FCC compliance for small FM transmitter

[[email protected]]

I know this question probably comes up a lot, as I've noticed from the
posts....

I want to build a very short range transmitter for operation in my
living room. Basically, I want to take the output of my subwoofer
channel from my home theater reciever and convert it to a digital
signal for broadcast to another part of the room. I'd like to be able
to hide the subwoofer and relocate it as needed without having to
reroute wires.

I think I can find enough info for the various circuits. However, I'm
pretty ignorant on FCC regs. I'm aware that the FCC requires that the
signal level be less than 250 microvolts at 13 meters from the antenna.
But, I have no idea how to test this. I understand this is typically
done in a lab, so of course, I wouldn't be able to test this. I also
understand that there is a formula for approximate conversion from the
transmitter power to signal strength:
(P*G)/(4*pi*D^2) = (E^2)/(120*pi)
P=Transmitter Power (Watts)
G=Antenna Gain
D=Distance (m)
E=Field Strength (V/m)

Ok, to make a long question even longer, I guess I'm asking if there's
an easy way to make sure I'm not violating FCC regs. I'd like to know
that by experimenting, I'm not going to get sued, fined or go to jail.
And also, I'd like to respect proper use of the spectrum.

I apologize for my ignorance. I'm a power engineer by education and am
trying to revive my long lost interest in elecronics. I can't imagine
a more useful thing to know.

 

[gwhite]

I know this question probably comes up a lot, as I've noticed from the
posts....

I want to build a very short range transmitter for operation in my
living room. Basically, I want to take the output of my subwoofer
channel from my home theater reciever and convert it to a digital
signal for broadcast to another part of the room. I'd like to be able
to hide the subwoofer and relocate it as needed without having to
reroute wires.

I think I can find enough info for the various circuits. However, I'm
pretty ignorant on FCC regs. I'm aware that the FCC requires that the
signal level be less than 250 microvolts at 13 meters from the antenna.
But, I have no idea how to test this. I understand this is typically
done in a lab, so of course, I wouldn't be able to test this. I also
understand that there is a formula for approximate conversion from the
transmitter power to signal strength:
(P*G)/(4*pi*D^2) = (E^2)/(120*pi)
P=Transmitter Power (Watts)
G=Antenna Gain
D=Distance (m)
E=Field Strength (V/m)

Ok, to make a long question even longer, I guess I'm asking if there's
an easy way to make sure I'm not violating FCC regs. I'd like to know
that by experimenting, I'm not going to get sued, fined or go to jail.
And also, I'd like to respect proper use of the spectrum.

I apologize for my ignorance. I'm a power engineer by education and am
trying to revive my long lost interest in elecronics. I can't imagine
a more useful thing to know.

Just don't do anything really stupid and you don't have to worry about
regulators if you are building a one-off "low" powered unit.

You don't even mention band.

 

[Mac]

I know this question probably comes up a lot, as I've noticed from the
posts....

I want to build a very short range transmitter for operation in my
living room. Basically, I want to take the output of my subwoofer
channel from my home theater reciever and convert it to a digital
signal for broadcast to another part of the room. I'd like to be able
to hide the subwoofer and relocate it as needed without having to
reroute wires.

I think I can find enough info for the various circuits. However, I'm
pretty ignorant on FCC regs. I'm aware that the FCC requires that the
signal level be less than 250 microvolts at 13 meters from the antenna.
But, I have no idea how to test this. I understand this is typically
done in a lab, so of course, I wouldn't be able to test this. I also
understand that there is a formula for approximate conversion from the
transmitter power to signal strength:
(P*G)/(4*pi*D^2) = (E^2)/(120*pi)
P=Transmitter Power (Watts)
G=Antenna Gain
D=Distance (m)
E=Field Strength (V/m)

Ok, to make a long question even longer, I guess I'm asking if there's
an easy way to make sure I'm not violating FCC regs. I'd like to know
that by experimenting, I'm not going to get sued, fined or go to jail.
And also, I'd like to respect proper use of the spectrum.

I apologize for my ignorance. I'm a power engineer by education and am
trying to revive my long lost interest in elecronics. I can't imagine
a more useful thing to know.

Maybe you should just use infrared. This will keep you outside the
purview of the FCC and help you maintain good karma vis-a-vis the RF
spectrum.

Barring that, if you are willing to stick to the bands used by cordless
phones and WiFi devices (~900 MHz and ~2.5 GHz) you might be able to buy
some kind of cordless phone chipset that would take care of most of the
work for you. I think this would be technically legal, but even if it is
not, I don't think it would cause anyone any problems.

Good luck!

--Mac

 

[mc]

An extremely simple way to be practically sure of compliance is that if in
an FM radio that is known to be in good working condition, you can't get a
solid signal from more than about 100 feet away, you're fine. That is, a
radio makes a good field-strength meter.

That is, make it no more powerful than it needs to be. (Reducing the
antenna is the easiest way to cut back.)

The old specification was to have a 0.1-watt final amplifier. That's a good
starting point.

 

[Jim]

Seems to me I remember some sort of FCC regulation that said:

Less than 100 mW DC to the final amplifier and less than 2 Ft antenna


================

It was something of that order, which when considering final efficiency
and the small radiating element would make the device meet the radiated
power spec for unliscenced operation.

In any case, a setup like that, particularly indoors, would be no problem
as far as the FCC is concerned. The RF signal isn't going to get very far.


Jim Pennell

 

[Rich Grise]

I know this question probably comes up a lot, as I've noticed from the
posts....

I want to build a very short range transmitter for operation in my
living room. Basically, I want to take the output of my subwoofer
channel from my home theater reciever and convert it to a digital
signal for broadcast to another part of the room. I'd like to be able
to hide the subwoofer and relocate it as needed without having to
reroute wires.

"Hide the subwoofer and relocate it as needed"?

1) Hide it from whom?
2) Do you even know what a "subwoofer" is?" If so,
3) Why would you want to relocate it in the first place?
4) As needed by whom? For what?

And once you answer all these questions, then throw away all of the
answers and use an inductive loop with a plain ol' they come in every
amplified PC speaker audio amp.

Cheers!
Rich

 

[[email protected]]

1) Hide it from whom?
Hide it from plain view... It's not the most attracive piece or
furniture.

2) Do you even know what a "subwoofer" is?" If so,
The ominidirectional base speaker.

3) Why would you want to relocate it in the first place?
I want to hide it behind an end-table or the couch. (See question 1.)

4) As needed by whom? For what?
As needed by ME. Maybe I won't like the way it sounds or looks where
it is.

Now, what is a "plain ol'...."? I apologize for my ignorance on this.

 

[Jim Thompson]

1) Hide it from whom?
Hide it from plain view... It's not the most attracive piece or
furniture.

2) Do you even know what a "subwoofer" is?" If so,
The ominidirectional base speaker.

3) Why would you want to relocate it in the first place?
I want to hide it behind an end-table or the couch. (See question 1.)

4) As needed by whom? For what?
As needed by ME. Maybe I won't like the way it sounds or looks where
it is.

Now, what is a "plain ol'...."? I apologize for my ignorance on this.

Personally I would look for pre-made transmitter-receiver pairs.

Keep us posted as to what you find... I'm interested also.

Nevermind some of the replies... some folk don't have a clue about
aesthetics ;-)

...Jim Thompson

 

[JeffM]

transmitter [to broadcast the] output of my subwoofer channel

Maybe you should just use infrared
Mac

Great--until someone walks across the room.

 

[JeffM]

ominidirectional base speaker

jondoeengineer

That's *bass*.

Now, what is a "plain ol'...."?

You chopped out the most important part of Grise's post.
Here it is again: inductive loop.
http://www.google.com/search?&q=hea...ive+cordless+OR+wireless+-Infrared+-Infra-red

ignorant on FCC regs

Grise's solution avoids that.
Mac also (wisely IMO) suggested avoiding RF.

 

[Mac]

transmitter [to broadcast the] output of my subwoofer channel
...to another part of the room
jondoeengineer

Maybe you should just use infrared
Mac

Great--until someone walks across the room.

If you use enough power, line of sight is not even remotely necessary. For
example, my remote control will operate my dvd player even when I am
behind the dvd player.

--Mac

 

[Rich Grise]

Personally I would look for pre-made transmitter-receiver pairs.

Keep us posted as to what you find... I'm interested also.

Nevermind some of the replies... some folk don't have a clue about
aesthetics ;-)

Mr. Thompson might or might not be surprised at how easy it is to hide an
induction coil.

BTW, "Plain ol'" is "plain old" as in plain old ordinary everyday, seen-
one-seen-'em-all, run-of-the-mill, whatever.

My point was, a set of amplified PC speakers has an amplifier already in
it. With speaker output. And it's probably stereo, so you could use them
in a bridge.

An induction coil is simply a coil of maybe a dozen turns of, say, #30 or
so magnet wire in a rectangle about the ceiling. You wind a corresponding
coil, maybe a dozen turns, maybe a hundred, around the base of the remote
woofer. Connect the two ends of the loop at the base of the woofer to the
mic input of the audio amp -

Oops! Amplified computer speaker amps probably aren't the best one for
using an induction coil. You need a mic input. But mic preamps are like
rice on the internet. See
http://www.google.com/search?q=mic+preamp+schematic
probably. I just typed that URL by hand, from memory. I'll go check if it
actually works...
Yup! :-D

Anyway, find a mic preamp, and just put the two ends of the speaker's
induction coil to its input, and its output to the speaker, of course. At
the source end, take the two ends of the big coil around the ceiling, and
connect them to the center channel speaker output.

The whole room becomes a transformer; the secondary is the winding at the
base of the speaker cab. This is how Mr. Tesla did his "power
transmission" demos, by the way. The whole room was inside the field of a
big transformer.

An RF link would involve considerably less ladder work, I must admit. ;-)

Cheers!
Rich

 

[Rich Grise]

That's *bass*.


You chopped out the most important part of Grise's post.
Here it is again: inductive loop.
http://www.google.com/search?&q=hea...ive+cordless+OR+wireless+-Infrared+-Infra-red


Grise's solution avoids that.
Mac also (wisely IMO) suggested avoiding RF.

Thanks for the honorable mention there. You can call me Rich.

I've described a home-made coil in another followup, but neglected to
follow up on my little barb, about how easy it is to hide. In the first
place, it's going to be in the corner between the ceiling and the wall,
where a little magnet-colored wire isn't that noticeable in the first
place, but it'd be really easy to touch it up with a little paint.

And, when you're not listening to the stuff that needs the subwoofer,
you could switch it over, and have it do double-duty as your DX receiver
antenna! ;-P

Cheers!
Rich

 

[Dave VanHorn]

I used this system years ago, for wireless headsets in a hamshack.
Pocket boxes with loopstick antenna and amplifier, into standard headphones.
Beware feedback coupling though!

Works wonderfully, until you cross the wire, then poof.. As expected.

 

[John Woodgate]

I read in sci.electronics.design that Rich Grise <[email protected]>

Anyway, find a mic preamp, and just put the two ends of the speaker's
induction coil to its input, and its output to the speaker, of course.
At the source end, take the two ends of the big coil around the ceiling,
and connect them to the center channel speaker output.

The whole room becomes a transformer; the secondary is the winding at
the base of the speaker cab. This is how Mr. Tesla did his "power
transmission" demos, by the way. The whole room was inside the field of
a big transformer.

An RF link would involve considerably less ladder work, I must admit.
;-)

But it wouldn't have a frequency response rising at 6 dB per octave, as
an induction loop coupling does. Not ideal for a sub-woofer channel!

The rising response turns over (3 dB point) to flat at the frequency
where the sending loop inductive reactance equals its resistance. Since
it's your sending loop design, I'll leave you to calculate that
frequency.

 

[Rich Grise]

I read in sci.electronics.design that Rich Grise <[email protected]>


But it wouldn't have a frequency response rising at 6 dB per octave, as
an induction loop coupling does. Not ideal for a sub-woofer channel!

The rising response turns over (3 dB point) to flat at the frequency
where the sending loop inductive reactance equals its resistance. Since
it's your sending loop design, I'll leave you to calculate that
frequency.

OK, I'll revise that.

You get 1000 pounds of magnet wire, and cover the walls of the room with
glue....

;-)

Thanks!
Rich