ad: Flexradio-1

Trapped Tubing Dipole for 10m/6m Apartment Array

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB6LWN, Nov 20, 2021.

ad: L-HROutlet
ad: l-rl
ad: Left-3
ad: Left-2
ad: L-MFJ
ad: L-Geochron
ad: abrind-2
  1. KB6LWN

    KB6LWN Ham Member QRZ Page

    Not much room to put an array, plus I'm on the ground floor, no space to the rear of complex.
    About the only place I have to put an array is under the outside ledge on the upper floor walkway.
    So I'm not expecting any great working pattern, and no doubt it will suffer massively in the winter when the redwood walkway is soaked, but anything would be better than what I would be able to erect indoors, and if I'm not mistaken the larger the conductor - the wider the bandwidth (which would be a good thing as I work phone and digital modes).

    I can use Plexiglas on some other insulator for mounting (hanging) supports along the length of the radiator. And I won't be feeding at any more than 100 watts PEP. There is about 26-27ft of horizontal space for the array.
    And I doubt there will be much EMF radiation impact, unless someone is in physical contact during transmit.
    I'm thinking 3/4" to 1" diameter element, and since I'm going to be supporting the length by hanging insulators, and don't plan on being able to steer the array, the center support should not need to be so hefty - just enough to hold the balun and the connectors solid, I'm thinking a solid plexiglas rod inside the elements... Also for lower visual impact, as it will be mounted above the central mailboxes where everyone walks. I don't want to scare anyone with big chunks of metal plate above their head. <see attached photo>

    Screenshot from 2021-11-19 15-10-00.png

    There is a recess of about 3" under the front edge of the decking, and I suppose it wouldn't be too much if the array hung down another 3 inches below the header-board of the walkway. The balun could be mounted up (vertically) between the walkway beams and be basically out of sight, and thereby keep the feed-line out of basic sight as well...

    I understand I'll be bidirectionally challenged as the building itself is offset about 15° front facing True South, but at least I'll be back OTA with a substantial radiator in the air, and should have SOME gain, even though the pattern is going to be skewed all over the place and ERP will be greatly reduced being that close (6") to structure, not to mention the tree on the East end of the array. IMHO, if I can keep the array nice and shiny it might look kind of cool...
    Have a 12th and 65th Edition of THE HANDBOOK as reference, so pointing to formulas there is OK, been a LONG time since I've done any brewing...

    Thanks in Advance for input, and especially references to something along these lines that might be in production and available in kit form already.

    73 - Bruce
  2. KB6LWN

    KB6LWN Ham Member QRZ Page

    Make that a 33rd Edition, not a 12th Edition...
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    At first glance I'd probably try to hide a vertical dipole in that pine tree with the coax feeder running straight to the railing and then snaked into your apartment with some good common mode choking along the coax run. You could build it as a fan dipole with 10m and 6m elements made from wire and have a very stealthy antenna that would likely out perform a horizontal antenna strung to the underside of the balcony. A 10m/6m fan dipole is only roughly 16 feet long and you'd maximize your SWR bandwidth on 10m by running a full length dipole on 10m that doesn't rely on traps.

    If you're dead set on a horizontal antenna under the railing then I'd still consider a fan dipole sort of arrangement with wire elements that will be a lot stealthier than a trapped antenna built out of tubing. The best reason to run tubing is so you can rotate the antenna but that doesn't seem like an option in your situation.
  4. AK5B

    AK5B Ham Member QRZ Page

    FWIW, I also second the nomination for a stealthy vertical dipole in the cedar or pine tree off to the side. Take care to run the coax perpendicular to the elements a ways and be sure to include a FT240-31 RG400-wound CMC at or very near the feedpoint. I sand my RG400 so flat paint will adhere better, too---not much sticks to Teflon.

    At such low heights and close to buildings I wouldn't worry about directionality much; just be glad you can get an antenna outside to get on the air. December is a good month for 10/6m Es, too. ;)


  5. KB6LWN

    KB6LWN Ham Member QRZ Page

    Thanks Jeff and David,

    Unfortunately, if I did something like that in the tree, I'd be out of here on my ear.
    Ideally would be topping the tree, lashing a mast to it that extends a decade from the top and mount the vertical array I have in storage on that, but I can't take the chance of losing my welcome here...
    I've been homeless for about a year now, and living in an RV on my burned out property ever since the fire in 2015. I need to get my feet under me solidly again before I can take chances like that. It was only through Grace and assist from VA that I got into this place and I need to watch my P's and Q's...

    Yes, Jeff... Getting an antenna outside and OTA is the goal.
    Had I done better in the draw, I would have had the top floor Apartment,
    but I'm just feeling lucky to have a roof and places to S, S, S, & Sleep..

    David, you mention a wire fan dipole. Am I incorrect that normally there is a wider bandwidth with the larger element ?
    Like I said, it's been a long time since I brewed anything...
    Like what you said about the prop in December... Lost all my MilRadios in the fire and I've seen a couple old VHF-Low RT's online that I'd like to get in operation on the bottom of 6m. And I did manage to save my TS-140s, so if it's still operational I have AC Pwr now to run that... All thee time I was in my RV I was on Solar... Not enough Umph for the 140...

    But I'll look up the fan dipole and see if that would be a possibility...

    Just looked up L/C traps in The Book, so I'm still looking for input on the trapped array.
    Also thought about a set of 2m traps in the design, but I think I'm getting into too much loss at that point, and IIRC the higher in freq one goes - the more finicky the design and tuning gets for traps.
    Maybe a ground level Co-linear coaxial stick in the corner of a room for that.
  6. KF4ZGZ

    KF4ZGZ Ham Member QRZ Page

    The wider bandwith with a larger element thing is true to some extent.
    That extent is INCHES.
    In order to get a usable wider bandwith in a wideband dipole you would typically build a cage dipole with elements that are seen by the antenna as 6"-8" in diameter. Anything else, just build a wire dipole and use a tuner.
    That looks to be a fence behind the pine tree, might be a good candidate for an endfed.
    Would you be lucky enough to be on the end?
    W4HMB and AK5B like this.
  7. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    A 10/6 meter band aluminum tubing dipole can be built as two coupled dipoles, no traps needed. I can post a parts list.

    16.0' x 0.75" (plus smaller for the telescoping end pieces) aluminum tubing dipole fed via coax
    8.2' x 0.75" or smaller diameter aluminum spaced tubing 4" away for 6 meters

    It looks good in EZNEC. DX Engineering has the parts to build it; Tubing, center insulator, plastic sheets.

    Center insulators that would work:

    Plastic mounting plates: plate

    Here is the SWR with it mounted 6' above medium ground (0.005S/13) and in the clear.

    Last edited: Nov 20, 2021
    AK5B likes this.
  8. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    Aluminum tubing for the deluxe no tubing cutting version is below. Some money can be saved by cutting two 72" tubes in half for the four 36" tubes. More can be saved by using the less expensive non-slit tubes and slitting them with a hacksaw. Spacing the 6 meter band element can be accomplished with plastic plates or by simply hanging element a few inches below the 10 meter band element using string.

    The two 72" x 0.875" tubes form the center of the 10 meter dipole. 36" x 0.750 tubes telescope into the ends and are adjusted for lowest SWR. Hose clamps secure the tubes. The 6 meter band dipole is open-sleeve coupled to the 10 meter element and is not split in the center for a feedline. The 6 meter band element uses one 72" x 0.500" tube for the center and two telescoping 36" x 0.375" tubes for adjustment.

    Qty 2, 72" x 0.875 OD, 0.058 wall, slit each end, DXEAT1208, $23.49 each
    QTY 2, 36" x 0.750 OD, 0.058 wall, no slit, DXE-AT1496, $10.99 each
    QTY 1, 72" x 0.500 OD, 0.058 wall, slit each end, DXE-AT
    1205, $18.49
    QTY 2, 36" x 0.375 OD, 0.058 wall, no slit, DXE-AT1240, $7.99

    Feedline choke:
    The feedline common-mode choke can take many forms and this is sure to start a discussion of the "best" ferrite material to use. As DX Engineering stocks only type 31 material in this size that is what the choke uses. A simple choke can be constructed using ten DXE-CSB-31-275-10 snap-on cores slipped over RG-8X or RG-58 coax at the dipole feedpoint. $31.99 for ten. This part appears to be this Fair-Rite part which is ~190 ohms at 28MHz and ~230 ohms at 50MHz. Ten cores form a ~2k ohm choke.
    Last edited: Nov 20, 2021
    AK5B likes this.
  9. K7TRF

    K7TRF Premium Subscriber QRZ Page

    A larger diameter element does give somewhat increased SWR bandwidth if all else is equal but when comparing a fan (or parasitically coupled as suggested above) dipole vs a trapped dipole made with larger diameter tubing you'll get wider bandwidth with the former. Trapping means coil loading the lower frequency element which substantially reduces the SWR bandwidth. So sure the trapped tubing version uses tubing but more importantly it's trapped and the lower band is shortened compared to the fan dipole and that shortening of the lower frequency band will cost you more SWR bandwidth than the larger diameter tubing will give you back.

    IOW, a full length dipole will just about always have a wider SWR bandwidth than a trapped dipole even if the trapped version is made with a larger diameter conductor.
    Last edited: Nov 20, 2021
  10. RW4HFN

    RW4HFN Ham Member QRZ Page

Share This Page