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

[Rob Matherly]

Yeesh Ed, thanks for keeping it simple :^)))))

72/73/oo
Rob, W�JRM
ARRL; FP Qrp -330; Live-Wire #442; IA QRP #143; SOC #497; QRPp-I #19

-----
Visit my website! http://www.qsl.net/kc0bom

AIM - kc0bom, jimrob4 ---  MSN - jimrob@jetnetinc.net
Y! - kc0bom --- ICQ - 114690148

"And by good night I mean shut up"

----- Original Message -----
From: Hare,Ed, W1RFI <w1rfi@arrl.org>
To: 'Rob Matherly' <kc0bom@arrl.net>; Flying Pigs <fpqrp-L@mpna.com>; SOC
List <soc@mailman.qth.net>
Sent: Friday, April 12, 2002 4:44 PM
Subject: [SOC] RE: [fpqrp] Unlicensed FM or AM brodcasting to my yard...


> 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) = -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) = -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 = -37.87 +20log(distance in feet) + 20log(FMHz)

Then, reverse engineer the required power by using:

Needed transmit power (dBm) = 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,
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...
>
>
> 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
>
> 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...
>
> 3. How the heck do I design something that only puts out 18.7nW? :^)))
> Talk about QRPpppppppppppppppppp :^)
>
> 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" :^)
>
> I suppose I could get a mp3 player, but where's the fun in that? ;^)
>
> 72/73/oo
> Rob, W�JRM
> ARRL; FP Qrp -330; Live-Wire #442; IA QRP #143; SOC #497; QRPp-I #19
>
> -----
> Visit my website! http://www.qsl.net/kc0bom
>
> AIM - kc0bom, jimrob4 ---  MSN - jimrob@jetnetinc.net
> Y! - kc0bom --- ICQ - 114690148
>
> "And by good night I mean shut up"
>
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