Help me decide - OCF dipole or random wire?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by G8GYW, Oct 1, 2019.

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  1. G8GYW

    G8GYW Ham Member QRZ Page

    OK, here's a sketch, I hope it's self explanatory.

    Coax Y drops vertically from the unun for about 1.5m then runs horizontally to the shack.

    I could run a wire from the shack end of the coax to ground but it would be about 6m long. A ground wire from the unun would be 5m.

  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    NEC requires you to input the (X,Y,Z) coordinates of each wire end, and here is my best interpretation of your layout. I am using a total of 6 wires, W1 from Pergola to Summer House and this is the one I vary to bring the system to resonance. W2 from Pergola to transformer on house. W3 is the coax from the transformer to the CMC, and it is also varied. Right below the CMC a down-wire W4 drops to a ground rod. W5 is coax running over to the shack. W6 is another down wire to earth below to simulate an incidental path to earth through your ham rig and its power supply.

    I set up the optimizer in AutoEz to find a length of W1 and W3 that minimizes SWR50 at 7.1MHz looking into the primary of a simulated 50:2450 Ohm Autotransformer. W3 ended up only 1.1m long, while the total wire length of W1+W2 is 20.1m. So far, I have not added the components in the long wire that would make this antenna resonate in the 20m, 15m and 10m at the same time it resonates on 7.15MHz.

    Here are the details:
    Row 11 is not a real wire. It is a construction wire from which I derive the real W1.

    The pink lines show the relative magnitude of the current that flows in each wire with a drive level of 100W. Note that the currents on W3 to W6 are quite small compared to W1+W2, as you want them to be...

    This is the detail of the current distribution. W1 is 40 segments, W2 is 100 segments, so the transformer is at seg 139. The CMC is at seg 149 (on coax W3). W4 is seg 150 to 165 and the rest of the wiring to the shack W5+W6 is seg 166 up. Again, notice that the peak current in the dipole is ~1.65A, while current in the transformer secondary is only ~0.2A.

    This shows the variation of Swr50, R, jX and gain across the 40m band. As you can imagine, the antenna is mounted too low to be great, but here are the patterns:


    I have optimized the antenna on 7.15MHz, without regard to aligning the swr minima on the higher bands. My suggestion is that you initially build it as described here, and after we see how close we come on 40m, we then "multiband" it with the addition of the compensating coil and/or capacitor. Make it so that you can easily lower the distal end of W1 so that it can be trimmed to length, and that is also where I will add the coil...

    I uploaded the .ez file for this antenna, but to overcome the limitation of QRZ.COM, I had to append a .txt to the file name (it is not a readable text file). To use it with EzNec, you will have to remove the .txt after downloading it.

    Attached Files:

    Last edited: Oct 8, 2019
  3. G8GYW

    G8GYW Ham Member QRZ Page

    That's great, thanks. I will put it together shortly and report back on SWR.

    I need clarification on grounding please. Where does W4 connect? Is it the coax shield on the shack side of the CMC?
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Yes, at the top. At the bottom, to a suitable ground rod. W4 can be the same kind of wire you make the antenna from. Not critical at all.
  5. G8GYW

    G8GYW Ham Member QRZ Page

    Thanks :)
  6. KA3CNC

    KA3CNC XML Subscriber QRZ Page

    Cant do a ground mounted vertical?
  7. G8GYW

    G8GYW Ham Member QRZ Page

    It needs to be as unobtrusive as possible because my wife has made our garden very attractive and she doesn't want it cluttered with poles.

    I'm starting with a wire and when she's got used to that I'll see what else I can get away with ;).
  8. G8GYW

    G8GYW Ham Member QRZ Page

    OK it stopped raining long enough for me to get the wire and unun up. The feeder and ground will have to wait until tomorrow.

    I've measured the VSWR at the unun. It doesn't look too bad for a first attempt.

  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    I'll take that Pilsner now....

    How long is the coax from the transformer to the VNA? Are you using a CMC?
  10. G8GYW

    G8GYW Ham Member QRZ Page

    You deserve it sir!

    The coax was just 1 metre. No CMC.

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