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Is my impedance too high?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W3ESX, Jul 26, 2021.

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  1. W3ESX

    W3ESX Ham Member QRZ Page


    I have an Arrow Antennas OSJ 140/440 J-pole antenna mounted in my attic. I have never really obtained the range I would like out of it, I am using it at 50W with a Yaesu FT-7900 on FM. I can hit the repeater about 2 blocks away just fine but further out I just cannot hit, and I've received reports that fellow hams in the same area can hit those repeaters just fine.

    In the feedline I have a 5-loop manual Balun by wrapping the coax around 5 times and zip-tying it to secure it. However, I realized yesterday that I also have another humongous loop in the line where the excess coax is wrapped around a large plastic dogbone/cable wrap prior to plugging into the transceiver.

    My question is: with the cable looping around the cable wrap, have I effectively created another huge balun that could be strongly affecting my signal and range?

    P.S. I have attached a diagram showing the setup I have. My brain had a "moment" and I drew in the KB9VBR J-pole antenna and not the Arrow, but they are both J-poles so I figure that is unimportant.

    Any advice appreciated.

    73de Jamison W3ESX


  2. W1VT

    W1VT Ham Member QRZ Page

    At VHF and UHF the loss from excess coax can have a significant impact on signal strength.
    W3ESX likes this.
  3. K8JD

    K8JD Ham Member QRZ Page

    IF...the antenna is a good 50 Ohm load, The rolled up excess coax can not do anything more than add the usual loss you get from a long coax run on VHF, !

    Attic antennas sometimes work fine for HF, have worked ops with them across the US and Canada.

    On VHF and UHF they can be more affected by metal wiring, HVAC pipes and metal backing on insulation. Is your attic "Clean" ?
    You can get multiple reflections off small metal items that can cancel your radiated signal in multiple directions.
    A disguised antenna above the roof can make a big difference. I have seen ones that are made to look like a vent pipe that can fool the landlord :D
    Last edited: Jul 26, 2021
  4. W3ESX

    W3ESX Ham Member QRZ Page

    Based on what W1VT and others have said, I'm definitely going to be shortening the coax run and seeing if that helps. Hoping it will.

    As for if my attic is clean...not entirely sure, but probably not. I went and took a peek after your comment, and there is a run of Romex electric wire perpendicular to the antenna elements. And also, the antenna is mounted not too far from the central furnace shaft, which is metallic. I'm not sure if the insulation is metal-backed, but it doesn't look like it. It looks like paper to me.

    I own my home so I'm less worried about Landlord problems than I am XYL problems :) I have generic approval for antenna construction as long as it's not "unsightly" hihi
  5. KG5RZ

    KG5RZ Ham Member QRZ Page

    Out of curiosity, what type of coax and how long?
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    J-poles are notorious for putting high CM current on the coax feeding them, especially if the j-pole is not mounted on a massive conductive pipe or tower. A "coax choke" is needed. Randomly winding up a few turns of coax and expecting that to provide the needed choking action rarely does anything useful. The choke has to be carefully "engineered" to be effective.
  7. WA9UAA

    WA9UAA Ham Member QRZ Page

    Better off to get the antenna outside the attic and you'll need the length of coax to get there.
  8. W3ESX

    W3ESX Ham Member QRZ Page

    RG-58, the cheap kind. Got it as a young ham. Probably a 50' length. I did some research last night and learned more about line loss with cheap coax. Very informative. Currently looking into a wattmeter+dummy load combo so that I can determine if my coax is bad.

    (probably couldn't hurt to just get some RG-8X or one of the other higher-quality coax's available - still researching)

    Good to know, thank you. I will look into this.

    This would be nice, but not currently an option for me :)

    Thanks everyone.

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