I bought the following toy transmitter to play with:
http://store.qkits.com/electronic-kits/radio-frequency/3v-fm-transmitter-kit.html
The description and schematic is in the attached k7.pdf file. It came with 160 cm of thin, multi-strand, insulated wire as a monopole antenna. I assembled it as instructed and it was working fine across all the 88-108 MHz range.
But, me being me, I could not resist tinkering with it. The instructions state:
"Connect a half or quarter wavelength antenna (length of wire) to the aerial point. At an FM frequency of 100 MHz these lengths are 150 cm and 75 cm respectively."
Well, I cut the antenna down to 75 cm expecting better gain and ... the oscillator stopped oscillating! The attached WAVE2.png shows the right BC547's emitter voltage. It shows the oscillator tries to oscillate but is quickly damped down. I tuned the trimmer all the way down to lowest capacitances and the oscillator started working at about 135 MHz and higher; see WAVE1.png. And again, when I lower the frequency below 135 MHz, the transmitter stops oscillating.
I disconnected the oscilloscope probe and monitored the FM signal with my SDR, confirming the same phenomenon.
I am perplexed. Why is this happening? Should't the quarter-length monopole resonate quite fine? Why this dramatic dampening effect? What is the theoretical explanation for this? I will be grateful for any insight from RF experts. Thank you.
P.S. I am posting to this forum for its expertise. This is not about FM transmitter design; I was just playing with this simple circuit and noticed this scientific curiosity. Please don't send me advices on how to design advanced FM transmitters; you would be wasting your ink...
http://store.qkits.com/electronic-kits/radio-frequency/3v-fm-transmitter-kit.html
The description and schematic is in the attached k7.pdf file. It came with 160 cm of thin, multi-strand, insulated wire as a monopole antenna. I assembled it as instructed and it was working fine across all the 88-108 MHz range.
But, me being me, I could not resist tinkering with it. The instructions state:
"Connect a half or quarter wavelength antenna (length of wire) to the aerial point. At an FM frequency of 100 MHz these lengths are 150 cm and 75 cm respectively."
Well, I cut the antenna down to 75 cm expecting better gain and ... the oscillator stopped oscillating! The attached WAVE2.png shows the right BC547's emitter voltage. It shows the oscillator tries to oscillate but is quickly damped down. I tuned the trimmer all the way down to lowest capacitances and the oscillator started working at about 135 MHz and higher; see WAVE1.png. And again, when I lower the frequency below 135 MHz, the transmitter stops oscillating.
I disconnected the oscilloscope probe and monitored the FM signal with my SDR, confirming the same phenomenon.
I am perplexed. Why is this happening? Should't the quarter-length monopole resonate quite fine? Why this dramatic dampening effect? What is the theoretical explanation for this? I will be grateful for any insight from RF experts. Thank you.
P.S. I am posting to this forum for its expertise. This is not about FM transmitter design; I was just playing with this simple circuit and noticed this scientific curiosity. Please don't send me advices on how to design advanced FM transmitters; you would be wasting your ink...
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