Richard Harrison wrote:
At high altitudes,
high potentials can easily produce corona and flashover.

That's why the cubical quad was invented. The voltage at
the voltage maximum points is in the ballpark of half that
of a dipole and those points can be insulated.
--
73, Cecil, W5DXP

Looks perfectly reasonable to me, Jim. Another perfectly reasonable thing to
ask is "what is the electric field strength near the antenna elements?" because
if the field strength is too great, you can get corona. And EZNEC (NEC2, etc)
will give you at least an estimate of that value. As you say, it can be
interesting to know if an insulator will be adequate, and that's a question
well worth asking.

Cheers,
Tom


Jim, K7JEB, wrote:

Well, I chose one particular configuration
and one particular integration path because
I was curious about the original question -
something about how much voltage would the
end insulator have to handle for 100 watts
of radiated power.

I chose a vertical, half-wave monopole fed
against perfect ground and looked at the
driving source data with EZNEC. The feed-
point impedance was 2188 +j66 ohms and a
driving current of .213 amps produced a
radiated power of 100 watts and a feedpoint
voltage of 466 volts. 1500 watts scales
that up to 1805 volts. Symmetry about the
ground would increase that to 3600 volts
for the free-space case. That is what I
would adopt as my design-to target for
end insulators.

I know it's crude, but I was just looking
for a ballpark figure.

Jim, K7JEB, wrote:
....
Again, this is just a special case of the general
problem. But it has a configuration that is easy to
implement in the EZNEC program and is quite
relevant to typical ham-radio, low-band dipole
installations.

It's also easy to get the electric field strength near the antenna from EZNEC.
I expect the field to be highest near wires, because of the shape the field
must take near the wires, so that's where I'd look first to get an idea about
possible breakdown of the air or insulators.

Cheers,
Tom

Really???


K9CUN

What we know as and call "voltage" is the potential difference between two
points. Give me access to the two points and an ideal voltmeter (AC or DC) and
I'll measure it for you. I am assuming sinusoidal steady state AC, isn't
everyone?

73 de Jack, K9CUN

In , (JDer8745)
wrote:


What we know as and call "voltage" is the potential difference between two
points. Give me access to the two points and an ideal voltmeter (AC or DC)
and
I'll measure it for you. I am assuming sinusoidal steady state AC, isn't
everyone?

73 de Jack, K9CUN


I'll bet you can. So I have this piece of clear plastic with two terminals
staked to it. Call them A and B, A about 2 inches above B as I look at it. I
have a 10k resistor wired from A to B and dressed an inch or so off to the
left. I also have a 1k resistor wired from A to B and dressed off to the
right, so the two resistors are a couple inches apart. There aren't any other
electrical connections to the two resistors. I have a pretty good AC
voltmeter, and using my best techniques, I measure one volt RMS across the 10k
resistor. Using a scope, I see it's a nice 100kHz sinewave, constant
amplitude. So what's the voltage across the 1k resistor?

73 de

Just one caution about this. I don't think EZNEC or other NEC-based
programs will give accurate field strength any closer to a wire than a
few wire diameters. So it probably wouldn't be good for predicting the
likelihood of corona discharge and the like. It might be possible,
though, to model a wire as a number of very much smaller parallel wires
arranged in a circle, to get good field strength values closer to the
real wire. I wouldn't completely trust the results, though, until at
least a few test cases were run which could be compared to theoretical
values.

Roy Lewallen, W7EL

K7ITM wrote:
Jim, K7JEB, wrote:
...

Again, this is just a special case of the general
problem. But it has a configuration that is easy to
implement in the EZNEC program and is quite
relevant to typical ham-radio, low-band dipole
installations.


It's also easy to get the electric field strength near the antenna from EZNEC.
I expect the field to be highest near wires, because of the shape the field
must take near the wires, so that's where I'd look first to get an idea about
possible breakdown of the air or insulators.

Cheers,
Tom

Roy, W7EL wrote:
"I don`t think EZNEC or other NEC-based programs will give accurate
field strength any closer to a wire than a few wire diameters."

The Antenna Section in Keith Henney`s "Radio Engineering Handbook" was
written by Edmund A. Laport, Chief Engineer of RCA`s International
Division at the time (around 1950).

On page 637 Ed writes:
"Where high power is to be transmitted, or at high altitudes, antenna
insulation and conductor designs require care to details. For h-f use,
only radial potential gradients need to be considered. At high
altitudes, pluming may occur with consequent damage to the system.
Fortunately in practice, high power is generally used with directive
antennas, and the power is divided among several dipole sections thus
tending to minimize the problem. A thin-wire dipole gives an end
potential of about 3,900 volts rms for 1000 watts input for a height of
0.25 wavelength. It will be higher for smaller heights, and falls to a
minimum of about 1,700 volts as height increases to 0.75 wavelength;
beyond this point it settles down to the free-space value of about 3,000
volts. Potentials vary as the square root of the power ratio and as the
inverse square root of the capacitance per unit length. For a potential
of 3,900 volts on a wire 0.101 in. in diameter (No, 10 B&S), the radial
gradient is of the order of 31 kv per cm. As a rough approximation for a
cage, the gradient for one wire is divided by the number of wires in the
cage."

The multiwire observation is important because, if the potential
gradient at any point in air becomes greater than 30,000 volts/cm., the
air becomes ionized and sparking or corona discharge will occur.

On page 645, Ed writes:
"With 100-kw carrier input (to an 8-dipole array), the end potential on
each dipole is 7,500 volts rms. A 3000-ft. 580-ohm two-wire balanced
feeder used with this antenna had an efficiency of 67 per cent.

I am comfortable with Ed`s experience which can be scaled for the power
and configuration of the antenna.

Best regards, Richard Harrison, KB5WZI

"So what's the voltage across the 1k resistor?"
------------------------------------------------------
It's whatever my ideal voltmeter reads when it's connected across the ends of
the 1-kOhm resistor.

Jack