

Help with TEM Cell Calculating Field in uV/m  RF Cafe Forums

Bill 
Post subject: Help with TEM Cell Calculating Field in uV/m
Posted: Mon Jan 10, 2005 2:47 pm



Hello
My expertise in RF is limited
I write code in C and assembly, design analog
circuitry to a 1 megahertz or so, and push a
lot of paper.
I am trying to calculate
the field strength in a TEM cell given a signal
generator output in dbM and the septum length
of the cell. The signal generator output impedance
is 50 ohm, the cell looks like a 50 ohm load,
the testing frequency of 930 Mhz is well within
the passband of the cell. Lets put aside cable
loss, adaptor loss, swr, and the accuracy of
my measuring instruments for the time being.
The TEM Cell manual gives a method for calculating
field strength in cell at test specimen location:
1. Record the emf applied at cell input 2.
Divide by the septum length 3. You get volts
per meter.
I hook up a hp sig gen
to the cell, and set the output to 86dBM
1. To calculate the emf applied to the cell,
I use the formula P=E^2/R, rearranging to E=
sqrt (RP). I use 50 ohms for R, and get P from
P= 10*log(Px/1mW) 2. I divide by the septum
length. Now I dont have the work in front of
me but I get something close to 4050 uV/M
3. I put a 50 ohm dipole antenna at the test
point (gain is +2.5dbi), but measure a 107
dbM power at the test point. 4. Using the
formulas from 1, I do not get 4050uV/m...I
get something substantially less
Whats
going on? Should I be using 377 ohms instead
of 50 ohms in step 4?
Please help this
victim of a lumped systems education make progress!
Thanks.





Guest 
Post subject:
Posted: Wed Jan 26, 2005 12:28 am



*bump*
Anyone?
Is "antenna" factor
responsible for the 20 db difference? What is
"antenna factor" for a dipole immersed parallel
to a theoretical perfect TEM wave Efield and
no higher modes?
Is "antenna factor"
the same as "gain of an antenna"?





Boomholzer 
Post subject:
Posted: Wed Feb 09, 2005 12:55 pm


Joined: Wed Feb 09, 2005 11:33 am
Posts: 3 Location: Cubicle 
What is the septum length?
I do not follow
how you are converting dBm to uV at the TEM
input. It looks like your starting with dBm
(power referenced to 1mW) , converting to the
same units and then sticking that number into
a Ohm's Law equation.
86dBm = 2.512pW
Now using your Ohms Law Calculation: P = E^2/R
, R=50 E=11.2uV
What do you get when
you take 11.2uV emf / septum length? 22.4uV
is the input emf, no load. 11.2uV is the
RF voltage in a 50Ohm system.
The DUT
antenna is nonisotropic, how are you orientating
it?





Guest 
Post subject:
Posted: Thu Feb 10, 2005 3:12 am



The septum length is 0.22 meters.
The
DUT antenna/PCB is clearly distorting the field.
We can put an isolated receiver in and get a
correct message 9/10 times @ 83 dbm. But when
we put the bottom circuit board in (the receiver
is mounted to it), we get the correct message
9/10 times @86 dbM indicated input power. Its
very repeatable +/ 1 dbM indicated input power.
I am told the reason why we measure
107 dbM with a 50 ohm dipole adjusted to the
proper dimensions is something called "antenna
factor" of a dipole. I am told my calculations
do not have to use 377 ohm. I have always thought
of this as "gain" of an antenna but I believe
my understanding is incomplete.





Guest 
Post subject:
Posted: Thu Feb 10, 2005 3:17 am



The DUT antenna is a loop antenna as in a 900
MHZ pager. It is aligned to be in parallel with
the E field of pure TEM mode wave, as the dipole
was.





Guest 
Post subject:
Posted: Thu Feb 10, 2005 3:23 am



Sorry for the multiple replies but I am consulting
my notes. Finally the DUT has been located in
the cell such that the long axis of the antenna
is half way between the septum edge and the
opposite wall of the chamber.





Boomholzer 
Post subject:
Posted: Thu Feb 10, 2005 12:47 pm


Joined: Wed Feb 09, 2005 11:33 am
Posts: 3 Location: Cubicle 
A loop antenna?? Is is a OEM antenna or something
you designed? If your sensitive to the H
field, then the 377Ohms would apply. However
I doubt that is the case at 900MHz. I
am not sure on your PCB and antenna DUT size.
I think the ruleofthumb is to never exceed
2/3rds the cavity height and leave 1/3 between
the DUT and septum open to prevent field distortion.
Here is a good read that I hope will answer
some of your questions:
http://www.tdkrfsolutions.com/DataPDFs/antenna_paper_part3.pdf





Guest 
Post subject:
Posted: Mon Apr 25, 2005 11:55 pm



In general, to calculate antenna factor, you
begin with the input to the TEM cell in dBm.
Subtract the reading on the spectrum analyzer
(coming from the probe output) in dBuV (which
is dBm plus 107). The result is your antenna
factor. To calulate the antenna factor for
HP 11940/11941A nearfield probes, use the formula:
104.3 Specan rdg(dBuV)TEM input (dBm).


Posted 11/12/2012



