On 21 Oct 2003 01:59:07 -0700, (Daniel) wrote:
What i want to simulate is a common EMC-Test. Put a PCB with the
circuitry in a shielded EMC-room (I dont know the english word) and
put a 10V/m E-Field around it in 3m distance from the sending antenna.
Frequency band is 80MHz to 1GHz

English is perfect. Specification is too.

I started with modeling a simple loop and a stub. I experienced no
problems for the loop, but the stub is driving me crazy:
In order to calculate the output voltage of a PCB antenna for a given
geometry and EM-Field at a desired frequency, I need to know the
output impedance for this antenna. I didn't find anything on the web
on calculating/extracting it.

This, in fact, is one of the easiest things to determine - if you know
the frequency and physical dimension.

This was the question! unfortunately
This, in fact, is one of the easiest things to determine - if you know
the frequency and physical dimension.
doesn't help me.... I know the physical dimension (PCB Layout, 93mm
length, 2mm Strip width, 0.035mm hight) The freqeuncy band is given
above.
The only thing antenna books are talking about (the ones i read) are
wire antennas, where a radius is given for calculating the
Waveresistance(? correct word ?)ZL. As i dont have a radius (stripline
is rectangular) I am not sure what to do. Is it possible to say the
area of the stripline is 0.035mm*2mm (crossection), and therefore
build up the equation 0.035mm*2mm=2*pi*r^2 and then follow for the
radius: r=sqrt((0.035mm*2mm)/(2*pi)). I think this is rather wrong,
how else can I determine the waveresistance and with it the Impedance
of the antenna?

You are working too hard. Round wire compared to flat stripline is
not that different. The difference, for the broad range of frequency,
will be overwhelmed by other things.

I was thinking of a 2D FEM in order determine the unit per length
parameters L' and C' and then i can calculate the waveresistance, too.
What do you think?

thanks

Daniel

Hi Daniel,

Think of doing this by parts. The first approximation will be close.
The second approximation will improve it a little. Being perfect will
not matter.

For overall dimension of dipole/monopole/loop, wavelengths less than 1
and more than 0.1 can be found in many antenna reference books.

Let's look a some simple approximations for a loop:
Ra = 320 · pi^6 · (radius / wavelength)^4
for
radius = 0.046 M
wavelength = 3 M
then
Ra = 0.017 Ohm

for
radius =0.046 M
wavelength = 0.3M
then
Ra = 170 Ohm

Let's look a some simple approximations for a dipole:
Ra = 80 · pi² · (length / wavelength)²
for
length = 0.186 M
wavelength = 3 M
then
Ra = 3 Ohm

for
length = 0.186 M
wavelength = 0.3M
then
Ra = 304 Ohm

Now, be warned. These formulas only work for length wavelength and
for radius wavelength. The last evaluation for a nearly halfwave
dipole reveals 304 Ohms for what is actually closer to 70 Ohms.

So for your range, for your largest dimension (93mm) the radiation
resistance will vary between roughly 1 Ohm and 100 Ohms (I am
discounting the loop too). This is for "free space." Your board will
limit that severely. This again suggests high evaluations will be
modified (and closer to my 1 - 100).

This is a first approximation. A second approximation will not alter
this more than 30%. Nearby components will significantly alter it
through proximity.

Trying to compute the reactances will meet similar issues.

73's
Richard Clark, KB7QHC