RF Coil Calculator

RF Coil Calculator - Calculates The Coil, Capacitance or Frequency for an FM Transmitter Circuit

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Coil Calculator for determining Inductance


Virtually any small adjustable FM Transmitter - can be set outside of the FM band ( 88 to 108 Mhz ) and up into ( Or Below ) the VHF band ( 144 To 174 Mhz ) - And Received on any Tunable FM or VHF Receiver with a Single Component Change, That Being It's Coil. The determining factor lay in its tank circuit. Of course a couple of other factors come into play such as it's feedback capacitor and also the internal characteristics of the transistor itself.

For the most part - Only the Coil - ( Usually Described as L1, Shown Below In The Example Schematic ) in it's Tank Circuit ( Tank Circuit = The Combination Of C2 and L1 ) has to be changed. Using the calculator below you can determine the New frequency you want by simply typing in the Desired FrequencyThe Fixed Capacitor  - and then "Hit" the  "Compute Button" and it will Calculate The New Value of Inductance you will need to make your transmitter- Operate At Your Desired Frequency. If By Some Chance Your New Frequency Doesn't Oscillate ( Transmit ) - Simply Increase or DecreaseThe Value Of The Capacitor Between The Emitter To The Collector ( Always Placed In The Position Of C3 Below ) - With A Lower Or A Higher Value. You Can Use A Small Trimmer Capacitor ( A Ceramic Trimmer Capacitor Is Shown Below ) In Place Of A Fixed One In The Place Of C2 To Adjust Your New Frequency "Right On The Money" And It Will Also Allow You To Adjust The Frequency Up And Down The Band.

The Coil Form I Use To Make This Conversion ( 3368K-ND ) Is Very Cheap and They Can Be Found Thru The Search Block Below. I have hand wound my custom coils for over 50 years. My coil standards are given below and are 100% accurate. A Sample of how I wind them is shown below.


FM Transmitter Circuit Coil Form
FM Transmitter Circuit Inductor coil
Ceramic Capactor for RF Transmitter Circuit


A Ceramic Trimmer Capacitor


Coil Form 3368K-ND ( Enlarged Views )

THE PRIMARY REASON - I use the small 3368K-ND Form shown above ( Enlarged View ) in all of my Professional Crystal Controlled RF transmitters on Bugplan.Com and wind them in a toroidal configuration is because of;

Toroidal inductors have higher Q factors and higher inductance than similarly constructed coils. This is due largely to the smaller number of turns required when the core provides a closed path. The flux in a toroid is largely confined to the core; the confinement reduces the energy that can be absorbed by nearby objects, so toroidal cores offer some self-shielding.

Another reason I wind them in this fashion is because they are extremely small, prevent a lot of instability and radiating into other coils and sections of the board. I always mount them 90 degrees to each other on the board itself. At the same time the high "Q" of these coils tend to cut down on the harmonics of the transmitter.

New Frequency Caculator

Enter Your Desired Frequency, Your Capacitor ( ie, 5pf, 10 pf, 20 pf and so on )
And then click on "COMPUTE" 
Look for your New Inductance Value - In The Coil Standards Below

Inductance (µH)
Capacity (pF)
Frequency (MHz)

If You Don't Find The "Exact" Value Of Inductance You Are Looking For, INTERPOLATE - ( ie, Remove or add a turn to the nearest value that matches your new value.

( Coil Form ) - Part Number:    3368K-ND    DigiKey Electronics

COIL WIRE (Magnet Wire): Size # 26
NOTES: Even turns means that the wire turns are spaced out even around the entire form. Tight turns
means that the turns are wound against each other. ( See The Example Photo Above ) For example take the first inductance .018 Microhenry.
2 Tight turns. This would be two turns wound right next to each other with about 1/4" lead coming out at
the bottom for soldering, etc. The distance across these two wires would be less than 1/32" with the rest
of the form being empty.
Now take the 2nd Inductance .025uh. This would be three turns, one on the left side of the round
washer, one at the top and one on the right side with the leads terminating out of the bottom. In other
words, the three turns are spaced even over the entire circle of the washer.
.018 uh  -  2 tight turns.
.025.uh  -  3 even spaced turns.
.034.uh  -  5 even spaced turns.
.046.uh  -  9 even spaced turns.
.049 uh  -  5 tight turns.
.053 uh  - 10even spaced turns.
.06 uh    -  8 tight turns.
.077 uh  - 10tight turns.
.09 uh    - 15tight turns.
.14 uh    - 20 tight turns.

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