[Lowfer] Part 15 power limit for Hifers

[w1tag at charter.net]

Garry,

Ahhh, but field strength is not specified by any characteristic of the 
receiving antenna. In fact, half-wave dipoles would almost never be used 
for FS measurements. At 13 MHz, the FCC (or anyone else) would typically 
use a small loop.

To correct one error, the current FCC Part 15 regulation specifies 
15,848 uV/m at a distance of 30 meters in that "Hifer" band. See FCC 
rules 15.225(a).

A half-wave dipole in free space produces a maximum electric field of:
E = SQR(49.2*P)/D , where
E = field in Volts/Meter
P = power in Watts
D = Distance in meters

So for 15848 uV/m at 30 meters, I get 4.6 milliwatts.

That E field is a characteristic of the transmitting antenna, the power 
and the environment. Field measurements are simply a sampling of the 
field strength at a point, and any significant coupling of power into 
the the receiving setup would render the measurement useless.

John, W1TAG


On Sat, Aug 6, 2011 at 3:16 PM, Garry Hess wrote:

> I recently activated a hifer beacon on 13.55544 MHz. I set the power 
> according to guidance at http://lwca.org/sitepage/part15/index.htm, 
> which indicates 4.8 mW is allowable for half-wave dipoles and 
> quarter-wave verticals over ground planes (I'm using the former). 
> However, I just calculated what 10,000 uV/m at 30 m represents and I 
> think that guidance is in error. The maximum field strength implies a 
> received power of 1.72 uW by a half-wave dipole. For free-space, 
> far-field propagation this would be produced by a transmit power of 
> 1.86 mW (2.7 dBm) using a half-wave dipole. If an isotropic radiator 
> is assumed then the answer is 3.05 mW, matching the suggested 4.8 mW. 
> Thus, it seems the lwca reference applies to an isotropic radiator, 
> not a dipole or vertical. Comments appreciated.
>
> 73, Garry, K3SIW, EN52ta, Elgin, IL
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