Connect with us

Dipoles and the rig's RF ground...

Discussion in 'Electronic Basics' started by billcalley, Jan 1, 2006.

Scroll to continue with content
  1. billcalley

    billcalley Guest

    I realize that dipoles are balanced antennas, but does the rig
    itself still need an RF ground too? (I know the radio always needs a DC
    ground, of course). How about if the dipole is being used as a
    non-loaded "all band" antenna (IE: RIG--TRANSMATCH--LADDER
    LINE--DIPOLE) -- would this affect the need for an RF ground on the rig
    for operation in the dipole's non-resonant bands? Or is no RF ground
    _at all_ required with a dipole; unlike when using random wires or
    verticals, and other such un-balanced antennas?

    Thanks!

    Bill
     
  2. With ladder line it is best to use a balun between the antenna tuner and the
    transmission line. An RF ground on the rig is then not required (you won't
    have RF voltages on the rig's chassis).

    The thing about an RF ground is that due to the length of the ground
    circuit, the rig is often not grounded anyway.
     
  3. Roy Lewallen

    Roy Lewallen Guest

    If your feedline is balanced, that is it has equal and opposite currents
    on the two conductors, then there's no current left over to flow to or
    from ground and no need for an RF ground connection. All the current
    from one conductor goes back on the other. Feedlines can be balanced
    even if they're coax and/or the antenna is unsymmetrical; they can be
    unbalanced even if the antenna and feedline are symmetrical.

    If the feedline isn't balanced, the difference current (that is, the
    difference between the currents on the two feedline conductors) will
    find its way to ground however it can. This often creates undesirable
    effects. But if you can't avoid it, it's better to provide a low
    impedance path for the ground current if possible. And that can
    sometimes be difficult to do.

    (I know the radio always needs a DC
    No, it doesn't. It needs an AC safety ground if connected to the mains,
    and a lightning ground if that's a possible hazard. But DC isn't important.

    How about if the dipole is being used as a
    The trick is to get the feedline balanced on all bands. That requires
    either a truly balanced tuner, or a combination of a good balun and
    impedances on all bands at the balun which the balun can handle.

    Roy Lewallen, W7EL
     
  4. Bill Turner

    Bill Turner Guest

    ORIGINAL MESSAGE:

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Any antenna which requires a connection to ground should be shown the
    trash can immediately. Ground (earth) is a lousy conductor and does
    nothing to help your signal. RF belongs up in the air, not down in the
    dirt.

    If you find that connecting a ground wire actually improves your
    signal, you have a SERIOUS problem in your antenna.

    73, Bill W6WRT
     



  5. Most modern shacks have to long distance between the rig and ground. It's
    not equal to an ungrounded rig but you might experience hf in your shack.

    This is best solved by a short ground cable to a proper ground rod.

    The next best thing is to buy or build an artificial ground. It's very
    simple and can be made to cover all ham bands easily and will always give
    you a perfect length of the earth cable... By cheating of course - but it
    works!


    Cheers

    M0DFI
     


  6. Get a virtual earth! They are easy to build for all ham bands!

    It's basically a phasing unit for the earth connection which can null the
    voltage on the earth at the RF Rig!


    Cheers

    M0DFI
     
  7. If the antenna is TRULY balanced and the feedline dressed well away from it
    at right angles you should have no common-mode currents on the feedline.
    That's the ideal case and in that ideal case you need no RF ground at the
    radio. The ideal case, however, rarely ever exists in practice.

    And end-fed wires can be a whole different ball game. I had to use a 16
    foot counterpoise once to "ground" a rig in a 2nd story location when I
    end-fed a very long wire with it.
     
  8. billcalley

    billcalley Guest

    Thanks Guys -- I really appreciate the clarifications on grounding! It
    sometimes gets a bit confusing for me.

    Best Regards,

    Bill
     
  9. Roy Lewallen

    Roy Lewallen Guest

    That only prevents one of the two ways common mode current can be
    created, by coupling. It can also be created by conduction. A common
    example is a coax-fed dipole, where the current in the outer feedline
    conductor splits between the antenna conductor and the outside of the
    coax. An equivalent problem can occur when a dipole is fed with
    symmetrical line such as ladder line, and one conductor of the line is
    connected to the rig's chassis at the rig end. The current on the inside
    of the chassis is equal to the current from the "hot" conductor, and
    this splits between the transmission line conductor and the outside of
    the chassis. A detailed explanation of conducted common mode current can
    be found at http://eznec.com/Amateur/Articles/Baluns.pdf.

    Roy Lewallen, W7EL
     
  10. Still, if the antenna is TRULY balanced (a situation that only rarely
    actually happens), you won't get common-mode currents. I've never had a
    problem with them with well-grounded (from an RF standpoint) ground-
    mounted verticals either.

    Essentially this is why I recommend using open wire or twinlead and
    feeding it through a proper balanced-line tuner. Years ago, I built an
    amplifier that literally had a balanced line output and fed a 600-ohm
    feeder direct off two taps on its output coil. That feedline was only
    ten feet long and I worked a TON of 80m DX an the inverted vee that it
    connected to. And I could always tap the coil so as to have ZERO RF in
    the shack (though my landlady's little 7.5 watt light bulbs used to light
    on some frequencies when the house wiring picked up direct from the
    antenna).
     
  11. chuck

    chuck Guest

    Is it not true that if the currents on the transmission line are unbalanced
    (i.e., unequal on the two conductors) then the transmitter must already be
    connected to ground? If not, what is the path of the differential current?

    Chuck
     
  12. Roy Lewallen

    Roy Lewallen Guest

    That's true only if by "balanced" you mean that the two feedline
    conductors carry equal and opposite currents. In that case, common mode
    current is zero by definition. But if you really mean symmetrical, as
    most amateurs do when they say "balanced", you certainly can have common
    mode current.

    A detailed explanation of how that happens is in the article at
    http://eznec.com/Amateur/Articles/Baluns.pdf, and the article by Walt
    Maxwell, W2DU at http://www.w2du/r2ch21.pdf which is referenced at the
    end of the first article? Note particularly figures 3 and 4 of the
    Baluns.pdf article.
    The reason this provides balanced feedline currents is that the
    impedance to ground at the base of the antenna is much less than the
    impedance looking back from the feedpoint down along the outside of the
    feedline. Consequently, the large majority of the current from the
    inside of the coax shield flows to ground rather than down the outside
    of the coax. And laying the coax on the ground keeps coupled common mode
    current down.
    That combination will produce a truly balanced system with no common
    mode current. But it's not the only way.
    Years ago, I built an
     
  13. No, I mean ELECTRICALLY balanced. And with the feedline at right angles
    to the antenna so that it doesn't pick up anything by induction. It's a
    tricky thing to do, yet back in the old days hams used to feed dipoles or
    extended double zepp antennas with open wire line and not get much RF in
    the shack. I know mine didn't. I was putting nearly 700 watts into the
    antenna and you could touch the amp chassis without any RF burns. Didn't
    have the fancy tools I have now for testing things, but still managed a
    good, clean and loud CW signal from an angled dipole. Worked a lot of DX
    on 75 with that antenna, including a nightly sked with Midway Is. for
    traffic (from Vancouver, BC.).
     
  14. Ran a Windom in Texas in 1965 (WA5KBO) with only 150W and burned a hole in
    my lip (no joke) with the RF on the metal ring around the microphone! The
    Windom was a good performer, but I could not effectively ground the rig. I
    was in student housing (College Station) and was not allowed antennas but
    improvised!
     
  15. Dave

    Dave Guest

    rf 'ground' is a real misunderstood thing. and things like this point out
    just how poorly understood it is. there is really no need for a radio to be
    'grounded' to prevent rf burns or to have an antenna work properly... the
    important thing is to remember that at that point where the rf leaves the
    radio on the center conductor of the coax connector the current there must
    be exactly balanced by a current going the opposite direction the inside of
    the shield of the coax connector. looking at the worst possible case, just
    stick a random wire in the coax connector and run the rig off a battery with
    a short cable and no other 'ground' wire. current flows out the center
    conductor of the connector into the exposed wire and somehow has to get back
    to the inside of the connector shell to balance it out... well, the only
    place for that current to come from is coupling from the antenna wire back
    onto the case of the radio and from there it flows back into the connector.
    now, put your hand on a metal part of the radio, or your lip if you are
    unlucky, and what happens?? you are much bigger than the case of the radio
    and you are fairly conductive, so now you provide a bigger collector for the
    current from the antenna so lots of it flows through you to get back to the
    radio connector... hence rf burns. how to stop it?? provide a lower
    impedance path for the rf to get back to the connector than through you.
    that can be a counterpoise wire, a 'ground' wire that collects current from
    the soil under the antenna, connect the case of the radio to your car and
    the car becomes the collector... OR add an equal sized second wire that
    'balances' the current from the wire in the center conductor... the critical
    point is that it mut be very nearly identical to the first one so the
    current in it is the same... a dipole that is symetric with respect to the
    feed point will work, but you have to watch out because the case of the
    radio is connected to the coax connector also, which tends to unbalance the
    equation since there is no equivalent lump of metal on the center conductor
    part of the antenna. to make this job of balancing the currents easier we
    normally add a length of coax (to get the antenna farther away from the
    radio) then add a balun to help force the currents in the two halves of the
    dipoles to be equal so there is no need for current to flow from the radio
    back into the inside of the shield. a choke on the outside of the feedline
    also can help, but the reason is different... a choke on the coax creates a
    very high impedance so that current has a hard time flowing from the case of
    the radio or outside of the coax back to the inside of the coax. enough
    rambling, just remember, rf ground is a myth, all you need to do is get
    those currents equal while preventing the path where they flow from being
    through you or something else that could be damaged by them.
     
  16. Cecil Moore

    Cecil Moore Guest

    Dang Charles, I did exactly the same thing in 1957. If you had
    asked me, I would have told you to watch out for those metal
    microphones when using a Windom.
     
  17. I only made that mistake once! That burn was very slow to heal, by the way.
    I clearly understood several principles after that. Close-talking the mic
    and over-modulation was the least of them.
     
  18. A true windom with a single wire feed, or one of the latter-day kind with
    twinlead?
     
  19. This is where those artificial ground things come in handy. But the end of
    the counterpoise needs to be where it can do no harm, as that's where the
    artificial ground sticks the voltage.
     
  20. Single wire feed. I was a student (living off of my wife) and used what I
    could get my hands on.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-