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Softrock Lite 6.2
Adventures in Electronics and Radio
Elecraft K2 and K3 Transceivers
Field - Toroid and Solenoid Inductors
04 November 2009. Original
18 December 2009. Added Q data and table of contents; added single turn section
Table of Contents
One bit of amateur radio lore is "a toroid inductor is
self-shielding" and hence does not have an external field to interact with other
inductors or with nearby conducting objects such as the enclosures.
Like many fables, there's some truth in this statement,
but it's far from being 100% correct. In building a prototype notch filter
recently, I ran across a case where there was not enough room to space toroid
inductors to minimize unwanted coupling and hence found shields between adjacent
inductor necessary. This lead me to make some simple measurements to demonstrate
the difference between the external fields of a toroid and solenoid inductor.
Inductor Coupling Data
The data I collected is not the most sophisticated, but it
shows the concept. The solenoid core at the left is 36 turns on a 0.5 inch
diameter Delrin core, and measures approximately 7uH. The toroid core at the
right is 25 turns on an T50-7 core, and measures 2.8 uH. The long object in the
center is the test coupling probe I use with a spectrum analyzer as a
"signal sniffer." It consists of a 5 turn shielded pickup loop constructed from
coaxial cable, with a BNC connector at the other end of the probe handle. The
two large black cores are high u ferrite cores that reduce coupling from outer
shield to the sensing coil.
I connected the inductor under test to the output port on an HP 8752B
vector network analyzer and the signal sniffer coil to the VNA's receiver port,
with the VNA in transmisison mode. Since these inductors are of a value and
construction that would typically be used in the MF and HF range, I set the
frequency range to cover 300 KHz to 30 MHz, with log sweep.
I made two pairs of measurements by positioning the sniffer
coil to be coaxial with the solenoid and with the toroid core, at approximately
1 inch and at 3 inches spacing. I also made a third pair of measurements
orienting sniffer coil to achieve maximum coupling at 10 MHz.
The VNA data shows the loss in dB versus frequency between
the sniffer coil and the inductor under test. This loss provides a quick and
dirty approximation of the external field of the two inductors and hence their
propensity to couple to other inductors or the environment. If the toroid is
truly self-shielding, there will be no signal pickup.
The solenoid inductor shows a worst case coupling of 37 dB
at 10 MHz and even with 3 inch spacing has about 63 dB loss.
The toroid inductor—while some 10-20 dB better than the solenoid—has a far from
zero external field.
The rather cluttered plot below shows all six test runs. The
data shows that at lower frequencies and greater spacing the toroid has perhaps
20 to 25 dB less external field than the solenoid inductor. However, at
higher frequencies and closer spacing, the external fields begin to converge.
In terms of the filter I'm working on, the photo below shows the experimental
layout with shields added. The shielding material is thin tin-plate stock
obtained at the local hobby shop. It's easy soldering material and can be cut
with tin snips or even heavy duty scissors. Although I oriented the three large
inductors to minimize coupling, given the size of the parts and the available
space, there's only so much that can be done to reduce unwanted coupling by
orientation and core-to-core spacing.
or not the external field of an inductor is a problem depends, of course, on the
application. In many circuits, coupling between inductors down 50 or 60 dB is
inconsequential. However, if you are working with a filter and wish to achieve
100 dB band stop rejection, great care must be taken to avoid unwanted coupling.
In the case of the Z10020 band reject filter, careful printed circuit board
layout allows very high stop band rejection to be achieved without shielding. In
the case of the experimental filter pictured below, shielding is helpful in
achieving target performance goals.
Single Turn Effect
Dr. Rudolf Rieder commented on my data:
fighting against pick-up from a switching DC-DC converter I ran into a note
(forgot the ref.) pointing out that although the toroid core does pretty
well include the magnetic field of the coil wound around it, the coil itself
forms a net 1-turn loop around the circumference of the torus. If you want a
(more) "quiet" switching power supply you ought to bring the ends of the
coils together by forming an external 1-turn backward-loop (or 2 1/2-turn
loops), thus compensating the field (effectively a "bifilar" design). At the
time this small change helped a lot.
I am therefore wondering whether a similar approach to RF-toroids wouldn't
have a similarly beneficial effect, i.e. a significant reduction of stray
fields and thus coupling. If you feel like it next time, please try it and
repeat the measurements. I'd be quite curious to read about it when next
visiting your page.
This is correct. The typical toroid
inductor can be considered to be two inductors. One is the traditional "circular
solenoid" where the magnetic flux follows the core and the second is the
one-turn loop mentioned by Dr. Rieder. Thus, even if 100% of the flux of the
"circular solenoid" is confined to the magnet core, the one-turn loop is
certainly not so confined.
I'll look at the one-turn effect in more
detail in the future.
Effect of Nearby
Shielding on Q
One consequence of flux
leakage is that the inductor loss—and hence Q—is influenced by nearby metallic
objects. The leakage flux induces current in nearby conductors and since the
energy lost in the conductors is provided by the inductor, the inductor's total
loss increases. Q is inversely proportional to loss, so increased loss
translates to lower Q.
This effect can be visualized in several ways. Perhaps the
simplest is to think of it as a transformer, with the inductor as the primary
and nearby conductors as multiple secondary windings. The secondary
windings have resistance and hence loss. The induced current and hence the level
of loss depends, in transformer terms, upon the coupling coefficient, which
relates primary flux and secondary flux. In a good transformer, primary and
secondary flux is tightly coupled, with a coupling coefficient very near 1.00.
In the case of a toroid inductor's field inducing currents into a nearby
conductor such as a shield, the coupling coefficient will be nearly, but not
A common example of induced field loss is seen when a
toroid is near a shield. This may be an intentional shield to reduce inter-stage
coupling, or it may be an inadvertent shield such as an aluminum enclosure wall.
Or, the toroid may be mounted horizontally on a printed circuit board with a
In order to demonstrate the effect of shielding on
inductor Q, I made a simple test using a small piece of thin aluminum and a 2.8
uH inductor wound on a T50-2 powdered iron core, as in the earlier leakage
test. The aluminum sheet is approximately 2" (50mm) x 4" (100mm) and is 0.050"
The toroid is held in place with an axial screw, and
spaced from the aluminum sheet with nylon washers, 0.1" (2.5mm) thick. I made
measurements with the inductor touching the shield, with 0.1" and 0.2 (5mm)
spacing, with a plastic (nylon) axial screw and a stainless steel screw.
Finally, I made measurements with an intentionally poor mounting method, one
that forms a shorted turn.
I measured Q and inductance with an HP 4342A Q-meter at
7.9 MHz. The "free space reference" is the measured Q without the aluminum
The plot shows a rather small change in Q when nylon
mounting hardware is used. Indeed, even with the core touching the aluminum
plate, the measured Q dropped by 10 points, from 246 to 236. A small gap of 0.1"
(2.5mm) reduces the Q by 5 points and increasing the gap to 0.2" (5mm) reduces Q
by 4 points.
In contrast, using a stainless steel mounting screw
results in a much greater Q loss. (The screw did not form a shorted turn; rather
it was just a single vertical element with a stainless steel washer on the top.
Shorted Turn Mounting
It's common knowledge—and if it's not, it should be—that a
toroid should not be mounted with conducting hardware that forms a "shorted
turn," i.e., a continuous conductor making a loop through the toroid
hole. The reason is that this configuration makes a rather efficient transformer
and couples energy from the inductor into the unintentional one-turn shorted
secondary winding formed by the mounting hardware.
To demonstrate this effect, I made a quick shorted turn
mounting, illustrated to the right. This is perhaps not a very good mount, but
the better the mount, the greater the loss and the lower the Q. A pair of
aluminum stand-offs, for example, one through the core and one adjacent to the
core edge, with a thick aluminum bar joining the two will have a greater Q
reducing effect than the long mounting strap I used.
The shorted turn mount reduced the measured Q from a
free space value of 246 to 152. It also reduced the measured inductance from 2.8
uH to 1.69 uH. The reason for the inductance reduction is that the secondary
inductance appears in parallel with the primary (the 2.8uH free space measured
I expected the inductance to be considerably lower than
1.69uH and the resulting Q to be worse than the 40% reduction.
With small cores such as the T50 series, only minimal loss of
Q results so long as the core is at least 0.1 inch from the nearest conducting
surface, where non-conducting hardware is used. Conducting hardware should not
be used under normal circumstances and if conducting hardware is used, it should
not be allowed to form a shorted turn.