[Elecraft] BL2 choking Impedence measurements?

[Ian White GM3SEK]

CUTTER DAVID wrote:

>Ian
>
>Our friends over the pond (K9YC etc) extoll the virtues of type 31 
>material but I have yet to see this in European stocks, (I was 
>disappointed to see  none at Friedsn) do you know of an easy source?

Some UK suppliers do stock beads in #31 material but mostly not the ones 
that interest us. I haven't checked in the past few weeks, but will need 
to refresh this information in time for the RSGB Convention.

 From my own measurements on cores obtained from the USA, the #31 
material is genuinely better than #43 in terms of delivering a high 
resistive impedance over a broad bandwidth. However, #43 cores are much 
more readily available in Europe.

>Reading through your latest choke missive, I notice that nice obround

(thank you for that new word :-)

>shaped core from Farnell but looking up the Fair-Rite catalogue I 
>notice they  have a slotted version made for multiple cable entry: 
>2643165151 which would lend itself even more to the quest for low 
>capacitance between turns.   Can you see anything against this?
>
>What is going through my mind is how the amateur fraternity could 
>persuade a distributor in EU to stock these and the type 31 cores.

That 4-hole core is an interesting thought, but it is probably quite 
expensive and still only #43 material (3rd and 4th digits of the type 
number). I would prefer to focus my efforts on the second task, trying 
to develop a supply route for more normal-shaped cores in #31.


>Further to your careful layout within the core, I'm sure I've seen a 
>guide that  would help to keep things in order - I think it's a 
>plumbing part, I will continue to look.  I think mounting the chokes on 
>a piece of ply with clamps would give  it rigidity and an easy mounting 
>method.  
>Ready-made cables would make these chokes very easy to put
>together but the nice microwave ones I bought surplus recently have
>mostly N type connectors: another reason for that slotted type of core.
>
That sounds like a good application for Jim's larger snap-on bead in 
#31.

>I haven't seen measurements done on a solenoid coil of coax which is
>very easy to do using up surplus lying around.  If the turns are slightly
>separated, that should help with self-capacitance. 
>
As my web article shows, air-wound chokes have the drawback that their 
impedance is almost entirely reactive, and therefore runs the risk of 
being canceled by opposite reactance up or down the line. At some 
frequency this series resonance is almost sure to occur, and there the 
impedance of the choke falls to almost zero.

To repeat: only ferrite loading can create a broadband and predominantly 
*resistive* impedance which is not subject to series resonances.

>  Follow the link below for a reasonably short summary, and then go
>  on to
>  read Jim's much longer treatise:
>  http://www.ifwtech.co.uk/g3sek/in-prac/inpr1005_ext_v1.pdf
>  --


[Eric, thank you for your indulgence in letting us continue this 
discussion. I hope that readers are finding it useful.]


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek