"Barett" wrote in
:

....
I only need about 3 of them I think, so the postage is a killer
unfortunately.

We experience the same problem here in VK. Local resellers charge very
high prices, and many US suppliers charge very high shipping on
magnetics.

Another option is to look on Ebay, Ebay sellers who really want to sell
internationally have reasonable prices... but be prepared for excessive
shipping quotes from some.


I have found some FT140-43's.

You could use these to make a common mode choke, but the physical size
limits the amount of heat that they can dissipate and the power rating.


Do you know if I stacked 3 Ferrite FT140-43's rings and wound 6
winding of RG58cu through, what would the maximum impedance Ohms could
I expect @ 3.5MHz?

Also how much impedance could I expect to gain every time I added
another FT140-43 ferrite onto the stack?

At low frequency, the impedance is close to proportional to the square of
turns, and proportional to the number of cores stacked but for a core of
this type, the inductor will probably be self resonant in the HF range
and stacking cores increases self capacitance and reduces the self
resonant frequency and the above approximation does not apply so well.

I should stress that self resonance isn't necessarily bad for these types
of lossy inductors, you should put aside Rules of Thumb (ROT) that say to
never use an inductor above its self resonant frequency.

I have written a short article entitled "A method for estimating the
impedance of a ferrite cored toroidal inductor at RF" which you may find
interesting, it is at http://vk1od.net/blog/?p=806 .

Is Neosid still selling product in the UK? I believe they stopped
manufacture, but they may be selling product.

In answer to the shipping cost issue, I went looking for a locally
available core that suited a general purpose HF common mode choke, and
Neoside produce a core larger than FT240 with a material that is a good
choice for the purpose, similar to Fair-rite #52. I describe the design
at http://vk1od.net/blog/?p=581 . Moving house has delayed completion of
this, the design is done, the materials are here, I just have to
construct one in a box and measure its common mode impedance to calibrate
the amount of equivalent shunt capacitance. An alternative would be a
stack of two FT240-52 cores.

Everybody, and every manufacturer has a recipe for common mode chokes,
but if you do not provide a plot of common mode impedance, preferably R
and X separately, then you might reasonably question the recipe.

None so more than the commercial purveyors of the magic Caronlina Windom
which calls out aspects many of us might regard as a disadvantages as
positive features (sheer marketing spin) and depends on undescribed
proprietary components.

Owen

Owen Duffy wrote in
:

....
Everybody, and every manufacturer has a recipe for common mode chokes,
but if you do not provide a plot of common mode impedance, preferably
R and X separately, then you might reasonably question the recipe.

Aargh!

Everybody, and every manufacturer has a recipe for common mode chokes, but
if *they* do not provide a plot of common mode impedance, preferably R and
X separately, then you might reasonably question the recipe.

In article ,
Barett wrote:

I have found some FT140-43's.

Do you know if I stacked 3 Ferrite FT140-43's rings and wound 6 winding of
RG58cu through, what would the maximum impedance Ohms could I expect @
3.5MHz?

I did something like that a few years ago, to build a common-mode
choke for our ARES/RACES ham shack's HF feedline.

I used off-the-shelf (surplus-store) ferrite tubes - very probably a
43 mix or something close to it, based on the simple inductance
measurements I did with an MFJ analyzer. They're roughly 1 inch long,
and large enough to allow three through-the-center passes of RG-8X coax.

I glued somewhere around six of them, end-to-end, to create a long
tube, and then ran the RG-8X through... creating a long three-turn
choke. Added N connectors to the ends of the coax and stuffed the
whole thing into a chunk of PVC tube with end-caps.

The results were gratifying. When I tried a common-mode measurement -
along the braid, from one end connector to the other - the impedance
at 3.5 MHz was too high for the MFJ meter to read it... it simply said
" 1500".

A normal (differential-mode) measurement through the coax, looking
into a 50-ohm dummy load, reads 50 ohms (1:1 SWR) as closely as
matters not (i.e. within the accuracy of the MFJ meter).

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Barett Wrote:



The Fair-Rite part number for the 2.40in #31 toroid is 2631803802. Paste
this number into the search fields at http://gb.mouser.com and
http://uk.farnell.com and search out the best deal.

[...]

I only need about 3 of them I think, so the postage is a killer
unfortunately.

I have found some FT140-43's.

Do you know if I stacked 3 Ferrite FT140-43's rings and wound 6 winding of
RG58cu through, what would the maximum impedance Ohms could I expect @
3.5MHz?


You're in luck, as I happen to have a few FT-140-43, and the measurement
equipment is out on the bench.

That 3-core choke has its best performance around 15MHz (peaking at 4000
ohms, broadband and predominantly resistive, which is quite a good
result) but at 3.5MHz it's only about 1900 ohms and mostly inductive.

Also how much impedance could I expect to gain every time I added another
FT140-43 ferrite onto the stack?

I had only one more to add, which moved the peak down to 12MHz, 5000
ohms, but only added a few hundred ohms at 3.5MHz.

Clearly these cores are not the best for your application.

I'm working on a better approach from the new ARRL 2010 handbook, which
uses cores that *are* available in the UK and gives the best "value for
ferrite" of anything I've seen. Publication is due in the May Radcom,
but I may be able to give some previews in the next few weeks.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Dave Platt wrote:

I used off-the-shelf (surplus-store) ferrite tubes - very probably a
43 mix or something close to it, based on the simple inductance
measurements I did with an MFJ analyzer. They're roughly 1 inch long,
and large enough to allow three through-the-center passes of RG-8X coax.

I glued somewhere around six of them, end-to-end, to create a long
tube, and then ran the RG-8X through... creating a long three-turn
choke. Added N connectors to the ends of the coax and stuffed the
whole thing into a chunk of PVC tube with end-caps.


You might do better breaking the thing up into multiple cores and
multiple windings. That is, rather than 3 turns through 6 cores, do 3
turns through 1 core, then 3 turns through another core, then 3 turns, etc.

The logic here is that the impedance scales as the number of cores, but
doing them all in one shot means that the input coax is very close to
the output coax, so you have a lot more capacitance coupling the input
to the output.

I think Jim K9YC actually did some comparisons using #31 2.4" cores
comparing stacking to stringing em out.

Hi all

I have been reading loads.

I have managed to borrow a Rigexpert AA-200. Is it possible to measure the
impedance with a AA-200 analyser and how would I do that?

And a big thank you for all the links and info.


"Ian White GM3SEK" wrote in message
...
Barett Wrote:



The Fair-Rite part number for the 2.40in #31 toroid is 2631803802. Paste
this number into the search fields at http://gb.mouser.com and
http://uk.farnell.com and search out the best deal.

[...]

I only need about 3 of them I think, so the postage is a killer
unfortunately.

I have found some FT140-43's.

Do you know if I stacked 3 Ferrite FT140-43's rings and wound 6 winding of
RG58cu through, what would the maximum impedance Ohms could I expect @
3.5MHz?


You're in luck, as I happen to have a few FT-140-43, and the measurement
equipment is out on the bench.

That 3-core choke has its best performance around 15MHz (peaking at 4000
ohms, broadband and predominantly resistive, which is quite a good result)
but at 3.5MHz it's only about 1900 ohms and mostly inductive.

Also how much impedance could I expect to gain every time I added another
FT140-43 ferrite onto the stack?

I had only one more to add, which moved the peak down to 12MHz, 5000 ohms,
but only added a few hundred ohms at 3.5MHz.

Clearly these cores are not the best for your application.

I'm working on a better approach from the new ARRL 2010 handbook, which
uses cores that *are* available in the UK and gives the best "value for
ferrite" of anything I've seen. Publication is due in the May Radcom, but
I may be able to give some previews in the next few weeks.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Barett wrote:

I have been reading loads.

Good for you!


I have managed to borrow a Rigexpert AA-200. Is it possible to measure
the impedance with a AA-200 analyser and how would I do that?

I don't have one of those particular analysers, but be VERY careful when
measuring the high impedances that you'll typically see in RF chokes.

Most antenna analysers are primarily designed to measure impedances
around 50 ohms, and they may be quite inaccurate for impedances of
several k-ohms. Check with a 4.7k or 10k chip resistor... you may have a
nasty surprise.

Also read Jim Brown's notes about the effects of the analyser's own
shunt capacitance. Choke resonances in the HF region are the product of
several uH of inductance but only a few pF of inter-winding capacitance,
so a few pF of additional shunt capacitance from the analyser can make a
big difference. The more advanced Vector Network Analysers have the
facility to "calibrate out" such errors before making measurements, but
the AA-200 doesn't seem to have that.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Ian White GM3SEK" wrote in message
...
Barett wrote:

I have been reading loads.

Good for you!


I have managed to borrow a Rigexpert AA-200. Is it possible to measure the
impedance with a AA-200 analyser and how would I do that?

I don't have one of those particular analysers, but be VERY careful when
measuring the high impedances that you'll typically see in RF chokes.

Most antenna analysers are primarily designed to measure impedances around
50 ohms, and they may be quite inaccurate for impedances of several
k-ohms. Check with a 4.7k or 10k chip resistor... you may have a nasty
surprise.

Also read Jim Brown's notes about the effects of the analyser's own shunt
capacitance. Choke resonances in the HF region are the product of several
uH of inductance but only a few pF of inter-winding capacitance, so a few
pF of additional shunt capacitance from the analyser can make a big
difference. The more advanced Vector Network Analysers have the facility
to "calibrate out" such errors before making measurements, but the AA-200
doesn't seem to have that.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ok Ian all noted.

Thanks

On Thu, 11 Feb 2010 11:32:35 -0000, "Barett"
wrote:

I have managed to borrow a Rigexpert AA-200. Is it possible to measure the
impedance with a AA-200 analyser and how would I do that?

Hi Barett,

Pass a single, short piece of wire through your core/bead. Connect it
to the analyzer. Read the Z. This will be enough to characterize
your ferrite material at any frequency within the range of the AA-200.
Now pass the wire (now longer) through your core/bead twice. Connect
it to the analyzer. Read the Z. You should observe four times the
earlier reading (you may have to perform complex math if the Z
contains substantial X).

Now pass the wire (now longer) through your core/bead three times.
Connect it to the analyzer. Read the Z. You should observe nine
times the earlier reading (you may have to perform complex math if the
Z contains substantial X).

Ferrite materials with one pass of wire will not exhibit a huge value
such as to lead to significant error, so as you progress through
successive passes of wire, you can be reasonably assured that the
square law will be observed within the capacity of the analyzer at low
HF frequencies (simply because the wire length being a significant
portion of a wavelength can confound measurements at higher
frequencies).

The single pass test will also help to select and compare previously
known and unknown ferrite materials.

73's
Richard Clark, KB7QHC

On Feb 9, 2:24*pm, Jim Lux wrote:
Dave Platt wrote:
I used off-the-shelf (surplus-store) ferrite tubes - very probably a
43 mix or something close to it, based on the simple inductance
measurements I did with an MFJ analyzer. *They're roughly 1 inch long,
and large enough to allow three through-the-center passes of RG-8X coax..

I glued somewhere around six of them, end-to-end, to create a long
tube, and then ran the RG-8X through... creating a long three-turn
choke. *Added N connectors to the ends of the coax and stuffed the
whole thing into a chunk of PVC tube with end-caps.

You might do better breaking the thing up into multiple cores and
multiple windings. *That is, rather than 3 turns through 6 cores, do 3
turns through 1 core, then 3 turns through another core, then 3 turns, etc.

The logic here is that the impedance scales as the number of cores, but
doing them all in one shot means that the input coax is very close to
the output coax, so you have a lot more capacitance coupling the input
to the output.

I think Jim K9YC actually did some comparisons using #31 2.4" cores
comparing stacking to stringing em out.

Jim's comment is a good one from another standpoint he didn't mention
explicitly too: if you put a single choke right at the antenna
feedpoint, and the feedline is in some sense resonant (e.g. a quarter-
wave long with the far end grounded), the fields from the antenna may
very well excite currents on the outside of the feedline anyway, even
with a "perfect" choke. By putting the separate chokes spaced out
along the line, say a quarter-wave apart at the highest operating
frequency, you break up resonances that might otherwise occur in the
line.

By the way, for a single-band choke on a band (10 meters) that was
extremely troublesome for me in one installation many years ago, I
wound (approx., from memory) 5 turns of the feedline coax into a
solenoid coil about 12cm diameter, secured so they were stable. That
made a coil of roughly 3 microhenries inductance, self-resonant just a
bit above 30MHz. A pure 3uH would only be a little over 500 ohms
reactance, but resonated with a wee bit of capacitance across the
coil, the result was a very high impedance that solved the problem
nicely. I've since found the technique valuable and easy to apply to
RG-58C/U type line used at 147MHz--much smaller coils, secured on
sections of PVC water pipe.

Cheers,
Tom