Talking House F/S

Set your spectrum analyzer to dBuV, instead of dBm. This will make it easier to add the Antenna Factor for the frequency you are measuring. Also, add in any losses due to coax, adapters, etc. Then, as a final step, you can convert to uV/m.

This way, you can directly compare your FIM against your SA. Then you can flip a coin to see which one you believe is accurate 🙂

This page explains and shows the conversion formula:

http://www.microvolt.com/table.html

The relevant text is this:

Field strengths in microvolts per meter (mV/m) can be calculated by taking the inverse log of the field strength measured in dBuV/m. As an example:

FS(uV/m) = 10 raised to the power of {(dBuV/m)/20}

where,

FS = Field Strength in decibels above a microvolt per meter or in microvolts per meter

Indicated Signal Level = signal level observed on the receiver (spectrum analyzer). Note: usually read in dBuV, or in dBm and converted to dBuV.

AF = Antenna Factor (signal loss) in dB

CL = Cable Loss (signal loss) in dB

i have one of those chinese tenna sa's not a hp or motorola

I ran an NEC analysis of a ground mounted antenna with 16 30ft radials over "average" ground and adjusted the power and coil resistance to get 5.5 mV at 30 meters, as you measured.

Your field strength at farther distances is  predicted to be:

100 meters: 1,468 uV/m

175 meters: 821 uV/m. Compare this to the 1,800 uV/m reading in the NOUO for Jerry Gaule.

Conclusion? Who knows? As long as your antenna is not visible to an FCC drive by, you could get a knock on the door for further evaluation of your setup. I don't think it is likely because I think your field strength is below their "trigger" level. And I don't think you are eligible for an NOUO even if they do knock.

As an aside, measuring field strength at 30 meters can give some squirrely results because 30 meters is just at the transition distance from reactive to near field at 1550 kHz. Basically, it's at an extremely near field distance. It would be better to measure at 100 meters or 175 meters to get you out away from the near field. See http://electronicdesign.com/energy/what-s-difference-between-em-near-field-and-far-field

This subject is not a simple one.

Here is a link  http://www.hpmemory.org/an/pdf/an_150-10.pdf  to an application note from Hewlett Packard covering this topic, based on a spectrum analyzer as the "receiver." 

The r-f voltage shown by the receiver display depends on the electrical characteristics and physical orientation of the receiving antenna at the frequency of interest, antenna system losses, and the impedance at the point of the conducted voltage measurement within the receiver.

Section B of the Appendix (second column, bottom of page 2) shows a a 1.75 dB difference between 50 ohm and 75 ohm systems -- a voltage difference of about 22%.

Commercial field intensity meters typically are supplied with an antenna and antenna system loss of known values, and the metering circuits and display read directly in units of field intensity (V/m, mV/m etc).

These commercial FI meters are calibrated in a "standard field," which typically is generated by a loop antenna using the hardware and process described in Part II of the paper linked below.

For FCC work, FI meters need to be re-calibrated as often as necessary to maintain their initial accuracy.  Some broadcast engineers/consultants do that routinely every two years or so, and it can cost as much as $1,500 when done by the manufacturer of the meter.

http://www.scott-inc.com/html/nrsc.htm

Rich,

Yes, the topic is certainly more complicated than you would think. In my research on the subject, I came across those two sources you mentioned, as well as quite a few others, which weren't as understandable. For myself, this is a learning experience.

I will be doing further reading and hopefully, some experimenting with loops on a spectrum analyzer. My goal is to be able to make some measurements that are "close enough" to professional equipment. We'll see how that pans out.

Joe