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RF lowpass filters and inductor orientation

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redhat:
I've been playing with filter design lately in the process of making PCB's for a few prototype transmitters and the old debate is raging.  This is what I found.

Among hams there is a notion that toroids are self-shielding, that is the magnetic flux of the winding is contained to within the core and thus very little outside influence can be expected.  With air-wound inductors, this is not true, and every pole in a filter is usually designed and build with adjacent coils at right angles to each other.

This lack of interaction in toroids is somewhat true I found, but measurable interaction does occur.

I built a 7 element MW lowpass filter of shunt type Chebyshev design with 0.1dB ripple and a cutoff frequency of 1.9 MHz.  In this design, all inductors were built  broadside to each other to save space, cores were T200-2 spaced about 0.6"

Anticipated rejection (Fc = 1720 KHz)
Fc -0.5dB
2Fc -50dB
3Fc -76dB

Measured rejection
Fc -1.9dB
2Fc -51dB
3Fc -77dB

I then changed the winding direction of the middle inductor and measured again.

Measured rejection
Fc -0.26
2Fc -51
3Fc -76

I found it interesting that the rejection of the harmonics was little affected, but reversing the winding direction had a large effect on the passband loss.  With the inductors wound the same way,  passband power loss was around 35%, meaning 100W in, 65W out!  With the inductor wound the other way, losses were reduced to just 6%.

All measurements were done with a HP signal generator and spectrum analyzer.  First, reference measurements were made with the generator set at 10dBm on each frequency, then the filter measured, and the loss calculated.

+-RH

ff:
Quite unexpected, Redhat.  It never occurred to me that the winding direction would affect LPF operation.  I have occasionally lashed one together and had a very disappointing amount of insertion loss for no apparent reason.  Thanks for the heads up about it.  Would it also be safe to assume that the interactions would increase as power through the LPF increases?

redhat:
No, insertion loss would remain at the same ratio regardless of power level.

SV1XV:

--- Quote from: redhat on February 21, 2016, 2002 UTC ---cores were T200-2 spaced about 0.6"
--- End quote ---
Your observations are reasonable: three 2 in cores, 0.6 in apart, with low μ material. It is reasonable to expect some coupling between windings under these conditions as not all magnetic flux is contained in the toroids. Coupling would be less if the inductors were of smaller size, further apart and made of higher μ material.

redhat:
Smaller sized cores unfortunately could not handle the power excursions that these filters will have to reliably endure.  This is further complicated by the relatively low inductance values which when coupled with higher mu core types leads to higher losses through core saturation.  The only real way to reduce coupling is to play games with inductor phasing, spacing, or throw in the towel and go with air inductors.

+-RH

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