UPDATE March 28, 2017: Do you have a quality antenna analyzer and are you using a base loaded vertical? Continue reading but also check out this link for the steps that I use to make measurements useful in determining EIRP.
…That’s a very good question and one that is difficult to answer. Theoretically the best way would be to set up the antenna you want to use and then take measurements of the radiated signal at a given distance, adjusting power to the coax accordingly to meet the required limits. Unfortunately there is no inexpensive, calibrated field strength measurement equipment available to the amateur and meters designed for the bottom end of the AM broadcast band just don’t work like expected because they don’t have the tuning range nor does the calibration curve extend down to 472 kHz. You could always try to build a field strength measurement system and then calibrate it. John Andrews, W1TAG, did just that and talks about the process here. That’s a lot of challenging work even for an experienced RF Engineer.
You could use modelling software to make the determination but only a handful of guys have unobstructed antenna fields that would yield useful and accurate model data. Forget about it for a backyard vertical on MF and LF. Of course the direct measurement method works well, making measurements of antenna base current and antenna resistance and then plugging the numbers into the I squared R formula to determine ERP. Current can be measured with an RF ammeter and Andrews details the construction of one here. I use this approach but calibration can be tricky and meters are never linear. Ever. Most of the higher end analysers and VNA’s give reasonably good values of antenna resistance. Be sure to disconnect the ground system and impedance matching from the circuit when measuring, however. This method still does not address the actual radiated power at 1-kilometer, only what should be squirting out of the antenna, and trees and houses will reduce radiated field strength.
I said previously that models are not terribly accurate for the common ham in most cases. But that information might give a reasonable rule of thumb by which one might form opinions and educated guesses. I expect the ARRL will have to come up with a few rules of thumb once 630-meter is implemented under Part-97 for the reasons specified above. They did something similar with 60-meters, specifying something to the effect that a half-wave dipole at X-number of feet above ground with 50-watts applied would result in an EIRP near the legally specified limit. Or something like that. Unfortunately it will be much tougher at 630-meters and perhaps the only saving grace for 2200-meters is the fact that only a few guys will be able to meet a 1-watt EIRP limit without burning their loading coils to the ground which discounts most of the average hams.
A few years ago the 600-meter research group had a link on their page that specified some data to be used as a rule of thumb for setting up transmit antennas that was developed by Ralph, W5JGV / WD2XSH/7. I can’t find the link today on their web page but I did have the forethought to print it out when I saw it for the first time and subsequently scanned the page for later use and archiving. Its presented below. Note that the web address at the top of the page does not appear to work.
Keep in mind that this information is specified for 500 kHz and its going to be different at 472 kHz but the ERP numbers should go down as you decrease in frequency and losses will go up so with that information the potential station builder can make some reasonably educated guesses. Note that the power numbers are for a 1-watt ERP limit. 630-meter should be 5-Watts EIRP.
There are several spreadsheets and web calculators available. Keep in mind that there are numerous approaches to making the same calculations and each has their own merit under certain conditions so its probably best to make a number of calculations and determine which is best under your situation. Each method may yield slightly different answers. Neil, W0YSE / WG2XSV, has a link on his website to a spreadsheet that he developed using a number of sources and can be downloaded here. John Molnar, WA3ETD / WG2XKA, also has a spreadsheet on his website that can be downloaded here. There are a few others but they tend to get complicated because of their handling or lack of handling of top loading so these probably paint a good enough picture for now.
Finally, a really good document that is published by Nautel for their NDB transmitters contains some good antenna descriptions and calculations that might be usable for the would-be stations builder. It can be found here.
It will be interesting to see what the ARRL comes up with for their rule of thumb and recommendations. The information above is a good starting point.