Last edited on Sept 4, 2026
---- Contents ----
Section 1 - How to
identify stocked ferrites that you accidentally mix up
Section 2 – The
#61 mix series resonant frequency issue
Section 3 –
FB-43-101 beads added to a discrete VHF amp to boost gain by 1.59
dB
Section 4 – NULL currently, however, we’ll hopefully add more
content over time
Section 1: How to identify stocked ferrites that you accidentally mix up
aka: I’ve got some loose, 'unlabelled' ferrites on my bench. How might I identify them?
This text assumes you own an LCR meter. We feel these are 1 of the most useful bench tools besides a VOM/multimeter and a DSO/oscilloscope.
If you know the
precise ferrites that you stock – for example #43, #61 and #77
mix-- and you accidentally mix up these ferrites on your bench --
its easy to sort them out. If you’re referring to some random
ferrite you got at a Ham festival, then this system won’t identify
the mix.
Using the wrong mix can really mess up a project, so we
routinely check the ferrites used in our projects before beginning
each build.
After warm-up, perform a 2 point calibration on your inductance meter (open & closed circuit). Connect a short (1 cm or so) piece of wire between the test leads and through the ferrite under question -- a 1 turn measure.
Above — Binocular core test. You may also measure by making a single loop through both holes – whatever way works better for you.
Above — Assessing a through-hole ferrite bead.
Above — Assessing a completed transmission line transformer (4:1 Z) wound on a FT37-43 toroid. The single turn method works whether windings exist or not.
Once you measure a sample of your ferrite collection, you’ll get a sense about the normal inductance range of values for each given part -- and may use these normal values to identify your ferrites whenever the mix feels unclear to you.
Above — 4 different measures of random FT37-43 toroids. The measure labelled X is that of the wound 4:1 transmission line transformer shown above.
Above — 4 different measures of random FT37-61 toroids.
Above — A measure of a random FT50-77 ferrite toroid donut.
Section 2 – The #61 mix series resonant frequency issue
In 2020, an emailer named Diego asked us to help him troubleshoot a broadband transmission line transformer wound on a FT50-61 that was behaving differently from simulation.We can’t recall every detail, however, the the transformer exhibited weird losses at lower middle HF frequencies. Diego sought to use a low loss transformer from ~ 5 to 30 MHz. In an attempt to get enough inductive reactance at the lower end, he wound 14 bifilar turns.
The mix was #61
(NiZn) and seems to offer reasonable Qul from ~ 0.2 to 15 or so MHz
-- and most importantly does not suffer the high losses associated
with the more common #43 mix.
We remembered sweeping
some toroidal #61 mix transmission line transformers and comparing
them to identical turns number transformers wound on #43 mix many
years ago. We also measured the QuL of some #61 and #43 mix coils.
In #61 mix of size 0.50 (Amidon) and 0.699 inches (TDK) toroids we
measured a Qul of 172 @ 1 MHz in the TDK products and a little less
in the Amidon coils. The QuL of size 0.50 Amidon #43 mix toroids = ~16-17 @ 1 MHz ---
low enough that sadly at HF and above ,we could not see a series
resonant frequency (SRF) in sweeps with our tracking generator plus
spectrum analyser . We observed a definite + sharp SRF in the
inductors wound on #61 material toroids.
We suggested he might consider stacking 2 to 4 binocular cores such as the BN-61-2402, or Fair-Rite 2861002402 so the transformer required less windings and hopefully that would increase the SRF to above 30 MHz. He eventually got a satisfactorily working design through bench experimentation.
Stacking ferrites - alternate example --- VHF 6 dB Combiner/ Return Loss Bridge:
Above — VHF return loss bridge/ 6 dB hybrid combiner illustrating
the stacked ferrite concept another way. 3 stacked BN-61-2402
ferrites with 4 turns of wire made a simple but reasonable return
loss bridge/ 6 dB hybrid combiner for lower VHF. #61 mix works great
at VHF --- 4 turns of wire kept the winding capacitance down to allow
the bridge to reach into 200 MHz with a directivity of 22.9 dB.
We
primarily use this this bridge from 25 to 150 MHz for 2-tone testing
and 50 Ω port return loss measurement.
Above — A photo of the 3 stacked binocular ferrites.
Section 3 – FB43-101 beads added to discrete a VHF amp to boost gain by 1.59 dB.
Experiment with 2 ferrite beads in our last-ever weather radio receiver:
Above — A discrete part LNA for a VHF weather channel receiver (Environment Canada ended its Weather radio service on March 16, 2026, after 50 years of operation). So this receiver is now garbage.
For this simple LNA,
we used a favorite cheap BJT – the 2SC3357 which is probably too
hot for ~ 162 MHz, however, to keep it stable, we just placed a
resistor as the collector load. Experiments showed that ~39 ohms
worked quite well. 40.1 ohms seems strange – but someone mailed us
a small bag of Vishay / Dale Precision Film resistors in through
hole -- military spec. & amazing devices. Of course, we had to
try one out! Mouser #: 71-RN65D40R1FB14 to get a datasheet.
Other
than the air wound coil [4 turns #20 AWG, 6 mm diameter, 1 cm long, tap 1 turn from ground], all the parts but the collector resistor
were SMT/SMD.
Above — A sweep of the LNA with no ferrite beads – just the 40.1
Ω collector resistor.
Above — A sweep of the LNA with 2 ferrite beads slipped over 1 wire
lead of the 40.1 Ω collector resistor. Gain increased by 1.59 dB
and the amp stayed rock stable. That was fun.
Normally we place
ferrite beads in stuff for RF/EMI filteration,or perhaps to surpress
parasitic oscillations by sticking them on BJT collector and/or base
leads – this proved a new use for us.
Musical inspiration for these bench notes came from the B52s. Under the chordal backdrop of Ricky Wilson's unique guitar work, Kate Pierson and Cindy Wilson's vocal leads & harmony simply thrills & amazes us.
Best to you!
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