Antenna Arcing at High Strap Levels

Discussion in 'Technical' started by KA4KOE, Jan 7, 2016.

  1. KA4KOE

    KA4KOE Guest

    With my anticipated future increase in power level, I am going to take down my non-resonant 180' long dipole, fed with 600 ohm open line, and install a resonant, coax fed single bander on 75 meters. I've been running the numbers and the voltages on the dipole ends are frighteningly high. Part of the antenna change is my matching network (Palstar BT1500) will smoke and I don't want to have an attic fire as my open wire line goes through a good part of my attic. A complicating factor is that my property has multiple trees and therefore impossible to run a wire free and in the clear. I've always used No. 14 AWG THHN/THWN stranded/insulated, with a 600V jacket. This wire was purely adequate in the past.

    Now, the rough calculated dipole end numbers are significantly in excess of the 600V jacket rating (several thousand volts). I've anticipated putting up insulated HV "GTO" neon sign wire for a dipole but that will get expensive fast. I understand that this voltage will decrease from the ends as one moves towards the center. Probably some sine/cosine function as a function of degrees times the starting voltage.

    Already planning on getting big ceramic insulators for the ends.....the big dog 12" long type.

    Opinions?

    Philip
     
  2. WD5JKO

    WD5JKO Member

    Phillip,

    Good questions! I too run my inverted V through some trees. The antenna is about 120' long, and the last 30' on each end is in the clear. The insulation at the ends is enhanced with insulating rope going the rest of the way. If I were to run a lot more power, I like you have reason for concern. Consider though, the 600v rating of the wire you are using is likely a rating for 60 Hz AC power. The rating at 4 Mhz might be a lot less. The dielectric constant of the plastic insulation can be quite high which effects the antenna resonance a bit as compared to bare wire only. Perhaps Teflon insulated wire with a dielectric constant of 2 would be better in regard to insulation that maintains itself over higher frequencies. Another idea is just use the center conductor of RG-58 (non foam), or better yet the Teflon equivalent for the antenna.

    Perhaps some tree trimmers can prune some branches to give your antenna wire a clear shot?

    Jim
    Wd5JKO
     
  3. KA4KOE

    KA4KOE Guest

    This is the most inexpensive 15KV GTO cable I have found. It's weather and UV resistant.

    http://www.jantecneon.com/AC-10-B.html

    Using formulas and some guesswork, I'm seeing 600Vp at the center and about 12,000Vp at the ends (not RMS), this was calculated at 4000W peak (assuming 1000W carrier, 100% modulated). I understand it is very difficult to calculate the end voltages due to a variety of factors.

    Also saw some foot long ceramic dogbones at Surplus Sales.

    I have not decided how I am going to make up the center insulator. The Budwig HQ1 seems a little light duty for the task at first guess.