[BC] FCC Rule question

[Jeff Glass]

In a message dated 2/13/2010 8:00:27 AM Central Standard Time, broadcast-request at radiolists.net writes:

>If you are referring to antenna pattern calculations the following applies:

>    47 CFR ?73.150 Directional Antenna Systems.

>    (b)(6) The values used in specifying the parameters which describe
>    the array must be specified to no greater precision than can be
>    achieved with available monitoring equipment. Use of greater
>    precision raises a rebuttable presumption of instability of the
>    array. Following are acceptable values of precision; greater
>    precision may be used only upon showing that the monitoring
>    equipment to be installed gives accurate readings with the specified
>    precision

Al:

The stuff you posted in response to my question is great!  Thank you very much; greatly helpful.

Specifically, I'm studyingt the eample in 73.185 (d) regarding skywave calculations. In that example, they give the 50% skywave as. 06217 mv/m and the corrected 10%skywave as .1616 mv/m as being 8.42 dB apart. I don't see how they get 8.42 dB. 20 log .(1616/.06217) = 8.297, which is close for practical purposes I would think, but not equal to what the FCC got.

I diddled around with various combinations of significant figures, and could only get it down to 8.4 dB. If this is good enough for the FCC, then why did they report 8.42 dB? The accuracy and precision are not compatible. I cannot see how they got the answer they got.

So maybe there is something in the rules that specifies the accuracy and precision for calculations expressed in dB?

Anyone know if there is an FCC  graph somewhere for 50% and 10% skywave for SS + 6 hours?  The only such graph I've found is one for SS + 2 hours. The SS + 6 hour version is the one used in the 73.185 (d) example, which was done by formulas.

Al, thanks again :-) If you or anyone else here can help me unravel this mystery, please let me know.  My experience so far has been that t is possible to get the same answers as the FCC gets, so long as the assumptions made are the same as the ones made by the FCC. Often, it's not easy to determine those FCC assumptions.

many thanks,

Jeff Glass
Northern Illinois University