On 4/10/14 5:34 AM, Patrick Greenlee wrote:
Just about everyone using a tower in Ham radio also uses ferrites, often
on the coax to/from the tower. OK hi-tek guys what is a good
experimental lash-up for testing ferrites? Typically the mix is not
labeled on the part and how do you know it is labeled right if it is? A
lot of kinds and sizes of ferrites available at all kinds of prices
typically not labeled. Even if you get what you pay for then a few
months later when they are mixed with some previous ferrite bargains
which is which?
What sort of test equipment do you presume having?
A couple turns though the core and a way to measure Z at RF frequencies
is a pretty good way to sort them out.
It's pretty idiosyncratic (everyone has a different style of winding,
etc, and the parasitics on your analyzer are different than someone
else's)..
But, if you have something like a MFJ, you can measure Z at 2 or 3
frequencies at something like 1-2 MHz, 10-20 MHz and 100 MHz and get a
feel for what the different mixes are like (with some knowns to check with).
the different mixes have VERY different mu, so the Z at a given
frequency will be radically different (for the same size core). And the
other thing is whether it's a powdered metal or ferrite core and whether
it has high loss or just high inductance.
I have a little chart in the box with my old MFJ 259, a PL259 to
cliplead adapter and 12" of wire. Mostly, I just measure when I know
what it is, write it down, and then in the future, compare.
I don't think identifying the mix is an important goal although you may
be able to make an educated guess after measuring performance attributes
of your ferrites. What would be good to know and would constitute
success would be to get a measure of their characteristics at various
frequencies. The preferred method of testing should not require a bunch
of lab gear not typically found in a modest shack. A minimal set of gear
would include your station's primary equipment, such things as your
transmitter, coax jumpers, dummy load,SWR meter, volt meter and watt
meter.
That would be tough.. you're going to have to do a lot of recabling.
What you really want is something like a RF bridge (e.g. a MFJ antenna
analyzer or equivalent).
The test may require putting a toroid on a length of coax and
then installing a connector (maybe repetitively.) Tests may require
winding an insulated conductor around/through a ferrite core.
Almost certainly.
Perhaps an antenna analyzer would be available for a different test
schema. Maybe there are other pieces of equipment that would be useful
and likely to be available. To be accessible to more hams the tests
should not be limited to only those requiring more sophisticated gear
such as my HP 8753D Network Analyzer with accompanying HP 85041 S
parameter accessory for testing from 300 Hz to 3 GHz.
One could probably build a "ferrite checker" for hamfest/junkbox use
with something like a canned crystal oscillator (e.g. used in
computers), a diode and a voltmeter. It's mostly a matter of calibration.
It's all about measuring the Z of the thing at a couple or three
frequencies. For "categorization" the Z measurement doesn't have to be
all that accurate.
How about it guys? What about a quick and dirty GOOD ENOUGH test to
select ferrites based on their in-shack measured performance?
Patrick NJ5G
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