of direct burial cable
    • Once at the tree that will be the center support I will hoist the antenna as high as i can get it. I have other trees to use as the end point of the two wire runs.
    • Up to this point I think I see the picture....
    Now what I don't see (this is where I would love to find a video)
    • is the feed line just simply dropping back down to ground level from the tree to make its underground run back to the house? Is it attached to the tree for some stability before going underground? Using what?
    • I am confused between some feed lines being ladder and some regular coax
    • What is the best type of feed line for this long run and at what length do I need to be concerned about high loss?
    • What else am I missing (open ended question for sure!)
    Thanks for any help - I am still a serious HF Noob

    73 - Rich
    N8SMG likes this.
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    If the coax is well supported at the dipole feedpoint using a good and strong center insulator, it doesn't need any other support and can free hang down to ground level just fine -- even if the dipole is up 100' above ground. If you want to use the tree trunk for additional support, you can certainly do that, but I'd support it "loosely" so it's not tightly bound to the tree and the supports just keep the feedline from blowing around much in the wind. Running coax through eyehooks screwed into the tree at a couple of heights can accomplish this just fine and not damage the tree.
    If you're installing a resonant, center-fed dual-band dipole, no reason to use ladder line. I'd strongly recommend coax. You can't "bury" ladder line.
    The length you need is enough to reach from the station to the antenna; there's no particular length. I'd use direct burial RG-213 type cable. It's not heavy nor expensive, can handle serious power, and is available in "direct bury" composition. Loss in 150' or so is very little.
    I don't endorse using three trees to support wire antennas unless the antenna is a loop and actually needs three or more support points. If you can pick one "high as possible" support, you can make an inverted vee; make the included angle between the wires at least 90 degrees (or wider), use rope extensions and good insulators, and tie off the ends to whatever is available to create the angle between the wires -- and keep both wire ends at least 10' above ground so nobody can touch them unless they're a giraffe. If you want to make a "flattop" dipole and not an inverted vee, use just two support points, one at each end, and make one end of that spring or weight loaded so the dipole won't break when the wind blows.

    In all cases when using trees for wire antenna supports, leave some slack in the wires intentionally (don't try to make them taut) so tree swaying is less likely to break anything.

    And get it up as high above ground as you possibly can -- the higher, the better. On 40m a dipole starts really working well at about 65' elevation above ground.
    N8SMG, KN4CQB and K7TRF like this.
  3. AC1CX

    AC1CX Ham Member QRZ Page

    Unless you mount as an NVIS
    KN4CQB likes this.
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Good advice above.

    As Steve points out ladder line is most often used to feed non-resonant antennas which can be tuned across multiple bands with a good tuner in the shack. If you'll actually have an antenna that's resonant on 20m and 40m and will work those bands then it's easiest to just feed it with coax.

    Are you thinking about some version of a parallel dipole (aka fan dipole) or some sort of 40m/20m trapped dipole? Either can be set up as a standard dipole or an inverted-V with a high center support but will allow easy feed with coax for a multiband antenna. The parallel dipole should have a wider usable SWR bandwidth on both bands and if you have the horizontal space (around 65' between your end supports or a bit less in an inverted-V configuration).

    Basically your plan is good, bury the long horizontal run of coax and run it up to your antenna in the woods. Other things to think about could include how you intend to raise the center and/or end points of your antenna. Lot's of ways to launch a line over high tree branches but think about maintenance, wind sway of the trees as Steve pointed out and also tuning the antenna.

    For instance, you might consider raising a pulley at each end point and then running the support lines for your antenna through those pulleys. That allows you to hang counter weights at each end to allow for tree sway in the wind and also allows easy raising and lowering of the antenna both to make initial antenna tuning easy and to help with any down the road maintenance. Similar idea if you go with an inverted-V approach, raise a pulley to the high point and run a line from your feedpoint insulator(or balun) through the pulley so you can easily lower and raise the antenna.
    KN4CQB and NH7RO like this.
  5. W1GHD

    W1GHD XML Subscriber QRZ Page

    I wouldn't just let the cable drop to the ground; too easy for it to get damaged hanging out in the open. Let it drop straight down from the antenna for a bit, than pull it over to one of the trees with a rope and then bring it down along the tree. As to attaching rope to trees, use a "tensionless knot" on the tree. That is simply four wraps of rope around the tree, with the loose end of the rope tied off. With four wraps, there is no longer tension on the rope. This is very strong, and doesn't damage the tree or stress the rope.

    My dipole is up about 60 feet. I have fifty-foot length of RG8X hanging down from the antenna, then transition to DXE 400MAX which is direct buried. The RG8X is lighter and less visible than the larger cable. The 400MAX is overkill for HF, but it is rated for direct bury and not much more expensive than lesser cable.

    The 400MAX comes out the ground directly to my single point ground with PolyPhaser lightning protection, then into the shack. Here's a couple of photos. The antenna isn't really that close to the trees, it's just the angle from the ground causing the illusion.

    dipole.JPG ground.jpg
    N8SMG, KN4CQB and NH7RO like this.
  6. KN4CQB

    KN4CQB XML Subscriber QRZ Page

    Thank you all SO Much for the fast and informative reply. Clears up the questions I needed help with. I guess I should have said that this is a dual band dipole I will install as Inverted V .... did not mention that.

    Big 73

  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Even for NVIS.

    65' is not too high to produce a powerful NVIS signal on 40m, if NVIS happens to be working at 7 MHz at the time; it often doesn't. I've seen lots of times lately when the critical frequency is around 5 MHz, so it misses the 40m band.
  8. K2CQW

    K2CQW Ham Member QRZ Page

    How sweet, now I know what NVIS means thanks to Google & Wikipedia. More lost brain cells.
  9. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    If you plot the radiation angle from a 1/2-wave inverted vee on 40m at 10', 20', 30', 40', 50' and 60' you'll see the vertical radiation comes down at higher elevations but the vertical radiation is still very useful even at 60 feet.

    "Most" don't really want or need NVIS unless the primary objective is checking into a very local net or something.

    Also record the critical frequency for NVIS hour to hour and day to day, and it's not terribly useful on 40m. Much more useful on 60m and 80m.

    "NVIS" has become a ham radio buzzword over the past several years, but I have no idea why. I don't find it the slightest bit useful.:p
    N8SMG, NH7RO and K2CQW like this.
  10. K2CQW

    K2CQW Ham Member QRZ Page

    I'm sure I could have used those brain cells and electrons for something useful. harhar

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