XPost: rec.radio.amateur.moderated
Ham radio ON7IR
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Low pass filter improved
Posted: 26 Apr 2022 12:54 AM PDT
https://on7ir.blogspot.com/2022/04/low-pass-filter-improved.html
As you can read in the previous blog article, measurements showed that the filter is not so well adapted to the frequencies to be filtered. Time to improve.
First I checked the 80m low pass. By measuring the capacitance it was
already clear that there was something wrong with one or more capacitors.
When the filter board is without voltage (the relays are not activated) the capacitors are in parallel and should have a total capacitance of 3140pF
(820pf + 1500pF + 820pF). However I measured just over 1700pF.
After removing all the toroids from the board (I wanted to measure them
all) it became clear that the capacitor that should be 1500pF is a 150pF
one. I replaced it with a 1500pF 2kV capacitor.The removed capacitor with
wrong valueThe right one
Next step was to measure the inductance of the toroids and compare them to
the values described in the QST article (see previous post) and KC0WOX' website. Inductance was easily measured with my MFJ-259C SWR Analyzer which
can also measure capacitance and inductance.The SWR analyzer can measure inductance, capacitance and coax loss as well.
15-10m
Both toroids are 0.429uH but should be 0.325uH. Removed two windings to get 0.338uH.
20-17m
Measured 0.44uH and 0.46uH, should be 0.575uH. Removed and rewind 8 turns
close to each other
Values now are 0.54uH and 0.57uH
40m
With 2.45uH and 2.57uH the inductance is too high. Removed 5 windings on
each toroid to get 1.476uH and 1.408uH. Close enough to 1.30uH
80m
Inductance too low. Measured 2.28uH and 2.31uH but it should be 2.48uH. All windings were removed and I used new magnetic wire with 0.6mm diameter
instead of the removed 0.75mm wire.
One toroid needed 21 turns to get 2.35uH but the other one has 20 turns to become an inductance of 2.42uH.
The miniVNA now revealed much better results.80m.
40m
20m - 17m
15m - 10m
Passthru
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