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.

I guess this all (well most anyway) makes sense. And while at first I
was glad that you all confirmed my thinking that you can't get more
out than you put in (although you can get what you put in out more
efficiently), Roy's part 15 comments are making rethink things.
Because I'm field strenght limited, I concievable could be over limit
in one direction. I don't think that I could accidentally rig an
antenna like in Reg's example, but just for piece of mind could you
give me an idea of how far from isotropic (or how close to
ultradirectional) a vertical piece of wire would be at 1/4, 1/2 and
not even close to being matched to wavelength? I'm not looking for
calculations or anything, a simple very, not very or somewhere in
between to isotropic would be fine.

TIA



On 09 Oct 2003 16:27:30 GMT, (Avery Fineman)
wrote:

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

I guess this all (well most anyway) makes sense. And while at first I
was glad that you all confirmed my thinking that you can't get more
out than you put in (although you can get what you put in out more
efficiently), Roy's part 15 comments are making rethink things.
Because I'm field strenght limited, I concievable could be over limit
in one direction. I don't think that I could accidentally rig an
antenna like in Reg's example, but just for piece of mind could you
give me an idea of how far from isotropic (or how close to
ultradirectional) a vertical piece of wire would be at 1/4, 1/2 and
not even close to being matched to wavelength? I'm not looking for
calculations or anything, a simple very, not very or somewhere in
between to isotropic would be fine.

TIA



On 09 Oct 2003 16:27:30 GMT, (Avery Fineman)
wrote:

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