[AMRadio] Low powered FM

[Jeff/W5OMR]

I found the answer, Brett...

The question was:

===================================================
Q:
I'm curious about something... Low-powered FM broadcast.
Somewhere in the FCC Rules and Regs, it says that anyone
can transmit anything, as long as it's below (_blank_)
amount of milliwatts, and the antenna is less than (_blank_)
feet, or has than (_blank_) ERP.  Can someone fill in the
blanks?

A:
In the United States, Title 47 of the Code of Federal
Regulations (Part 15, subpart C) indicates that unlicensed
open-air broadcasting is limited to microscopic power levels.
The power limit for unlicensed FM transmissions is a signal
strength of 250 microvolts per meter, measured 3 meters from
the transmitting antenna. At this power level, stereo reception
with a good signal to noise ratio is only possible within a 100 foot
radius, and an average car radio can barely detect the signal at a
distance of 100 meters.
=======================================================

That's for Low Power Frequency Modulation.
The answer continues:

=======================================================
On the AM band, the limit is 0.1 watt and an antenna system no more than 3
meters long; in most
cases this provides a range of 3 or 4 city blocks for cheap receivers, farther
for high-quality radios.
=======================================================

----- Original Message -----
From: "Brett Gazdzinski"
Sent: Thursday, January 17, 2002 14:09
Subject: RE: [AMRadio] Low powered FM



: It would be interesting to be able to key, talk and listen
: remotely while mowing the lawn say...
:
[8< snip >8]
:
: You could do the cordless headphones and wireless mike on vox
: using headphones with a boom mike......

John/WA5BXO and Larry/WD5CFJ hooked up an FM radio one
time to John's 32V exciter.  He then hauled his reciever to the
remote feild-day site, for traditional Sunday morning AM QSO
on 75m.

How they accomplished this, was to take the output of the detector
in the FM rig, and apply the recieved signal at to the base of a
2N3904 (something quite generic) to key the 32V, which was the
exciter at the time, as I recall, of his 304TH rig.  The audio was
then taken off of the discriminator, and fed into an input on the
speech-amp.

Gains were adjusted, and the setup was tested, and it all worked
nicely, but John was a nervous as a cat on a hot, tin roof about
leaving his big rig on, and unattended.

This was back in the days of 1kWDC Input to the final and the VariAc
had been reduced to a level of around 300w, carrier output.

It's been done before, yes.  And, with +/-5kc of deviation, there's 10kc
of audio, theoretically, available.

As I said, as long as the speech-amp has low-end roll off, somewhere
around, oh - I'd say 80cycles and no more than around 3.1kc, to prevent
buckshot and splatter, respectivly.

Passing audio down to 30cycles, and beyond 5kc may sound nice when
you're 90-gazillion db over S-9 and have the receiver in the 16kc
position, but out in the distance, the audio doesn't make it.

The "power robbing lows" sound mushy, and too many
highs, just irrates people up to 7kc up the band.

73 = Best Regards,
-=Jeff/W5OMR=-


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