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What keeps twin lead from becoming radiators?

Discussion in 'Ham Radio Discussions' started by KE7ZNV, Jan 14, 2022 at 6:42 PM.

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  1. KE7ZNV

    KE7ZNV Ham Member QRZ Page

    I feel like I'm either missing, or forgetting, something stupid simple here, and I just can't think of the correct words to Google to figure this out...

    I've set up an OCFD out in the back yard as a strictly RX only antenna for SWL/HF. The plan was to connect it through a 4:1 balun to coax, then coax all the way back to the house and receiver. I understand that the balun is what helps to curtail common mode currents on the coax due to the impedance mismatch, thus keeps the coax from becoming part of the radiating elements as well as curtailing noise pickup on the coax.

    But then I've read things where it's said that ideally, coax shouldn't be used at all on antennas like dipoles, long wire, random wire, etc., and that the twin lead or ladder line should be used as a feedline all the way back to the receiver or transmitter. Which I also think I understand as keeping impedance mismatches in check by doing so.

    But what then keeps the twin lead or ladder line from appearing to be part of the antenna and then making the dipole MUCH longer/larger? I understand that my OCFD set up is naturally an unbalanced antenna, and I think that's where I would be using a 1:1 balun? But if I were to make it center fed dipole, that would then be a balanced antenna, fed by balanced twin lead or ladder line. I don't think I would be using a balun, or an unun here, and I can't recall ever seeing anything listed as a balbal...what then keeps the feedline from just appearing to be an extension of the dipole?
  2. G0TNU

    G0TNU Ham Member QRZ Page

    KS2G and KA4DPO like this.
  3. N0NC

    N0NC Premium Subscriber QRZ Page

    W4II, KS2G and KA4DPO like this.
  4. NQ6M

    NQ6M Premium Subscriber QRZ Page

    Edited- See articles above
  5. KE7ZNV

    KE7ZNV Ham Member QRZ Page

    Oh...duh..."balanced". I KNEW I was missing something stupid simple, lol :facepalm:. Thanks guys! ;)
  6. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    I have several stealth antennas, so ladder line is my feedline of choice. I also use an Icom AH4 on one particular antenna, and it works great, but I limit power to 80 watts CW just to be certain that I don't push it. There is not a discernable difference between 80 and 100 watts at the receiving end anyway.

    I use 300 ohm window line since it is much smaller and less visible than 450 ohm ladder line. It can also handle a kilowatt of power if it is properly matched, since I never run more than 100 watts, it really doesn't matter anyway.

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