AM antenna

cs/antennas.html

I forgot to mention I added 100 ohm's of series resistance, before adding the R to ground.

George H.

I don't have a clue about your problem, but here are two more amps to look at.

Mikek

Some of the electronics textbooks published during the war are interesting, especially the military training manuals. The magnetron is mentioned as a wimpy, pretty much useless novelty. Disinformation!

John

You saw a gain in signal strength because your antenna is very short compared to a wavelength, which basically makes it into a capacitive probe into the aether*. Since it appears as a fairly low-value capacitor to ground, sampling the voltage with a way high impedance made the gain higher.

I'm not exactly sure what adding the inductors did -- it set up a resonant circuit, and that worked against the op-amp's coupling between inputs to make the whole thing sing. As to the exact mechanism -- I dunno.

You probably don't want to actually tune such a small antenna: you'd need an enormous coil, when you did get it all tuned you'd be losing most of your signal in the coil (or you'd be picking up more signal with the coil than the whip), and the impedance looking into that series-tuned LC circuit would be absurdly low (instead of absurdly high, as you are now with your capacitive whip).

The suggestion to use a coil is a good one -- by using a directive antenna you'll reduce the amount of atmosphere "seen" by the antenna in comparison to the signal source, which will increase your signal to (atmospheric) noise.

• Remember that stuff?

You may have overload problems with such broad band amplifier.

In addition man made noise is also easily capacitively connected to the whip.

One method to interface the extremely high impedance reactive whip to a low impedance (50-75 ohm) receiver input is to use a parallel resonant circuit as the input and connect the antenna with a small capacitor to the top of the resonator. From the tap in the coil close to the ground, feed the signal to the receiver directly or through an emitter/source follower. Of course this requires tuning the front end when changing stations, but it also solves a lot of intermodulation problems.

An electrostatic shield (with a gap at the top) around the coil will also reduce the capacitive pickup from nearby manmade sources.

Thanks all, Mike, Mark, Tim.

Playing around a bit more today. (There=92s hardly anyone at work.)

The oscillations (singing) was basically at the slew rate limit of the opamp.

I wonder if when I tune out the antenna C with a bit of L if I just have to supply more current to the antenna. With too large an R on the input to my non-inverting buffer the antenna tries to draw current from the opamp. (?)

Anyway the circuit seemed to have no loss of signal with a 10kohm on the input to ground. And by adding ~1 mH in series, the signal strength went up by another 3-4 dB. (Sure I could just add more gain later... but why)

Here=92s the spectrum of a large signal.

I=92m tuned 33k Hz above =91cause I=92ve got the signal running into a Q=3D=

I also ripped a ferrite loop stick out of a radio. The radio couldn=92t find any AM signals in the room (BTW). And when I hooked up the loop stick to a differential amp. I could barely hear anything either. (L =3D 300 uH)

Thanks again for the help/ ideas.

George H.

Yeah I don't know enough about what was important back then to know what might be left out.

There was not very much about loop antenna's for instance.

George H.

Run the numbers. You're not going to tune out the capacitance of an antenna that's 1/100 a wavelength long with "a bit of L". You're going to tune it out with a honkin' BIG bit of L, and when you try you're going to find that you can't achieve that L -- you can only achieve a resonant circuit that's slightly inductive after tuning, and whose resistive losses predominate over the radiative losses of the antenna.

I rather suspect that you're doing more to peak the response of the op- amp circuit than the antenna, but I'm too lazy to go farther than that.

If you're going to use a loop antenna of any kind you need to tune it. Just a big inductance, without a big capacitance to resonate it at your desired frequency, isn't going to respond strongly to your desired signal.

AM radios have at least a two-section capacitor, with one section controlling the oscillator and another section controlling the antenna circuit (of which the loop stick is a part).

At slew-rate limit or during current limiting, the loop is _broken_, so all bets are off for good behavior. Many designers don't worry about limiting conditions, but I do. I evaluate all my OpAmp designs for stability while current limiting.... particularly a rail-to-rail one that limits at 100mA... as I did whilst in NY ;-)

...Jim Thompson

George,

A 10ft wire will have about 0.1644 - J 7221 ohms impedance at 1mHz when measured about 10% away from the earthed end.

John S

There is another consideration for tuning a short antenna for MF frequencies: When the antenna is correctly tuned, (assuming a large and good enough coil), the bandwidth of the tuner/antenna combination is too narrow to pass useful modulation sidebands.

For aviation NDB beacons (around 300 kHz), we are using

For listening, a voltage follower with a whip is the way to go, unless you have space for a looong wire.

you

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I did 'run the numbers'.I was guessing a capacitance of something like

Oh that's OK. I appreciate the ideas. The opamp is just a buffer.

Q=3D5 BP

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L =3D

Hmm OK that makes sense. I'd rather a more broad band antenna. Did you see the loop circuit on C. Wenzels antenna page. He's got a loop driving a differential video amp. (MC1590? I think.)

George H.

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=A0 =A0 ...Jim Thompson

=A0 =A0| =A0 =A0mens =A0 =A0 |

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Hmm, I'm not driving the circuit to the slew rate limit. With a 10 kohm resistor from the buffer input to ground the signal is a few mV. But with 100k ohm the series LC circuit breaks into oscillation at ~1-2 MHz depending on the L.

George H.

quoted text -

Aha! I misread. When it oscillates it's at the slew rate limit? ...Jim Thompson

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Thanks John, 7 kohm is in the 'ball park' of what I've been guessing. I'm not sure of where the ground is.

George h.

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Yeah that is not what I want. A farily broad band would be better. But I still see pelnty of channels as I move the LO around, so I can't be that narrow band.

Cool, and after reading Terman, I have some clue as to what a top hat is.

I'm starting to believe it!

George H.

The ground is connected to your circuit "ground". You have a power supply maybe or some other capacitance to the earth? Is there nothing except your headphones and head? That's capacitance, you know.

Think about the complete circuit. The circuit goes from antenna through

nics/antennas.html

Any idea how to calculate the gain of a loop antenna based on size? Seems that larger is better, but how much better is a 6 foot square loop compared to a 3 foot square loop at broadcast AM frequencies?

-Bill

Any idea how to calculate the gain of a loop antenna based on size? Seems that larger is better, but how much better is a 6 foot square loop compared to a 3 foot square loop at broadcast AM frequencies?

-Bill

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