| Microwave
Circulators Using Ceramic and NdFeB Magnets
R. Billings 2/25/00
M2Global, San Antonio TX
The purpose of this discussion is to compare briefly
magnetic and mechanical properties of Ceramic and
Neodymium Iron Boron (‘Neo’) Biasing Magnets
when used in a Drop-in Circulator operating above
Gyromagnetic Resonance mounted on a 90°C base-plate
with 40 watts of through-power continuously applied.
The Ferrites being biased well above saturation are
magnetically equivalent to an air gap, 2-2.5 mm long,
sustaining a Magnetic Flux Density in the order of
2300 Gauss (.23 T). For each Magnet type
the Magnetic Circuit is individually configured to
apply the same Bias Field to the Ferrite stack. A
single Magnet is disposed above the Ferrite stack
in the Ceramic Magnetic Circuit, while in the Neo
Magnetic Circuit three Magnets are disposed along
flats cut into the Ferrite edges with 120° symmetry. The
Ceramic Magnetic Circuit produces magnetic field lines
which are more nearly uniform and orthogonal to the
plane of the Ferrite than does the Neo Circuit. This
results in a lower Insertion Loss Circulator.
Temperature Compensation is achieved when each Magnet’s
temperature coefficient compensates the de-tuning
effect of the thermal gradient of the Saturation Magnetization
(4piMs) of the Ferrite material. These
naturally balance in the PCN/PCS bands with Ceramic
Magnets and an Aluminum doped Ferrite material.
Other combinations use Temperature Compensation Steels
(30% Nickel) to balance these off. Thus the higher
Temperature Coefficient of Ceramic Magnets (-.20°C)
as against Neos (-.13°C) does not lead to degraded
electrical performance at temperature.
Historically, Ceramic Magnets appeared first, in the
late ‘40’s. They were originally
Barium (later Strontium) Ferrites having similar manufacturing
methods and mechanical properties to Microwave Ferrites. Neo
Magnets appeared later, in the early ‘80’s
and became very popular as their price was comparable
to Ceramics and they had a Maximum Energy Product
an order of magnitude higher.
Salient magnetic properties are:
• Type BHmax
(MGOe) Tc (°C) Tmax (°C)
• Ceramic
3.45 460 300
• NdFeB 20-40
310 150
(Data courtesy
of Dexter Corp.)
The Energy Product, BHMax in MegaGauss-Oersteds (1
MGOe = 7956 J/m^3), is a measure of the Maximum Energy
Density (proportional to Magnetic Field Strength squared)
that can be produced in an given air gap. The
Curie Temperature, Tc in °C, is the temperature
at which the material loses its magnetic properties.
Maximum Service Temperature, Tmax in °C, is the
maximum long term operating temperature.
It can be seen from the above data that while the
Energy Product of Neo magnets is much higher than
Ceramics (allowing the design of thin side-magnet
devices ), the Curie Temperature is much lower and
severely limits the ambient temperature at which stable
long term operation may be expected. It
should be noted that Tmax is a maximum temperature. It
is attainable with only certain grades of Neo, ‘pre-aged’
(this involves demagnetizing the fully charged Magnet
by a predetermined amount), and enclosed in a Magnetic
Circuit biased for optimal temperature stability. Under
‘real world’ conditions, Tmax often is
115 degrees or even lower.
Mechanically, both materials are hard and brittle,
the Neos slightly less so. They should
not be and are not used as structural components within
the Circulator. Ceramics are very strong
in compression, having similar mechanical properties
to the ferrites which they bias. The packaging
technique used with Ceramic units is very robust and
has evolved over 35 years. Many millions
of units have been produced, some capable of withstanding
even pyrotechnic shock. Neos reside to
the side of the ferrite stack. Their major
vulnerability is to lateral shock which can be minimized
with the use of appropriate adhesives.
A more insidious difference is a susceptibility to
oxidation and corrosion of Neo Magnets. Over
time a rust-like coating grows on an unprotected surface
and penetrates into the material. The oxidized
material has a lower Coercive Force which ‘shunts’
the remaining Magnet, further reducing the Flux Density. This
metallurgical deterioration is irreversible and exacerbates
the low Curie Temperature problem in providing sufficient
magnetic bias to the Ferrites at elevated temperature. This
leads to a runaway condition, eventually demagnetizing
the Circulator. Certain coatings or Nickel
plating can eliminate most of the difficulty, but
the Neo surface is notoriously difficult to clean
and process. Any imperfection in the protective
layer will initiate the deterioration cycle. Ceramics
do not oxidize or corrode.
In conclusion then, a Circulator optimally designed
with Neo or Ceramic Magnets will exhibit no major
electrical difference, other than loss, under low
power conditions over the specified temperature range. The
Neo unit will be thinner and lighter. However,
under the power and temperature conditions met in
a High Power Amplifier application, the long term
effects described above have in the past led to demagnetization
of the Neo Magnet units. Ensuing high reflected
power causes catastrophic failure of the Amplifier
and other stages.
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