GENERAL DISCUSSION OF ACTIONS OF VARIOUS DEVICES
This page is all about experimenting and trying it for yourself. The devices described are quite crude and basic but work. Instead of believeing all the theortical stuff someone has made up, try it yourself and see what happens. At lower frequencies, the concepts are the same but things are big so it is best to try things using 50MHz or more.
Inductors store energy in a magnetic field. This is true of coils and of a single straight piece of wire. A single piece of wire is really just a coil with a single turn. It also produces a magnetic field around it which is built up with increasing current and collapses with decreasing current.
The inductance of a straight piece of wire is dependant on frequency, thickness and material but generally ranges somewhere between 400 and 700 nH/m (nano Henries/metre or nH m-1). In these examples, to keep things simple, a constant value of 500nH/m is used. The skin effect is partly to blame but for copper the conductivity is high enough that for 1mm wire, skin effect doesn't really play a large significant role in transformers up to 1GHz.
At lower frequencies the inductance per unit length is higher but the impedance decreases at a much higher rate. (Xl=ωL) At higher frequencies, the inductance per unit length decreases but not as much as the impedance increases with frequency.
SO - for all these examples an inductance of 500nH/m will be assumed. We can talk about the rest later.
DIAGRAMATIC REPRESENTATION OF TRANSFORMERS
At right are two simple transformers (a) and (b). Both are isolating transformers because there is no reference between the input and output. (a) uses air as a core and (b) some sort of magnetic material indicated by the lines between the windings. The dots indicate the starting point of the windings both made in the same direction and sense. If phase is relevant in the circuit they are used in, dots can be left in as in (a) or if phase is irrelevant, as in (b), they can be omitted. In either case, whether phase is relevant or not, they are the same transformers.
(c) and (d) are not transformers. They are simply an example of different diagrammatic representations of exactly the same thing. They are the same device shown in two different ways.
For good transformer action, the inductive reactance of each winding should be at least 3 times, preferably 5 times or more, the resistive load/input impedance. We can talk about that later as well.
EXPERIMENTS WITH TRANSFORMERS TO TRY YOURSELF
If you don't believe me, try it for yourself. Seeing is believing. You will need some coax coming from various RF sources, a suitable dummy load, a couple of RF cores (good ones if you like but anything really), some wire and an output socket.
The experiments shown below were done when I was needing a new BNC-BNC patch cord. I cut the coax just a little long so I could play around before I added the second plug. Same for N connectors or whatever.
Transformer action usually involves transferring energy from one circuit to another. This is usually done by some sort of magnetic transfer. If AC is passed through one of two parallel wires, the rising and falling magnetic field around that wire will induce a current in the other. The closer they are the greater the transfer of energy.
BE CAREFUL NOT TO BLOW UP A RADIO
Some experiments below may damage radios so you do all of them at your own risk. For the most part, a 10watt radio will probably not suffer too much damage if short circuted for only a few seconds when turned down to 1 watt so this approach is highly recommended. If it bothers you, put a 25 ohm resistor in series with the radio.
|1)||All dummy loads are 50 ohms unless otherwise stated.|
|2)||Although it is frequency dependant, an inductance per unit length of 500 nHm-1 for a straight piece of wire has been used throughout. Impedance change will be greater as a result of frequency change than per unit length change.|
|3)||If you are worried about your radio, don't do the experiments.|
|4)||None of these things are necessarily useable devices although, if made carefully, there is no reason why not. They are described only to illustrate various concepts.|
EXPERIMENT 1 - (ALMOST) STRAIGHT WIRE TRANSFORMER
Pick some frequency band and get about 1/5th wavelength of dual wire (speaker wire or similar). I used a strip of computer ribbon cable and it worked quite well. For 144MHz this will be about 400mm.
(a) Connect one of the wires across the RF source and the other across the dummy load as shown. Notice the only direct connection between the source and the load is the coax outer. Leave the wire in one big loop. Set the SWR meter on forward (must be a micro-stripline type or similar) and calibrate it to full scale on forward while pressing PTT. Don't worry about reflected.
(b) Leaving the SWR meter calibrated for the first position and leaving it on FWD reconnect as shown and again put some RF through it. Take no notice of the SWR reading, this is only being used to measure relative forward power.
When connected as in (B), you should notice the meter deflection will go somewhere near 80%. This shows a reasonable energy transfer.
WHY: The twin wire is acting as a single turn air core transformer. The reactive input and output impedance of the device will be about 180 ohms based on the 500nH/m. This is less than ideal and a longer piece of wire will work better but this should demonstrate the point.
EXPERIMENT 2 - ADDING SOME TURNS
Using all the same equipment as above, cut the wire in half (200mm for 144MHz). This time instead of a single loop, make it a double loop and repeat experiment 1.
It will depend on the frequency and type of wire but generally the transfer characteristic should be better.
WHY: By halving the length of wire, the inductance (and hence impedance) of the wire will halve BUT mutual inductance now takes effect multiplying the inductance by 4. For the 500nH/m approximation, the input and output reactive impedance should now be about 360 ohms or double the reactance of the first example.
The same things works on 160 metres or any frequency but this translates to 32 metres of wire. As it turns out, it isn't that much because, at the lower frequencies, the inductance per metre of a straight piece of wire is more like 650nH/m reducing the 32 metres to 24. Each time you double the number of turns though, the impedance multiplies by 4. At right is a simple 1:1 air-core isolating transformer (balun or otherwise) made of 2 x 1 metre lengths of single wire made into 8 turns by wrapping around a medicine bottle. It is not really ideal but, as an experiment, I managed to get better than 85% transfer on 3.5MHz.
EXPERIMENT 3 - EFFECTS OF CORE MAGNETISATION
Do not try this directly unless your radio is immune to very high SWRs.
At right (top) is a typical mains/audio style transformer. There are two separate and distinct windings. The primary winding magnetises the core and this magnetisation is transferred to the secondary winding via the core. In this case, the two windings are close together but need not be.
So try it with RF. It won't make any difference how many turns, what frequency or what material, if you can measure anything on the output you are doing better than me. Any forward power you can manage to send out with the SWR meter on the radio side will be converted to heat by the resistance of the primary winding. This won't be much and is NOT very good for radios to do it for too long.
SO WHAT'S THE CORE FOR? For single band operation, you mostly don't need one for transformers. Even for broad band operation, devices like the Johnson Matchbox ™ work very well using an air cored transformer. The air-cored transformer in the Johnson Matchbox ™ is adjustable though. Cores are an advantage when a simple 1:4 or 1:whatever match is required over a wide bandwidth such as an HF amplifier output.
At the higher frequencies, the core has very little effect. As the frequency decreases, so does the impedance of a winding. Remember a minimum of 3 times the load impedance is required - better 5. At the lower frequencies the core takes more and more effect thus taking the impedance of the winding back up. Thats all it does.
This experiment demonstrates that, at RF, anything happening on one side of the core has no effect at all on what is happening on the other. The idea that there is a difference between winding a Guanella balun on two cores or one is a load of bollox.
AIR V MAGNETIC MATERIAL (CORES)
Core wound devices can have shorter wire lengths (thus less series resistance) for the same inductive reactance. On the other hand, air has one definite advantage. AIR CAN'T BE SATURATED. The small extra resistance in the wire (for the same impedance) is more than compensated for by not turning so much of the signal's energy into odd harmonics. What you lose on the swings you more than gain on the roundabouts.
EXPERIMENT 4 - ACTION OF A CURRENT BALUN
Get some computer ribbon cable or even 3 pieces of single wire taped together as in (a) at right. You only need a few centimetres. Connect as shown in (b) and (c). The "coils" shown are actually only straight pieces of wire. Put some RF through it and compare the two readings of RF volts.
There will be a small reading in (b) because of stray RF around the place and inconsistencies in the wire but there will be a much higher reading in (c).
This is NOT a transformer. The current balun consists of only wires 1-4 and 3-6. The sense wire, 2-5, is only there to demonstrate the magnetic field strength. "Is" is the sense current and, in the case of (b), Iw is the normal working currents. These are currents in opposite directions for a current balun.
It the case of (c), "Ic" are common mode currents. That's pretty obvious because they are in the same sense and of the same magnitude.
EXPERIMENT 5 - IMPEDANCE, VOLTAGE AND CURRENT TRANSFORMATION
A couple of reminders:-
Faraday's law of induction. Induced EMF in any closed circuit is equal to the rate of change of the magnetic flux through the circuit.
Ohms law - restated. Voltage, current and impedance are inextricably interlinked. It is impossible to tansform one without taking the others with it according to the relationship V=IR.
Take a longer piece of triple wire as shown in experiment 4(a) above. For 144MHz, this should be about 400mm as in experiment 1. This time measure the SWR with the 50 ohm dummy load. There are stray capacitances, inductances and RF so it may not be perfect but it should be reasonable. Notice one of the wires (green) is just left hanging. Before moving on, try it with the green wire connected to itself around the loop. You will find the results interesting.
Now reconnect the red and green wires (or whatever colours you use) as shown in (b). Again, because of stray RF, the reading might change a bit but not much. When I did it there was no difference at all.
If you used radio with a 75 ohm output impedance, the value of the dummy load should be 300 ohms. If you want, you can even try it with a 4 wire transformer (1 primary 3 secondary) but in this case the dummy load impedance (resistance) should 450 ohms. Funny, I seem to have seen these numbers somewhere before.
WHY: It might seem a little strange that doubling the number of turns quadruples the impedance but it isn't so strange. Remember Faraday's law of induction. A change in magnetic flux density is generated by the primary (mauve). This produces an EMF in one of the secondaries (red). Because mauve and red are close to each other there must be the same, or near the same, rate of change in magnetic flux density in both wires. The induced EMF in red must therefore be the same as that applied to the mauve wire. The same can be said of the green wire. When green and red are placed in series you end up with double (or near double) the volts.
In the end it is energy (power) that is transfered between primary and secondary. Power, W = V2/R or R = V2/W. Both resistive (power consuming) and reactive (non-power consuming) currents are included in this so instead say apparent power aW = V2/Z or Z = V2/aW
Thus for the same apparent power, double the volts gives 4 times the impedance. To increase the power through some resistance, the voltage must increase. If the impedance changes, so does the power and therefore the volts and/or current. It is impossible to wind any sort of transformer that transforms a voltage and not something else.
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