In almost all the radio receivers whether FM or AM, custom wire-wound coils are used that are wrapped on a ferrite core usually (not always) of 100 mm x 10 mm dimensions. Is it possible to use fixed inductor in place of such a custom coil? If no, why? What difference will it make or what will be its demerits compared to ferrite wrapped coil?
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\$\begingroup\$ You should at least include pictures of what you are asking about. \$\endgroup\$– Edin FifićApr 27 at 14:34
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4 Answers
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The ferrite rod acts like an antenna, and that's why you need to turn your radio when listening to AM stations - the radio signal enters into the rod, is concentrated by it while side signals (coming in at 90 degrees to the rod) are greatly reduced.
If you replace it with a tuning inductor, you will need an external LONG antenna to achieve the same sensitivity/reception.
As for the FM radio coils/inductors, they are custom AIR wound (no core) to give exact inductance values AND, very important, to make them of a very high Q (quality) factor which means better selectivity and reception.
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2\$\begingroup\$ I'd say the radio signal is "concentrated" in the ferrite rod, rather than "amplified" -- amplification implies an increase in power. Also, the ferrite rod is most sensitive to signals arriving perpendicular to it. Both the E- and H- fields of an arriving wave are perpendicular to the direction of travel. The rod antenna is most sensitive when its long axis is parallel to the H-field. \$\endgroup\$ Apr 27 at 15:02
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\$\begingroup\$ @MarkLeavitt You're right, I have used the wrong word. I will edit it. I also couldn't remember the word perpendicular, but that's why we have the option for helpful comments like yours. Thank you. \$\endgroup\$ Apr 27 at 15:17
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You'll only see that in an AM receiver (or the AM part of an AM/FM combo), not FM.
And no, it won't work (well) if you replace it with a conventional inductor. Its properties as an antenna depend on its physical size as well as its inductance. If you replace it with an inductor many times smaller, the received signal will be far less — or maybe nothing at all, depending on the construction of the inductor.
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Is it possible to use fixed inductor in place of such a custom coil? If no, why? What difference will it make or what will be its demerits compared to ferrite wrapped coil?
Think about what happens to the lines of magnetic flux as the ferrite rod gets longer. Flux lines are red: -
And, the more lines of flux that pass through the rod, the greater the induced voltage in the coil and the bigger the signal you get.
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The ferrite rod wound with wire yields a small-volume antenna having the right inductance.
This antenna coil inductance is tuned with a parallel capacitor so that the antenna is resonant at the desired frequency. A common variable-capacitor has maximum capacitance of 365 picofarads...this should resonate with the inductor at the lowest frequency on the AM broadcast band: 540kHz in North America.
Antenna coil inductance would be \$ {1}\over{(2\times\pi\times540000)^2\times365\times10^{-12}}\$ henries.
This coil can be wound in air as a loop antenna. A small-area loop antenna nevertheless should be wound with a large capture area...fewer turns are required if circumference is large.
Since this kind of air-wound antenna is cumbersome, few radios use it, unless its enclosure is large enough to accommodate its large size.
TLDR: An air-wound inductor's demerit is its large size compared to a coil wound on a ferrite rod.
