Choking KPA500 and testing a box of unknown chokes

[Michael Walker]

All

I am doing some work on a box of chokes I received and I am trying to
figure out just exactly what mix they are.

Since I run a remote station, I choke everything.  I know the chokes I buy
and what they are, but it is the unknown box that needs testing.

I have a tracking generator, so I am trying to figure out  a test jig.  Do
I just wrap the choke in fleedline, have a common ground and then use the
shield of the feedline to feed my signal over looking for the attenuation
on the receiving end of the generator?

Thoughts?

Mike va3mw
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[Jim Brown-10]

On Mon,5/22/2017 6:30 AM, Michael Walker wrote:
> I have a tracking generator, so I am trying to figure out  a test jig.  Do
> I just wrap the choke in fleedline, have a common ground and then use the
> shield of the feedline to feed my signal over looking for the attenuation
> on the receiving end of the generator?

Nothing like that. Study k9yc.com/RFI-Ham.pdf and the slides for a talk
on coax chokes at k9yc.com/publish.htm  Lots of detail there.

73, Jim K9YC


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[Don Wilhelm]

Mike,

You may be over-thinking it.

Wrap 10 turns around the core and measure its inductance.

 From that data, you can calculate the AL value.  Match that AL value
against a chart of the values for various cores.

73,
Don W3FPR

On 5/22/2017 9:30 AM, Michael Walker wrote:

> All
>
> I am doing some work on a box of chokes I received and I am trying to
> figure out just exactly what mix they are.
>
> I have a tracking generator, so I am trying to figure out  a test jig.  Do
> I just wrap the choke in fleedline, have a common ground and then use the
> shield of the feedline to feed my signal over looking for the attenuation
> on the receiving end of the generator?
>

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[Jim Brown-10]

This only works at low frequencies, well below resonance of the choke
that you've wound.  AND -- it only yields the AL value at low
frequencies. Many materials with very different mu and mu' curves have
similar low frequency performance. Common mode chokes are used near
resonance, which is far more difficult to measure accurately.

There's one rather practical way to identify #31 core material with a
simple VOM. Measuring between two points on the core on a medium to high
ohms scale should read resistance values in the range of 50K - 200 K
ohms, because its bulk resistivity is fairly low. Most other materials
have much greater bulk resistivity. This characteristic is shown in a
table on early pages of the Fair-Rite catalog (downloadable from their
website).

73, Jim K9YC

On Mon,5/22/2017 1:08 PM, Don Wilhelm wrote:

> Mike,
>
> You may be over-thinking it.
>
> Wrap 10 turns around the core and measure its inductance.
>
> From that data, you can calculate the AL value.  Match that AL value
> against a chart of the values for various cores.
>
> 73,
> Don W3FPR
>
> On 5/22/2017 9:30 AM, Michael Walker wrote:
>> All
>>
>> I am doing some work on a box of chokes I received and I am trying to
>> figure out just exactly what mix they are.
>>
>> I have a tracking generator, so I am trying to figure out  a test
>> jig.  Do
>> I just wrap the choke in fleedline, have a common ground and then use
>> the
>> shield of the feedline to feed my signal over looking for the
>> attenuation
>> on the receiving end of the generator?
>>
> ______________________________________________________________
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> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]
>

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[Wes Stewart-2]

Really?

On 5/22/2017 4:25 PM, Jim Brown wrote:
> .... Common mode chokes are used near resonance...
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[Jim Brown-10]

Yes, really.

Jim

On Mon,5/22/2017 10:15 PM, Wes Stewart wrote:
> Really?
>
> On 5/22/2017 4:25 PM, Jim Brown wrote:
>> .... Common mode chokes are used near resonance...


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[Wes Stewart-2]

Well, I would prefer one that presented a high resistive impedance with no
resonance at all:-)

Wes  N7WS

On 5/22/2017 10:54 PM, Jim Brown wrote:
> Yes, really.
>
> Jim
>
> On Mon,5/22/2017 10:15 PM, Wes Stewart wrote:
>> Really?
>>
>> On 5/22/2017 4:25 PM, Jim Brown wrote:
>>> .... Common mode chokes are used near resonance...

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[Bill Leonard N0CU]

Mike,

Wes raises one of the many interesting aspects of balun design, and not surprisingly, one that there doesn't appear to be a consensus on. Two of the main trade offs are bandwidth and choking impedance. The  highest choking BW is usually obtain with an air core choke (coax looped with or without some form like PVC). However, this approach is very high Q, which yields relatively low R and narrow BW. I recently tested on of these that was centered at 14 MHz (by adjusting the number of turns and loop diameter) and the BW was only around 2-3 MHz. At the other end of the design spectrum is the sleeve balun, which has a very broad BW (1-30 MHz), but it has a relatively low impedance, which is almost entirely resistive.

Some say to go for the maximum reactance and minimum resistance, others say go for a net impedance of greater than 1K (or 5K depending on the "expert). Then there is also the issue of power dissipation which I haven't even touched on.

So, which balun type you need is driven by what you want the balun to do. Feel free to contact me off line if you would like some more details.

Bill N0CU

[Jim Brown-10]

On Wed,5/24/2017 9:39 AM, Bill Leonard N0CU wrote:
> Two of the
> main trade offs are bandwidth and choking impedance. The  highest choking BW
> is usually obtain with an air core choke (coax looped with or without some
> form like PVC). However, this approach is very high Q, which yields
> relatively low R and narrow BW.

Bill,

This simple-minded analysis is WRONG because it fails to realize the
fact that the choke is adding inductance to a part of the antenna (the
transmission line as a common mode conductor). It is, in fact, no
different from adding a loading coil to an antenna to resonate it! At
some frequencies, that transmission line will look capacitive, and at
those frequencies, the added inductance causes the line to resonate,
INCREASING common mode current rather than reducing it.

Think about it -- how is this air core inductor different from a loading
coil that we add to the base of a short vertical to resonate it?  Using
your logic, that loading coil ought to block the current, but we all
know that it does not.

THIS fundamental principle is why air core chokes DON'T work in many
systems. That is, they don't choke common mode current. Now, the ANTENNA
works to the extent that it radiates (and may yield a fine SWR, which is
NOT a measure of how well the antenna works), so users think the choke
is fine.

The virtue of chokes wound on lossy ferrite cores is that by using a
suitable number of turns, the choke, with the stray C between turns and
the loss (resistance) coupled from the core forms a parallel resonant
circuit with a very low Q, and with a high value of resistance at
resonance. And the low Q (typically on the order of 0.5) makes that
resonance quite broad so that it can cover multiple ham bands. In
addition, #31 material have a second dimensional resonance below 3 MHz
that broadens the choking Z curve much as did stagger-tuned" IF stages.
That large value resistance reduces common mode current -- it can never
"resonate" with the antenna (transmission line) into which it is inserted.

This, and other concepts associated with common mode chokes are
articulated in a tutorial that is on my website, and was added to the
ARRL Handbook in 2010.

73, Jim K9YC

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[Eric Swartz - WA6HHQ]

Folks - we are now at 10 posts in a storp period on this OT thread.

Let's close the thread now in the interest of lowering email overload for our
other readers. Please take it to direct email as needed.

73,
Eric
Moderator
/elecraft.com/


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[Bill Leonard N0CU]

Jim,

I apologize: I made a typo in my post. It should have said "...the highest choking IMPEDANCE is usually obtained with an air core choke (coax looped with or without some form like PVC)". Maybe this is an over-statement, but of the few chokes I have tested, air core chokes made with just standard coax have shown the highest net impedance, but also the narrowest useable BW because of the high Q. I have seen these baluns used in countless QST articles over the years, and even as part of some commercial antenna designs.

In my previous posts, I tried to emphasize that choosing a choke to reduce common mode current needs to be a system level process. Because of the importance of the resonance problem, I explicitly stated that adding a choke can either reduce, have no effect on, or increase the common mode current. I appreciate the points you make regarding the use of a ferrite core that yields wide BW and high resistance. However, there are those that argue against this approach by claiming that there is no need to burn up the common mode energy in the choke resistance, when you could just as easily use a high reactance choke to reflect it and re-radiate it out the antenna. However, the proponents of this approach either don’t discuss the potential resonance problem, or state that as long as the choke’s resistive component is greater than 5x the antenna feed impedance, you don’t need to worry about it.

Bill  N0CU