Is there a way to theorize an approximate effect using the difference between two EIRP values?
If, for example, a dish is getting an SNR of 80 and a BER of 0 from a satellite reported to have an EIRP value of 42 for the area can an SNR and BER for an adjacent satellite with a reported EIRP of 37 be guessed with any kind of hope of being correct? I am not trying to calculate actual numbers but rather guesstimating the likelihood of capturing a usable signal.
EIRP is an essential tool in determining your ability to receive reception. Both the ITU and the FCC specify the format and method of presenting the data. Those regulatory agencies provide that the dBw data used be submitted using a zero (0) offset for the center point. This makes the maps contour power variances between two EIRP curves valid even when the power level is changed Typically the data is provided for at least the -2,-4,-6,-8,-10,-15, and -20 dBw contours. The peak value is substituted for the zero (0) level when publishing the curves and offsets correspondingly calculated.
If you have a reference signal (you do in your case) and the geographic location of your reference, it does not matter if the transmitter power has been dialed down to a lower level since all calculations are based on an offset value from the center point. For example, if the center point EIRP is reduced by 6 dBw, all of the other contours are also reduced by 6 dbw and the relative difference in dBw values remains constant.
Remember, all values have a logarithmic relationship and a dBw change of 3 represents either double or half power, a dBw change of 6 represents 4 times or 1/4 power, etc.
In your case, your signal at the 42 EIRP map point is about 3.15 times that which you would see at the 37 EIRP location (a change of 5 dbw). And since signal strength is proportional to antenna surface area, you would need an antenna with 3.15 times the surface area to receive an equivalent signal at the 37 site as compared to your 42 site.
