Tom W8JI 2023/09/26 08:02
Hi Dave,
They would be very close even in production because the required load to
create a current transformer also swamps out small impedance
irregularities. Each is terminated, as they must be.
They must have some capacitance intervention between winding and
transmission line conductor, plus high voltage isolation if used live.
They aren't a problem.
Any common mode current will show as unbalance, but since the standing
wave points of common mode can be different than the differential, to
get a good idea at one point the voltage balance (including phase) also
must be checked.
I have a long video or article I started on balance and common mode. I
feel it is critically important because we are off on "a high impedance
choke fixes everything" trip that is largely false. I just lack time.
My best balanced antenna is a sloppy 75M low dipole near buildings and
other junk, different leg heights. Without any balun or choke it has
virtual no CMC. If a choke is added it actually becomes worse.
My worst balance dipole is 130ft high, in the clear perfectly, with coax
dropped straight down to ground. It horrible without a choke yet a
modest choke cleans it up about the same as a nose bleed choke.
The is mostly because of electrical length from source (antenna
terminals) to to feedline ground and resulting ability to drive common
mode onto the cable, not antenna symmetry.
73 Tom
On 9/25/2023 12:54 PM, W0LEV wrote:
> Tom, thanks for the "whack-on-the-side-of-the-head"! I knew this, but
> was so motivated in building the little device that I failed to visit
> phase relationships. Again, thank you!
>
> Thinking further, if I had two identical current sensors (several
> turns of conductor wound identically on identical cores and connected
> in parallel should sum to zero if the phases and amplitudes are
> identical. However, finding two toroidal cores of identical
> characteristics might be the devil in the details as ferrites are
> typically controlled to only 20%. Thoughts?
>
> Or, with one current sensor - same design - if I passed both leads
> through the sensor, if phases were opposite and equal in amplitude, I
> should get zero results. Non-zero results would indicate either :
> 1) phase relationships not 0 and 180-degrees
> 2) amplitudes not equal with proper phasing
> 3) lots of CM current.
> Of course, any combination of the three above as well.
>
> Of course, the two halves of my 450-foot doublet are certainly not
> truly balanced by any means! That would take a monumental effort to
> prepare the soil for a goodly area beneath the wires. Also assure the
> wires are deployed in a perfectly identical manner. None of this is
> within the efforts of the typical amateur, myself included. so.........
>
> Gotta think this over a bit more in depth.😊🙂
>
> Dave - WØLEV
>
>
> On Mon, Sep 25, 2023 at 4:58 AM Tom W8JI <w8ji@w8ji.com> wrote:
>
> Hi Dave,
>
> If the scalar current is compared, it will not confirm balance. It
> can only tell you if it might possibly be balanced or can't be
> balanced. It can only prove unbalance, it cannot prove balance.
>
> Consider this example. If the lines were simply parallel and
> excited in phase, 100% unbalance and all common mode, the scalar
> currents would be exactly equal but the line would be all common
> mode and no differential mode at all.
>
> To confirm balance, the current transformers have to be matched
> and connected in series at RF before detection. Then, by inverting
> phase of one transformer by 180 degrees and reading the new sum,
> one phase will give you the total common mode current and the
> reversal give the net differential current. From that you sorta
> know balance. But you would have to confirm this at two points
> separated by a large fraction of a wavelength.
>
> Any balance measurement must include phase.
>
> As a matter of fact at any single point in the line, to be
> balanced, the line must have:
>
> 1.) Equal and opposite phase currents
> 2.) Equal and opposite phase voltage to a neutral plane around
> the line
>
> Attached is my line balanced test fixture. I use it with a dual
> channel vector voltmeter or a four port vector network analyzer.
> It has two current samples and two voltage samples and requires
> external detection. This allows a single point measurement of
> balance.
>
> 73 Tom
>
> On 9/24/2023 1:05 PM, W0LEV wrote:
>> Since I use open wire for my transmission line to/from the
>> antenna feedpoint, I built a little box for the purpose of
>> reading current on each of the two conductors. Each of the two
>> conductors is outfitted with a current probe similar to that
>> illustrated in the reference. I can switch between the two
>> current probes. I have tested and calibrated each using a
>> non-inductive resistor across the "output" to read equal values.
>> In place on the feedline, if both current probes read the same, I
>> know my common mode choke is doing the job.
>>
>> Dave - WØLEV
>>
>> On Fri, Sep 22, 2023 at 6:28 PM The Greene Family
>> <cvgreene@snet.net> wrote:
>>
>> As W8JI said, these small VNAs are really great for a lot of
>> applications, but they are limited once you get way outside
>> the 50 Ohm impedance region. Lots of articles have been
>> written about that. Even the very expensive VNAs are
>> limited, but to a lesser degree. I guess you do get
>> something for 20 dB or so more money.
>>
>> And, as Tom has suggested, the actual performance of common
>> mode chokes depends largely on the environment they are used in.
>>
>> So, sticking with W8JI, have you considered using something
>> like this to measure common mode currents on your feedline?
>>
>> Current Meter <https://www.w8ji.com/building_a_current_meter.htm>
>>
>> Test your chokes in actual use.
>>
>> 73,
>>
>> Clarke K1JX
>>
>>
>>
>> --
>> *Dave - WØLEV
>> *
>>
>>
>
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> --
> *Dave - WØLEV
> *
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