How many MURZ farz in the woods?

[Lscott]

1 hour ago, SteveShannon said:

Because RF is emitted in all directions equally from an isentropic antenna, one way to compare the range of different power outputs is to think in terms of the volume of a sphere:

Actually the antennas respond to the "E" field. Look at the sensitivity rating of a radio's receiver. It's specified in microvolts.

To get a ruff idea the power density varies as the inverse square of the distance since the same power is spread out over a larger surface area of the sphere around the isotropic source.

So if the power density is:

PD=(watts)/(sphere's surface area)

Expressing sphere's surface area as a function of the radius:

sphere's surface area=Pi*radius^2

PD=(watts)/(Pi*radius^2)

And watts can be expressed as:

watts = (Volts^2)/(Resistance of free space), which is about 120*Pi, or 377 ohms.

So your power density becomes:

PD=[(Volts^2)/(377 ohms)]/[(Pi*radius^2)]

Or after a bit of rearrangement:

PD=[(volts/radius)^2/[(Pi*377 ohms)]

That will vary as the surface area of the sphere, or the inverse square of the distance. If the radius is in meters then you have the "E" field intensity in Volts/Meter.

PD=(E^2)/(Pi*377 ohms)

Note that antenna simulation software will specify the antenna's "E" field in volts per meter. Also be careful you want the "Far Field" number.

This also leads to another topic about "path loss" and how there is is a difference between VHF verses UHF for signal strength given antennas of equal gain. It turns out you need much more power at UHF to get the same signal strength you would see on VHF for the same power. Part of the gain you get on VHF is ruined due to the short stubby VHF antenna gains on HT's, reportedly around -5 to -6 db, which almost cancels out the reduction in path loss reduction. Just can't win.

[SteveShannon]

5 hours ago, marcspaz said:

 

The information above is technically accurate, but should we caveat that for the woods?  Since the forest isn't consistent in density, levels of foliage change, the amount of water in the trees vary, etc.  actual results may vary.

Yes!  Sorry, I thought I included the phrase “all other things being equal.“

I meant to anyway. 

[WRPG745]

The math is a great explanation of why more power doesn't always deliver what a user expects. It reminded me of a joke where the punch line includes "spherical cows in a vacuum" (iykyk)

[AdmiralCochrane]

13 hours ago, WRPG745 said:

The math is a great explanation of why more power doesn't always deliver what a user expects. It reminded me of a joke where the punch line includes "spherical cows in a vacuum" (iykyk)

The famous bovine derivative of the Schwarzchild metric