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OCFD noise at higher elevation (+tuning Q) — newbie disappointment... ;-/

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AI7DR, Feb 24, 2021.

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  1. AI7DR

    AI7DR XML Subscriber QRZ Page

    Hi All,

    Finally built myself a 33’ tilt tower (which I am kind of proud of!) and hang one end of my OCFD from a tree branch at 15’ to that height.

    The result is that I am picking up much more noise — my radio S-meter used to show -120dB floor, now it is more like -100dB, if not -95!

    Of course, when I pick up a station, what used to be S7, is now +10, sometimes going to something crazy like full scale +50, and, if I find someone to make a QSO, I get very favorable reports, 59s, once even +10 too, at my barefoot 20W.

    But I hear much fewer stations now! Working Japan in the afternoon used to be routine from my QTH in WA, now I almost never hear them (granted, it hangs at slightly different E-W angle, so, maybe, that was the luck of its previous orientation). There are just too few station at the dial now... :(

    Is that much of a change to be expected? I understand that I pick more signals, and they add to noise, but is not it a bit too excessive?

    Yes, now it passes over the roof of my house, at before it was maybe 30’ to the side, but I could not find much noise sources inside before, and live in a very quiet these days neighborhood...

    I guess I need to continue along my plan to put a beam on top of that tower, but still, this dipole has served me so well when hanging so low! ;-/

    Also, a tuning question: it is designed for 80m and higher, now at 80m it is too long, on 40 dead on, and on 20/17/10 too short! It is homebrew classic 1/3 design OCF with 4:1 balun, I have a degree of freedom to tune two branches independently, can I use that to make all bands better centered? If not, I would sacrifice 80m to center the higher ones, but is there a better game I can play there?


    Paul B. AI7DR
  2. SM0GLD

    SM0GLD Ham Member QRZ Page

    Make it longer and 40m will be slightly low and the higher bands may be acceptable.
    Now 80m will be very low but you can fix that by inserting a capacitor loading at the center of the antenna. This will move 80m resonance up in frequency.
    The capacitor on my OCFD was about 400pF.

    Moving a tuned OCFD to a higher position will change.....everything.
    AI7DR likes this.
  3. AI7DR

    AI7DR XML Subscriber QRZ Page

    Thanks for the shunting idea! Let me see if I understand it correctly: capacitor across feedpoint will mostly affect (trim extra L) 80m, since there is the only one resonance on the line at 80, and for higher bands it’s effect will be averaged down over other nodes available? Right?

    Now, where I live, running to electronics store is not an option, and DigiKey for just one cap is a bit silly , can I use a piece of coax instead? Hmm...

    Thanks again,

    Paul B.
  4. W4EAE

    W4EAE XML Subscriber QRZ Page

    I feel your pain. I also have an 80m OCF dipole.

    Tune and test with the antenna in place. As mentioned by SM0GLD, tuning while it is low than then raising pretty much guarantees it will be out when you raise it.

    As to the noise, I solved that problem with this:

    It is cheap, dead simple, out of the way, and works like a champ. If your rig has a separate connection for a receive antenna, its a no-brainer. Mine does not, but I have been quite successful switching between RX and TX with one hand and an appropriately mounted coax switch. It has become second nature.
    AI7DR likes this.
  5. SM0GLD

    SM0GLD Ham Member QRZ Page

    No, not shunting.
    A series connected capacitor at the center of the antenna wire (dipole total length /2)
    This will compensate for the inductive reactance at 80m.

    For the noise problem: Do you have a 1:1 choke in series with the 4:1?
    AI7DR likes this.
  6. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Turn off your preamp or turn down the RF gain in situations like this.
    AI7DR likes this.
  7. N3PM

    N3PM Ham Member QRZ Page

    Mike N3PM
  8. AI7DR

    AI7DR XML Subscriber QRZ Page

    Got it Re: C! Now I understand your proposal better!

    As to the choke, after 4:1 balun I made about 6 turns of coax on a FT240-31 ferrite ring, just in case, was thinking of adding the same at the shack entrance too... Does it really help though?
  9. AI7DR

    AI7DR XML Subscriber QRZ Page

    Already did, at least to have consistent S-readings! ;) But I do not think it did much to the variety of stations I can hear...

    Having a separate listening antenna, as suggested by others, also crossed my mind, but before going these, I wanted to know if what I am seeing is real and common — sounds like it is! ;-/
  10. SM0GLD

    SM0GLD Ham Member QRZ Page

    This is a good approach. I also had to put an extra choke further down the line.
    You now, OCF dipoles are unbalanced witch can create problems.
    Last edited: Feb 24, 2021
    AI7DR likes this.

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