Please read the description/textbox first. Circuit of a Dipmeter (Test oscillator) usable for frequencies between 20 MC and (say) 110 MC.
The oscillation is set via the biasing potentiometer of 22 K (can also be 25 K).
The LED indicator must be set to its proper working point via the 500 K potentiometer or 500 K trimmer. Not very difficult. More info in the video.
It is an elaboration of the circuit in this (18 July 2022) video: https://youtu.be/SUXYCphn2EU
Corrections to the video:
• On 5.00 in the video I talk of a “green” (color) trimmer, but of course it is a yellow (kind of) trimmer
• On 5.59 in the video I talk about “someone on the carbon layer”, but it is of course “somewhere” on the carbon layer of the trimmer. Hilarious.
APPLICATIONS OF THIS RADIO CIRCUIT (THE TEST OSCILLATOR/DIPMETER FOR 20MC-110MC)
1. Find the oscillation frequency or bandwidth of unknown coils or unknown coils with a capacitor parallel to it (between 20 MC-110 MC): connect the coil to the test oscillator, set the coil into oscillation with the 22 K (or 25 K) potentiometer, read out the frequency on the scope or frequency counter.
Please note: that oscillation frequency never comes “alone”. It could be that the coil is effected (damped) by the way it is wired into an electronic circuit.
A rule of thumb: when it is wired to the gate of a FET or the grid of a radio tube their (grid or gate) impedance is so high that the effect on the frequency of the coil or the tank circuit (L-C) will be very small. When a coil or a tank circuit (L-C) is loaded with (input-output) resistance, the frequency drops in general, just like the Q. The bandwidth will/can go much broader, it can loose its quality to be sharp tuned to a certain frequency (physics & radio theory).
2. Test the Q (quality factor) of unknown coils in the 20 MC-110 MC band: when the oscillator does not want to start the Q is (too) low.
3. Test the frequency (oscillation frequency) in another way: let the oscillator oscillate. With a coil of X windings, say between 6 and 15 turns on a paper or plastic form of 7 mm diameter, it will go between 20 MC and 110 MC. Readout the frequency. Make another tank circuit (L-C) and lay it parallel to the oscillator coil (must be parallel otherwise the coils don’t couple fierce in the electromagnetic HF/AC way). Turn the C (trimmer/tuning capacitor) of the external coil to the frequency where it sucks out the maximum HF energy out of the oscillator coil. This is the resonance frequency (see also 1 when you want to wire that coil into a radio circuit, the input-output impedance issue).
These are only a few applications.
You can also use it as test oscillator for a radio that you have made, in the frequency band of 20 MC-110 MC.
My You Tube channel trailer is here: https://youtu.be/xbgQ8T3oqh4 When you search, search always “NEWEST FIRST” to get the right overview. You can also search via the “looking glass” on my Channel trailer via keywords like ”audio”, “radio”, “amplifier”, “filter”, “Shortwave”, “transistor”, “FET”, “oscillator”, “generator”, “switch”, “schmitt trigger” etc; so the electronic subject you are interested in. My books about electronics & analog radio technology are available via the website of "LULU”, search for author “Ko Tilman” there.
https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=Ko+Tilman
I keep all my YT videos constant actual, so the original video’s with the most recent information are always on YouTube. Search there, and avoid my circuits that are republished, re-arranged, re-edited on other websites, giving not probable re-wiring, etc. Some persons try to find gold via my circuits. I take distance from all these fake claims. I cannot help that these things happen. Upload 20 July 2022.
MORE INFO? When you want to know more about dipmeter (s): go to the www (Google) or to my channel trailer, “Radiofun232 on You Tube”. Go on my channel trailer to the “looking glass” and type keywords like “dipmeter” or “oscillation” or “resonance” or “coil” or “radio coil”.
Pin connections of the BC 549 c are the same as BC 547.
As drawn in the picture, from left to right: E-B-C.
My book "Retro Radio" (DUTCH TEXT) is availabe via the website of Elektor Netherlands. It is/was sold out, so the printed version is no longer available. ISBN number is 978-90-5381-234-1. First print december 2008.