[TowerTalk] Voltage at ends of yogi parasitic elements?

[jimlux]

On 10/17/16 11:11 AM, Steve, W3AHL wrote:
> 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

That is not a valid assumption - each piece of the antenna both radiates 
and absorbs energy from every other piece (that's what the whole method 
of moments is based on).  You can also have a lot of "reactive" power 
circulating - antennas are resonant, so, in general Vrms*Irms is not 
constant everywhere (any more than it is in a lumped LC)

The other thing is that voltage isn't well defined in an antenna - at 
least in a way that can be measured, you wind up with a "voltage 
compared to what?  the ground that's infinitely far away? some other 
part of the antenna? etc".  What you CAN do in NEC (and other modeling 
codes) is look at the electric field, and that will tell you if 
breakdown is likely.

You can also (with some tricks) calculate the voltage drop along each 
segment of the antenna, then sum them all. You have to sum allowing for 
the phase of the voltage, too.   (put a wire with a large resistor load 
in parallel with each segment, using an NT card, then look at the 
currents in those wires... you can also do the same thing, sort of, to 
connect a "non radiating" wire from a node to the center of the element, 
but I'm not sure it actually works right)







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.

Actually, this is not a valid calculation.. Consider a tesla coil - the 
toroidal electrode on the top is like your ground radials - the voltage 
everywhere is about the same.

For the radial case, the current is evenly divided among the radials, 
but the voltage remains the same on all radial. (so the "power" entering 
each radial is 1/N times the total power)


I only have rev 20 of the antenna book, so I can't look up the figure 
you're referring to.



>
> 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.

Is that inserted in the end segment, or between the end segment and the 
center?

A 1:2 ratio between parasitic and driven is about what I would expect 
for most 3 element designs..




>