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How to Calulate Received Field Strength 1

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kingtutley

kingtutley

Electrical
May 11, 2007
49
Ok. First, I am new to the field of electromagnetic compatibility. I am an EE who was voluntold to take over the EMI/RFI lab where I work. No experience.

I have the basic knowledge after reading a lot of the IEC and MIL-STD documents on EMC, but I am at a loss on some of this and I need some help.

I have been gong through the lab equipment and trying to establish a baseline for the current configuration. What I am doing is this. I use a function generator with a set amplitude and frequency and a passive antenna with known antenna factors to send energy to my active receiver antenna positioned 1 meter away from the transmitter.

What I am trying to determine is what my receiver antenna SHOULD be receiving in terms of v/m at 1 meter. I have my spectrum analyzer reading the values from the receiver antenna, but I don't know how to tell if my readings are correct as I have no idea how to calculate the field strength from my transmitter from the inputs that I know.

I hope this is not too convoluted or confusing. If anyone can provide any assistance, please help me. I'm in way over my head here and don't even know where to really begin.
 
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I probably can't help you in total, but here's some thoughts.

You've already found antenna factor. Antennas have a relationship between the surrounding field strength and the voltage they provide at their terminals (with all the usual disclaimers). For antenna used in EMC work, the antenna factor will be explictly documented and calibrated. There should literally be a certificate documenting the relationship.

But the relationship works in both directions (reciprocity). i.e. for the (hopefully) calibrated transmitter antenna too. Voltage applied to the terminals leads to a given field strength.

In the middle is the pathloss. But over only 1m distance, you're almost certainly going to be in the near field. So the famous "32.45 dB" pathloss equation won't be directly usable. Outside my knowledge how to deal with near field.

There is a three-antenna method to calibrate antennas yourself. But I'd expect that most labs would send their equipment to a calibration lab.
 
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MoM software such as NEC can be used to calculate e-field intensities including the near field for the conditions set in the NEC model, which then can be plotted.

The graphic below is one example, showing that the transition from near-field to far-field conditions exists at an h-distance of about 30 meters from that radiating system.

GW_F.I._vs_H-_Distance_from_Part_15_AM_Xmt_System.jpg
 
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