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 Post subject: Small RF amp for signal generator
PostPosted: Jan Mon 01, 2018 3:40 am 
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Joined: Aug Sun 02, 2015 7:34 pm
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Location: Yukon, OK USA
Disclaimer: I am not an engineer so don't ding me for my experiment. Tnx

Got to playing in the shop and thought that my signal generator was just an under powered AM transmitter and got to looking at some designs for transmitters and noticed the Vectronics 1290K just fed an oscillator signal to a 2N3904 and an LM386 to modulate it. That set me to thinking (sometimes a dangerous thing) that a small RF amp might be possible. So I broke out my stash of 2N3904s and a breadboard. Using a variable power supply and a pot to adjust the bias I watched the output signal for distortion on my scope and found a point at which I got a pretty good signal out that covered my house. I have not really worked on the antenna tuning yet. Audio was good after setting the modulation at the signal generator. I just fed an iPhone into the external audio input of the generator to play music.

Not only a frequency agile VFO but I have a frequency meter that monitors my generator so I know exactly what my output frequency is and can keep track of any drifting.

Kind of fun way to spend indoor time when the weather is in the 20's. Had pretty much everything I needed so cost was about zero. Thought I would throw this out and ask if anyone else has given this a try?

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 Post subject: Re: Small RF amp for signal generator
PostPosted: Jan Mon 01, 2018 3:34 pm 
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Joined: Jun Wed 08, 2011 2:33 am
Posts: 6996
Location: Ohio 45177
No but I once had a URM tube type RF generator. URM 25? Not sure what version, but it would certainly crank out enough RF at max to jam FM BC to quiet down noisy dorm neighbors for awhile. But if they used tapes, SOL.


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 Post subject: Re: Small RF amp for signal generator
PostPosted: Jan Mon 01, 2018 5:50 pm 
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Joined: Aug Sun 02, 2015 7:34 pm
Posts: 883
Location: Yukon, OK USA
Had a interesting experience with this setup. Since the drive is coming off my signal generator I have an output control. When I reduced the RF to the amp the audio sounded better. Don't think it was distortion from the transmitter because I have been watching the modulation on my scope while working with this. Appears that the AGC may have some impact on the audio quality on this radio and it was getting too much signal. Now I can put the RF just above the noise level but not swamp the receiver.

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 Post subject: Re: Small RF amp for signal generator
PostPosted: Jan Fri 05, 2018 3:20 am 
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Joined: Dec Sat 24, 2011 9:17 pm
Posts: 2641
Location: Vancouver Island, Canada
I've used my old Heathkit SG-8 signal generator for a transmitter. It uses 2 tubes to do the magic. For the antenna, I used a 10' strand of wire.

Yes, I found my more powerful AA6 Stromberg Carlsons would not receive as well as the weaker AA5's. Turning the output down might be the answer, maybe a shorter antenna? Before I got anywhere with adjustments I had to pack it away for ongoing renos.
That wallpaper is history. 8)


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