Tunable RF coils question

[Ziddik]

ppl.

Is it possible to replace the variable capacitors in an fm transmitter circuit with tunable rf coils? I got plenty of them in my junkbox so i just want to know as i don't have any variable caps left now. These are some pics of the tunable rf coils.

 

[duffy]

Those are probably tanks, tuned to an "intermediate" frequency. Take one apart, and look for a capacitor soldered to the inside at the bottom. Also, you may find there are 2 windings, these are often transformers.

You would need to remove the capacitor and completely unwind the tiny bobbin inside the tunable cap. Then try rewinding it with just 1 or 2 turns, and see if that gets you in the frequency range you want.

 

[Ziddik]

so it is possible to work an fm transmitter without variable caps, right? But if i removed those caps and used only an inductor, then will i get stability while using the transmitter?

 

[duffy]

You still need a cap, but it will be a fixed cap. You can make a tunable tank circuit with either a variable cap and a fixed inductor, or a variable inductor and a fixed cap.

 

[Ziddik]

You still need a cap, but it will be a fixed cap. You can make a tunable tank circuit with either a variable cap and a fixed inductor, or a variable inductor and a fixed cap.

Thanx duffy, and what is the accurate value of the fixed cap for 94.00 or between 88-100mhz(there are stations in rest of the frequencies after 100) mhz fm band? I tried to use a ceramic one written "15" on it, but it doesn't work!. And i know the frequency will change when spread the 6 turns inductor so i don't need a variable cap, all i want to know is the value of the ceramic cap for fm and anywhere on 88-100mhz

 

[duffy]

You don't want the full 6 turns, the ferrite has a higher permeability than air.

I used 10 pf caps when winding the coil around a pencil.

 

[Ziddik]

You don't want the full 6 turns, the ferrite has a higher permeability than air.

I used 10 pf caps when winding the coil around a pencil.

good, but i did not get what you meant with 10pf!. I mean, does a fixed 10pf cap become a replace for a variable capacitor?. And is using a ferrite cored inductor better than an air cored inductor? Will the transmitter's range increase if i used a ferrite cored inductor? Thanx alot for helping me

 

[mvs sarma]

@Ziddik,

A stuffed aircore coil is always better. the stuffing is generally by non capacitive, material, many FM front end coils can be seen locked with a small foam stuffer.

Most modern method of tuning is Varactor (Varicap) based. at the FM frequencies in the worldwide 76 to 110MHz range. better to study and emulate it.

However, if you had ever opened a Car radio and seen, you can find that the designs employ tunable inductors. In the frequency range you are talking about, resonant inductor needed for a known capacitor can be calculated from basic parallel resonant LC circuit. F=1/(2*pi*sqrt(L*C))
load this formula into excel sheet and fix the value of C. set a frequency and see what is value for L and tabulate perhaps you need a 0.1uH coil approximately for the antenna tuning.

Then make an air-cored coil, with thicker wire for say 100MHz. then use a brass slug inside slowly screwed under a fixed support plate. and measure the inductance with an accurate LC meter. the brass slug helps minor changes in L values.

This way within a day you can make these coils yourself.

 

[duffy]

does a fixed 10pf cap become a replace for a variable capacitor?. And is using a ferrite cored inductor better than an air cored inductor? Will the transmitter's range increase if i used a ferrite cored inductor?

Yes, the fixed 10pf replaces the variable one. The ferrite is not really "better" (as mvs sarma mentions) at least not in the sense of improving range. The ferrite will suffer eddy current and magnetostrictive losses, which can rob energy from the tank. The aircore can have the same problem if it's external field interacts with nearby metal - the capped ferrite coils have the field contained internally. They both suffer from resistive losses (magnified by "skin effect") but thicker wire can reduce that. The aircore can also transmit noise from vibration if it is made as an open spring.

 

[mvs sarma]

Good post Duffy !.
that is where air-cored coil should be locked from vibration, mostly foam stuffer is used.
One can also use insulating bobbins. generally when more coils are in close vicinity, they are arranged in isolating chambers as sections and / or mounted at right angles.

 

[colin55]

For 88-108MHz FM you need a 5 or 6 turn coil 0.25mm wire on a 2.5mm air core. You need about 33p to obtain the correct LC ratio. That's all you need to know.
Generally a ferrite material has to be F29 for 100MHz The core in the cans operate at a lower frequency and cannot be used.
Brass slugs simply reduce the Q and change the inductance from 1 to less than 1.
Do you want me to go through and correct any of the above submissions or can you work out the incorrections.

 

[Ziddik]

For 88-108MHz FM you need a 5 or 6 turn coil 0.25mm wire on a 2.5mm air core. You need about 33p to obtain the correct LC ratio. That's all you need to know.
Generally a ferrite material has to be F29 for 100MHz The core in the cans operate at a lower frequency and cannot be used.
Brass slugs simply reduce the Q and change the inductance from 1 to less than 1.
Do you want me to go through and correct any of the above submissions or can you work out the incorrections.

Heya collin,
33pf? Does a 33pf replace a variable cap?

 

[colin55]

yes. . .

 

[Ziddik]

yes. . .

ok, one more question please, i think it is written "332" for a 33pf cap right collin? I got some ceramic caps

 

[duffy]

No, 332 = .0033µf, or 3300 pf. A 33 pf is generally just stamped with a 33.

Note that Colin's coil has a 2.5mm I.D. This is a lot smaller than a pencil - so the inductance goes down, and then the capacitance needs to go up for the same frequency. That's how tank circuits work.

 

[colin55]

Sorry, I made a huge mistake. The coil is wound on a 3mm screwdriver or twist drill.

 

[Ziddik]

it works even with a 15 pf cap, but the range of the transmitter is very low (only 10M!).i used a 2n3904 and all the resistors which i used in this project are SMDs , has anyone ever made a 1Km transmitter? It's hard like hell to make a transmitter

I used colin's transmitters
and changed component values as:

Supply volt - 3.7V

10p feedback cap changed to 5p (written only "5" on the cap)

470R emitter resist. Changed to 100R

used only 47K instead of 150K for final stage transistor

.did not add 5-30p cap in final stage and used only a 7turn 0.3mm inductor, 3mm dia

bc547 is used as mic amp and two 2N3904

did not use +1u cap, instead i used a poly cap (a green, sack like cap written 223 on it)

Used 1mm thick copper wire to make L1 6turn coil

Result:

stability is not bad, but the range is only 10M!

The transmitter won't work when i added the last 2n3904 transistor (3rd transistor) so i tested the transistor without 3rd 2n3904,

Hope, collin can understand the mistakes i made,

 

[colin55]

You have to read ALL the pages on RF transmitter on my site to save yourself making all the mistakes you have made.

You need 10p for 3v
100R too low. You are over-driving the transistor.
.did not add 5-30p cap in final stage and used only a 7turn 0.3mm inductor, 3mm dia
The final stage is TUNED. You have to tune it
2N3904 cannot be used for RF stage. It has no gain at 100MHz. It is only a 250MHz transistor

The circuit will transmit 27km from the top of a mountain as stated by one of my customers.

 

[Ziddik]

You have to read ALL the pages on RF transmitter on my site to save yourself making all the mistakes you have made.

You need 10p for 3v
100R too low. You are over-driving the transistor.
.did not add 5-30p cap in final stage and used only a 7turn 0.3mm inductor, 3mm dia
The final stage is TUNED. You have to tune it
2N3904 cannot be used for RF stage. It has no gain at 100MHz. It is only a 250MHz transistor

The circuit will transmit 27km from the top of a mountain as stated by one of my customers.

thanks collin, so what is the replacement transistor for 2n3563?
I had built the same circuit about 1 year ago, but i disassembled it later and forgot how i made it and what replacements i have made,

 

[mvs sarma]

try to use 2N3866 locally available . but biasing would change. please check pinout while mounting