[SOC] Unlicensed FM or AM brodcasting to my yard...

[JMcAulay]

At 02:36 PM 04/12/2002 -0500, Rob, W=D8JRM wrote:

>I'm wanting to whip something up to listen to stuff (mostly OTR shows)
>that's played with my computer out in the yard and around the house on an
>AM or FM radio.  I've been checking around on maximum allowable unlicensed
>part 15 (whew) power levels to make sure I don't get busted for anything.
>At FM it's 250uV@3m (according to one site that works out to 18.7nW to an
>isotropic) and on AM it uses a formula of XuV per meter measured 30m from
>the antenna, where X is figured up with the formula 24000/F(kHz) (the 30m
>is extended to the edge of the campus of educational places).
>
>Here's the point of all this:
>
>1. How do I convert uV/meter to wattage

You don't bother.  See below instead.

>2. I have a big yard (about 3 acres)... if I was to go with FM part 15
>would 18.7nW cover all that?  The cordless phone works fine all over the
>yard, how much power do those put out?  I could compare the two I
>suppose...

FM broadcast band requirements for unlicensed operation and cordless phones
are like night and day.  If you do compare them, it would not mean anything
anyway.  If you decide to use FM, be sure to get your transmitting antenna
way high up, or you won't get out worth diddly-squat.


>3. How the heck do I design something that only puts out 18.7nW? :^)))
>Talk about QRPpppppppppppppppppp :^)

Easy way is to design for more power, pad it down, then use a very short
e-probe for an antenna.  =20

>Btw, checked into stuff that you buy.  I don't want to spend 70 bucks on
>something that just plugs into my soundcard :^)  Also checked into LPFM...
>doesn't look like they'll be accepting new licenses anytime soon.  Plus I'm
>not an organization... unless I create something called: "The Society For
>Broadcasting of Old Time Radio and Whatever Else I Feel Like Listening To
>Out In The Yard" :^)

Forget LPFM, even after they start the next round of windows.  That is,
unless you want to guarantee operation for X hours per day, buy a
receiver/decoder for EANS, plus meet a few other obnoxious regulation
requirements.  No, you do not really *have* to operate that many hours, but
as soon as you don't, you're wide open for a license challenge from some
clown who swears that his organization can and will provide more service
than you, and with a history of not doing it, you will lose.  If you even
bother to go to the hearing and spend all that money.  Yecchhh.

>I suppose I could get a mp3 player, but where's the fun in that? ;^)

You know it.

The formula you cited does apply in that region of trhe spectrum, except
part of the rules provide for a special allowance if you operate between
510 and 1705 kHz.  See Sec. 15.219.  The rules allow operation in that part
of the spectrum with 100 mW total maximum input to the final amplifier
(excluding filament power), and antenna/transmission line/ground lead
combined length of 3 meters maximum.  And any emission outside that band
must be 20dB down from the unmodulated carrier.  I've been able to pick up
a rather poorly designed and installed Part 15 AM transmitter for more than
a quarter mile.

There is also Sec. 15.225, "Operation within the band 13.553-13.567 MHz."
This allows you to radiate 10,000 microvolts per meter at 30 meters.  This
means in any or all directions.  This obviously equals 100 microvolts per
meter at 300 meters.  It's not bad.  Of course, you'd require a short-wave
receiver, but I'll bet you have one or two of those lyinmg around.

73
John WA6QPL  SOC 263