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Buddipole (dipole) Use in Attic

Discussion in 'General Technical Questions and Answers' started by KB1IDH, Sep 29, 2004.

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  1. KB1IDH

    KB1IDH Ham Member QRZ Page

    Running 100W, how far aware from objects should the antenna (buddipole) be to prevent any arcing?

    Buddipole link

    How do you calculate voltage at the end of a dipole antenna?

    Thank you
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    You can't. You'd have to know the actual impedance at the tip of the antenna to calculate the voltage, and this is difficult to measure, or even to estimate.

    However, I've used a conservative rule-of-thumb that the end-point Z of my wire antennas is 50,000 Ohms. That's probably a bit on the high side, which means I'm erring on the side of safety, because my voltage calculations will come out high.

    With 100W transmitter power, this would yield a terminal end voltage of 2236Vrms or 3162Vpk. Sounds a bit high, but I'm not so sure. I've seen 100W cause burns to tree limbs and stuff, plenty of times when the wire end touched or came very close to touching the limb.

    As such, I'd stay as far away as possible, and in any case a few inches.

  3. WD8OQX

    WD8OQX Ham Member QRZ Page

    How close are you talking about here? What is the ant. arcing to - grounded metal?

    I'd be more concerned about the arcing than anything else as there is definitely a problem there somewhere.
  4. AB8TM

    AB8TM Ham Member QRZ Page

    I'm not really sure if this is a practical issue for you to worry about. Nevertheless, it would be good to be mindful of it and keep it as far away from objects as you practically can.

    I know there are a lot of Buddipole (and similar) attic configurations on the air everday.

    Also, W3EE in my experience is very good about answering his emails and seems like a very nice gentleman. Maybe the best guy to ask would be him, assuming he gets feedback from his customers.
  5. K7JBQ

    K7JBQ Moderator Volunteer Moderator QRZ Page


    Absolutely right, except that the call is W3FF:

  6. AB8TM

    AB8TM Ham Member QRZ Page

    Oops, thanks for the correction Bill!
  7. XV2PS

    XV2PS Ham Member QRZ Page

    Arcing? I would worry more about swr than arcing. Can it arc? Plus voltage between what and what?
  8. AG3Y

    AG3Y Guest

    RF is funny stuff, it can arc from any conductor with a higher potential to any conductor with a lower potential.  I have seen examples of arcs forming from the end of an antenna to nail points that were sticking through from the shingles on the other side of the roof itself!  I have also heard of arcs from the ends of antennas to any electrical wire running close by.  

    My own personal experience was not with arcing ( I took many measures to make sure it did not happen ) but voltages induced into the power line from the antenna.  I was running a "slinky dipole" within a couple of feet of the wires in the ceiling of our upstairs bedroom.  Every time I keyed the CW key, the light in the bedroom would light up !  I "cured" the problem by bypassing the light with a capacitor, but after considering that the RF voltage was still flowing around in the house wiring, I decided to put the antenna outside where it really belonged !

    Stay below 100 watts, keep the antenna a fair distance away from anything that is grounded ( like at least 3 feet, I would say ) and follow some of the other guys advice like constantly monitoring your SWR for any changes that would come about by an end letting go, etc.

    (added with edit function )
    OH, BTW, I wanted to mention that an FM station that I once worked at had its tower just outside of the building containing the studios. The coax line looped over a flourescent lamp and then out the wall to the tower. Even with that lamp turned off, there would be a glow coming from the area of the tube immediately above the coax line! And this was in a professionally installed, fairly low powered FM facility!

    Good luck!   73 from Jim AG3Y
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