Liam,

You are absolutely right that the antenna cannot radiate more power
than you feed it. They are probably talking about "effective ratiated
power", which is closely related to antenna gain. You get more power
at the receivers, but only in the directions where radiation is
maximized. For example, a Yagi directs the power more in one
direction, and a "vertical collinear" directs it in a horizontal
plane. Each does so at the "expense" of power radiated in other
directions, but if you don't care about reaching receivers in those
other directions, it's not a problem. But be careful: do the rules
limit the maximum power, or the maximum ERP (effective radiated
power)? (The same thing works for receiving, too. You can use
antenna directivity to increase the received signal level, and to
reject signals or noise coming from directions you don't care about
receiving.)

Cheers,
Tom

Liam Ness wrote in message . ..
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

Thank you to everyone who responded to my question. And thank you for
making what I thought I had a handle on so much clearer to me.

Liam Ness

Thank you to everyone who responded to my question. And thank you for
making what I thought I had a handle on so much clearer to me.

Liam Ness

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

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

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

In article , "Reg Edwards"
writes:

Antennas have directional properties.

It the total available power is concentrated in one direction then, as far
as the receiver is concerned, the APPARENT power of the transmitter has
increased.

But receivers in less-favoured directions from the transmitter will
experience an APPARENT reduction in the transmitter's output power.

Quite true, Reg.

To get even more basic for new folks to radio theory, assume the
ideal isotropic antenna, one that radiates equally in all directions.
It creates an EM field of the same density of RF energy per square
area all around an ideal sphere enclosing the isotropic antenna.

A half-wave dipole that is very high (elevated above ground) has an
antenna pattern (of RF energy per square area) that is maximum
perpendicular to the axis of the wires. That RF energy is minimum
along the wire axes.

An ideal dipole has a "gain" of about 2.4 db over the ideal isotropic
antenna and such gain is referred to as "2.4 dbi" with the little "i"
indicating the reference to the isotropic.

Because it is difficult to build a reference antenna that can perform
like the ideal isotropic, many more complex antennas reference their
gain to the half-wave dipole and those gains, in db, are labeled as
"dbd" with the little "d" suffix refering to a dipole.

FM and TV broadcast antennas are usually designed for antenna
patterns that are almost omnidirectional in the horizontal plane and
have very little RF energy at elevations above or below horizontal.
The term "ERP" for Effective Radiated Power was first used with
FM and TV broadcasting to indicate the basic power output of the
transmitter multiplied by the antenna gain. For broadcast listeners
they would "hear" a signal as if the station's power output was as
strong as the ERP value.

Directional antennas simply focus the RF energy in certain
directions. What would be an equal value of RF in all directions
with an ideal isotropic antenna now increases above ideal in the
direction of maximum RF energy. That is the "gain."

If one were to plot RF energy density per square whatever in a 3-D
graph (almost always the case in antenna analysis programs or on
antenna range receiver plotters), the "shape" formed is what everyone
intuitively describes as the "pattern." [it could be called an "isopower"
plot, I would guess]

For an ideal isotropic antenna, the "pattern" is a sphere. For a good
half-wave dipole very high above ground, the pattern looks like a torus
or doughnut shape. A typical FM or TV antenna pattern looks like
a fat pizza. A very high gain parabolic reflector radar antenna pattern
looks sort of like a long breadstick. Complex wire antennas start
looking like the outline of the contents of a spaghetti bowl.

It's about 10:30 PM local time here and I'm getting hungry...bye...:-)

Len Anderson
retired (from regular hours) electronic engineer person

In article , "Reg Edwards"
writes:

Antennas have directional properties.

It the total available power is concentrated in one direction then, as far
as the receiver is concerned, the APPARENT power of the transmitter has
increased.

But receivers in less-favoured directions from the transmitter will
experience an APPARENT reduction in the transmitter's output power.

Quite true, Reg.

To get even more basic for new folks to radio theory, assume the
ideal isotropic antenna, one that radiates equally in all directions.
It creates an EM field of the same density of RF energy per square
area all around an ideal sphere enclosing the isotropic antenna.

A half-wave dipole that is very high (elevated above ground) has an
antenna pattern (of RF energy per square area) that is maximum
perpendicular to the axis of the wires. That RF energy is minimum
along the wire axes.

An ideal dipole has a "gain" of about 2.4 db over the ideal isotropic
antenna and such gain is referred to as "2.4 dbi" with the little "i"
indicating the reference to the isotropic.

Because it is difficult to build a reference antenna that can perform
like the ideal isotropic, many more complex antennas reference their
gain to the half-wave dipole and those gains, in db, are labeled as
"dbd" with the little "d" suffix refering to a dipole.

FM and TV broadcast antennas are usually designed for antenna
patterns that are almost omnidirectional in the horizontal plane and
have very little RF energy at elevations above or below horizontal.
The term "ERP" for Effective Radiated Power was first used with
FM and TV broadcasting to indicate the basic power output of the
transmitter multiplied by the antenna gain. For broadcast listeners
they would "hear" a signal as if the station's power output was as
strong as the ERP value.

Directional antennas simply focus the RF energy in certain
directions. What would be an equal value of RF in all directions
with an ideal isotropic antenna now increases above ideal in the
direction of maximum RF energy. That is the "gain."

If one were to plot RF energy density per square whatever in a 3-D
graph (almost always the case in antenna analysis programs or on
antenna range receiver plotters), the "shape" formed is what everyone
intuitively describes as the "pattern." [it could be called an "isopower"
plot, I would guess]

For an ideal isotropic antenna, the "pattern" is a sphere. For a good
half-wave dipole very high above ground, the pattern looks like a torus
or doughnut shape. A typical FM or TV antenna pattern looks like
a fat pizza. A very high gain parabolic reflector radar antenna pattern
looks sort of like a long breadstick. Complex wire antennas start
looking like the outline of the contents of a spaghetti bowl.

It's about 10:30 PM local time here and I'm getting hungry...bye...:-)

Len Anderson
retired (from regular hours) electronic engineer person

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.