The following are some formulas that are handy for antenna work.
This is a main beam power density estimation program intended for use as part of a routine evaluation of RF safety compliance with FCC regulations. Amateur Radio operators licensed by the Federal Communications Commission of the United States of America are required to do a "routine evaluation" of the strength of the RF fields around their stations, subject to certain exemptions. These rules can be found in the FCC's ET Docket No. 9362. A calculator can be found on the N5XU web site. 

A typical RF field or power density measurement device is composed of a probe, leads and metering instrumentation. The probe is used to detect the field. It can either be a conventional antenna or another type of sensor. The performance and the application of the measuring instrument as a whole depend to a large extent on the design and characteristics of the probe. See the FCC's paper on Measurements of Environmental Fields at Amateur Radio Stations. (PDF) 

SIMPLE FORMULAS
Abbreviations
 E =VOLTS or (V = VOLTS)
 P =WATTS or (W = WATTS)
 R = OHMS or (R = RESISTANCE)
 I =AMPERES or (A = AMPERES)
 HP = HORSEPOWER
 PF = POWER FACTOR
 kW = KILOWATTS
 kWh = KILOWATT HOUR
 VA = VOLTAMPERES
 kVA = KILOVOLTAMPERES
 C = CAPACITANCE
 EFF = EFFICIENCY (expressed as a decimal)
DIRECT CURRENT
 AMPERAGE .......... I = P ÷ E
 WATTS ............... P = E x I
 VOLTS ................ E = P ÷ I
 HORSEPOWER .. (V x A x EFF) ÷ 746
 EFFICIENCY ...... (746 x HP) ÷ (V x A)

AC SINGLE PHASE or 1ø
 AMPERAGE ............ I = P ÷ (E x PF)
 WATTS .................... P = E x I x PF
 VOLTAGE ............... E = P ÷ I
 VOLTAMPS ........... VA = E x I
 HORSEPOWER ....... (V x A x EFF x PF) ÷ 746
 POWER FACTOR ... INPUT WATTS ÷ (V x A)
 EFFICIENCY ......... (746 x HP) ÷ (V x A x PF)

Frederick R. Vobbe, W8HDU
