Measurements using field probe
From: terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
Sent: Monday, June 29, 1998 12:46 PM
To: Tesla List
Subject: Re: Measurements using field probe
Since so many people are interested in building the VI Antenna Probe, I
will add more to how it can be calibrated.
There are three things that can be adjusted to calibrate the probe:
1. The area of the antennas.
2. The capacitive load on the voltage antenna.
3. The distance from the coil.
Although not adjustable once the antenna is built, the area of the probe
antennas can be changed to fit different circumstances. The area can be
optimized for very large or very small coils or various distances as needed.
Also, the area can be changed to give a convenient scale factor at a given
distance. If you need to change the area drastically, please contact me
with the details. There are some signal to noise ratio and E-field concerns
that my need to be addressed.
The only real adjustment to the antenna itself is the voltage antenna load
capacitance. This is adjusted so that at a given distance, the voltage
scale and the current scale are comparable. Once adjusted, this should
never have to be changed. The load is adjusted by adding more or less
capacitance to the load capacitor.
The distance from the antenna to the center of the top terminal determines
the gain of the antennas. Once the voltage antenna's capacitive load is
set, the distance is the only adjustment you can make to the antenna. I
use a small hook on a string to measure the distance easily. From the
following probe equations it can be seen that if you double the distance,
both output voltages reduce to 1/4 their original values. ( Ca is
proportional to 1 / R^2 ) Obviously, the antenna must always be placed
safely away from the coil's arcs.
These are the equations that govern the voltage and current probe outputs:
Vov = ( Ca / Cl ) Vs
Voc = Rl ( Aa / (pi x R^2) ) ) Is
Vov = Voltage probe output to scope.
Ca = Capacitance between the coil's top terminal and the voltage antenna.
Cl = Voltage antenna's load capacitance.
Vs = Tesla coil's secondary voltage.
Voc = Current probe output to scope.
R1 = Current probe load resistance (must be 50 ohms to match coax).
Aa = Area of current probe antenna (same units as R).
pi = 3.14159...
R = Distance from antenna to center of top terminal. (same units as Aa)
Is = Secondary current.
x = Multiply.
/ = Divide.
There are three typical ways to calibrate the probe to real values:
1. Know the secondary current with some type of high bandwidth current probe
on the ground lead of the secondary and then adjust the distance of the
antenna until the readings match (given a scale factor). Then the voltage
probe can be adjusted by calculating the voltage with the equation:
Vs = 2 x pi x Ls x Fo x Is
Ls = Secondary inductance.
Fo = Resonant frequency.
Note that this measurement should be made at low power with no breakout and
with proper quenching. This will give good stable readings that will match
the equations very well.
2. If one knows the primary voltage accurately (high voltage probe) and the
secondary capacitance, then the secondary voltage under no breakout and good
quenching can be found using the equation:
Vs = SQRT ( Cp / Cs)
The secondary current can be found by:
Is = Vs / ( 2 x pi x Fo x Ls )
SQRT = Square root function
Cp = Primary capacitance
Cs = Secondary capacitance
3. If no instruments are available, the physics of the current probe alone
can be used.
The former equation for the current probe gives good results and can be used
alone to calibrate the current probe:
Voc = Rl x ( Aa / (pi x R^2) ) ) x Is
The voltage probe can then be adjusted using the equation:
Vs = 2 x pi x Ls x Fo x Is
Note that the area between the coil and the antennas should be free from
large objects and such that may disturb the electrostatic fields being
Once the probe is calibrated you can turn the coil back up to normal levels
and go for it.
At 11:11 PM 6/28/98 -0500, you wrote:
>From: Barton B. Anderson [SMTP:mopar-at-uswest-dot-net]
>Sent: Saturday, June 27, 1998 10:35 PM
>To: Tesla List
>Subject: Re: Measurements using field probe
>Just prior to flying out for Toronto last Monday, I printed out your HTML
>of the antenna writeup for measuring currents and voltages. I read it on
>ride. I definately have this project prioritized high on my "things to do
>am wondering though, it appeared to me that to correctly calibrate the current
>array, it is necessary to set the current to a fiberobtic probe measurement and
>then the voltage array can be calibrate via calculation through other
>values. Is this correct? Do we need to setup a current probe and calibrate
>BTW, great write up!!! The antenna appears very easy to construct.