Power vs Range
I have been playing with a pair of COBRA model PR3850-2 WX EVP units rated at 12 mile range and have found this much to date.
The power consumption from the batteries with the loaded voltage at 5.68 volts is 275 MA in the low power mode and 733 MA in the high power mode. Using this information as a starting point the following assumptions are made concerning the output power.
Using Ohms law, P=E X I, 5.68 X .275 = 1.562 watts. This is the total power required in order for all of the electronics to operate in the “LOW POWER” transmit mode. Assuming that this unit would be outputting 1/2 watt (500 mw) as specified for FRS maximum power one could approximate the relative RF power by subtracting the 1/2 watt assumed output power from the additional power consumed.
1.562 - .5 = 1.062 watts consumed by all of the electronics other then the actual RF power needed for the output stage.
Remembering that in the high power setting the current was 733 ma. 5.68 X .733 = 4.16344 watts minus the 1.062 = 3.10144 watts of approximate input power. Most class “C” R.F. output stages operate with an efficiency of 80% typically so by subtracting ~ 20% we have a remaining final output power of 2.48 watts.
To confirm this I modified the stock antenna with a BNC connecter and connected it to a watt meter with a 50 Ohm termination and on the 10 watt range I could see a reading of ~ 2.5 watts. This was confirmed on two other meters as well.
I hope to order from you this week a set of MIDLAND GXT710VP3 rated at 26 miles and do the same. Hopefully within a few weeks I should see how closely this emulation works.
One other assumption I am making on the mileage ratings within the same manufactures clams is that if the power rating given is the same 5 watts (regardless of the actual power) over another model the receiver sensitivity and or signal to noise ratio must be better, another specification never mentioned. --mike shearer–