Selecting a Core

Basic Materials  ...

• Ferrite (like FT50) ... Used for making broadband inductors and transformers

• Powdered Iron (like T50) ... Used for making tuned circuits

Sizes ...  Outer Diameter

• 0.375" diameter ... T37

• 0.500" diameter ... T50

• 0.690" diameter ... T68

• 1.300" diameter ... T130

Binocular Cores

• Compact way to make a broadband transformer in a small/limited space.

• Why?  There's more cross-section core material being covered by the winding

• There is no coupling between each half (this is good)

• Equivalent of stacking two conventional cores together and winding around both at the same time

 

Permeability (AL = nH /N²) = ____________.  A rough indicator of the useful frequency range, and more specifically a measure of relative magnetic field concentration or compression relative to air.

• Mix 2

• Mix 6

• Mix 10

• 43

• 61

Chart from NA5N masterpiece publication: Data Book for Homebrewers and QRPers

 (contact Bill N8ET of KangaUS for copies of this great book)

 

 

Winding the Toroid

Wire Size & Types

• Any wire from 26-30 gauge magnet wire will work

• Mostly selected for reasons of being able to fit the desired number of turns onto a given core size

• General rule of thumb:
     ... Good to have the turns span 2/3 to 3/4 around the core
     ... Can spread/separate turns to slightly adjust inductance, as this changes the flux coupling.  (Because of mfg permeability tolerances.)
     ... Bringing the ends together increases the stray capacitance, reduces Q

• Magnet wire has enamel coating
     ... Need to strip, scrape or sand off the ends before soldering into your circuit
     ... Or better, use "Soldereze"-like heat-strippable wire
    
   

   

Craft shop magnet wire, various colors.  Price around $2.50-$3.00
Sizes: 20 thru 26 ga
The wire has thermal strip insulation ... good for multiwinding xfmrs

 

 

The dreaded Winding Process

 

 

 

Creating taps in the inductor

 

 

Identifying different windings on a toroid ... for later (proper) connection

 

 

 

 

The winding of a single-winding toroidal inductor is fairly straight-forward. Each pass through the center counts as a turn. The windings should be spaced evenly around the circumference of the toroid, ideally leaving about a 15 degree "wedge" between the beginning and ending of the winding. The image below is of a 22 turn coil wound on a toroid.

Occasionally, you may find that there is not enough room on the toroid to place all of the windings without having to go back and add a layer of winding. Tony Parks suggests that you overlap some turns as you put on windings around the circumference of the core so that all turns are on the core by the time you get back to the start end of the winding. This should have negligible effect on the coil's performance in the radio.

Leonard, KC0WOX, has an excellent (if large - 183 Mb!) video showing the winding of a 26 turn coil on a T-37 core. While the turns andcore size in the video are not used in any of the Softrock kits, the technique is essentially the same. The actual process of winding the core begins at about 8.5 minutes into the video.

 

A Meditation on Winding Toroids by K6JEB:

 

Interesting technique using chopstick in a vise, sliding the toroid core up/down while sliding wire through to create turns.

See live video of this technique at  http://www.youtube.com/watch?v=GT44U10WqRA

 

 

 

Here's a great website illustrating winding toroids, with pics and videos .... http://golddredgervideo.com/kc0wox/softrock/opamprx.htm

 

 

 

Making Transformers with Toroids

 

 

Bifilar

 

   

 

 

              Dot notation for a 1:4 impedance transformation

 

Cautions When Working With Toroids

• Using toroidal baluns in some ATUs ... When the impedance of the antenna is far outside the performance range of the balun, it becomes real lossy ... wasting power, getting hot, giving misleading readings ... and can ultimately destroy the balun!

• Physical Delicacy ... Toroid cores are usually formed by pressing magnetic materials tightly together.  So if they experience a shock (like dropping on the floor), or even just get a chip on the edge while grouped together in a plastic parts bag, the internal stress will tend to quickly crack the core, rendering it unusable.

• Sharp Edges ... Sometimes sharp edges occur during the manufacturing process of some of the larger ferrite cores, and when winding the magnet wire through and around the core the enamel on the wire may scrape off and ultimately short to adjacent turns.

• Power Handling Capability ... Watch out for the condition of a toroid getting too warm during operation.  This could indicate an abnormal condition (high SWR, dramatically varying impedances, DC components flowing in the toroid, etc.  The venerable HW-8 experiences this condition.  How to know? ... heat, use calculator, copy someone else's design as a starting point(!)

• Material selection ... Use the right material for the right frequency

• Mounting ... Do not mount a toroid laying down flat on a pcb surface with ground plane ... will dramatically lower the Q

• Proper Lead Tinning ... Be sure that you fully strip back the enamel on the ends of the winding.  Sometimes one doesn't strip back and tin far enough, and when you pull the wire tight in the pcb mounting holes (which nicely snugs up the toroid), you expose the unstripped part of the wire and the solder does not electrically connect to the wire = open circuit!

• Measure after winding ... Measure the toiroid with your AADE L/C Meter II to double check that you have the intended inductance.  This step will also let you tweak the inductance a bit by spreading/compressing the windings.

• Do NOT use RTV to pot or glue the toroid in place! ... The Acetic acid will corrode the junctions, create unwanted losses and eventually destroy your circuit operation.

• Do Not use epoxy ... Too brittle afterwards, and when the components expand/contract afterwards the epoxy will likely crack, along with brittle components inside (like a toroid core)

• Do use Q-Dope ... Available from General Cement, Mouser, DK, etc.  See Clifton Lab's page for excellent discussion of "coil retaining compounds", plus a way to create your own special Q-Dope! ... http://www.cliftonlaboratories.com/coil_coating_compounds.htm#General_Cement_Q-Dope

 

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ANALYZE THIS!!!

 

 

This is the output amplifier/LPF for the Softrock RXTX 3.6 ... an excellent case study in use of all the toroids we've been discussing tonight.

 

QUIZ:   If we said to build this for 40m operation, can you identify the preferred type (ferrite or powdered iron), permeability mix (red, yellow, white), standard toroid or binocular, bifilar winding or single wire ... for each of the 6 inductors shown ion this schematic?

 

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THIS WEEK'S PROJECT ... The "N2CX Choke Balun"

 

Here is a handy project that illustrates many of the principles we've covered this week ... the Choke Balun.  When coax is used as a feedline for a dipole, or other types of balanced antennas, it is ideal to maintain the e-field inside the coax, between the center conductor and the inner side of the coax shield.  If some mechanism is not present to prevent the signal from also traveling along the outside of the coax shield, that signal will radiate and in general present an unintended/undesirable match to the radio-antenna system.

 

This simple project provides a way to limit, or "choke" off the the signal flowing along the outside part of the coax shield, thus preventing the feedline from radiating, and thus you get more of the intended signal to/from the antenna. 

 

 

The N2CX Choke Balun consists of a toroid wound with 10-turns of RG-174 coax, located right up at the feedpoint of the dipole antenna.  The coax feedline connects to one side, and the legs of the dipole antenna are connected to the BNC connector on the other side of the box (one side of the antenna goes to the center conductor; the other side goes to the "ground" side of the BNC connector.)

 

 

The schematic couldn't be simpler ... just strip and connect the ends of the coax to the BNC connector on either side of the toroid.

 

 

We used a small plastic box from radio Shack ... any small, lightweight enclosure would work equally well.  (It'll be helpful to ultimately make the enclosure water-tight to reduce the effects of changing climates.

 

 

When connecting a BNC to a panel of some sort, a handy trick to do is to use an old BNC adapter on the busy-end of the BNC connector being attached so you don't bend/deform the barrel of the connector.  This also provides a handy way to grasp the connector while tightening.

 

 

Here's what the enclosure looks like with the two BNC connectors installed.

 

 

This FT-114-43 core is 1.14" in outer diameter, and it has a permeability mix of 43, which is quite suitable for our use at HF.

 

 

RG-174 coax is very handy to have on the workbench, as we make use of about 20" here.  It's lots easier to prepare the ends before you start winding, rather than afterward.

 

 

 

When putting the 10 turns of the coax on the toroid core, you count just as with standard single-wire windings - each time through the core counts as a turn. 
You'll need to keep a firm grasp on the coax as you make the windings.

 

 

 

Then, when complete, use a tie-wrap or a piece of electrical tape to hold the ends together.

 

 

Next, simple connect each end of the coax to the BNC connectors as you would normally expect: center conductor to center pin, and shield to the "ground" of the connector.

 

 

Tuck the toroid down inside the box in preparation for buttoning things up.

 

 

But before you do close up the box, let's measure the actual inductance of the choke.
IMPORTANT:  Measure from the "ground/shield" sides of each BNC connector ... after all, the coax's shield is actually the issue here, right?!
With our handy-dandy AADE L/C Meter II  (you DO have one, don't you?!), we measure a shield inductance of about 44 uH.

 

 

Lastly now, let's look at the result of using this choke balun in a coax feedline to a dipole antenna being used on 80m.
We see that the reactive impedance resulting from the choke is 967 ohms ... which happens to be enough to stifle the RF current flowing all along the outside of the coax
thus making our antenna system perform a whole lot better! 

 

 

PS:  Within days, we'll have a parts kit together for CWTD attendees to order
... you too can then make this nifty N2CX Choke Balun!