In reference to recent postings on this reflector regarding receiving
loops, the following article by Frank Gentges which appeared in the
current issue of the AMRAD Newsletter, may be of interest.
73
Andre' N4ICK
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Making a Carbonyl Iron Rod
RF magnetics are made from either powdered iron or ferrite. Raw
ferrite powder is not suitable for RF magnetics as it needs to be
fired in a kiln using special recipes to convert the raw material into
the RF magnetics we are familiar with. Raw powdered iron is mixed with
some binder matrix and pressed under high pressure (50,000 psi) into a
mold to make RF magnetics-like cup cores, toroid cores and rods we are
seeing from places like Amidon.
The powdered iron used in these applications is made with the carbonyl
process where the iron particles are precipitated out in very small
sizes around 1 micron. In addition the process can coat the individual
particles with a non-conducting iron compound. This coating limits
particle-to-particle conduction, which could lead to large eddy
currents much as a solid piece of iron would have. This iron powder is
called carbonyl iron powder.
It is made for several uses. It is made, of course, for RF magnetic
devices. Also it has been used in copy machines as "developer" where
the powder coats a magnetic rod and acts as a very soft brush to clean
toner from a photo drum surface. Here it is just a soft brush held
together with a magnetic field. Carbonyl iron can also be pressed into
objects and fired at high temperatures into solid objects where the
individual particles fuse into a sold piece in a sintering process.
Long powdered iron or ferrite rods of over a foot could provide high
sensitivity LF loopstick antennas. However, there is limited demand
and the costs can exceed a thousand dollars for a rod of 2 feet. The
obvious question is whether one could make a rod at home.
We looked at this and selected carbonyl iron over ferrite because it
could all be done at room temperature.
Pressing the carbonyl iron at 50000 psi is somewhat impractical at
home so instead, a PVC pipe was filled with the carbonyl iron powder
and it showed improvement over an equivalent air core coil at LF.
The powder was made by GAF and is called "IRON POWDER C". This powder
was
obtained some 15 years ago on another project and enough left over for
this job.
A 16-inch piece of 1-inch schedule 40 PVC pipe was capped on one end
with a PVC cap and the PVC cement. A portion of the powder was poured
into the pipe and tamped with a broomstick using a hammer to compress
it and force out the air. This force was much less that the 50,000
pounds of pressure used in industry. Repeated pouring and tamping
eventually filled the pipe full. When full the other end was capped.
This worked well in antenna tests so a 36- inch piece of 1-inch pipe
was filled. This worked even better as a LOWFer receiving antenna. It
was wound and a piece of aluminum flashing was put around it as a
shield. The whole assembly was put into a larger 2-inch piece of
schedule 40 PVC pipe and capped to seal it for outdoor use.
This powder is very fine and seems to go places you don't intend and
you will get dirty doing this so use old clothes and be prepared to
shower after filling. After capping the pipe can be cleaned and be
ready for winding.
Iron powder can react with moisture and turn to rust, which will
degrade the RF performance so if the end caps fully seal the rod can
be used for years.
The type C powder used is not a high permeability material and when in
a rod
configuration the overall inductance of a coil will not dramatically
increase over a similar air wound one. You may only see an increase of
two- or three-fold but this is to be expected.
Sources for the powder are limited since most powder suitable for this
rod is sold to large manufacturers of powdered iron components. GAF
seems to be in the roofing business and their web page at www.gaf.com
does not indicate other products. Hopefully we can find sources of the
powder in less than carload lots for amateur LF use. A second article
will cover the electrical design and use of the carbonyl iron antenna.
Frank Gentges, KØBRA
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