BL2 choking Impedence measurements?

[Jim AB3CV]

Does anyone have choking impedence measurements for a BL2 balun from
160m through 20m?

tnx
jim ab3cv
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[Dennis KB7ST]

Hello Jim,

I have the AIM 4170 antenna analyzer ( w5big.com ).  I could sweep the balun in 1:1 and 4:1 mode is you'd like.  I'm not sure it will give you the choking impedance but will do some checking with the designer of the AIM.  Let me know if that is of interest.  You can see what some AIM sweeps look like at http://antennagraphs.com 

[Jim Brown-10]

On Wed, 25 Aug 2010 11:19:15 -0700 (PDT), Dennis KB7ST wrote:

>I have the AIM 4170 antenna analyzer ( w5big.com ).  I could sweep the balun
>in 1:1 and 4:1 mode is you'd like.  I'm not sure it will give you the
>choking impedance but will do some checking with the designer of the AIM.

I don't own an AIM, but I've seen choke measurements made by an AIM that had
serious errors. A good common mode choke should have a high resistive
impedance at the frequencies where it is used, and the AIM is best suited for
measuring impedances relatively close to 50 ohms (that is, within 1:5 or
5:1). It's fundamentally unsuitable for measuring the high impedances of a
good choke.

73, Jim Brown K9YC



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[Bob Naumann W5OV]

Jim,

Please allow me to add some facts about the AIM4170C to this discussion.

First, the AIM4170C is not designed only for "measuring impedances
relatively close to 50 ohms".

>From WX0B:

The AIM4170C can measure 50K ohms R and +/- 50K ohms X easily.

The fact is is that the higher the impedance, as you know from your work
with the Type 31 cores, even 1pf or .5pf of stray capacitance can
drastically affect the measurement.

So the test fixture must be carefully built and calibrated to get a
precise measurement. The AIM4170C or even an HP 8753 would have trouble
with a choking reactance measurement in a 5K+ environment.

Jay

73,

Bob W5OV
Array Solutions
(P.S. Both Jay and I own K3s now).

> On Wed, 25 Aug 2010 11:19:15 -0700 (PDT), Dennis KB7ST wrote:
>>I have the AIM 4170 antenna analyzer ( w5big.com ).  I could sweep the
>> balun
>>in 1:1 and 4:1 mode is you'd like.  I'm not sure it will give you the
choking impedance but will do some checking with the designer of the AIM.
> I don't own an AIM, but I've seen choke measurements made by an AIM that
had
> serious errors. A good common mode choke should have a high resistive
impedance at the frequencies where it is used, and the AIM is best suited
> for
> measuring impedances relatively close to 50 ohms (that is, within 1:5 or
5:1). It's fundamentally unsuitable for measuring the high impedances of a
> good choke.
> 73, Jim Brown K9YC
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[gm3sek]

 W5OV wrote:

>> I don't own an AIM, but I've seen choke measurements made by an
>>AIM that had  serious errors. A good common mode choke should
>>have a high resistive impedance at the frequencies where it is used,
>>and the AIM is best suited  for  measuring impedances relatively close
>>to 50 ohms (that is, within 1:5 or 5:1). It's fundamentally unsuitable for
>>measuring the high impedances of a  good choke.
>> 73, Jim Brown K9YC

>Jim,
>
>Please allow me to add some facts about the AIM4170C to this discussion.
>
>First, the AIM4170C is not designed only for "measuring impedances
>relatively close to 50 ohms".
>
>>From WX0B:
>
>The AIM4170C can measure 50K ohms R and +/- 50K ohms X easily.
>
>The fact is is that the higher the impedance, as you know from your work
>with the Type 31 cores, even 1pf or .5pf of stray capacitance can
>drastically affect the measurement.
>
>So the test fixture must be carefully built and calibrated to get a
>precise measurement. The AIM4170C or even an HP 8753 would have trouble
>with a choking reactance measurement in a 5K+ environment.
>

I don't own an AIM4170 either, but have made many measurements on
feedline chokes with a N2PK VNA and have traded comparison data with an
AIM4170 owner.

The key point that WX0B makes is that the instrument MUST be calibrated
to include the effects of the connecting leads attached to the device
under test (DUT). This is not an option, it's a MUST!

If the instrument is not capable of being calibrated in this way, then
I'd agree with Jim that it is totally useless for measuring high
impedances. This categorically rules out all the simple antenna
analysers like the MFJ-259B, Autek etc.

However, the AIM4170 is a higher-level instrument that is capable of
being re-calibrated to include the effects of an attached test fixture
(which then becomes "part of the instrument"). With care, it seems that
the 4170 can be used to make quite accurate vector impedance
measurements on chokes. The next step, having calibrated the instrument,
is to check how well it measures a test load like a 5-10k chip resistor.
With appropriate precautions, a pre-calibrated N2PK VNA can measure a
10k resistor within a few percent up to at least 30MHz, which is "plenty
good enough for choke work".

Everyone agrees that even quite small variations in the test fixture can
drastically affect some aspects of measurements on feedline chokes,
particularly the resonant frequency. But that is also a criticism of the
choke itself: if its performance depends on a narrow resonance that is
difficult to measure, even under controlled conditions in the lab, then
that same choke will also be extremely sensitive to the way it is
installed in real-life situations and is unlikely to deliver consistent
performance.

For consistent performance in a wide range of installations, a feedline
choke needs to deliver a high *resistive* impedance over a broad
bandwidth. As Jim Brown has so often said, resistive loss in a feedline
choke is *good*, provided the resistance is high enough! This requires
quite careful design based on a specific ferrite core.

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





--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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[David Cutter]

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?

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

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.

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.

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.

73

David
G3UNA


> 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
>
> --
>
> 73 from Ian GM3SEK
> http://www.ifwtech.co.uk/g3sek
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[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
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[David Woolley (E.L)]

Ian White GM3SEK wrote:
> CUTTER DAVID wrote:
>
> 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

I'm fairly sure that Farnell did some #31 material, bigger than beads,
but smaller than typical for amateur radio use.  I seem to remember you
had to read between the manufacturer's catalogue number to work out it
was #31.

> to refresh this information in time for the RSGB Convention.

I tried writing to the RSGB's EMC contact email addresses a few months
ago, enquiring about sources of #31 material (and twisted pair speaker
cables in less than whole drums, although I've found the latter and I'm
waiting to see if it helps).  However the first address I tried bounced
and the second one never replied, so I gave up.

I think the big problem is one of education.  Most people in the hobby
believe that toroids are good, but probably don't even realise they have
to be ferrite.  The RSGB's filter collection doesn't seem to have
changed in a couple of decades, and the big traders, presumably
successfully, sell EMC toroids without any indication of what they are
made from.

>> 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.

In terms of the big electronics distributors, you probably have to
convince them that there is a large commercial shortwave RF engineering
industry in the UK, which may not be true any longer.  For the amateur
equipment suppliers, you need to educate the market so they refuse to
buy anything else.

--
David Woolley
Emails are not formal business letters, whatever businesses may want.
RFC1855 says there should be an address here, but, in a world of spam,
that is no longer good advice, as archive address hiding may not work.
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[Jim Brown-10]

On Thu, 26 Aug 2010 12:51:06 +0100, CUTTER DAVID wrote:

>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.

I suspect that all it takes is a large order to get their attention. Many
of the folks who work in these facilities are living the deep past, and
stock what they used to sell. The #31 material is a relatively new
material, developed around the turn of the century (2000) and first showed
up in the 14th Edition of the Fair-Rite catalog. Not all sizes of cores
were listed. When I was doing my initial work on the use of ferrite cores
for RFI suppression in audio applications, I ordered a lot of samples from
that catalog. Early on in my work, I called and requested #31 cores in the
2.4-inch o.d. size, and was told they were not available. A month later the
rep called me back and said he had five samples for me!  

If you look at the current catalog (15th edition), you will see that #31 is
now listed as their standard mix for suppression at HF, for the most part,
replacing other materials.

Here in the US, over a period of about five years, we've organized a dozen
or so large group purchases of a few selected sizes of cores to get a good
quantity price from distributors. The first order we did was for more than
1,000 pieces, and was combined from clubs in Chicago, Minneapolis,
Indianapolis, and Los Angeles. We paid about $4 each for the 2.4-inch
toroids in #31 material. The next order I was a part of was nearly three
times that size, and the price we negotiated held for one year, during
which time we were able to add to the order. As a result, main line
industrial electronics distributors like Newark, Allied, and DigiKey are
now stocking some of these parts and offer decent prices, even for much
smaller quantities. Fair-Rite was happy to deal with us directly if we
wanted, and we did for part of a couple of orders. But in some cases, we
got better prices from distributors.

There is a list of suggested distributors in Appendix One of my tutorial,
along with a list of suggested parts to buy, and a worksheet to help
estimate how many you should buy. The worksheet assumes you are buying in
quantity and buying for use over a period of 3-5 years for general RFI
suppression and making transmitting chokes, and is based on your own
personal hamming activity.

http://audiosystemsgroup.com/RFI-Ham.pdf 

It's a sure thing that if you walk up to a distributor with an order for
500 ferrite cores you'll get them, and at a good price. The parts that I
recommend that you order are the 2.4-inch o.d. toroids, the "biggest clamp-
on," and the clamp-on that fits on RG8. You won't use it for RG8, but it is
quite useful for wrapping multiple turns of small wires attached to noise
generators like power supplies, computers, and the like, and around audio
cables that are picking up RFI in your stereo (or a neighbor's). By
limiting yourself to those three parts, you have the most useful, and you
build quantity.

73, Jim Brown K9YC



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[Jim Brown-10]

On Thu, 26 Aug 2010 21:15:34 +0100, David Woolley wrote:

>I'm fairly sure that Farnell did some #31 material, bigger than beads,
>but smaller than typical for amateur radio use.  I seem to remember you
>had to read between the manufacturer's catalogue number to work out it
>was #31.

DO NOT order from the distributor catalog. Give them the Fair-Rite part
numbers listed in my Appendix! You know what you want, insist that they
supply that, not something that they might have ordered for another
customer, or that some salesman told them to buy.

You can't be passive about this if you're to get what you want. Think like
an agressive yank! :)

73, Jim K9YC


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[Jim Brown-10]

On Thu, 26 Aug 2010 08:08:16 +0100, Ian White GM3SEK wrote:

>However, the AIM4170 is a higher-level instrument that is capable of
>being re-calibrated to include the effects of an attached test fixture
>(which then becomes "part of the instrument"). With care, it seems that
>the 4170 can be used to make quite accurate vector impedance
>measurements on chokes. The next step, having calibrated the instrument,

Yes, the AIM is a high quality instrument, but one tool does not fit all
needs. It is, in essence, measuring S11, which means that very small
errors in the measurement can cause very large errors in the result. The
only GOOD way to measure a choke is by measuring S21 -- that is, as the
series element of a voltage divider.

The errors I have seen in choke measurements show up as lead capacitance
that is not precisely cancelled, which results in an incorrect
determination of the resonant frequency. The error is small for low
frequency, low-Q chokes, but can get very large at increasing frequency,
and for higher Q chokes.

In the materials on my website, there are graphs showing the effect of
the 0.4 pF stray capacitance of my own test fixture on a very low Q choke
(Q=0.4) wound to cover a tri-band beam (20-10M). It shifts the resonant
frequency about 25%. The error is insignificant for #31 chokes wound for
40M and below.

The error becomes very bad with a higher Q choke, like one wound with
Fair-Rite #61 or #67 (Q around 10). The AIM data I've seen shifted the
resonant frequency of a choke like that by a factor of nearly 2:1. The
AIM measured the resonance of choke as being around 16 MHz that was
actually resonant above 30 MHz. And, because the resonance moved and the
loss of the material varies with frequency, the peak in the choking
impedance also was in error, but by a smaller amount.

73, Jim K9YC


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[gm3sek]

Jim Brown wrote:

>On Thu, 26 Aug 2010 08:08:16 +0100, Ian White GM3SEK wrote:
>
>>However, the AIM4170 is a higher-level instrument that is capable of
>>being re-calibrated to include the effects of an attached test fixture
>>(which then becomes "part of the instrument"). With care, it seems that
>>the 4170 can be used to make quite accurate vector impedance
>>measurements on chokes. The next step, having calibrated the instrument,
>
>Yes, the AIM is a high quality instrument, but one tool does not fit all
>needs. It is, in essence, measuring S11, which means that very small
>errors in the measurement can cause very large errors in the result. The
>only GOOD way to measure a choke is by measuring S21 -- that is, as the
>series element of a voltage divider.
>

I measured the same chokes in both types of test jig, reflection and
transmission, and neither method has any clear advantage over the other.
Both methods have potential problems with variations in series
inductance and shunt capacitance (the latter in parallel with the
choke). In both cases, everything depends on the care taken to maintain
the test jig in exactly the same geometry, first for calibration and
then for all subsequent measurements.

Among other things, this requires the construction of special reference
standards which allow the instrument to be calibrated without changing
the geometry of the test jig.


>The errors I have seen in choke measurements show up as lead capacitance
>that is not precisely cancelled, which results in an incorrect
>determination of the resonant frequency. The error is small for low
>frequency, low-Q chokes, but can get very large at increasing frequency,
>and for higher Q chokes.
>

As I said previously, any choke that is liable to this kind of
measurement error will have exactly the same problems in a practical
installation - only far, far worse - resulting in unreliable
performance. That is a very good reason to avoid high-Q (frequency
selective) chokes and choose a more broadband design.

>In the materials on my website, there are graphs showing the effect of
>the 0.4 pF stray capacitance of my own test fixture on a very low Q choke
>(Q=0.4) wound to cover a tri-band beam (20-10M). It shifts the resonant
>frequency about 25%.
>The error is insignificant for #31 chokes wound for
>40M and below.
>
>The error becomes very bad with a higher Q choke, like one wound with
>Fair-Rite #61 or #67 (Q around 10).

That is correct: on 14MHz and above (and especially at VHF), even a tiny
change in shunt capacitance can move the resonant frequency by a large
amount. That creates an "interesting" measurement problem - but much
more important is the effect of uncontrolled stray capacitance when that
same choke is installed on the antenna.

In practical installations, it means that we cannot rely on resonance
alone to provide a large choking impedance, because the resonant
frequency of the choke will move when the choke is installed.


>The AIM data I've seen shifted the
>resonant frequency of a choke like that by a factor of nearly 2:1. The
>AIM measured the resonance of choke as being around 16 MHz that was
>actually resonant above 30 MHz. And, because the resonance moved and the
>loss of the material varies with frequency, the peak in the choking
>impedance also was in error, but by a smaller amount.
>

There is no reason for the AIM to have that problem. Either the user was
allowing the test setup to vary, or the AIM had never been correctly
calibrated with the jig in place.




--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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[David Cutter]

I like measuring things in the lab and doing the calculations, so we can
avoid blowing things up, wasted time and disappointment, but I also like to
put the thing into practice and do relevant measurements in situ.

I'm building yet another current meter (others have grown legs and wondered
off somewhere) to "measure" braid current with various chokes.  I've ordered
some of those nice obrounds from Farnell.

One thing that interests me is that we never used chokes and baluns "in the
old days"  circa 50s and 60s - how did we live without them?  Not many beams
around my way in those days; I'm more interested in keeping noise out of the
receiver.

David
G3UNA

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[gm3sek]

David Cutter wrote:
>I like measuring things in the lab and doing the calculations, so we
>can avoid blowing things up, wasted time and disappointment, but I also
>like to put the thing into practice and do relevant measurements in situ.
>
>I'm building yet another current meter (others have grown legs and
>wondered off somewhere) to "measure" braid current with various chokes.

A clamp-on RF current meter is the *only* way to measure how effectively
a choke is working in the actual installation. It's one of those things
that is so easy to make, and then you wonder where it had been all your
life!  (That is also why borrowed current meters don't come back.)

>
>One thing that interests me is that we never used chokes and baluns "in
>the old days"  circa 50s and 60s - how did we live without them?

I think we lived in ignorance... but also feedline radiation/pickup was
far less important in those days because there was far less electronics
of other kinds.

>Not many beams around my way in those days; I'm more interested in
>keeping noise out of the receiver.
>
These days, that is often the most important reason to use feedline
chokes.


--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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[Jim Brown-10]

On Sat, 28 Aug 2010 11:14:33 +0100, Ian White GM3SEK wrote:

>>Not many beams around my way in those days; I'm more interested in
>>keeping noise out of the receiver.
>>
>These days, that is often the most important reason to use feedline
>chokes.

YES! And keeping RF out of the shack, and out of your neighbor's living
room high futility system.

>As I said previously, any choke that is liable to this kind of
>measurement error will have exactly the same problems in a practical
>installation - only far, far worse - resulting in unreliable
>performance. That is a very good reason to avoid high-Q (frequency
>selective) chokes and choose a more broadband design.

YES. This is the fundamental problem with #61 as a core material.

73, Jim K9YC



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[n7ws]

I am reminded of a time in my career when I was a member of a Components and Materials Department. We had experts in a variety of fields including, failure analysis, NDT, chemists, materials scientists, metallurgists, etc.  I was the RF/uW guy and had a “private” lab with over a mega-buck worth of HP test equipment in a shielded room.

The components guys were of course experts in that field and were also the guys who wrote a lot of specifications. I often got roped into this as well when RF parts were at issue.

It drove me bonkers when a missile program would say for example, a feed-thru filter needed to be specified over the frequency range of DC to X-band, simply because the missile operated at X-band.  If you want to know frustration, try telling a manager, or worse a government contract person, that measuring a power filter with foot-long shielded leads at 12 GHz is a really dumb idea.

Equally silly (and applicable here) is measuring a device in a 50-ohm environment and trying to determine attenuation, when in actual operation, neither the source or load impedance is known.

--- On Sat, 8/28/10, Ian White GM3SEK <[hidden email]> wrote:
 

> I measured the same chokes in both types of test jig,
> reflection and
> transmission, and neither method has any clear advantage
> over the other.
> Both methods have potential problems with variations in
> series
> inductance and shunt capacitance (the latter in parallel
> with the
> choke). In both cases, everything depends on the care taken
> to maintain
> the test jig in exactly the same geometry, first for
> calibration and
> then for all subsequent measurements.

     

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[Jim Brown-10]

On Sat, 28 Aug 2010 11:59:21 -0700 (PDT), Wes Stewart wrote:

>Equally silly (and applicable here) is measuring a device in a 50-ohm
>environment and trying to determine attenuation, when in actual operation,
>neither the source or load impedance is known.

Exactly. It is the 50 ohm environment that is the source of much of the
measurement error. Measuring S21, by one means or another, is an effective
solution, and the one I've used.

Measuring shield current, as David has suggested, is an indication of the
effectiveness of a choke IN A PARTICULAR CIRCUIT -- for example, a feedline
onto which it is inserted, the antenna to which the feedline is connected,
and the termination of the feedline at the receiver. The effectiveness of a
choke in any given application depends upon the common mode impedances
within that particular circuit.

These are two VERY different problems, and two very different measurements.
Both are useful. Measuring the choke on the bench tells us about the
properties of the choke. Knowing that, we can optimize the choke for a
given frequency range and use, AND we can insert the properties of a choke
into a model and, if the model knows enough about the circuit of interest,
predict the effectiveness of the choke in reducing common mode current.

There is, of course, another set of unknowns. They relate to the source(s)
of the noise, their location, polarization, directivity, proximity to the
feedline and to the antenna, the directivity of the antenna, etc. The choke
will have little effect on the component of noise picked up on the antenna
-- it can suppress noise picked up only on the feedline. In general, the
choke will be most effective in suppressing noise that is much closer to
the feedline than to the antenna, and/or is rejected by antenna
directivity.

73, Jim K9YC


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[David Cutter]

This brings me nicely to the final bit of the puzzle:  say, in a given
location, you want to set up your own personal outdoor lab to make these
measurements comparing various solutions.  How do you create a constant
noise source *which will most effectively demonstrate* the performance of
one choke against another?

I suspect that, at the end of the day, this is probably going to be the
cheapest and most conclusive of all tests.

David
G3UNA

>
>>Equally silly (and applicable here) is measuring a device in a 50-ohm
>>environment and trying to determine attenuation, when in actual operation,
>>neither the source or load impedance is known.
>
> Exactly. It is the 50 ohm environment that is the source of much of the
> measurement error. Measuring S21, by one means or another, is an effective
> solution, and the one I've used.
>
> Measuring shield current, as David has suggested, is an indication of the
> effectiveness of a choke IN A PARTICULAR CIRCUIT -- for example, a
> feedline
> onto which it is inserted, the antenna to which the feedline is connected,
> and the termination of the feedline at the receiver. The effectiveness of
> a
> choke in any given application depends upon the common mode impedances
> within that particular circuit.
>
> These are two VERY different problems, and two very different
> measurements.
> Both are useful. Measuring the choke on the bench tells us about the
> properties of the choke. Knowing that, we can optimize the choke for a
> given frequency range and use, AND we can insert the properties of a choke
> into a model and, if the model knows enough about the circuit of interest,
> predict the effectiveness of the choke in reducing common mode current.
>
> There is, of course, another set of unknowns. They relate to the source(s)
> of the noise, their location, polarization, directivity, proximity to the
> feedline and to the antenna, the directivity of the antenna, etc. The
> choke
> will have little effect on the component of noise picked up on the antenna
> -- it can suppress noise picked up only on the feedline. In general, the
> choke will be most effective in suppressing noise that is much closer to
> the feedline than to the antenna, and/or is rejected by antenna
> directivity.
>
> 73, Jim K9YC
>
>

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[Jim Brown-10]

On Sat, 28 Aug 2010 22:41:47 +0100, David Cutter wrote:

>This brings me nicely to the final bit of the puzzle:  say, in a given
>location, you want to set up your own personal outdoor lab to make these
>measurements comparing various solutions.  How do you create a constant
>noise source *which will most effectively demonstrate* the performance of
>one choke against another?

The choking impedance measurement can give you a first approximation of
the parallel equivalent circuit, certainly sufficient for this purpose.
You add that choke (or any other choke of known characteristics) to a
circuit and observe the noise reduction (if any). You take that reduction,
in dB, and work backwards to compute the initial series impedance of the
common mode circuit. If the choking Z is much higher (5:1) than the
impedance of the common mode circuit, reactance doesn't cause much error.
Now, knowing the common mode impedance of that particular circuit, you can
predict what more or less choking Z will do.

THAT'S why I've always placed so much emphasis on determining the
equivalent circuit values for the choke. And THAT'S why it was learned at
least 60 years ago that RESISTANCE is the key to suppression. This
principle can be found in old references from the 50s and 60s (I know,
because in the process of publishing my work as an AES paper stating that
principle, I did a literature search and found some references to it), but
somehow it never found its way into modern literature, including ARRL
publications.

73, Jim K9YC



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[David Woolley (E.L)]

The cable does seem to have helped.  That's another of Jim's suggestions
that the UK amateur radio trade and RSGB shop have failed to take note
of.  I used Van Damme Blue "Studio Grade", as it was available in cut
lengths and not too over-priced.  You can get it even cheaper (from
Rapid) if you can justify 100m, but I bought it from a specialist,
online, hi-fi cable shop.  Rapid's catalogue didn't even mention that it
was twisted pair.

(I'm in a flat, with antenna covenants, and I wanted to be able to
demonstrate a clean house in terms of QRP interference to domestic
equipment.)

David Woolley wrote:

>  (and twisted pair speaker
> cables in less than whole drums, although I've found the latter and I'm
> waiting to see if it helps).



--
David Woolley
"we do not overly restrict the subject matter on the list, and we
encourage postings on a wide range of amateur radio related topics"
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