# AM antenna

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You have to know the total loss resistance (including skin effect, losses in variable capacitors) and the radiation resistance. The radiation resistance is easily calculated from the size (at least for a single turn loop).

The radiation resistance (well below 1 ohm at these frequencies and sizes) are in series with the loss resistance, so the losses (and hence gain) depends e.g. on the conductor size (10 mm copper tubing or some thin wire).

I whiled guess for a loop antenna is -10 .. -40 dB, while the gain for a typical ferrite antenna might be -60 dB or even worse. This should be compared to the huge atmospheric noise levels at those bands.

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The -10 dB is a bit optimistic for the MW band 200-600 m.

The -10 dB I was referring to was for a 2 m single turn transmitter loop at 80 made of copper tubing and the estimated efficiency was somewhere 1-10 % and hence gain in the -20 to -10 dB range

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Show the circuit here. How exactly were the chokes, antennas, etc. all connected? It makes a difference.

Thanks, Rich

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Sure not much to it,

+------+ | |\ | 10' piece +-|-\ | of wire 1mH | >-+--out

---------+-LLL--+---|+/ unknown R |/ distance R ------- R ground | GND (circuit ground)

So circuit ground is different from whatever ground the piece of wire sees. (spatially different)

It's lying on the 4th floor of an old cement industrial building. 20 years ago it was a Trico auto wiper plant.

If R's 10k ohm it's fine, at 100k and above it oscillates. I've got a opa2134 plugged in at the moment... (similar behavior with other opamps.) With the values shown it oscillates at about 1 Meg Hz, but it's so big it's hitting the negative power rail and 'getting ugly'. With only 100uH of L it rings with smaller amplitude at something like 2.2 MHz. (but it's a RC / slew rate looking shape. and it's not constant, but still being dragged about a little bit by the input signal.)

George H.

Had my fill of Turkey and all the fixin's

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Yeah, there are different grounds involved. Is that the problem?

George H.

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ronics/antennas.html

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Hmm I made a ~1 foot diameter loop, 5 turns 22AWG, L~14uH. With one side grounded it seemed to have more electro static pickup than magnetic. I should try it with a differential input.

Any idea how big a coil I need to match the same signal level as a 10 foot piece of wire? (Leaving off the 'coupling' inductor)

George H.

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rlt-n4ywk.htm

That's nice! Thanks,

George H.

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Try a longer antenna and connect your circuit common to a water pipe.

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PVC or ABS?

Always use a name brand placebo, never a generic one.

--
Many thanks,

Don Lancaster                          voice phone: (928)428-4073
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Yes. Sometimes I forget I am so old that metal water pipes are gone.

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rlt-n4ywk.htm

So, it appears using a fixed length of wire and doubling the diameter with half the turns would not change the gain much since the gain goes up with diameter and down inversely with turns? Also the inductance drops as the diameter increases for the same length. This seems to indicate more wire is an advantage regardless of area except for wire losses?

-Bill

# For a fixed wire length:

"Sensitivity goes up as the loop diameter, and down inversely as the number of turns."

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