[Lowfer] FS measurement question #2

[John Andrews]

Andy,

> **** Question #2 : If I understand this correctly...

No, sorry. Let's say for a moment that FCC Part 15.217 did not exist. 
And let's say that you have an interest in running a transmitter on 
185.300 kHz. Without the 15.217 rules, you would turn to FCC Part 15.209 
and, as you have correctly observed, the maximum permitted field 
strength limit is 2400/185.3 = 12.95 microvolts/meter at a distance of 
300 meters. That specifies the electric field, which is measured in 
volts/meter. It has nothing to do with what sort of setup produced it. 
So there's no multiplier for a transmitting antenna.

The following is a gross simplification, as 300 meters is well within 
the near-field at 185.3 kHz, but: A small vertical (and 15 meters is 
less than .001 wavelength) fed with 1 watt of power, and over a 
perfectly conducting ground, with a no-loss coupling system will produce 
32 millivolts/meter at 300 meters. You would have to drop the 
transmitter power to 168 nanowatts to meet your 12.95 microvolt/meter limit!

You can see why we are happy to have 15.217 available to us. It allows 1 
watt into a 15 meter antenna (no feedline) with NO limit on field 
strength. If you are curious about your actual field strength, you can 
measure it as described in the article that Jay referenced, but the 
information would only be for your education, not to satisfy a regulation.

John A.