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Old 19th May 2017, 5:33 pm   #1
astral highway
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Default MW Loop antenna bizarre 'scope waveform??!! Ideas please...

I'm getting this strange waveform across a loop antenna under construction to match my pantry transmitter.

This works brilliantly with a wire antenna and matching inductor/ capacitor, but I do want it to run with the loop antenna whose physical construction is now finished.

I don't have access to another calibrated signal source yet, but this is what the scope sees when I feed it the 1MHz carrier from my pantry transmitter. The output of that swings around 65V, and is clean, symmetrical, and pretty much perfect, both modulated and unmodulated.

So, this signal is highly attenuated, showing the mismatch I suspected. 1MHz is bang on the money but this isn't a waveform showing a resonant antenna, so what is it?

Any ideas, please? or is it just possibly an artefact of the 'scope?

I'll have access to a calibrated signal source soon, but not today, and I am to too intrigued to wait!

Cheers folks
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Old 19th May 2017, 5:49 pm   #2
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Default Re: MW Loop antenna bizarre 'scope waveform??!! Ideas please...

A higher harmonic of the 1MHz resonating with the length of the loop?
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Old 19th May 2017, 6:19 pm   #3
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Default Re: MW Loop antenna bizarre 'scope waveform??!! Ideas please...

I like this MM, only, what's with the notch thing going on there? And can
I just assume the loop is way off tune?
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Old 19th May 2017, 7:54 pm   #4
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Default Re: MW Loop antenna bizarre 'scope waveform??!! Ideas please...

The loop may well be in tune regards the L and C, the length of wire will have it's own resonance due to the mere length. Say 3m long and a quarter wave, that is 12m which is 25MHz, your 'spikes' are about 1/25th over a whole cycle. All this is very much guessing but may well have a bit of truth in it.
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Old 20th May 2017, 9:11 am   #5
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Question Re: MW Loop antenna bizarre 'scope waveform??!! Ideas please...

Those spikes don't look like they are being produced from a linear analogue source. Is the PSU that powers your transmitter an SMPS? If so, those spikes could be coming from that. Those spikes are what I would expect to see from a relatively low-freq. square-wave after it had been passed through an HPF.

Al.
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Old 20th May 2017, 10:52 am   #6
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Default Re: MW Loop antenna bizarre 'scope waveform??!! Ideas please...

Quote:
Originally Posted by Skywave View Post
Is the PSU that powers your transmitter an SMPS? If so, those spikes could be coming from that. Those spikes are what I would expect to see from a relatively low-freq. square-wave after it had been passed through an HPF.
Hey Al, the same thought about the stripped-out square wave did occur to me. But there is no such signal source in the electronics here. I built a super-regulated linear power supply.

The square wave is likely from the living room lights (subject of another thread on mains-borne interference) and the antenna itself must be the high pass filter...so the antenna is directly picking up the signal from the lights , perhaps ...
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Old 20th May 2017, 10:56 am   #7
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Default Re: MW Loop antenna bizarre 'scope waveform??!! Ideas please...

With the the loop antenna connected what's the waveform like at the PA's collector?

Lawrence.
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Old 20th May 2017, 6:25 pm   #8
astral highway
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Default Re: MW Loop antenna bizarre 'scope waveform??!! Ideas please...

Problem solved. I have a calibrated frequency source now and the looop antenna was +700 KHz out of tune (by calculation using a Java applet!) I have now hand-wound another bespoke inductor to put in series and the whole thing is now tuned to resonance.

Mods, please close thread.

Thank you.
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