BNC to Binding Post antenna configurations?

Discussion in 'QRP Corner' started by VE7JBX, Aug 30, 2017.

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  1. VE7JBX

    VE7JBX Ham Member QRZ Page

    This has probably been dealt with before, but I haven't been able to dig it up so at the risk of engendering the wrath of some folks, I'll ask it again.

    I picked up a little BNC to two binding post adaptor, and carry it along in my QRP kit bag. I was planning to try it out this weekend and compare it against my usual coax fed inverted-V I use. My idea was to just put one dipole arm on each post, and toss the far ends up in trees or something as far as possible - sort of an upside down inv-V with no feedline.

    I thought I'd ask though, in case there was any other or better options for how to configure this. One wire run vertical, other along the ground? Or what about using paired speaker wire, separate two dipole arms to resonant length and leave a paired 'feedline' section back down to the radio?

    All things I will probably try, but if there's a clever (and efficient) configuration for this direct to binding posts antenna, I'd be interested to hear.
    KA0USE likes this.
  2. KU3X

    KU3X Ham Member QRZ Page

    I don't see a problem with what you are going to do. Elecraft suggests doing the same thing with their kx3 and kx2.
    There are always better ideas for performance but your plan will get you on the air.
  3. KM4DYX

    KM4DYX XML Subscriber QRZ Page

    I've played with this concept a bit, not a whole lot, though. "An upside down inverted V" That's kinda funny.

    This very concept was utilized by military HF rigs, many generations (of rigs) ago. The antenna wires were attached directly to the rig in the V configuration.

    I tried it, first making sure the antenna was resonant with my AA-30. It worked, not great, but it worked.

    I didn't try running the "hot" wire vertical and the "ground" wire on the ground, but if you do then you'll want to point the ground wire at your "target".

    Let us know how it works out.

    KA0USE likes this.
  4. KU3X

    KU3X Ham Member QRZ Page

    Both legs are hot legs. It's AC, both legs are equal.
    Some people say one leg is plus and there other minus. They are both plus and both minus. Just because the one leg is connected to the case of the radio does not
    mean it does not carry RF. Both wires carry the same amount of RF.
    KA0USE, KC8VWM and AI3V like this.
  5. VE6NS

    VE6NS Platinum Subscriber Platinum Subscriber QRZ Page

    I use a BNC to two binding post adaptor with a 28' piece of wire hung in a tree and a 16' piece of wire laying on the ground. KX1 tunes it no problem on 20-30-40.
    KA0USE likes this.
  6. KM4DYX

    KM4DYX XML Subscriber QRZ Page

    You probably know a lot more about this than me. Please correct me where I am wrong:

    The output from a modern rig is unbalanced.
    In the case of a dipole, a balanced antenna, we ensure that the shield of the coax is connected to a 1/4 wave wire to ensure low impedance.
    In the case of a tuner we ensure that the "hot" connection goes to the radiating element and the "ground" connection goes to a RF ground.
    In the situation posed by the OP I'm not sure if it matters what wire is which, but wouldn't it matter in the case of a rig with internal tuner, or an external tuner for that matter?

    Trying to get smarter.


    P.S. EDIT: In the case of an unbalanced antenna like a ground plane we always hook the shield of the coax - the ground side - to the ground plane elements. Wouldn't that be the same as running one side of the antenna on the ground and putting the other side vertical?
    Thanks for any info that you have to make this clear to me.
    Last edited: Aug 30, 2017
    KA0USE likes this.
  7. VE7JBX

    VE7JBX Ham Member QRZ Page

    Well, I will give it a try in next few days and report back. I did understand that both sides of the BNC connector / both binding posts carry equal and inverse phase values of RF energy at all times; that kind of why I was thinking it might work better to have the two 1/4 wave radiating elements coming right off, than running coax (inherently unbalanced) to a dipole feed. Sans RF choke, I sometimes get

    In spite of 'both legs being hot', many antenna designs treat the two sides differently - e.g. verticals with one or more counterpoises.

    I'll just try a few things and watch RBN to see what it shows. (I have some old Spilsbury SBX-11 radios in the shack as well.. all they have for antenna out is two binding posts on front panel. Seen lots of photos of them in use with two wires just thrown on top of snow)

    My whole idea in carrying the BNC to binding post adaptor, was as a fallback if my coax fails in the field (or heck, even if I forget the antenna elements or lose them or break them or...) Ability to just find a couple of lengths of conductor, estimate resonant length, and couple on, has some appeal for this sort of tiny QRP rig I carry along on camping/ hunting / fishing / motorcycle touring outings.
  8. KM4DYX

    KM4DYX XML Subscriber QRZ Page

    Your thinking is in line with mine on all accounts. Please do report back, and I truly would like an explanation of where my understanding is wrong, if anyone would care to chime in.

    I heading outside to do a little "backyard portable." Look for my spots on WSPR if you are at all interested in what 1 watt through a "Poor Man's 20 M Yagi" up about 50 ft will do from NC. (I'll understand if you decide to pass on that exciting opportunity.)

  9. KM4DYX

    KM4DYX XML Subscriber QRZ Page

    P.S.: The WSPR worked so well that I decided to dust off the mic and try a little phone into Europe. S51DX, Slovenia (5/9) and LZ55UPB Bulgaria (5/5)!

    5 Watts/battery power/2-element wire Yagi Uda inverted V at about 50 ft.
    KC7JNJ likes this.
  10. KB2SMS

    KB2SMS Ham Member QRZ Page

    Yup, my KX2 tunes that up fast on 40-10m. I use 25' speaker wires.
    KC0LR and VE6NS like this.

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