AM antenna

AM antenna. (proving yet again that a little knowledge can be a dangerous thing.)

I've been reading Terman's "Radio Engineer's Handbook" * on antenna's, and Charles Wenzel's web site,

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(The only useful thing I could find on the web.)

My previous AM antenna was a ~10 foot piece of stranded wire (0.05" diam.) stuck into an inverting opamp. (Rin =1 kohm, gain =10, opa228, ) Today I made a non-inverting preamp out of a opa2134 and stuck the same piece of wire into it. (1 meg to ground for bias current) I got about a 3-4 dB increase in signal. I then tried adding some inductance right at the input. I strung three 100uH's in a row and clipped my antenna to the different taps.

Sang like the dickens, An RC, saw-toothie looking thing. about (~2.2 MHz to 1.7 MHz) More L leads to lower freq as expected.

100k across the input didn't stop it, but 10k did. (10k the worlds favorite resistor?) What's going on? My antenna needs more current?

George H.

*published 1943, first edition. There's a blurb in the preface about some things being left out because of the war.... it always makes me wonder as I read.
Reply to
George Herold
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cs/antennas.html

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

George H.

Reply to
George Herold

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

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Mikek

Reply to
amdx

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

Reply to
John Larkin

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).

20 years ago, which was the last time I was professionally involved with MF radio, "the" way to establish a good omnidirectional antenna was to use a short (1 meter or so) capacitive whip driving the gate of a FET source follower. Because atmospheric noise is so prevalent, you don't need a longer antenna to pull in more signal -- you just get more noise along with it. You only need enough antenna so that the atmospheric noise is 3 - 12dB above your reciever's inherent noise -- after that, you're just pointlessly wasting receiver dynamic range.

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?
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My liberal friends think I'm a conservative kook.
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Reply to
Tim Wescott

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.

Reply to
upsidedown

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.

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

5 BP filter at 33kHz.

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.

Reply to
George Herold

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.

Reply to
George Herold

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).

--
Tim Wescott
Control system and signal processing consulting
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Reply to
Tim

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

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
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

Reply to
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

20 m long ground plane antennas with 6 m capacitive top loading hat. The combination does not pass 1050 Hz identifier modulation (Morse) sidebands, so the ID is modulated using 400 Hz tone. The total efficiency of the antenna is around 1 %.

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

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Tauno Voipio
Reply to
Tauno Voipio

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

30pF. At 1 meg Hz, that's something like 5k ohm. But that C guess seems a bit low since the 10k ohm didn't load it that much. The 1 mH inductor at 1 Meg is about the impedance.
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Oh that's OK. I appreciate the ideas. The opamp is just a buffer.

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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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Reply to
George Herold

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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.

Reply to
George Herold

quoted text -

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

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
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.

10k ohm doesn't seem to load it very much... but I'm measuring the power in a single channel after it's gone through the multiplier. I'll have to think about how that works.... (I think the output is still linear, provided I'm not clipping somewhere.)

George h.

Reply to
George Herold

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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.

Reply to
George Herold

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

*something* to earth (at these frequencies). It may be through the capacitance of a power supply to earth or, if battery powered, through your body capacitance to earth.
Reply to
John S

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

Reply to
Bill Bowden

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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bw

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