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I am trying to construct a radio and, at the first stage, I have finally detected circuit a signal. My previous attempts have failed and I have discovered that it is because it was not properly grounded.

Originally I was using the ground from my oscilloscope, but it wasn't until I accidentally touched the grounding wire to a my aluminum window frame that all the noise disappeared and a much cleaner, oscillating wave, appeared.

So I would like to know, how do compact AM Radio sets establish a ground? I tried grounding to the wall, grounding to the Earth, and even a medium sized sheet of copper and nothing worked except when I attached it to my aluminium window (which is quite large).

For a while, I wasn't sure if my window was actually acting as an antenna, but, after switching things around (the antenna and the ground), the circuit did not work.

Therefore, I would like to know how compact radios establish a ground? I mean, they don't have a water pipe to connect to so how do they do it? Please do not make references to heterodyning since I am building a simpler version of an AM Radio.

My circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

EDIT: Note, I have also tried connecting my ground to the wall ground and it only slightly better than grounding to Earth.

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A "compact AM receiver" does not "establish a ground" at all, at least with respect to the earth. Instead, it uses the tuning coil itself as the antenna, which detects only the magnetic part of the electromagnetic wave. Your antenna is instead relying on the electric part of the electromagnetic wave, which requires a solid ground reference, or at least two connections that have very different responses to the E field.

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  • \$\begingroup\$ What about those compact radios which have a telescopic antenna in addition to their tank circuit? In my office I have one that is powered by a hand crank (no wires going anywhere). \$\endgroup\$
    – Klik
    Oct 9 '15 at 2:23
  • \$\begingroup\$ The telescoping antenna is not used for the AM band. It's only used for FM (or maybe shortwave). At higher frequencies, the rest of the radio circuitry provides enough of a counterpoise (essentially, capacitance) for the whip antenna to work adequately. \$\endgroup\$
    – Dave Tweed
    Oct 9 '15 at 2:29
  • \$\begingroup\$ Dave, I've been told that the reason that AM radio antennas use an iron core is because it helps to pick up a signal--I think they inferred that it acts like a monopole antenna. Would you know if this is the case or if the iron is just used to increase inductance? \$\endgroup\$
    – Klik
    Oct 9 '15 at 15:54
  • \$\begingroup\$ See: stormwise.com/ambarcore.JPG. \$\endgroup\$
    – Klik
    Oct 9 '15 at 15:55
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    \$\begingroup\$ Yes, the ferrite is used to increase the inductance to a useful value while maintaining a compact size. A commonly-used value is 240 uH, which resonates with a 40 to 365 pF variable capacitor to cover the AM band (540 to 1600 kHz). The antenna(coil) is still a magnetic dipole, however, not a monopole. \$\endgroup\$
    – Dave Tweed
    Oct 9 '15 at 16:21
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So I would like to know, how do compact AM Radio sets establish a ground?

Firstly, consider the transmission from a spacecraft to another spacecraft - they don't need a ground wire because their antennas are designed not to need a ground wire (which is pretty lucky because "ground" might be a million or so miles away). It's not science fiction ala star trek by the way.

"Ground" is a convenience for such antennas as monopoles aka rubber ducks. They are half of one half of one wavelength long i.e. half the length of the archetypal antenna known as a dipole. Dipoles don't need an earth but they do need a balanced feed in or feed out and this costs a few cents more than a quarter wave monopole.

But, as Dave Tweed has said, cheap AM radios can steal the magnetic part of the electromagnetic transmission and reconstitute that into a pretty good signal and a coil receiving an alternating mag field needs no ground.

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  • \$\begingroup\$ If ground is not necessary, then how does one filter out the bad signals? In my tuner circuit, if there was no "ground" then where would the bad signals be "moved"? \$\endgroup\$
    – Klik
    Oct 9 '15 at 2:19
  • \$\begingroup\$ The filtered-out signals end up cancelling within the LC loop. Only the signals that are at or close to the tuned frequency of the LC loop add to each other to produce a detectable signal. \$\endgroup\$
    – pjc50
    Oct 9 '15 at 9:07
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    \$\begingroup\$ @Klik If you are using a monopole antenna (as per your circuit) then it needs a ground connection or the tuned circuit (L and C) will not filter effectively. If you are using a dipole then you need a different "front-end" circuit because a dipole will only work properly with a "balanced" input circuit. \$\endgroup\$
    – Andy aka
    Oct 9 '15 at 9:33
  • \$\begingroup\$ @Andyaka I'd really like to know why the window worked as a better ground than the Earth, the wall outlet, or the ground from the oscilloscope. \$\endgroup\$
    – Klik
    Oct 9 '15 at 15:41
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    \$\begingroup\$ @Klik the best ground is via the shortest connection. Real ground, due to its massive size produces no local emf from a radio signal so a short connection to that point is best. However, if you could have pointed your wire antenna in a totally different direction you might get double the signal because the antenna signal would reverse phase and might act as a dipole with the earth wire. Just speculation of course. \$\endgroup\$
    – Andy aka
    Oct 9 '15 at 16:42

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