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I am trying to amplify the FM band (88-110 MHz) from a dipole antenna but my amplifier has no output signal.

My antenna is two lengths of wire that are half-wave resonant at ~94 MHz. One wire (not connected in picture) is connected directly to emitter and the other (purple) is connected directly to ground. No transmission line, everything is point to point.

When antenna wires are isolated and probed: one wire touches probe, other wire is connected to probe ground clip. There is a 50 mV peak to peak signal at 94 MHz and some AM noise; antenna works.

When the antenna is connected to rest of circuit, GND clip to circuit ground, I see the same 50 mV signal at the emitter pin of the BJT and base pin of the BJT. Collector/C3 has no VHF signal, but FFT shows massive peaks at AM frequencies. Physical construction uses same schematic and labelled part values as simulation. What could be responsible for this behavior? I suspect I may have created a low pass filter via an input/base/output capacitor and BJT parasitics or a resistor that is causing this collector behavior.

On the backside of the perfboard, there are two lengths of copper tape that serve as (+) and (-) rails. These are connected to the power and ground rails of a 5V USB power supply. The large electrolytic capacitor smooths out the noise from the power supply.

What can I change/fix with this circuit to make it work as simulated?

Schematic

enter image description here

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    \$\begingroup\$ Have you studied RF amplifiers with feedback from the ARRL book, chapter 5, yet? From the book, "This is a circuit with two forms of negative feedback with (usually) a single transistor to obtain wide bandwidth, well-controlled gain and well-controlled, stable input and output resistances." \$\endgroup\$
    – periblepsis
    May 8 at 6:10
  • \$\begingroup\$ @periblepsis I have the 2005 edition of the ARRL handbook book but I cant find schematic you are referencing \$\endgroup\$
    – Yousif Alniemi
    May 8 at 15:28
  • \$\begingroup\$ It's more about studying the section. The information there provides ideas that are important to understand. But it doesn't provide specific designs you can use, with part values. Instead, five topologies that are suggestive but not limiting. I just wanted to know if you were familiar with the presented ideas there. Look for a section called "RF Amplifiers with Feedback". \$\endgroup\$
    – periblepsis
    May 8 at 18:43

2 Answers 2

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You should use very "shorter" wires ... and changes some capacitor's value.
Good decoupling with 1 nF // 100 nF (in place of 1000 uF).

Here is what I get.
NB: we lost some dB between Vg and Vem. Load (R6) should be higher than some kOhm.

enter image description here

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  • \$\begingroup\$ Thanks for the suggestions. I will try changing the capacitor values first; Your schematic shows R6 at 10k, would a high-Z oscilloscope probe be an equivalent load? \$\endgroup\$
    – Yousif Alniemi
    May 8 at 14:10
  • \$\begingroup\$ Yes. It should be. Don't set an R6 lower. \$\endgroup\$
    – Antonio51
    May 8 at 15:59
  • \$\begingroup\$ I tried changing the capacitor values to the values in your schematic and still encounter the same issue, my FFT shows a large peak at 1-5 Mhz but only small blips at VHF. Initially I wanted to keep C3 on your schematic small to avoid amplifying AM frequencies, but I might run a high pass filter from antenna to amplifier instead. \$\endgroup\$
    – Yousif Alniemi
    May 8 at 16:06
  • \$\begingroup\$ Question: would parasitic Cobo form a low pass filter with Rc? Cobo for a 2N3904 is 4pF max, and with Rc being 2k it looks like if a filter were to form, it would impede frequencies near 20Mhz and beyond. \$\endgroup\$
    – Yousif Alniemi
    May 8 at 16:14
  • \$\begingroup\$ Decoupling capacitors very near to components should be very important. Your FFT should be showing the "peak" of the power supply (analog or PWM). How are you taking this FFT? Running a high-pass filter should be a good idea also. \$\endgroup\$
    – Antonio51
    May 8 at 20:06
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My oscilloscope picks up 100 mV RMS in the VHF range when the probe and ground clip are touching each other. This drops to 50mV RMS when they are not touching each other. All of my readings were likely influenced by these false values; I will find a way to zero my oscilloscope and re-measure my circuit to see what is actually happening in my circuit and go from there.

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  • \$\begingroup\$ Yousif Alniemi - Hi, You wrote this as an answer, but isn't clear to me how this answers your original question. As the OP, you would only write an answer if you solved the problem on your own and the topic can be closed. So either: (a) You correctly wrote it as an answer, because it really does answer your original question. In that case, please clearly state that this is the final solution, and "áccept" your choice of the "best" answer. (Self-answers can only be accepted once 48 hours has elapsed since asking the question. Answers by someone else can be áccepted any time.) \$\endgroup\$
    – SamGibson ♦
    May 9 at 14:45
  • \$\begingroup\$ (continued) Or: (b) You mistakenly wrote this as an answer, but in fact you still want more responses. In which case, this is an update and not an answer, and should be "edited into" your original question e.g. click "Edit" under the question, add this new information at the bottom, then delete this "answer". Or (c) You are trying to reply to someone in a comment and wrote an answer by mistake. In this case, please delete this "answer" and write the comment in the correct place. || Which applies here, (a), (b) or (c)? Thanks. \$\endgroup\$
    – SamGibson ♦
    May 9 at 14:45
  • \$\begingroup\$ I would like to accept my answer as the final answer; I used noisy equipment to make measurements so I do not trust any of my own results that prompted me to ask the question initially. \$\endgroup\$
    – Yousif Alniemi
    May 9 at 14:53
  • \$\begingroup\$ Yousif Alniemi - Hi, Thanks for explaining. As I said, you can áccept your own answer once 48 hours has elapsed since you asked the question. That means you can áccept your answer after 04:39 UTC on 10 May (tomorrow). || FYI although you included a polite "thank you" in an edit of your answer, site rules here say not to do that. Also, that user will not receive any notification since @replies don't work in answers, only in comments. The site rules also say not to even add a comment with a thank you, but if you really want to, please add a temporary comment to that answer. \$\endgroup\$
    – SamGibson ♦
    May 9 at 15:02
  • \$\begingroup\$ Thanks for the information, I revised my answer to better comply with site rules. \$\endgroup\$
    – Yousif Alniemi
    May 9 at 18:19

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