grin


"Matt Barrow" wrote in message
...
|
| "RST Engineering" wrote in
message
| ...
| Macklin, I like you, you are a nice guy. But sometimes
you give advice
| that is way the hell out of your depth. Like this one.
|
| Jim, I don't know you except by what you post here, but
you've gotta be the
| most pompous PITA in the Western Hemisphere.
|
|
|
|
|
| "Jim Macklin"
wrote in message
| news:AVTtg.67782$ZW3.26133@dukeread04...
| 104.1 FM is below the aircraft navigation and below the
| communication assigned frequencies. But there are
harmonic
| signals that you cheap FM transmitter is not screening
out.
| Are you using a wired connection from the PC to the FM
| transmitter or a WiFi or other radio? Most likely
you're
| getting the interference in that way.
|
| You may be in violation of FCC rules.
|
| Probably not, with a store-boughten transmitter. And
harmonics (which are
| integral multiples of a fundamental signal) probably
have little to do
| with it.
|
| Consider the most probable cause. An FM receiver at
104.1 has a local
| oscillator at 114.8 MHz to produce an IF of 10.7 MHz.
Not only will that
| beat with 104.1 on the low side of the LO, it will beat
with 125.5 on the
| high side. Most FM receivers have at least a +/- 50
kHz. wide IF strip to
| allow the stereo subcarrier at 38 kHz. to come through,
so 125.45, 125.5,
| and 125.55 will come through as well.
|
| The OP said that he doesn't get any interference when
his transmitter is
| off, so my best place to start looking is the front end
of the receiver,
| where the normal filtering of any decent FM receiver
should take care of
| the "image" problem. However, when a very strong signal
(like from a 100
| mW legal transmitter) comes blowing into the front end
of the receiver
| from a few feet away, crossmod and intermod are NOT your
friend, and no
| designer in this world can make a brick wall filter that
will take care of
| it.
|
| Having said all that, the real problem is to keep
whatever small amount of
| aircraft band com energy is present at the front end
from getting into the
| receiver.
|
| And, before we start off on a wild goose chase, I'd
advise the OP to get a
| small handheld aircraft band transceiver or scanner and
see if the real
| transmitter is somewhere around 125.5 MHz.. It is
always good to be able
| to do a math calculation to see exactly what is getting
into what before
| spending a lot of time chasing your tail.
|
| For those who say "you can't hear AM on an FM receiver",
I say
| horsefeathers. It may be reduced in volume, it may be
distorted, but it
| will get through.
|
| Do the test, tell me what frequency from 118-136.975
MHz. the aircraft is
| on, and we'll go from there.
|
| Jim
|
|
|
|
|