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Suggest a multiband antenna for 75, 40, 20m.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA7ARK, Aug 9, 2018.

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  1. WA7ARK

    WA7ARK Ham Member QRZ Page

    My wife and I are building a getaway house up North to get away from the AZ summer heat. I put up a Buckmaster OCF that I bought at a hamfest for $10 (sure glad I didn't pay the new price). It sort-of works, but I am looking for something better. I will explain the "sort-of"...

    Here is a picture of the existing installation, looking West.
    OCFa.png
    The line to the right (North) is just a guy rope. The long leg of the OCF from ESE is in the foreground; the short leg runs WSW. The apex is 39ft agl. The angle of the support ropes puts the short end about 20ft agl and the long end about 15ft agl.

    The vertical support is a light-duty TV antenna tower about 25ft tall, with an aluminum "painters-pole" at the top, for a total height of 39ft. The pole requires guying, so the OCF is a bit out of plane and pulls the tip of the pole to the South which is offset by the single rope guy pulling North.

    Now for the "sort-of" comment: I first put it up by connecting the 100ft RG8 feedline directly to the Buckmaster OCF transformer. The SWR on various HF bands looked good (on my RigExpert AA-600). Transmitting at 100W works ok, but when I used ~800W, all hell broke loose.

    First, I tripped 4 of the fifteen AFCI breakers in the house main electrical panel (including the one powering the 100W rig). I also set off the wired smoke alarms in the house. The manufactured house is new, build to latest electrical codes which require that most of the breakers be AFCIs. Code also requires cross-wired smoke alarms.

    It occurred to me that the OCF has lots of common-mode current on the feedline, which brings the RF into the house, so I put a 100bead common mode choke right below the OCF transformer. First I re-measured the SWR. I was a bit surprised to find that the SWR on all bands, and especially on 3700 MHz, got a lot worse, so much so that I now had to use an antenna tuner on 75m and 40m. 20m and 17m would still work without a tuner, but the SWR increased there, too.

    With the CM choke in the feedline, at 800W I wasn't tripping the AFCIs, but it still caused the smoke alarm closest to the end of the house where the tower is to go off, which I can live with by removing that one alarm (there are still four others active in the house).

    However, this does not put the Buckmaster OCF in a good light. The #$%$#$ thing relies on the feedline radiation (aka common-mode current on the feedline) to achieve an acceptable SWR on 75 and 40m. Blocking that by putting a good CM choke right below its transformer radically changes the feed point impedance, and a tuner is required.

    So, to improve my system, I want to get rid of the OCF and its common-mode current problems and replace it with symmetric wire antennas.

    Here are the requirements:
    1. Cover 75m, 40m, 20m and possibly 17m.
    2. Retain the existing 39ft center-support.
    3. The antenna wires should act as guys for the vertical pole (ie. run in either three or four directions)
    4. Coax feed, no tuner required on 40, 20, and 17m. Tuner ok on 75m.
    5. The 40m antenna should have best radiation due South; dont care about the other bands.
    6. Minimize common-mode current on the feedline, by antenna symmetry and CM choke.

    I'm thinking two inverted-Vs, about 90degrees apart, sharing one feedline and CM choke, one for 40m (my primary band), and the other for 75m/20m with 20m parallel-tuned traps. Any other suggestions?
     
  2. WN1MB

    WN1MB Ham Member QRZ Page

    Before any antenna suggestions get entertained...

    ...tell us all about your grounding system.
     
    KK7NAZ and KJ4ADN like this.
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    How about something like a Hustler 6BTV with elevated radials up on top of that tower?
     
    K9ASE and NH7RO like this.
  4. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Expectations vs. reality. Regardless of the advertising, an OCF is not going to provide lows (<2:1 SWR) impedance on all the ham bands. An OCF offers lower-but-easily matched impedance on several bands, and may have higher impedance on the non-harmonically related bands. You still need a tuner to obtain optimum matching.

    If a single antenna presents low SWR on all or most bands that is probably an indication it is lossy and inefficient. Low SWR is not proof of a efficient antenna. Unfortunately many hams fall for enthusiastic advertising (and always have).

    If the antenna meets your installation and operating needs, then get a tuner and be happy. If not get a true multiband antenna like one with band traps or a fan-type. Using a tuner is not difficult, nor indication of a bad antenna system. b.
     
    K0OKS, WY6H and NH7RO like this.
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    Good farmland, about 6in of black topsoil on clay below, quite wet.

    Tower stands on 36" steel base legs with concrete around the three legs, about 2ft South of the South end of house.
    One 8ft copper-plated steel ground rod within 12in of the tower base, mainly for lightning protection. Ground rod is bonded (#6awg copper) to tower, coax shield, and using less than 2ft of wire to the 16ftx64ft steel beam frame that runs the full length of the manufactured home.

    The electrical service panel about 40ft North of the tower, and is bonded to the house frame (by the manufacturer) and to a buried main grounding plate under the house, per code.

    The hamshack is at the North end of the house, about 65ft from the tower. The radio, tuner, and amp are "grounded" with about five ft of #10AWG solid copper to the steel beam frame under the house. The shield of the coax that runs between the rig and tower are effectively "grounded" at two places; once to the frame at the North end of the house, and a second time at the base of the tower. The coax runs under the full length of the house, laying on the dry gravel underneath the house for the full 64ft length of the house, coming up through the floor to attach to the rig, and under the house skirting to the tower base.
     
    WN1MB, WY6H and NH7RO like this.
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    Not helpful...

    I had no expectations; I only paid $10 for the damn thing...

    It worked as advertised (without needing a tuner) UNTIL I put a common-mode choke under its feed point, which tells me that feedline radiation is part of its design, or that Buckmaster doesn't understand the bad effects of common-mode currents on the feedline.
     
    Last edited: Aug 9, 2018
  7. NH7RO

    NH7RO Ham Member QRZ Page

    HyGain HyTower and a gazillion radials; looks like the near-perfect QTH for Marconi fans.

    Speaking of fans, why not a splayed fan dipole for something quick, easy and only requires one coax?

    Whatever you do it looks like big fun to me!

    73,

    Jeff
     
    Last edited: Aug 9, 2018
    VK2DMH, WZ7U and N4FZ like this.
  8. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    DX Engineering comments on this antenna:
    "These antennas are fed off-center with one leg 90 feet in length and the other is 45 feet in length. An antenna tuner may be required for complete all-band functionality at the absolute lowest SWR."
    https://www.dxengineering.com/parts/bmt-dx-ocf-hp
     
  9. W5DXP

    W5DXP Ham Member QRZ Page

    I don't use tuners and here's my antenna for 75m, 40m, 20m, 17m, 12m, and 10m. The series capacitor really works well on 75m and can be left in the system for the other bands.

    http://www.w5dxp.com/ZS6BKW80/ZS6BKW80.HTM

    [​IMG]
     
  10. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Yes, a close reading of the Buckmaster Instruction Sheet states it uses an 'Autotransformer" for impedance matching. This will not prevent feedline radiation like a Current Type Balun would. No surprise for better educated hams who are familiar with Balun types and how they function.
    https://static.dxengineering.com/global/images/instructions/bmt-dx-ocf.pdf

    -A learning point about antennas and baluns/transformers.

    Is this antenna a bad antenna? "No". It fits the needs of hams who want an All Band antenna that provides a <2:1 SWR without needing a tuner. Tradeoffs: It radiates from the feedline; may cause RFI in the shack, and doesn't necessarily have a true dipole Figure 8 radiation pattern. It is easy to operate, and doesn't require the expense of an external tuner. shrug.
     
    Last edited: Aug 9, 2018

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