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

    I have limited room to install my first HF antenna, and as the garden is my XYL's pride and joy I cannot deface it with towers, beams, guy ropes or radials :(.

    I'm trying to decide whether to go with a Windom antenna (25 +41 feet) or a 53ft random wire with 9:1 unun.

    The antenna will only be 12 feet above ground (don't laugh, it's the best I can do). Which one should I go for?

    Mike
    G8GYW
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Remember that the 9:1 random length end fed requires radials which might exclude it from the running. An end fed half wave antenna with a 49:1 or 64:1 matching transformer cut for 40m as its lowest band could work and only requires a very short piece of counterpoise wire as in something roughly three and a half feet long but the main element stretches from 53' to roughly 65' but that's not much different than your OCFD plan.
     
  3. N3PM

    N3PM Ham Member QRZ Page

    The location of the antenna's feedpoint may have a bearing on your choice.
    Mike N3PM
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Mounted that low, your proposed OCF dipole fed with coax has a low swr only on 40m. Its feedpoint impedance is about 60 Ohms, so just use 50 Ohm coax and a CommonModeChoke (no transformer). The next best band is 20m, where the feedpoint impedance at 14.2MHz is 176-j160 Ohms, for an swr50 of 6.5. The higher bands are hopeless.

    I played with the wire length on either side of the feedpoint, and changed the coax to RG59 and came up with the following combination:
    Make the short wire 23ft, make the long wire 47.7ft, and make the coax feedline RG59/U exactly 40.3ft long. That makes the SWR50 looking into the 75 OHm coax on 7 and 14MHz as shown below, likely the two most useful bands at the current state of the Solar cycle.

    G8.png

    If you need to lengthen the feedline, add RG58 or 8 coax, but do not shorten the RG59. To minimize RFI from inside your house getting into your receiver, I still would be adding a CMC somewhere along the coax.
     
    WB5YUZ and K0UO like this.
  5. N8AFT

    N8AFT Subscriber QRZ Page

    9:1 and long wire
    counterpoise wire is important...
     
    K0UO likes this.
  6. G8GYW

    G8GYW Ham Member QRZ Page

    Thanks everyone, that's all really helpful advice. The feedpoint for eaither antenna will be near the house, with 25 feet of RG58 to the shack. If I feed the far end of the random wire it would be 100 feet which I think is too long and I don't want to use heavier cable.

    If I go with the random wire I was going to try using the coax shield as the counterpoise with a CMC at the entrance to the shack (and there's another limitation, it's upstairs in the house so no easy ground).

    Mike G8GYW
     
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    With your added caveats, I would definitely look into an EFHW-4010 (Google it). It is quite easy to homebrew one using a self-made 1:49 transformer. Write back if you want help with that.
     
    WA8FOZ and G5TM like this.
  8. G8GYW

    G8GYW Ham Member QRZ Page

    Yes that looks like something I could make, although I see that a ground is recommended so I'd need to make it as an inverted L. Would a ground stake be sufficient?

    Mike G8GYW
     
    WB5YUZ likes this.
  9. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Given your height limitations, that is probably the best thing you can do!
     
  10. G8GYW

    G8GYW Ham Member QRZ Page

    So is an EFHW with 49:1 unun better than an end fed random wire with 9:1 unun?
     
    G5TM likes this.

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