Re: Filter problem

Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:

Bill Sloman <bill.sloman@ieee.org> wrote:
 

On 12/06/2025 7:06 pm, Liz Tuddenham wrote:
 
<snip>
 

I have some ferrite toroids that could be used to match the valve output
impedance to the filter characteristic impedance.

 
Do you know the nature of the ferrite in the toroids?
 
Manganese-zinc ferrites tend to be lossy at frequencies above a few
hundred kHz. Nickel-zinc ferrites  have a higher resistance but get
lossy above a few MHz.
 
If there are specialised ferrites for the 150MHz range I've yet to hear
of them.

 
They are FT-37-43, which are claimed to be good from  25 to 300 Mc/s.
 
 

Type 43 is okay.  Type 62 is better for chokes and baluns, where you don’t
care about loss.

For a one-off, I’d get some 50-cm micro coax jumpers with U.FL connectors
and make open-circuit shunt stubs.  154 MHz is just under 2 metres, so at a
velocity factor of 0.67, a quarter wave is 32.4 cm.

You’ll need a bit of series resistance between each stub and the next,
because otherwise you’ll get a parallel resonance between stubs of slightly
different lengths.

It’ll add some loss, and may need a transformer on each end to keep the
notches narrow enough. ( You might want to be hanging 50Ω stubs off a 5-Ω
point, for instance.)

This is easy to dork in LTspice. It isn’t the lowest-loss thing in the
world, but you can tune it with dikes.

If you don’t mind using half-wave shorted stubs, you can tune them by
sticking a sewing needle through the jacket into the center conductor,
which lets you adjust in both directions. I use thumbtacks in RG-58 like
that fairly often. Good Medicine.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics