[BC] Part 15 LPAM Questions?

[Richard Fry]

In a message dated 09/11/2006 12:28:55 PM Eastern Daylight Time,

>Ok, what about the "cage" idea? Suppose you used 8 copper pipes,
>etc etc

The r-f characteristics of this cage would be very similar to the 25" OD 
cylindrical radiator that I posted earlier.  Its elevation pattern and peak 
gain would be the same as the 25" cylinder, and the same as from the 0.5" 
OD radiator, for that matter.

I don't and can't speak for the FCC, but from the point of view of physics, 
either of these fat antennas would comply with Part 15 as long as the total 
of its height with the length of exposed conductor from the tx to the 
buried r-f ground did not exceed 3 meters.

>I am considering the feasability of making a P15-compliant antenna
>and attaching it to the wellhead in my yard. (That should be a *really*
>good ground: solid steel pipe, many dozens of feet straight down
>to the water table).

That may sound like a good r-f ground, but actually it will be poor. 
Radiation from the antenna induces r-f current in the top layer of earth 
out to at least 0.5 wavelength from the radiator.  That current needs to be 
collected by the r-f ground system and returned to the antenna system.  The 
resistance in that path is what determines the r-f loss in the ground 
system.

The earth is not a good conductor, so if the induced currents have to 
travel as much as a 1/2 wavelength through it to get to your wellhead pipe, 
the damage is already done.

Better to install a radial ground system that can capture the ground 
currents where they originate, and provide a low-loss path back to the tx 
system

RF