Modulated EMF waves emited from antenna

[Driverj30t9]

Give this video a go. Perhaps the first minute might help with AM modulation. At 2:37 is helpful as well because it shows demodulation of the AM signal. FM is later in the video.

Fantastic video Hans, thankyou for that.

 

[Hans13]

Fantastic video Hans, thankyou for that.

You're welcome.

 

[Lauri-CoyoteFrostbite]

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It is all about the physics- the differences between audio and RF... plus how these signals are received. The physiology of the human ear and the science behind an RF detector. I have some of my lecture notes, somewhere, on coherer receivers, a good example- how they work for spark-pseudo-cw only, and other such.... if I can find them.....
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Meanwhile, here's a photo of a neat Steam Punkish 'phone transmitter. Its a spark gap array using a carbon element microphone- place directly into the antenna! That long horn is something, No ?-
....to capture all the voice, or to keep the speaker at a safer distance from that arc'ing monster ? !
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Lauri
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[RFI-EMI-GUY]

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It is all about the physics- the differences between audio and RF... plus how these signals are received. The physiology of the human ear and the science behind an RF detector. I have some of my lecture notes, somewhere, on coherer receivers, a good example- how they work for spark-pseudo-cw only, and other such.... if I can find them.....
.
.
Meanwhile, here's a photo of a neat Steam Punkish 'phone transmitter. Its a spark gap array using a carbon element microphone- place directly into the antenna! That long horn is something, No ?-
....to capture all the voice, or to keep the speaker at a safer distance from that arc'ing monster ? !
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Lauri
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Safety? Not safety, clearly that thing would have to extend into the next county to be "safe".

 

[Driverj30t9]

There are many ways of describing an amplitude modulated RF carrier once it has left the antenna - Fourier analysis, rotating vectors and so on. But one of the easiest ways of illustrating how it is produced is to consider a simple tube amplifier and modulation transformer in the plate circuit - an audio signal is applied to the primary of the step-up transformer, B+ or HT+ is connected to the plate of the tube through the secondary winding and for example consider the input audio as low frequency audio sine wave.. An RF signal is applied to the grid of the tube and the resultant current comes out of the plate. However one of the tube parameters is "gain versus plate voltage". As the plate voltage is varying from the applied audio signal via the secondary winding, the 'gain' of the tube varies, thus on the positive peaks of the audio sine wave the tube gain is high and conversely when the audio sine wave is at a negative peak the gain is low and so the amplitude of the RF carrier varies in sympathy. The now modulated RF carrier is picked off from the plate of the tube via a capacitor to stop the HT+ voltage and gets passed on to the antenna via a Pi-coupler or something to match impedances.

Vacuum tubes are an undiscovered species to me but I"ll go over what you said though and try to make sense of it.

It used to be enough to tell guys that AM goes up and down, and FM goes back and forth. *sigh*

Good thing were not remaining that simple, might help technology progress more having more people who understand the way things work. Or can't be a bad thing at least.

There's a guy with a website, his name is C. Nyack, who used to have some wonderful interactive applets that you could see an oscilloscope representation of all sorts of mathematical functions, Fourier transforms, various modulation schemes including AM, etc. It would be perfect for this discussion, but alas, he's converting the java applets to javascript-html-css format, and most of them are down.

There might be others who have done the same. Will have to do a google search.

If I might make a suggestion, perhaps a $15 investment into an SDR dongle and the free associated software might also help.

That sounds like a good way to go. I was looking at free spectrum analyzer software to download, Not sure if I would need a mic or speaker or something to get received signals into my computer for the software?Don't know how all that works yet. I do have a MFJ sound card interface. Would using an SDR be better?

"Okay, my wise Yoda Masters (I was a Smart A** even in those days ) - " If I take an RF carrier and vary its amplitude, should I not get just a signal that varies in strength, but does not convey any audio qualities ?"

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"Like, if I whistle (an audio signal,) but only vary the strength, not the tone (frequency,) is that not an Amplitude variation ?.. Combine that with a hum (will call it a carrier) only the over-all strength of the whistle should vary in the amplitude. If I vary the whistle's tone, but not its amplitude with respect to the hum, there will be no variation in the amplitude, but the tone will be heard to vary in frequency, right ? Is this the same as AM radio modulation, and if so why is it different for radio but doesn't work for audio ?.... or does it ?"

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"Making our heads hurt, you do" was the response.

Haha, I can see why, I can.
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Years late I grok'd it all, but Driver, I understand your question now.

Glad you get it now and someone can relate.

Oh, and Driver, you ask'd about my UK license- I was living there and sitting the exam was no problem for a Yank... ")

That's good..

You are still conveying "information" in the variation of signal strength.

Yep, the envelope of the modulated carrier. Maybe that equates to different binary numbers or something based on the voltage amplitudes?

And the problem becomes one of physics and mathematics, which require that information being conveyed occupies bandwidth. So, the process of varying the amplitude at a specific information rate adds sidebands containing the information.

Even on-off keyed morse code occupies bandwidth greater than the single pure carrier frequency. Don't believe it? Watch it on a spectrum analyzer. The faster the code speed, the wider the occupied bandwidth. You can dampen the rise and fall times of the CW, and narrow the bandwidth, but then it limits the rate it can be keyed.

The "problem" (it's not really a problem) is that by altering any characteristic of a pure perfect sine wave, be it frequency, phase, or amplitude, for that instant, it's no longer a pure perfect sine wave. And if it's not a pure and perfect sine wave, it's got other frequency components in it, and that increases the bandwidth

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That's all very interesting and will need some looking into.

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It is all about the physics- the differences between audio and RF... plus how these signals are received. The physiology of the human ear and the science behind an RF detector. I have some of my lecture notes, somewhere, on coherer receivers, a good example- how they work for spark-pseudo-cw only, and other such.... if I can find them.....

.
.

Meanwhile, here's a photo of a neat Steam Punkish 'phone transmitter. Its a spark gap array using a carbon element microphone- place directly into the antenna! That long horn is something, No ?-....to capture all the voice, or to keep the speaker at a safer distance from that arc'ing monster ?!

That is one of the most steam punkish contraptions I have laid eyes on. That's pretty crazy and strange to have the mic in the antenna. The horn is impressive, probably longer to do both, keep you a little safer and capture voice well. Neat piece of history, I wonder how many people still run those things. Would make for an interesting ham shack.

 

[RFI-EMI-GUY]

snip
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Yep, the envelope of the modulated carrier. Maybe that equates to different binary numbers or something based on the voltage amplitudes?



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Snip.

You could actually measure the amplitude variations in binary. The path loss and multipath would affect the overall level, but the source variations could be tracked to some extent.

Also, correction to your post. Zz should get credit for his quoted two astute observations about Morse CW keying and it's sinewave observed in a scope or spectrum analyser. Not me, me not dat smart!

Sent from my SM-T350 using Tapatalk

 

[Driverj30t9]

Oops, my bad zz0468. Lots of quotes to put on these posts.

 

[Murphy625]

That sounds like a good way to go. I was looking at free spectrum analyzer software to download, Not sure if I would need a mic or speaker or something to get received signals into my computer for the software?Don't know how all that works yet. I do have a MFJ sound card interface. Would using an SDR be better?

Nope, you don't need any of that extra stuff.. You need the SDR dongle, and a free USB port, and an antenna to connect, and maybe some coax to extend the antenna outside... it doesn't even matter what kind of antenna, it will work with a long piece of wire if needed. The sound card in your computer will work just fine.

The only catch to the antenna is that you'll get better reception if the antenna is designed for the frequency range you'll be listening to and its really advantageous to get as high as you can. Can't say enough about how important altitude is.

Basically, for us newbie amateurs, it goes like this... You need one antenna for HF bands and one antenna for VHF/UHF bands. That will cover most of what you'll want to listen to and even talk on with a transceiver. For the SDR Dongle, any HF dipole designed for 40 meters will cover almost all the frequencies up into the 70cm (440Mhz) range. I'd recommend a G5RV variant. No, it won't be as good as an antenna designed for the VHF/UHF, but if you're just listening, its more than good enough.

Using the RTL-SDR dongle and a G5RV @ around 60 feet up, I listen to HAMS on HF all over the world, and I can hear VHF repeaters over 60 miles away as well as all the ambulances, cops, and even kids on the little Walmart radios occasionally. I hear the transit buses that move people around, the fire truck guys, and official sounding stuff I have yet to identify. Planes too!

I got really lucky with my HAM radio.. I jumped in with both feet and purchased a used ICOM 746 that has performed flawlessly. But knowing what I know now, I would have started with an SDR dongle and the G5RV antenna first. In the end, it didn't matter because of luck, but I could have easily made a mistake and have missed a good experience and been done with HAM radio because of it.

One other piece of advice, if you do purchase coax to get your antenna outdoors, get the best stuff you can afford. Coax is kind of like the weak link in a long chain of stuff. You can have the best radio and antenna in the world but if your coax sucks, your experience will suck as well. For runs over 30 feet, I'd highly recommend at least an RG8/U.. the bigger the diameter of the coax, generally the better it is, and the higher the frequency range you're listening to, the more important it becomes.