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Why is this signal stronger on 50 Ohm input impedance ?

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MathWizard:
I have a 20ft wire strung up to try and measure AM radio strength, and test a tapped, air coil  inductor I made. 1 end of the wire goes to the 495uH tapped inductor, and scope gnd is on the other end.

The strongest station is 800kHz, looking at it with FFT on a SDS2204x-plus, when using cheap coax w/ alligator leads, and the scope on 50R input, the strongest signal I can get is -13dB. And it's also on the center tap . It measures ~200uH on either side. And the amplitude increases as I move up or down towards the center tap.

But with a probe and scope set to 10x, and 1M input on the scope, I get -23.5dB on the middle tap. And it's not the strongest signal tap I can measure with the 10x probe. The voltage increases as I move up the coil taps, like a voltage divider.


I'm just studying a chapter on transformers and mutual inductance, but what's going on there, how can I measure a stronger signal, with a 50R resistance in parallel to that tap ? The coil resistance is only 4.5R total, so the 50R barely makes a difference anyways, and 10M certainly doesn't. Was is something to do with the input capacitance ?

What am I seeing ?

vk6zgo:

--- Quote from: MathWizard on August 04, 2023, 12:36:49 am ---I have a 20ft wire strung up to try and measure AM radio strength, and test a tapped, air coil  inductor I made. 1 end of the wire goes to the 495uH tapped inductor, and scope gnd is on the other end.

The strongest station is 800kHz, looking at it with FFT on a SDS2204x-plus, when using cheap coax w/ alligator leads, and the scope on 50R input, the strongest signal I can get is -13dB. And it's also on the center tap . It measures ~200uH on either side. And the amplitude increases as I move up or down towards the center tap.

But with a probe and scope set to 10x, and 1M input on the scope, I get -23.5dB on the middle tap. And it's not the strongest signal tap I can measure with the 10x probe. The voltage increases as I move up the coil taps, like a voltage divider.


I'm just studying a chapter on transformers and mutual inductance, but what's going on there, how can I measure a stronger signal, with a 50R resistance in parallel to that tap ? The coil resistance is only 4.5R total, so the 50R barely makes a difference anyways, and 10M certainly doesn't. Was is something to do with the input capacitance ?

What am I seeing ?

--- End quote ---

The 20ft wire has significant capacitance to earth, so you aren't looking at a simple inductive or resistive circuit.
It might be fun to look at your setup with a NanoVna.

T3sl4co1l:
What is the antenna capacitance? Is it tuning with the coil as you vary the impedance on it (namely where the 50R tap connects)?

Tim

MathWizard:
Ok thanks, I'll try an equation I just saw for wire capacitance, in a sim. On the outer taps of either end, it did measure around -25dB with the basic cable. The probe measured around -30 and -18dB iirc, on the end taps, it seemed fairly linear across the taps, so I'll use it's values for making models of the radio front-end.

I forgot how strong these local signals are.

T3sl4co1l:
That's not a great plan: capacitance depends on everything nearby.  You can't just measure it (e.g. DMM with capacitance range)?

Tim

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