# Need Help Calculating Field in TEM Cell

Discussion in 'General Electronics' started by StructureUnionWarrior, Jan 18, 2005.

1. ### StructureUnionWarriorGuest

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 too much 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. I believe
the power indicated by the digital display is the power delivered into a

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 (0.22 Meters). Now I dont have the work in
front of me but I get something close to 40-50 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, using a 50 ohm input spectrum
analyzer (albeit a cheap one)
4. Using the formulas from 1, I do not get 40-50uV/m...I get something
substantially less

Whats going on? Should I be using 377 ohms instead of 50 ohms in step 4?