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

[Hare,Ed, W1RFI]

> 1. How do I convert uV/meter to wattage

Not as easy as it sounds.  The conversion requires factors for =
frequency and
distance from the radiating source.

The first number you may want to know is the amount of received power =
that
an antenna will pick up from a specific field. This can be determined =
from
the formula:

Power (dBm) =3D -77 +20log(E) - 20log(F) + Gain(dBi) where E is in
microvolts/meter, F is in MHz and the gain is the gain of the antenna =
in the
direction of the radiating source.

If you are starting with a field of 250 uV/m at 3 meters, then this =
would
assume that the antenna is 3 meters from the radiating source.

You can then further correct that power for other distances by using =
the
formula:

P(dBm) =3D -77 +20log(E) - 20log(F) +gain(dBi) -20log(actual
distance/reference distance)  where the reference distance is the =
distance
at which the field is known, as in 3 meters for the 250 uV/m at 3 =
meters
specification.

Most FM BC receivers will have a minimum sensitivity of about -100 dBm =
or
so, by my rough calculation.

To determine how much power it would take to create this at a receiver, =
you
would need to first determine the path loss and antenna gains between =
the
two antennas involved.

Pathloss =3D -37.87 +20log(distance in feet) + 20log(FMHz)

Then, reverse engineer the required power by using:

Needed transmit power (dBm) =3D needed receive power (dBm) +pathloss -
transmit antenna gain - receive antenna gain

I hope I did all the math right; I cobbled this together pretty =
quickly.

73,=20
Ed Hare, W1RFI
ARRL Lab
225 Main St
Newington, CT 06111
Tel: 860-594-0318
Internet: w1rfi@arrl.org
Web: http://www.arrl.org/tis

> -----Original Message-----
> From: Rob Matherly [mailto:kc0bom@arrl.net]
> Sent: Friday, April 12, 2002 3:36 PM
> To: Flying Pigs; SOC List
> Subject: [fpqrp] Unlicensed FM or AM brodcasting to my yard...
>=20
>=20
> 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=20
> house on an
> AM or FM radio.  I've been checking around on maximum=20
> allowable unlicensed
> part 15 (whew) power levels to make sure I don't get busted=20
> for anything.
> At FM it's 250uV@3m (according to one site that works out to=20
> 18.7nW to an
> isotropic) and on AM it uses a formula of XuV per meter=20
> measured 30m from
> the antenna, where X is figured up with the formula=20
> 24000/F(kHz) (the 30m
> is extended to the edge of the campus of educational places).
>=20
> Here's the point of all this:
>=20
> 1. How do I convert uV/meter to wattage
>=20
> 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=20
> all over the
> yard, how much power do those put out?  I could compare the two I
> suppose...
>=20
> 3. How the heck do I design something that only puts out 18.7nW? =
:^)))
> Talk about QRPpppppppppppppppppp :^)
>=20
> Btw, checked into stuff that you buy.  I don't want to spend=20
> 70 bucks on
> something that just plugs into my soundcard :^)  Also checked=20
> into LPFM...
> doesn't look like they'll be accepting new licenses anytime=20
> soon.  Plus I'm
> not an organization... unless I create something called: "The=20
> Society For
> Broadcasting of Old Time Radio and Whatever Else I Feel Like=20
> Listening To
> Out In The Yard" :^)
>=20
> I suppose I could get a mp3 player, but where's the fun in that? ;^)
>=20
> 72/73/oo
> Rob, W=D8JRM
> ARRL; FP Qrp -330; Live-Wire #442; IA QRP #143; SOC #497; QRPp-I #19
>=20
> -----
> Visit my website! http://www.qsl.net/kc0bom
>=20
> AIM - kc0bom, jimrob4 ---  MSN - jimrob@jetnetinc.net
> Y! - kc0bom --- ICQ - 114690148
>=20
> "And by good night I mean shut up"
>=20
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>=20