Hi Sanjib.A, you pose several questions and I'll answer the best I can. As I'm not completely familiar with GTEMs, except from what I've read, you may need to consult other experts in the field.
I believe there is a "best" lower limit with GTEMS, depending on their size. So assuming the product you're testing does not exceed the size recommendation and the frequencies in question are - say - above 150 MHz, I've read they can be accurate enough for pre-compliance testing. Please correct me if I'm wrong...
As for your second question, there are "sources" of RF energy, such as devices producing fast edges (oscillators, buffers, ASICs, A/D, D/A, etc.) and these sources can couple via conductive, radiative, inductive or capacitive means to "radiating structures", such as cables, enclosure seams, etc.
Typically, it is common-mode currents that radiate via these radiating structures. As long as the loop area between the differential mode source and return currents is small, then DM emissions is usually a second-order effect.
A horizontal emission might come from a horizontal cable or a vertical seam, and visa versa with vertical emissions. You can also have a combination of the two.
I missed the webinar, but thanks for sharing the link and Q&A. I have the following questions for which I have not got satisfactory answers:
1. Is it okay to use a GTEM cell to do the pre-compliance testing? I saw a big difference in the results when we tried to fix a power supply issue in GTEM cell vs when we tried the same fix in an anechoic chamber. Would you suggest avoiding GTEM cell for pre-compliance testing?
2. How to interpret about the source of the noise (PCB or Cable; Common-mode or diff mode) if a frequency component is appears in Horizontal scan but not in the vertical scan or vice-versa or appears in both?