[BC] Part 15 LPAM Questions?

[Robert Meuser]

Of course, if the antenna were fat enough, it could be installed under a 
metal 'roof'. Since the solid closed top is of substantial diameter, 
that 'roof' would now become a capacative top load.

R


Richard Fry wrote:

> 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
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