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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Filter problem
Date: Thu, 12 Jun 2025 21:20:32 -0000 (UTC)
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Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
> 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.

The stuff I’m talking about is 0.050” diameter or thereabouts. Widely
available for cheap on AliExpress.  You can coil it as tight as you like. 

> 
>> 
>> 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?

I don’t know what your plate Z is at 2 metres, but it isn’t super high. 
I’d say a 1:10 transformer on each end (500Ω->5Ω), plus three OC stubs with
two  2Ω resistors to make a 3-section pi network, would be a good place to
start spicing. 

> 
> 
>> 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.
> 
You don’t want much conductance at the open end. Just the featureless cut
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
> 

Cheers 

Phil Hobbs 

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