There are a lot of useful calculators online these days. Here is one that I liked from Compliance Engineering: http://www.compeng.com.au/emc_conversion_tables.asp
There are a lot of useful calculators online these days. Here is one that I liked from Compliance Engineering: http://www.compeng.com.au/emc_conversion_tables.asp
This calculator will allow you to enter measured field strength and determine EIRP, or the other way around. For example, data from the test report for the new ChezRadio transmitter shows a field strength of 75 dBuV/m at 10 M. You can enter this value in the calculator to determine the EIRP in mW. Try it– You might be surprised! One caveat: this was apparently a near-field measurement, so accuracy might not be very good. I am a little surprised they didn’t perform this test outdoors, on an Open Air Test Site (OATS). Of course, you do have to be concerned about the ambient RF environment in that case. From this calculation, the predicted field strength at 1 km would be 40 dB lower, or about 56 uV/m. Not earth-shattering, but certainly audible on a good car radio.
Here’s another page with lots more interesting calculators: http://www.rfcafe.com/references/calculator_links.htm
RE: On-line Field Calculator
WEAK-AM wrote: “There are a lot of useful calculators online these days. Here is one that I liked… This calculator will allow you to enter measured field strength and determine EIRP, or the other way around. For example, data from the test report for the new ChezRadio transmitter shows a field strength of 75 dBuV/m at 10 M. You can enter this value in the calculator to determine the EIRP in mW. Try it– You might be surprised!”
Just a cautionary note – that calculator is accurate only for a spherical volume of space. Monopole antennas do not operate in such an environment, so the answer given by that calculator for such systems has a large amount of error in it.
One caveat: this was apparently a near-field measurement, so accuracy might not be very good.
In antenna engineering textbooks the near-field boundary is taken to be 2*(L^2) / lambda, where L is the greatest dimension of the antenna in meters, and lambda is the wavelength in meters. So measurement of the 1500 kHz field radiated from a 3-meter antenna at a distance of 10 meters turns out to be a far-field measurement.
Rgds,
Rich
Correct value
Perhaps someone would care to supply what they feel would be the correct value for the radiated power, based on the measurement in the test report.
WEAK-AM
Classical Music and More!
Radiated Power (AMTX100 Test Report)
If the test chamber truly was anechoic in the entire radiation sphere, and the field measurement was taken in the direction of the peak gain of the monopole radiation pattern, then the effective radiated power in that direction was 64.29 µW.
This is the same value as produced by the online calculator link above, but derived by other means. The point remains, though, that this value is inaccurate for this system when it is installed in any other environment (such as a typical Part 15 AM setup).
The amount of output power needed from the transmitter to produce that radiated power was a function of the usual factors: frequency, antenna length/width and ohmic loss, matching coil loss, and ground system loss.
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