Construction of 'Tapped' Air Core Coils

This is the construction of the 'tapped' air-core coils needed for the 30mW FM Transmitter, the 200mW FM Transmitter and the 7 Watt FM Transmitter. The construction approach of these coils will warrant the manipulation of squeezing and stretching the device many many times, without it coming apart at it's 'tapped' soldered point; as do many, when periodic opening and closing of the coil is needed. So, whenever you may need a 'tapped' air-core coil, the sturdiness of this inductor is the best I have found.

I will take you thru a step-by-step procedure (with pictures) in order for you to understand how a 'tapped' leg can be applied without having to solder a 'separate' piece of wire to the coil itself.

Let us being...

Begin by taking 12 inches of un-insulated 18 gauge solid copper wire. From one end, measure out 3 and 1/2 inches. At that point, bend the wire as shown in the picture below.



So now you have the upper end of the wire being 3 and 1/2 inches and the lower end of the wire being 8 and 1/2 inches.

Starting from the left end of the wire in the picture, take a pair of pliers and sqeeze the upper and lower ends together real close so that they touch each other. Back off your pliers a bit so that you leave the doubled-up wire extend 'out' from the pliers about 3/8 to 1/2 of an inch making sure that you keep both portions of the wire (upper and lower)' tightly together. It is in this area (3/8" to 1/2") that you will solder the wires (upper and lower) together. Take a look at the picture below. The picture shows how the pliers are positioned, so as to leave the said distance hanging out of the pliers. Even though the picture shows the extension to be slightly longer than 1/2 inch, 3/8 of an inch would be the perfect distance.



The picture below shows the area to be soldered... with solder, soldering gun and positioning of the wire in the pliers to be soldered



The picture below shows the portion of the wire that was soldered. It was soldered, then bent at 90 degrees (each wire) away from each other.



Now take your 5/16 of an inch standard size threaded bolt and place wire into the threads of the bolt, as shown in the picture below.


Begin making your wraps with the extra wire. The picture below gives you an idea on how this is done. As for the tapped air-core coils for the three transmitter projects (mentioned above), one wire will be wrapped 3 complete times around the bolt, and the other wire will be wrapped one time around the bolt. Count each revolution as 'one', everytime you pass the soldered 'tapped' leg. As you can see in the picture below, the left wire was wrapped '3' times.



Once the proper wraps have been made (on both sides of the tapped leg), you may now bend each wire to a 90 degree fold. These two 90 degree bends will form the 'two outer legs' of the coil. Look at the picture below for a representation of the two bends.



Next, cut both wires to the same distance as the center (tapped) leg. It should be right at 3/8 of an inch. The picture below shows the three legs to be longer than 3/8 of an inch, but that was because the tapped leg was 'actually' soldered for a distance of 9/16 of an inch . That was my mistake. (see the picture that has the ruler in it for verification of my mistake).



Carefully back out the coil from the bolt. Next, stretch the complete coil to a distance of exactly 7/16 of an inch. Now the overall height of the coil should be right at 3/4 of an inch, from the bottom of all of the legs to the top of the coil itself. If too long, trim the bottom of the legs accordingly. If too short, make another coil and make sure to add more distance to all three legs. Keep all legs straight down (even though the picture below shows the tapped leg bent out) for the 30mW FM transmitter project...the PCB template used in that project calls for all the legs coming straight down. The 'tapped' leg does NOT need to be bend out.




...and let the project continue!


...your friend, Patrick



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