knoledge challenge

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luk42

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my name is Jim...would like to say hi to all an thanks for having such a great venue...yes I am new to the community....I have a vast knowledge of electronics an book reference...just not a lot of practical application of most of my knowledge...so with that being said...bear with me...as I hope we venture forward together...I am seeking help with now what I am deeming a project....I am receiving a signal...definitely not commercial...I am not sure of its broadcast standard...RF for sure or just stray RF
How i am receiving this signal is through my mic on my laptop...mic is acting as antenna picking it up
now I understand that such things can happen from a lack of proper grounding an shielding of the mic circuitry...but I believe this is not the cause of how...because I can also pic it up on my phone my desktop...I can get the signal at home 50 miles from my home...so it is defiantly open air...I simply turn on sound recorder or any app that can utilize the mic an record...when playing it back...yes there is discernible audio...I have been wanting to hook up..SDR an dongle to see if I can isolate the signal...but would truly like to know more about the signals characteristics first so as to design forward with receiving antenna then tuning....heres my true thoughts...if it is not a straight RF broadcast then maybe I am picking up a harmonic of the broadcast....I have dialed through the band of an old AM radio an can hear it faintly about six different times across the band...that's what makes me think it may be a harmonic.
i would truly like if you all don't mind to please throw some ideas at me as to how what software i may try to identify this anomaly...now i do for see one problem so correct me if i am wrong...now when this signal hits my device i understand that it gets demodulated immediately...if this is true then how will i be able to
identify its modulation frequency....whos up for a challenge to see if we can backward engineer this signal

respectfully
luk42
 

prcguy

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So Cal - Richardson, TX - Tewksbury, MA
Its not uncommon to have an AM broadcast station do what you are seeing when your very close to the transmitter site. AM modulation is also the most likely culprit if your are hearing a clear voice or music coming from your non radio appliance. FM and other modulation will generally not be clearly understood when leaked into equipment.

The fact that you hear it several places in the AM broadcast band kind of points to the AM broadcast band as the source of interference. If you are really close to a transmitter it can show up in several places across the band and it can also be distorted on every frequency except its fundamental.

Rusted metal fences, corroded antennas and similar can rectify high level signals from a close by transmitter and generate Intermodulation products, or ghost signals on other frequencies. That could explain picking up the interference in several places on an AM receiver but not getting into your mic circuits or computer.

Any harmonic from a broadcast station is going to be very small and insignificant when discussing a signal leaking into a mic circuit. A harmonic will probably be at least 1 million times less than the fundamental signal broadcast from the transmitter.

A spectrum analyzer or a dongle receiver that covers the AM broadcast band would be useful for finding the problem, it should be the largest signal anywhere on the display.
prcguy
 

luk42

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Mar 21, 2017
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st.louis,mo
prcguy

thanks for your response...i did forget to mention that i believe that the source is over modulating
is why i get the signal anyway...if you have any other thoughts i would appreciate them
 
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