0
\$\begingroup\$

I have been trying really hard to understand the purpose of L2, L3 and C6 in this CW transmitter, especially L3 and C6.

I have received many conflicting answers such as that L3 is acting as a loading coil for the rather small antenna, that C6 & L3 is acting as a LC filter for RF harmonics or that it is 'amplifying' the voltage from the BJT PA.

Project Link

Thank you in advance!

enter image description here

\$\endgroup\$
2
\$\begingroup\$
  • L2 has an impedance of over 300 ohms at 27 MHz so it's fairly certain that this is being used as an inductor that allows the AC signal on the transistor collector to be freely developed whilst passing a DC current to said transistor.
  • L3 is to base load the 12" (300 mm) antenna and make it resonant at 27 MHz. A normal quarter wave monopole antenna would be a quarter wave long and, at 27 MHz this would be nearly 3 metres so, L3 helps make the "short" monopole resonate at 27 MHz.
  • C6 is just to prevent DC voltages on T2's collector being shorted in case the antenna touched ground.

So, for a short monopole look at this diagram of impedance: -

enter image description here

There is a red dot that approximates to 12" (300 mm) and that tells us that a short monopole of this length will have a capacitive reactance of about 1800 ohms. In terms of actual capacitance this is about 3.3 pF i.e. the monopole will look like 3.3 pF. This is series resonated by the 10 uH (L3) at this frequency: -

\$f = \dfrac{1}{2\pi\sqrt{LC}}\$ = 27.7 MHz

i.e. L3 is about the right ball-park for base-tuning a short monopole.

\$\endgroup\$
  • \$\begingroup\$ The impedance of L2 just happens to be very close (375) to the the magical vacuum number of 377 at that frequency. \$\endgroup\$ – jonk Feb 14 '18 at 14:30
  • \$\begingroup\$ @jonk yes it does!!! \$\endgroup\$ – Andy aka Feb 14 '18 at 14:31
  • \$\begingroup\$ Thought I'd just bring that up. ;) \$\endgroup\$ – jonk Feb 14 '18 at 14:32
  • \$\begingroup\$ Hi, thank you for your answer. Points 2 & 3 clear up L3 and C6. However could you please elaborate on point 1, specifically when you mention that L2 "allows the AC signal on the transistor collector to be freely developed". If I understand this correctly, L2 is basically acting as a RF choke, redirecting the AC signal to the antenna? \$\endgroup\$ – rfBug Feb 14 '18 at 14:38
  • \$\begingroup\$ The signal on the collector is still present even though it pushes power to the antenna. You could use a resistive collector load of circa 300 ohms and achieve about the same result. It's a bit of a sloppy design however because without proper current limiting from the 9 volt rail you could take excessive current through the transistor with just an inductor in the collector. It seems to rely on the 22 kohm base resistor to set collector current and this is not a great way of doing things. \$\endgroup\$ – Andy aka Feb 14 '18 at 14:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.