Fw: FCC Seizes Radio Equipment

Dan.Strassberg dan.strassberg@att.net
Fri May 27 16:09:02 EDT 2011

That's true, but... The local oscillator (LO) of a superheterodyne FM
receiver oscillates at the frequency to which the receiver is tuned
plus the receiver's IF frequency (by law, 10.7 MHz). For example, one
of the reasons why WCRB can't move very close to downtown Boston is
that WERS on 88.9 transmits from One Financial Center downtown. 88.9 +
10.7 = 99.6, which is only 100 kHz from 99.5. Without a filter ahead
of the receiver's demodulator, a signal at 99.6 + 10.7 = 110.3 MHz
could be received by a receiver tuned to 88.9, but radiation from the
LO itself is centered at 99.6 MHz and if the LO signal radiates, it
can interfere with WCRB's signal at 99.5. Radiation from the LO of FM
receivers tuned to stations in the upper half (approximately) of the
FM band has the potential of interfering with aircraft communications.

Dan Strassberg (dan.strassberg@att.net)
eFax 1-707-215-6367

> ----- Original Message ----- 
> From: "Larry Weil" <kc1ih@mac.com>
> To: "Sid Schweiger" <sid@wrko.com>
> Cc: <boston-radio-interest@lists.BostonRadio.org>
> Sent: Friday, May 27, 2011 2:35 PM
> Subject: Re: FCC Seizes Radio Equipment
>> If you are thinking of image reception, the image frequency is
>> the I.F. frequency above or below the tuned frequency, thus 21.4
>> above or below on a common FM receiver.
>> Larry Weil
>> Lake Wobegone, NH
>> Sent from my iPhone
>> Big freekin deal!
>> On May 27, 2011, at 7:01 AM, Sid Schweiger <sid@wrko.com> wrote:
>>> "The pirate was an FM signal, and last I heard aviation
>>> communications are done in AM.  So how did the pirate interfere
>>> with air traffic?"
>>> One theory:  99.7 + 10.7 (the IF commonly used in FM receivers) =
>>> 110.4, square in the middle of the aviation band.  Another theory:
>>> Spurious radiations from a cheap transmitter with likely little or
>>> no filtering and poor frequency control.  In any case, even if the
>>> interfering signal uses a different modulation type, just the
>>> presence of the signal is enough to interfere, and when commercial
>>> aviation is involved, the FCC doesn't hesitate to act.
>>> Sid Schweiger
>>> IT Manager, Entercom New England
>>> 20 Guest St / 3d Floor
>>> Brighton MA  02135-2040

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