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RC transmitter antenna to p.c. soundcard - Part 1

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As will become obvious as you read this post, I'm a beginner with electronics.

I've done some simple experiments with breadboards, TTL IC's, passive components,

and I know some of the very basics about d.c. and a.c. theory, but even a

fairly simple circuit / principle can quickly get me "dazed and confused."

I have a Futaba T6xAs transmitter and I am interested in viewing the transmitted waveforms on a p.c/soundcard oscilloscope. Actually, I'm interested in viewing the output of other circuits that I want to build as well, but I thought I'd start with something already built and working :)

This is the software I'm using : http://www.zeitnitz.de/Christian/Scope/Scope_en.html

The software is working with a mic plugged in to the soundcard, or with a sound source into the line in port.

Ok, so don't laugh too hard, but I naively tried hooking the antenna directly to the mic input (not really expecting it to work), by wrapping two wires around the tip and center conductors of "guitar" cord type connector, then wrapping the "tip wire" around the antenna. I tried putting the "center wire" on the (external) base of the transmitter antenna, and then tried "grounding" it against one of the metal parts of the face of the radio. Neither worked.

I also tried just plugging headphones into the mic jack of the soundcard, and holding the leads of the headphones near the transmitter antenna, thinking that the headphone leads might act as an antenna

So I have a few guesses on why it doesn't work. Please confirm / refute / ridicule / expand on my ideas.

1) Antennas transmit through air / space / vacum, not by direct contact

Any "direct" connect is doomed to fail.

2) A "direct" connection IS possible, but something a bit more sophisticated

than a bare cable touching the antenna is required.

If so, what would be required?

2a) I assume the (literally) micro / nano ? voltages generated by an antenna

can quickly be "killed" by a crude hookup like I was attempting.

Any tips, links greatly appreciated.

I have the ARRL handbook, and I've been googling antennas, soundcards, impdance

matching and everything else I can think of, but I can't find specific answers

as I assume that the answer is contained from knowing how all those (and others)

principles interact.


P.S. I'm starting to try and build an antenna / interface / amplifier / receiver, that might even work :), I will start a separate thread on questions about that project. So far, I've found information on calculating the values for a resonant circuit, and have wound a coil and put a trim cap on a breadboard. :)

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The answer is in the first paragraph, third sentence, of the link you provided.

"The frequency range depends on the sound card, but 20-20000Hz should be possible with all modern cards. "

Ok, 79.90 mhz is a "bit" higher than 20K ;), but wouldn't the signal show up as "aliased" ? (Possibly on a "real" scope, rather than soundcard/sofware one ....if there is a difference.

If not, then if transmitter freq was ~15 - 20K, would anything be conducted? I.e. is it possible to directly connect to the antenna?


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If the trans freq was in the audio range you would see a signal on the "scope". \

You could build a mixer with input of the signal you want to see and an oscilator to mix with that freq with the sum or difference freq in the 20 - 20,000 kc range.

You could also try a demodulator circuit just for the fun of it....

Good luck


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but wouldn't the signal show up as "aliased" ?

No (because of the limited analog bandwidth of the soundcard). On the other hand, the RF energy of the R/C Tx is high enough that a direct connection could cause damage to the Tx or PC. So, it is not something that I would recommend.

is it possible to directly connect to the antenna?

Yes it is possible, but not good practice. Keep in mind that the Tx's power amp can be stressed when you hang things on the antenna. And, those direct connections could transfer a lot of RF energy and harm your PC.

If you are interested in looking at the R/C signal then use your soundcard to measure the low voltage PPM signal on the trainer connector. It's safe and has all the important details you need to see if you want to study R/C signal encoding.

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