Power in Perspective

[C2]

Hey Forum,

I wanted to make a chart of real world statistical data on groundwave transmission distances at various powers.

I'm NOT interested in skip distances.

If anyone wants to help me collect the data, that would be great. All I am looking for is two parameters: Power and Distance.

For example, when I had my CB setup, I was able to talk 20 miles with a 4 watt carrier output. With a 100W carrier, I could talk near 70 miles.


I wanted to get some additional data and generate some graphs to help formulate a best guess at transmission distances with various power levels, and to perform some statistical analysis on the data.

Whatever mix of setup, conditions, antenna, and all those variable is just fine and not a concern. I don't care about the SWR, how much gain your antenna has, how sensitive your receiver was, or how that would affect your results. There is a reason that don't matter.

And thanks for the help.

 

[ken white]

The electric field strength of a ground wave signal is a function of the transmitting antenna current and distance from it. So if you know these values, or can estimate signal strength from an equation, and you pick a baseline receiver sensitivity, you should be good to go.

Electric Field Strength (E) = 60 (a resistance constant of the earth for the ground wave) x Antenna Current (Ia) / distance in meters from the antenna (d)

E = 60xIa/d

So if we estimate/calculate the antenna current from power out and antenna feedpoint impedance, Ia = (Pwr/50)^0.5

Or E = 60x(Pwr/50)^0.5/d which has units of volts per meter. This will translate to receiver sensitivity.

So the field strength is directly proportional to the antenna current and inversely proportional to the distance from the transmitting antenna.

Or d = 60x(Pwr/50)^0.5/E

So for every doubling of the distance at the same signal strength will require 4 times the power.

These calculated results should be fairly close to the empirical data you are collecting.

BTW, I only talk with 4W out of my CB and the antenna is mounted on a tire carrier on the back of my Jeep, so I don't have any good data, or range...

:shock:

 

[C2]

Yea, but someone thinks that it can't be done that way:

"there are many more factors involved in these calculations besides transmitter power...receiver sensitivity, antenna polarization, soil properties, transmitting and receiving antenna heights or gain let alone system losses at both ends of the "circuit""

So I figured there would be another way, that would be through statistical analysis of actual real-world data.

But thanks for the formulae, very nice!

 

[ken white]

..."there are many more factors involved in these calculations besides transmitter power...receiver sensitivity, antenna polarization, soil properties, transmitting and receiving antenna heights or gain let alone system losses at both ends of the "circuit"...

Well technically the person you quoted is correct since there are many variables that can be included in any model and tweeked for "1" specific case.

But to be honest, engineering is about creating a mathematical model of a system output that will give relatively accurate results for known inputs. The model has to be simple enough to do the math, while providing results that enforce the theory behind the model, and are close to the predicted value.

So, can a better model be made for almost every situation? The answer is yes but then the person doing the model would have to perform numerous tests to validate "all" of the variables in the model and then it will not provide general information anymore. Most times this increased effort doesn't produce any great insight and the original model is found to be more than sufficient.


Have fun with your data harvesting and don't take this stuff too serious.



EDIT: As a side note when I was active duty in the Coast Guard, we used a slightly more sophisticated model than the one I posted above to predict field strength for the CG LORAN Station signals, which are primarily ground wave, and the results were pretty accurate...