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This circuit is from an old Dick Smith Electronics project book. It’s an FM transmitter.

BJT based FM transmitter

I’m wondering why the antenna is connected at a tap of the coil, rather than at the collector. Is it to impedance match the output of the circuit with the impedance of the antenna?

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  • \$\begingroup\$ You haven't described the antenna. If it were a half-wavelength long wire, you probably could tie it to the coil-end, whereas a quarter-wavelength long wire would be best tapped-down on the coil. In any case, it would carry a DC voltage from the battery. \$\endgroup\$
    – glen_geek
    Commented Nov 11 at 14:41

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Is it to impedance match the output of the circuit with the impedance of the antenna?

Yes, actually, that is some excellent deduction!

The output impedance at the collector of the transistor is typically quite high, while the antenna usually has a much lower impedance (usually about 50 ohms for regular antennas). Directly connecting the antenna to the collector would result in a significant mismatch, leading to inefficient power transfer and reduced signal strength. By tapping the coil, the circuit essentially creates an impedance transformation, allowing for a better match between the transistor's output and the antenna.

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  • \$\begingroup\$ Thanks @Colin. Since that's a series inductance, and the impedance is being lowered, would that mean the BJT's output impedance is capacitive? A Maximum Power Transfer Theorem matchup going on... \$\endgroup\$
    – Greg Bell
    Commented Nov 12 at 1:42
  • \$\begingroup\$ Or is it to base load the antenna, cancelling the electrically short antenna's capacitance for better power transfer? I would also think the output impedance to be pretty low, since the top of the coil forms an impedance in parallel with the collector of the BJT. \$\endgroup\$
    – Greg Bell
    Commented Nov 12 at 2:12
  • \$\begingroup\$ @GregBell You're welcome. If there are capacitances in the output impedance, it would be internal junction capacitance, which I will admit I'm not really an expert in because that is a very low level topic, something extremely nuanced that frankly I never really consider too much. But I do know that the inductor L1 is will dominate your impedance characteristics since your collector is connected to the tank circuit... Next, by tapping the antenna partway down the coil, you are indeed creating an impedance transformation that can provide a better match to the antenna's impedance. \$\endgroup\$
    – Colin
    Commented Nov 12 at 3:10
  • \$\begingroup\$ (cont.) Electrically short antennas, such as the small whip likely used here, exhibit capacitive reactance. The tapped connection to the coil effectively loads the antenna, which could help in compensating for this capacitive reactance, providing a more suitable impedance match. You’re correct in noting that the top of the coil forms a parallel impedance with the collector of the BJT. Because the antenna tap point is not at the very top of the coil, it experiences a lower impedance than the collector, which is beneficial. \$\endgroup\$
    – Colin
    Commented Nov 12 at 3:11
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This is a common-base Colpitts oscillator modulated from the base of TR1. If connected directly to the collector node, the (typically 50 or 75 Ohm) antenna would de-Q the circuit and hinder its self-oscillation. Tapping the coil lets you avoid the de-Qing while still delivering some signal power to the antenna.

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    \$\begingroup\$ Also, note that a quarter wave at 100 MHz has a wave length of 3 meters (about 10 feet). If your pirate radio station has an antenna shorter than 30 inches is sure to have a capacitive component to its impedance. \$\endgroup\$
    – richard1941
    Commented Nov 15 at 0:07

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