On Wed, 08 Oct 2003 23:29:46 -0700, Roy Lewallen
wrote:

You've gotten some good advice some others. I'll just add that most Part
15 devices are specified in terms of field strength at some distance
from the antenna, depending on frequency, and not in terms of power or
ERP. There might be some sections with other criteria, but if there are,
field strength specification is by far the most common. The FCC does cut
some slack in testing for home-built devices (not marketed, not
constructed from a kit, and built in quantities of five or less for
personal use), in section 15.23. My copy is nearly ten years old now, so
I suggest checking a newer copy of Part 15. It's likely on the Web these
days.

Roy Lewallen, W7EL


Yea, thanks Roy, you are right. I know that Part 15 limits are field
strength based, and there isn't a direct corrolation with output
power. I have been relying in the 15.23 home built portion, which
provides some lee way to tinkerers. 15.23 acknowledges that home
builders probably don't have test equipment, and instead it is
directed at design. My stuff is super low power, at least that is
what my spice program tells me, so I'm not really worried. If I do
screw up, and an FCC representive contacts me (the first highly
likely, the second much less so) I'll push section 15.23's
applicability, stop using the transmitter and redesign per section
15.5.

Even though I'm in a really rural, really mountainous location, I
spent a good amount of time going over part 15 so I could justify that
anything I do is done in a good faith effort to be in compliance. You
are also right that part 15 is now web based. They just released a
new version of the regulations in August and you can d/l it as a PDF
from the FCC website.

Liam Ness wrote:

I've been homebrewing some simple part 15 transmitters and have always
thought that I was safely within part 15 by controling the RF output.
I use a spice program to estimate my output levels. I just read a web
page that suggests a antenna can increase the RF output power and I
wanted advice if that is true. It was suggested that output could be
increased from 30milliwatts to 60milliwatts by using this antenna. I
understand how you could increase voltage with a decrease in amperage
and vice versa, but I was under the assumption that you couldn't
increase total power without adding more power. I thought it would
violate one of the laws of thermodymanics otherwise. They didn't seem
to be talking about more effieciently radiating the transmitters
power, but actually increasing it above what is present at the antenna
port.

Could someone confirm whether it is posible to increase the power
output of an RF transmitter above the total presented to the antenna.

Yes, IF the present antenna presents a bad impedance match
to the transmitter. Two factors come into play: 1) maximum
power transfer occurs when the source (transmitter) and load
(antenna) impedances match; and 2) the efficiency of the output
stage of the transmitter may be compromised when connected to
a mismatched load, resulting in less RF power produced.

But I doubt whether that was the point. I think that all they
were trying to say was that you would get more effective
radiated power. And the claim of doubling the power raises
suspicion. If memory serves me, part 15 radiations are severely
limited. That may preclude 3db gain antennas.



If it is, I'd appreciate any pointers to information about this. I
don't want put myself out of part 15 by a poor antenna choice. (even
though I still can't believe that it is possible, it sounds to much
like perpetual motion)

TIA

You've gotten some good advice some others. I'll just add that most Part
15 devices are specified in terms of field strength at some distance
from the antenna, depending on frequency, and not in terms of power or
ERP. There might be some sections with other criteria, but if there are,
field strength specification is by far the most common. The FCC does cut
some slack in testing for home-built devices (not marketed, not
constructed from a kit, and built in quantities of five or less for
personal use), in section 15.23. My copy is nearly ten years old now, so
I suggest checking a newer copy of Part 15. It's likely on the Web these
days.

Roy Lewallen, W7EL

Liam Ness wrote:
I've been homebrewing some simple part 15 transmitters and have always
thought that I was safely within part 15 by controling the RF output.
I use a spice program to estimate my output levels. I just read a web
page that suggests a antenna can increase the RF output power and I
wanted advice if that is true. It was suggested that output could be
increased from 30milliwatts to 60milliwatts by using this antenna. I
understand how you could increase voltage with a decrease in amperage
and vice versa, but I was under the assumption that you couldn't
increase total power without adding more power. I thought it would
violate one of the laws of thermodymanics otherwise. They didn't seem
to be talking about more effieciently radiating the transmitters
power, but actually increasing it above what is present at the antenna
port.

Could someone confirm whether it is posible to increase the power
output of an RF transmitter above the total presented to the antenna.
If it is, I'd appreciate any pointers to information about this. I
don't want put myself out of part 15 by a poor antenna choice. (even
though I still can't believe that it is possible, it sounds to much
like perpetual motion)

TIA