[TowerTalk] Voltage at ends of yogi parasitic elements?

[Steve, W3AHL]

Rob,

As an upper limit, the voltage on the ends of the DE should be about 3700 volts RMS or 5200 volts peak.  This is based upon a 40M wire Yagi EZNEC model with a source power of 1500 watts, feed point voltage of 271.7 volts and current of 5.54 amps.  Assuming the power remains constant along the length of the element the end segment of the model shows a current of 0.41 amps, which would require 3658 volts RMS for 1500 watts.  That is the average current along the entire length of that end segment in the model, so it doesn’t represent the worst case at the very end, but it is close enough for your purposes.  Round up...!

In the ARRL Antenna Book, 22nd edition, page 3-14, figure 3.28, there is a graph showing the voltage on the ends of elevated radials of a vertical.  For 4 radial configuration the voltage is 1800 volts RMS at the ends.  For 12 radials the voltage is only 600 volts, since the power radiated  by each wire is less.  Extrapolating for a single radial yields 5400 volts RMS.

For the parasitic elements of the 40M Yagi the center segment’s current is only 2.55 amps, with 0.185 amps at the ends.  If I insert a zero current current source (acts as a voltmeter in EZNEC) near the end (2% from end) of the reflector element, it shows the voltage 2140 vRMS or 3026 vPeak.  Doing the same for the DE yields 4058 vRMS or 5038 vPeak.  Same caveat applies as given in the first paragraph.  Inserting the sources as voltmeters slightly changes the antenna behavior, as would using a voltmeter to actually measure it.

Good luck with whatever you decide to try!

Steve, W3AHL
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I’m playing with some wire yagis, and I?d like to put relays at the ends of the parasitic wires to switch the director to a reflector (and vice versa).  If I?m driving the antennas with a full 1.5 KW, what would be the maximum voltage that I can expect at the ends of the PARASITIC elements?  Even a rough estimate would be useful, I just need to get a sense of what kind of a relay I would need to use.

73,
   Rob  K4OV