[BC] Part 15 FM Power/Radiation Limits

[Ron Castro]

Unless the whip had a gain of -60 dB, the FI would most likely surpass the 
legal limit.  I doubt that it would actually have a maximum lobe of less 
than -3 dB including transmission line losses, so a 57 dB attenuator would 
be needed on the output of the 18 mW transmitter.

Ron Castro
Chief Technical Officer
Results Radio, LLC


----- Original Message ----- 
From: "WLOYPROD WLOYPROD" <WLOYPROD at loyola.edu>
To: <broadcast at radiolists.net>
Sent: Thursday, July 07, 2005 9:31 AM
Subject: Re: [BC] Part 15 FM Power/Radiation Limits


>I understand the calculations, but I was pulling the 18mW from
> recollection of testing various "kit" transmitters and Ramsey units and
> things with a conventional car-type replacement antenna (32" was the
> length I recall).  I'm using long fried brain cells, but IIRC that was
> the approximate power that could be used with that type of antenna, NOT
> a dipole.  I was referring to the common configuration used by the
> various tiny transmitters that are often incorrectly setup with 100mW
> for FM, when they are at the max around 18mW in their typical
> configuration.  Those type of units often use conventional car whips as
> their antenna (sometimes they use even shorter ones) or whatever is
> handy at a Radio Shack.  I didn't mean something that was built more
> conventionally ;)
>
> John
>
>>>> rfry at adams.net 07/07/05 9:05 AM >>>
> "John" (WLOYPROD at loyola.edu ) wrote:
>
>> ... 250uV/m @ 3m for FM, under 15.239 - works out to about
>>  18mW into a conventional 32" whip...
> ____________________
>
> Sorry, John, but that conclusion is a bit off.
>
> Although only field strength matters here, the input power for
> unlicensed
> FMs of ~11.5 nanowatts (NOT milliwatts) into a 1/2-wave dipole has been
>
> posted here already--see the thread "FM and AM/slight drift" on the
> June,
> 2005 BC list.  Depending on the length/configuration of the tx ground
> lead
> (if any), using a 32" whip antenna might allow increasing that antenna
> input
> power by a fraction of a decibel without exceeding the field limit at 3
>
> meters, but nowhere near to reaching 18mW.
>
> Here is the applicable equation:
>
> Field Strength in dB above 1 microvolt/meter =
>     104.77 + ERPi - 20 log (D)
>
>  where
>    ERPi = Effective Radiated Power in dB referenced
>                   to 1kW from an isotropic radiator
>    D = Distance in kilometers
>
> RF
>
>
>
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