So my question is, if there is a way to use a FM transmitter and send a signal on the same frequency as the local Radio, so that radios only play my signal. Or will it just be interference. In other words: Are FM recivers able to choose a stronger signal over a weaker one, or will they display both? Thanks in advance. PS: I know that Personal use would be Illegal.
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4\$\begingroup\$ We should probably make a note here of the HIGHLY ILLEGAL nature of actually attempting to do this! \$\endgroup\$– Robherc KV5ROBCommented Apr 26, 2017 at 17:29
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\$\begingroup\$ Good luck dealing with the cops. \$\endgroup\$– Enric BlancoCommented Apr 26, 2017 at 18:01
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\$\begingroup\$ Its actually not illegal if you have a small signal, personal FM radio transmitters do this all the time, but the range is usually under 100ft. If you turn up the power, there is a good chance you'll get a talking to by the FCC \$\endgroup\$– Voltage Spike ♦Commented Apr 26, 2017 at 18:51
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\$\begingroup\$ well if the FCC wants to swing by Germany thats cool by me, But i do get your point :) \$\endgroup\$– user6003203Commented Apr 26, 2017 at 21:12
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\$\begingroup\$ @user6003203 Don't worry, you certainly have a Bundes-CC of some sort. \$\endgroup\$– Dmitry GrigoryevCommented Apr 28, 2017 at 12:54
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3 Answers
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FM receivers have a "capture effect" with the stronger signal overriding the weaker signal.
answered Apr 26, 2017 at 17:26
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\$\begingroup\$ Awesome, is there a general threshold, or is it reciver to reciver kind of thing? \$\endgroup\$ Commented Apr 26, 2017 at 17:32
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1\$\begingroup\$ It's based on signal strength at the location of the receiver. Also, this is, as mentioned above, very illegal. \$\endgroup\$– HearthCommented Apr 26, 2017 at 17:35
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\$\begingroup\$ It only needs a few dBs difference in received signal strength. This means that if the level is just enough for the radios in your house to capture your signal, the rest of the area will capture the broadcast signal in preference to yours. \$\endgroup\$– Neil_UKCommented Apr 26, 2017 at 17:38
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\$\begingroup\$ Thank you. The Illegal part is fine, I´m not into making my own Radio. I though it might be a practical tool for emergency personnel to inform people in certain situations. \$\endgroup\$ Commented Apr 26, 2017 at 17:41
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\$\begingroup\$ @user6003203 - bcos ofc you can broadcast on all frequencies all at the same time ... not ... ;) \$\endgroup\$– brhansCommented Apr 26, 2017 at 17:47
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As others have mentioned FM receivers lock-in to the strongest signal in the vicinity, within limits. This is different from AM where one channel will just mix with the other.
This is one of the reasons FM if much more prevalent than AM. You can pack way more stations into a given bandwidth and the stations with the same wavelength can be a geographically a lot closer together.
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In simple terms, if there are two competing FM carriers, then the one that will theoretically "win" has to be at least 6 dB greater in amplitude at the receiver antenna than the other. This is because an FM receiver gets rid of amplitude noise by hard-limiting the received carrier and, an interfering signal can be regarded as amplitude noise because it isn't coherent with the larger signal.
With one signal at 6 dB greater amplitude than the other, the resulting limited composite signal will contain only the modulated carrier of the larger signal. Consider two similar amplitude FM carriers and the composite wave they produce when added: -
Clearly there is destructive interference at the "difference" frequency and this cannot be got rid of by a limiter because the signal disappears. However, as the amplitude of one of those signals rises there becomes a point when a limiter can work: -
This is simplistic and theoretical. In practise, a few dB more is usually necessary for decent FM listener quality.
answered Apr 26, 2017 at 17:54
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\$\begingroup\$ Commercial FM tuners generally cite capture rations of a couple of dB. \$\endgroup\$ Commented Apr 26, 2017 at 20:41
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\$\begingroup\$ How about some maths on why only couple dB suffice? Are several cascaded limiters required? What about bandwidths? \$\endgroup\$ Commented Apr 27, 2017 at 3:43
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1\$\begingroup\$ Feel free to make a more mathematically oriented answer. \$\endgroup\$– Andy akaCommented Apr 27, 2017 at 8:43
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\$\begingroup\$ It actually depends on the effectiveness of the limiting amplifier in the RX. Better limiters require less difference for capture. \$\endgroup\$ Commented Apr 27, 2017 at 10:05