Although the rule states that the winding should exhibit a minimum of four times the source impedance at the lowest frequency, it should be noted that if too many turns are wound on the core, troubles can arise at the high frequency end of our wideband transformer. It would be wrong to put 40 turns on the primary and 20 turns on the secondary in our previous example.
In a wideband transformer it is best to wind the core by using a "multi-filar" type of winding. The bundle of wires can be wound on the core at one time by laying the wires "side-by-side". An example of a quadrifilar winding is shown in the diagram. The advantages are twofold: all wires are of equal length and the interwinding coupling is improved. The disadvantage is that the turns ratio must always be a whole number (1,2,3,4 etc), which means that the impedance ratio will be 1,4,9,16 etc. Where you require a fractional ratio (1.5, 2.8 etc) you must use a "standard" winding technique, where the windings are placed seperately on the core. This "standard" winding always produces a narrower bandwidth than the multi-filar technique.
It is often a good idea to wind a layer of PTFE tape (plumbers thread sealing tape) on the core before winding, so that any sharp edges do not cut through the insulation on the wire. This is particularly true when winding a high power transformer where high rf voltages can cause breakdown. Always use wire with a suitable insulation coating for the peak voltages expected and use PTFE insulated wire for any high power RF transformers. For low voltage applications, wire with poly-urethane insulation is useful as it is self fluxing when heated with a soldering iron and does not need to be scraped first.
Iron powder cores are normally used in these applications, where
the important properties are inductance, Q and temperature effects. Having
selected a material suitable for the frequency of operation and a core size
to suit the application the initial winding should be evenly spaced to cover about
270 deg. of the circumference. This allows for some adjustment of the final
inductance by closing up the turns (increases inductance), or spacing out
the turns (decreases inductance). The effect on inductance
is shown for a typical winding spread over portions
of the circumference of a toroid core. If any adjustment compresses the
turns to less than 200 deg. it is better to add some turns and respace the
winding to cover about 270-300 deg. Conversely, any adjustment which
results in spacing the turns over 360 deg. means that the total turns should
be reduced and the winding respaced to cover 270-300 deg. Never exceed
about 330 deg. as the capacitance between the ends of the winding will increase
and cause unwanted resonance. Where winding information is shown in
circuit diagrams it is assumed that the winding will cover 300 deg. unless
Any link coupling should be placed over the "cold" end of the winding.
Although toroid core windings have very little inductive coupling to other circuit elements, try to orient adjacent cores by 90 deg. to keep the effect to a minimum, particularly where space is limited.
Plots showing the effect of different turns vs Q on a T-50-6 core are provided. The permeability temperature coefficient should be considered for any oscillator inductor, and depending upon the frequency, you should use the materials showing low values in the table Iron Powder Material Properties.
The same insulation requirements apply to cores used in narrowband applications as with wideband use.
Ferrites are roughly divided into two groups. Those with permeabilities up to
850 are usually made from nickel-zinc material and have high volume resistivity
ranging from 1x105 to 1x108. Higher permeability ferrites
are usually made from maganese-zinc material and have volume resistivity ranging
from 0.1x102 to 1x102. Iron powder cores are usually colour
coded and have very high volume resistivity. An initial test of the material can be made
by checking the dc resistance between opposite faces/sides of a core. Low readings indicate a high
permeability material. If you can measure inductance at a low frequency (10-100kHz),
wind 10 turns of wire on the core and measure the inductance. You can then work back from
the formula and calculate the AL
value, which can be compared with the tables of known cores of the same physical dimensions
and so come up with a reasonable match. If 10 turns does not give a measureable reading try 20 or 30 turns.
Getting the AL value and other useful data is easy using the program "minirk" located at http://www.dl5swb.de.
RF power rating can be roughly checked by using two exactly similar cores each wound with the same primary and secondary turns (say 10 turns each on primary and secondary) and then connecting the cores back to back as shown. This arrangement provides a 1:1 equivalent so that the transmitter sees the correct load. Losses are doubled by using two transformers, but this does not matter for the test. Set the transmitter to the desired frequency and reduce the rf power output to a minimum. Increase the power output in small steps (say 5 -10W per step) holding each setting for 30 seconds then checking the temperature of each transformer. The transformers should only get warm to touch but NEVER hot. When the final temperature of each transformer has reached about 40 deg.C you can say that you have reached the power limit for that particular core. Some cores will get hot at very low power. You have to make a value judgement about the core physical size versus the power rating achieved.