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100uH "rf choke"

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adnan012

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i want to construct 100uH "rf choke" for rf amplifier at 30 Mhz.
i have vk200 type ferrite beads.
is there any formula for coil turns,diameter etc
 
Usually the manufacturers provide information like that. Amidon, Palomar, Ferroxcube might have what you need. My Amidon handbook has a table that lists impedance at various frequencies for beads that are similar to the vk200.
 
If you can't find design information, you can also use the "cut and try" method. Just wind a coil then measure its inductance and iterate from there. Of course, this means you have to have an easy way to measure inductance, but this can be very simple if you have a sine oscillator and an oscilloscope or AC voltmeter. Just setup the oscillator to output a sine wave at a known voltage and frequency, put a known resistor in series with the unknown inductor so you have a simple series circuit of oscillator, resistor, inductor. Then measure the AC voltage across the inductor or resistor. You can then calculate the impedance of the inductor using simple ohms law. From this you can simply calculate the inductance knowing the frequency of the AC signal. Watch out for errors such as mixing peak vs RMS ac voltages (use either but be consistent). Go ahead and vary the frequency and resistor value in order to get a practical voltage drop across the inductor, but keep the frequency as low as is practical to avoid errors due to hookup. Also, be sure to include the oscillator's equivalent output resistance in your ohms law calculation.
 
Another thing worth mentioning. RF chokes are often not just inductance. For example, many ferrite based chokes have significant series resistance along with inductance. It is also common practice to wind wire onto a resistor to create a choke, so obviously the equivalent circuit of such a component has lots of parallel resistance. This has a practical purpose in reducing the Q of the choke, a useful thing.

So, inductors used for impedance transformation (matching) should have low loss, but inductors used as chokes may be better when they are more lossy. A choke wound on a VK200 bead would probably have significant series resistance as well as series inductance.

A formula for a simple air wound coil would not give you the correct answer for a coil of the same number of turns wound on a resistor or ferrite core. So, you have to rely on data from the manufacturer of the ferrite core.
 
thanks

actually i want to construct the following circuit with small modification.

for 28Mhz rfc choke is 10uH.

in the diagram other coils are wounded on T25-6 toroid. is it critical?
 

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adnan012 said:
thanks

actually i want to construct the following circuit with small modification.

for 28Mhz rfc choke is 10uH.

in the diagram other coils are wounded on T25-6 toroid. is it critical?

Yes, it is critical. If you make your own coils with other type of toroid or some other core, you will have to tune the number of turns.
 
it means that inductance is important. specified inductane for 20 turns on t-25-6 troid is 1.02 henry.

problem is that i don't have toroids. can i make coil by using tune able slug former with same inductance?
 
Yes, but then you'll have to shield the two "T25-6" coils from each other and the outside world as toroids are self shielding. You can turn the two coils 90 degrees to each other to minimize coupling. RF projects need a shielded enclosure anyway.
 
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