Aug 7, 2012
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Toroids ... Selection, Construction and Use
A continuation in our Component Selection
Many times when we see plans for a good project we think
"Hey, I'd like to make this project but ..."
we don't have the exact parts called for. But if we dig around in the junk
box for parts we do have
we can often use them just fine by altering the project specs or schematic
This week's program focuses on ... Toroids!
Are you a bit fearful, or at least hesitant, when it comes
to dealing with toroids in your homebrew circuits? C'mon, tell the truth!
Whether it's confusion about the right size to use, the right color (err ... the
"mix" or "permeability", whatever that is), how to mount that wiggly component
on the board, or the numb fingertips from wrapping endless turns around that
little FT37 core ... then losing count after 40 and either counting them while
looking through a 100x magnifier or ripping all that wire out to start over.
The homebrewing joy never stops with this component!
Well, you're in for a real treat with this episode of CWTD.
We'll go through all of this and provide a solid reference that you can
bookmark for later use through the years. We also give you some good tips
& tricks to use the next time you build a Softrock or an oscillator, and even
provide a real simple-and-handy project using the principles discussed during
73, George N2APB & Joe N2CX
to the MP3 podcast)
<20:05:50> "Todd K7TFC": Sorry! Newbie, here. Is the "white
board" the same as the "Session notes" page?
<20:06:28> "Joe N2CX": Yes Todd it is.
<20:06:49> "Todd K7TFC": Thanks!!
<20:08:15> "Joe N2CX": BTW all, since George updated the page during the
beginning of the session be sure to refresh the whilteboard page to get the
latest up to date version..
<20:25:54> "Todd K7TFC": I don't get making a transformer with bin. cores if
there's no coupling????
<20:26:32> "Joe N2CX": The coupling is between turns within each hole
<20:27:59> "Todd K7TFC": Wound bi/tri-filar?
<20:29:46> "Todd K7TFC": Got it!!!
<20:40:42> "Todd K7TFC": Could you clarify what A-sub-L characterizes?
<20:41:34> "Joe N2CX": Don't know the name for A sub L but it has units of uh or
mu per turn squared
<20:44:31> "Todd K7TFC": Fly-tying clamps might be good to hold small toroids.
Handheld Jewler's vises, too.
<20:48:29> "Joe N2CX": Soldereze is a trade name general term is heat strippable
insulation it is a plastic coating, not enamel
<20:49:52> "Joe N2CX": it is important also to tin the wire up to the core
<20:53:22> "Todd K7TFC": How important is evenly spacing the turns?
<20:55:35> "Todd K7TFC": The two yellow toroids just under "dreaded": is that
<20:57:09> "Joe N2CX": Even spacing gives nominal inductance and the ability to
later spread or compress the windings to tweak exact inductance
<21:07:01> "Terry WB4JFI": Larry - try selecting another mic PTT key. I had that
happen using several different assigned keys
<21:10:51> "Todd K7TFC": Shouldn't the core symbol be between the pri. & sec.?
Does that matter?
<21:11:38> "Joe N2CX": Strictly speaking you are right. It's often left to the
imagination of the viewer if it is relatively obvious
<21:12:15> "Joe N2CX": In other words it depends on the symbol det in your
<21:16:38> "George - N2APB": Great to have you with us here tonight Nancy!!!
<21:17:22> "Todd K7TFC": I think rtv for aquariums doesn't have acetic acid. I
may be wrong.
<21:23:15> "Todd K7TFC": Is building the choke balun in a metallic enclosure
<21:23:54> "George - N2APB": Yep, but it will make a heavier assy ... remember
that the balun gets located up at the junction of the two legs of a dipole.
<21:25:00> "Todd K7TFC": Altoids tin, perhaps.
<21:25:28> "Nancy - NJ8B": George, I'm glad to be here too. Much depends on
Carolyn, but I'm very glad to be here. I finally figured out how to get
TeamSpeak to work.
<21:25:48> "George - N2APB": Ooops, oh yeah, you'd have to isolate the grounds
of each BNC connector for it to work right!! Sorry about that.
<21:27:22> "Joe N2CX": HI Nancy!
<21:29:41> "Nancy - NJ8B": Hi Joe!
<21:32:19> "Joe N2CX": unbalanced feed of a dipole is discussed in some detail
in the reference "Some Aspects of the Balun Problem by W2DU
<21:33:01> "Todd K7TFC": The best 90 minutes for the last week! Are George and I
the only ones vain enough to have a photo avatar?
<21:33:46> "Larry - W2HHV": I need to used a different PTT key.
<21:35:40> "Terry WB4JFI": grok
<21:36:00> "Joe N2CX": grokked! from Heinlein!
inductors and transformers are electronic components, typically
consisting of a circular ring-shaped
of iron powder,
or other material around which
coiled to make an
Toroidal coils are used in a broad range of applications, such
as high-frequency coils and
can have higher
than similarly constructed
coils. This is due largely to the smaller number of turns
required when the core provides a closed magnetic path. The
in a high permeability toroid is largely confined to the core;
the confinement reduces the energy that can be absorbed by
nearby objects, so toroidal cores offer some self-shielding. In
the geometry of
magnetic fields, the
flux direction threads the "donut hole" in the center of the
torus, while the
flux direction is parallel the core of the torus
... Outer Diameter
0.375" diameter ... T37
0.500" diameter ... T50
0.690" diameter ... T68
1.300" diameter ... T130
Compact way to make a broadband transformer in a small/limited
There's more cross-section core material being covered by the
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
= 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.
from NA5N masterpiece publication: Data Book for Homebrewers and QRPers
Bill N8ET of KangaUS for copies of this great book)
Size & Types
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
... Bringing the ends together increases the stray capacitance,
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
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
dreaded Winding Process
Creating taps in the inductor
Identifying different windings on a toroid ... for later (proper)
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
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:
technique using chopstick in a vise, sliding the toroid core up/down while
sliding wire through to create turns.
video of this technique at
great website illustrating winding toroids, with pics and videos ....
Making Transformers with Toroids
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
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
Material selection ... Use the right material for the right
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.
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! ...
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?
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
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
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.
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,
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.
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!
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