Inductance - self and mutual

The law of inertia states a body at rest wants to remain at rest and a body in motion wants to remain in motion. This law applied to current flow in a wire is inductance. We only have inductance when the system is changing state. The rate of change effects reactance. It is harder to produce a rapid change than a slow change. Inductance is referenced to a given condition and reactance is referenced to the operating frequency. Now how does this apply to a coil?
 
Take a piece of wire 1 meter long and stretch it out straight. Measure the inductance. What would you expect to measure?

 The calculated value 1500 nH.

Here is another calculator. About 1500 nH again.

The thing to remember is that a straight piece of wire has inductance. If it is suspended in free space it should match the calculated value. Lay it on the ground or a metal desk top and the reading will vary. The math is designed to determine the inductance of the wire ignoring in perfect surroundings. In a coil there are other factors which must be addressed.

Now look at a transformer. Connect power to the primary with an open secondary and the primary draws a small current. The primary current is limited by the inductance of the primary. Now place a load on the secondary. The primary current will increase. The increase of current is caused by the reduction of primary inductance as the secondary draws energy from it. Transformer action depends on the coupling between the winding. This is mutual inductance. For a more detailed explanation follow the link.

mutual-inductance-effect-on-transformer


Now you should see that inductance is not as simple  as you might wish. The wire has inductance based on the laws of inertia. A coil will have an effect on itself this is self inductance. A transformer has mutual inductance between the winding's.

If you are not familiar with the right hand rule and the left hand rule you should do some research on them. The simple matter is that inductance is opposition to change. The basic rule for generator and motors or electromagnetic action are based on magnetic fields and electric fields and motion.  Our power source applies EMF. This EMF produces current flow. The current flow produces a magnetic field.Motion produced when a magnetic field expands or contracts will produce opposition. This opposition is Counter EMF.

The law of interia again. The EMF produces a change. The Counter EMF opposing  the change in the wire is inductance. As the field expands it passes through other windings in the same coil by the process of induction produces Counter EMF this is self inductance.  In a transformer the coupling is through mutual inductance , magnetic coupling between the primary and secondary  winding. Stray capacitance in the circuit will have a similiar effect as the secondary of a transformer. Any current which can bypass the windings will reduce the self inductance. At resonance this effect will become null because the stray capacitance and the circuit capacitance combine to produce the capacitance needed to produce resonance.

I said all that to get to this one point. When you wind a coil all the factors come into play and the simple answer isn't always the best. Sure capacitance comes into play as does self inductance and wire resistance and skin effect.

Consider a bank wound coil.

 The applied EMF sees the wire as a linear resistance and therefore the drop will produce a linear potential between windings. (allowing for a small difference in length as the coil builds) For the sake of easy math assume the 4 layer coils has 18 millivolts across it. now compare the difference of potential between adjacent winding. With 18mv applied and 1 mv per turn turn 15 and 16 have 1 mv difference of potential. Turns 18 and 14 have 4 mv difference. The e-field between the plates of a capacitor determine it charge which directly relates to its capacitance. The disorientated e-field of the bank wound coil produces an effective capacitance of the two outer layers. This would be less than a straight wound coil.

If the coil were wound straight down the form and back the first two rows would be:

   7     8    9   10  11  12
1    2    3    4    5    6   

The bank wound give a difference of 1 to 4 all the way up to 18 which is the only 5. The common wound coil gives 6 straight down the row. These winding still have the same magnetic coupling between windings and an e-field of higher value. The e-field bypassing a portion of the signal reduces the measured inductance. 

The basket weave adds one other function. The inductive coupling would be eliminated if we could wind our coil such that the winding were perpendicular. With the basket weave the winding are at such an angle as to reduce the inductive coupling.

Observe the angle the wires approach each other and draw your own conclusion. Can it be that by reducing the inter winding inductive coupling increase inductance or the inductance was already there but was being reduced by inductive coupling between turns?

*Actually the basket weave was developed to reduce the proximity effect. Which raises the RF resistance reducing Q. That's another story. Subject for another post.

** If the field expands and contracts from the inter circle ( center of the core ) it passes through the turns. This is (self) inductance. The induced currents passing through adjacent loops produces mutual inductance. This produces an effect called proximity effect.

*** The (self) inductance is what we desire. Mutual inductance is only desired in a transformer. I emphasize SELF as opposed to mutual because there seems to be some confusion which had lead to the assumption that their are the same.

4 transistor 80 meter receiver

My friend Pat Pending sent me this one. It is worth a look. I would have to make some changes to build it with my materials on hand. Diode tuning mainly.

op amp radio update

My first post was an op amp radio. It used a LM741 which is a 1Mhz amp. I built it with a NE5532 which is rated at 5Mhz. It was designed to drive a small speaker and it will. I'm using it with an earbud  and have to be careful to keep the volume down. The NE5532 is a dual amp chip so why not add a RF amp? The only circuit modification for the NE5532 was the pin out. Here is the modified circuit.

 The original to compare.

The NE5532 output is pin 1 . The NE5532 power is pin 8. Changing those two pins is all that's needed.  Pin 5, 6 and 7 are the second stage. They could be used for an AF preamp, IF or Rf stage. This circuit gain is controlled by the ratio of R3 to R6. 1,000,000 / 10,000 = 100. the circuit gain could be adjusted by varying R6. If we change R6 to 1,000 ohms then 1,000,000 / 1,000 = 1000. The gain would be 1000. If we leave R6 at 1,000 ohm and add a 10,000 ohm pot with the wiper tied to one end between R6 and the tie point between R6 and R3 we would have a gain or volume control. It would adjust between R6 + 0 = gain of 1000 as just calculated to R6 + 10,000 = 11,000 ohm. which gives 1,000,000 /11,000 = 90.9.  Adding a 100KOhm pot would allow you to adjust the gain down to around 10. Because the op amp is so flexible I'm abandoning the hearing aid amp for the time being. If you only want a low frequency amp you could also use a LMx58. The LM358s are about a dime apiece and are rated 1 MHz.

playing with the balanced mixer again. The Gilbert cell actually.

A slick little circuit made with 6 transistors, 2 resistors, and a transformer. It is available as an IC but what's the fun in buying a chip when you can build your own? Don't laugh at my breadboard, I made it too. Here's the chip version.

If you could find them it would probably be better than my home built but it's only 6 transistor so here we go.

Same circuit but the IO ports are labeled.

Not much to look at. All the jumpers cover it up. Let's see what it does.  With some signals applied. I don't have a lab but I have a little digital function generator and a grid dip meter. I put the function generator on the LO input and coupled the dip meter to the IF.

You can see the signals are interacting.

I changed the signal a little and it look good considering I don't have lab grade equipment here.

I turned on the frequency display The purple spikes are the input signals. One is dead center, the other to the right. Nice envelope being produced.

This looks much the same but note I have lowered the signal. The spike is to the left now.

My scope is not the best I can only step through the adjustment but here you are seeing the same signal as before but the time base is changed. Note the frequency reading is the difference.

Here I really made some changes to the display same inputs.

Cleaning up a little here.

I pulled the clip leads away and this is the circuit. The round red thing is the transformer I had on the negative supply lead. The junction between the two resistors are the positive supply point. That's it 6 transistor, 2 resistor, and 1 transformer = Gilbert cell.
My project at this time is an 80 meter receiver I might build that circuit into it?

update:
I made a couple of long tailed amps using 2SK2539s and they worked quite well. Now I'm looking at a Gilbert's cell using the JFETs.

Examining unknown toroid core.

10 for $5.65 0.87" OD doesn't sound bad BUT what are they and what can they be used for? The reviewers give good reviews but no one knows the material. First I download Mini Circuits mini ring program. Second I wind 40 turns on one of the cores. Third I measure the inductor I just wound. Fourth I insert the data in the program.

After entering the data I press the button at the upper right screen.  Done with that step. Now how to use the data obtained.

From unknown to usable. Back in the main program click copy AL from tool and click Copy Dimensions from Tool. The data is set enter desired inductance, in this case 121 uh and click the =>. The program tells you how long the wire needs to be and what the max size can be.

All that's left is to put a couple of turns for a primary and send a signal through the transformer formed to observe the frequency response. It looks very promising if it will work at the frequency range desired.

As my final test I did put 7 turns on a core and it checked good. It would be good to check frequency response.

TTFD antenna

The TTFD was developed by the navy during WWII.
It seems "Buck" Rodgers developed it in 1958. His version with info can be found here.

EDIT: the link broke. A search for TTFD will take you to some good info. The diagram above with the formulas is basically all you need to build one. It has been made under several names. A search for FLAG antenna will find some good info also. The main difference between a TTFD and a FLAG is the way they are mounted.

BROKE LINK...........


Another option for the apartment dweller?

My sidewise flag. It's the noise factor that sold me.

My antenna is a sideways flag. I have a 1.2KOhm resistor between the two blue dots. The antenna transformer is 20T:9T on a 1/2" yellow toroid. We have a low power local (about 50 miles) that I couldn't receive in the house or shop. I could receiver it on the car radio with noise and it would be swallowed up by the noise as I travel around town. With this antenna I am receiving it. It is a little noisy but steady. I have also been listening to the ham net roll call on 80M and 40M. I'm listening to ABC Australia as I write this. It is a design worth trying because of the RFI improvements. With a simple wire antenna I have much to high a noise level.