Re: Filter problem

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

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.

That was my estimate - it would fit in the overall housing (though not
in a die-cast box of the type I am using to make the modules).  It might
even work if it were simply threaded into the loom with all the other
wires.

 
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.

Would it be better to use inductive or capacitive coupling, to reduce
the losses?

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.)

The impedances are high because it is in the anode circuit of a valve.
I could use a tapped resonant circuit or a ferrite transformer to bring
down the impedance to 50-ohms, which would make it easy to test with a
VNA. 

Perhaps a quarter-wave line could be used as part of  the impedance
transformation?  Audio screened cable has a characteristic impedance of
about 120 ohms, so a termination of 50 ohms at the outpute end of a
quarter wavelength would appear as 288 ohms at the input end.

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.

I wonder how tightly you could roll it up without destroying the effect?

Some of the circuits I have already built for this project are at:
http://www.poppyrecords.co.uk/Radio/G8HEH/2metretransceiver.htm

--
~ Liz Tuddenham ~
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www.poppyrecords.co.uk