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As a follow up to my previous question about measuring antenna performance via amplitude at 88 MHz to 108 MHz on a DSOX1102G oscilloscope, can I trust these new readings? Measurement channel is on 1:1 mode. 680mV peak to peak with a passive antenna seems absurdly high and too good to be true. Does this result seem reasonable for a 1/4 monopole?

I am using a BNC tee, 50 ohm terminator and a SMA to BNC for my 50 ohm coax. Antenna has only mechanical friction connections, no solder.

enter image description here

Yousif Alniemi
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    Could you be picking up AM here? I wonder if the scope horizontal setting is really for some something like 50n per horizontal division? – ee_student Dec 16 '22 at 07:58
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    Why do you think the reading is absurdly high? – Andy aka Dec 16 '22 at 08:31
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    @Andyaka I do not have any experience in Antenna design and have been using random wires until now without any effort put into impedance matching. I used to call 70mV peak to peak high. I might be able to use a buffer instead of an amplifier with signals this strong. That idea is stunning to me... – Yousif Alniemi Dec 16 '22 at 15:17
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    @ee_student If I zoom all the way out to the kilohertz range the FFT shows AM very clearly and waveform looks different. Datasheet says max horizontal zoom is 5ns/div and resolution is 2.5ps. I would like to think this is displaying a VHF signal. – Yousif Alniemi Dec 16 '22 at 15:24
  • I just saw on the top it says 50ms or something like that, so I was wondering if maybe the zoom was too wide. 5ns per div is more than enough, but perhaps could double check the zoom level where this waveform appears? – ee_student Dec 16 '22 at 17:21
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    @ee_student I see what you mean, can confirm it says 50 ns; also visible if the posted pic is zoomed into – Yousif Alniemi Dec 16 '22 at 17:30
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    Oh I see - that's good then, but in the same time, no idea about the waveform. One option is try what happens with a basic FM antenna or an amplified Terk antenna. For the basic, seems like you can connect a 50 ohm resistor underneath, and sample the voltage between the antenna and the resistor (with the scope's ground below the resistor). But something is not quite right, because I've built AM radios and I don't think it ever was more than 50 mV, and even that's usually more than what I normally got. – ee_student Dec 16 '22 at 22:33
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    @ee_student I have had similar experiences with random wire antennas; I posted this update as a question because these measurements contradict the previous performance of anything I have built...Good point with the waveform, I will play around with that and edit the post with my findings. Thanks for the idea! – Yousif Alniemi Dec 17 '22 at 03:27
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    Good luck w/ your project. I'm also doing an FM radio project now, little less ambitiously. I've ordered a Terk amplified antenna which also has a filter. Then a RF signal generator (that one expensive at $70), & finally an AD831 based mixer. The idea is to downconvert the FM to 1 MHz or so & then I can proceed like in my AM projects - my own transistor design. With AM, I've built 6 different transistor radios all from scratch without any chips to help me with amplification or demodulation. A good antenna for AM is AN-200 on Amazon. It has no amplification, so you'd still have to build AM amp. – ee_student Dec 17 '22 at 10:00
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    One thing I learned batteries work million times better than 9V power supplies that connect to wall. With batteries practically no noise, while power supplies tend have like 50mV noise that drowns the signal (you amplify the signal, but you also amplify the noise). But even with batteries, I put a 10uF capacitor between power and ground. – ee_student Dec 17 '22 at 10:03
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    @ee_student Nice to meet a fellow homebrewer! I made a superhet a while ago with a dipole antenna and a VHF colpitts oscillator that really liked to drift. I mixed down to 10.7 directly with a 2N3904 BJT as a mixer + ceramic BP filter, wish I knew more about impedance matching at the time. Demodulation was a slope detector. My go-to power supply is a 120 to 9V transformer in series with a full bridge rectifier and a lm7805. I had some luck with 5V phone chargers with an RF clip-on choke but a linear design beats it by far. Good luck with your radio! – Yousif Alniemi Dec 17 '22 at 17:46
  • Wow you're more advanced than me! Great projects. – ee_student Dec 17 '22 at 18:20
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    Yousif, if you don't mind my sharing, against difficult odds the design worked. I even recorded a long video of it working to post on YouTube as proof. What didn't work is the AD831 mixer - too noisy (requiring + and - 5V). So instead I've bought a passive mixer (ADE-25) & it worked! Terk amplified antenna + RF signal generator (MAX2870) + mixer (ADE-25) go to a breadboard at 5 MHz. At the breadboard there's slope detector, transistor buffer, envelope detector - I've done my self all 3 of these. After the envelope detector, the signal goes to LM386 audio amplifier (sitting on the breadboard). – ee_student Dec 22 '22 at 07:51
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    (LM386 has DIP8 package so it sits on the breadboard with the rest of the demodulator, and its output is an 8 ohm speaker.) – ee_student Dec 22 '22 at 07:57
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    @ee_student Congrats! If you have the link handy, I'd be excited to see your project in action! – Yousif Alniemi Dec 22 '22 at 22:30
  • Thanks for saying that. I'll try to upload it today & paste the link here. – ee_student Dec 22 '22 at 23:04
  • Here it is, thanks again for your nice comment (the station is 88.5 KQED in San Francisco area): https://youtu.be/jzR9s2imoGk – ee_student Dec 24 '22 at 03:29
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    @ee_student Nice! Enjoy your new FM radio – Yousif Alniemi Dec 24 '22 at 16:49
  • Thanks I'm just glad something works.. – ee_student Dec 24 '22 at 21:53

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