Balun measurements

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by VK2FXXX, Feb 13, 2011.

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  1. 4L4AGU

    4L4AGU Ham Member QRZ Page

    It may sound weird, but...

    As I understand, balun forms (or at least should form), pure resistive load for stray RF currents (in maximally wide range of frequencis), right? Lossing these RF currents in resistive load, means generating heat, right? So, if that power should be lost anyways, why should not recycle it? Say, arrange balun in that way, that most of stray RF is induced into coil, from where it will be rectified, and received DC current used for any practical purpose, such as battery charging and so on?
     
  2. M1ENI

    M1ENI Ham Member QRZ Page

    Been thinking about this with a load of rx antennas could you charge a battery and run your lights of it !!!!!

    73 John m1eni
     
  3. VK2FXXX

    VK2FXXX Ham Member QRZ Page

    4L4AGU
    Ok Im not 100% sure but here goes.
    Remember a transmission line connected to its characteristic impedance will deliver ~100% power to the load. If the load is not the same as the Z of the feedline reflections will occur ,producing standing waves.
    So If a balun has an impedance of say 1000+j0 and the impedance of the common mode presented to it is say 100+0 this would be a swr of 10-1 and 66% of the current would be reflected from the balun back to the antenna element where it belongs?( This may be absolute rubbish)
    Ideally a balun would be infinite R +j0 across all frequencies of use ,therefore 100% of power reflected from the balun to antenna element.
    So basically the CM current takes the easiest path ,which happens to be the antenna element/s.
    Please remember I am a novice at all this and these are my thoughts on your statement /question above.
    Its not as simple as I have made it sound of course (reactance of CM and balun) ,and this is only one way to describe what happens,its probably not even correct!
    Anyway something to think about
    Brendan
     
    Last edited: Feb 19, 2011
  4. VK1OD

    VK1OD Ham Member QRZ Page

    Hi Brendan,

    That is the way the re-re-Reflections crowd would put it.

    It is true that if a transmission line is not terminated in a load equal to its characteristic impedance (at that frequency), that there will be a reflected wave and standing waves develop on the line. (Some people state that the load must be the conjugate of Zo for zero reflection, but they are wrong.)

    Assuming that a transmission line is purely a transmission line (eg that it is not radiating EM energy), in the steady state the power delivered by ALL transmission lines to the load (irrespective of load impedance) is the input power less that converted to heat in conductor loss and dielectric loss within the line.

    It is likely that the common mode Zo is much higher than the figure you mention and probably not uniform throughout its length, but dealing with the principle of what you have said...

    You method of analysis is only valid where one terminal of the balun is directly connected to the ground reference for the common mode line. That is typically not the case where the balun is employed near the feed point of a dipole.

    If you treat the common mode 'conductor' as a pure transmission line with some kind of termination, and attach that to the feed point of say a dipole, aren't you ignoring the coupling that exists between ALL conductors near the antenna due to their shared electric and magnetic fields?

    Here is an interesting thing to explore. If you take an 80m half wave dipole, and insert a pair of insulators in the middle of each leg, then use it on 40m, do you think there is zero current flowing in the outboard conductors?

    There isn't, the infinite R insulators do not assure zero current, and the reason being that the conductors are coupled through shared fields.

    The same issue exists for a common mode choke, even if of extremely high impedance does force zero current on the other side.

    You have been tempted to apply simple linear circuit analysis techniques to a problem that is better explained as coupled conductors (as does NEC for instance).

    Current usually doesn't "take the easiest path", that implies ALL of it flows by the easiest path. For lienar circuits, a variation on ohms law is the I=EY, so where circuit elements are in parallel, they share the same E, and their currents are proportional to Y (or 1/R), so the total current 'divides' according to Y, each element carries the fraction Y/Yt of It.

    Owen
     
  5. W5DXP

    W5DXP Ham Member QRZ Page

    Not only that, but there is non-zero current from millions of received signals some of which originate 93 million miles away.
     
  6. VK1OD

    VK1OD Ham Member QRZ Page

    If your point was to compare the current in the isolated conductor due to the fed section of the antenna with the "current from millions of received signals", you will no doubt be surprised to learn that it is considerable larger. Try to model this in NEC and you will get an idea.

    Owen
     
  7. W5DXP

    W5DXP Ham Member QRZ Page

    Who cares how large it is when the received signals from 93 million miles away are wiping out all HF communications? (Hint: It was humor of the irony variety.)
     
  8. VK2FXXX

    VK2FXXX Ham Member QRZ Page

    I stopped at one

    Owen ,you will notice I stopped at one reflection ! :D
    I should have continued with : which creates a standing wave in a steady state ,,which increases the amplitude of voltage and current on the line,which in turn increases dielectric and conductor losses. etc.

    I neglected that after the initial 66% reflection (10:1 SWR) statement.oops


    What would happen if we made a balun that has a choking impedance very close to the common mode impedance . Would the balun be totally ineffective? would all the power be dissipated in the balun ?

    Would these be differential mode currents? Can you reduce these with a balun?
    Are they simply added to common mode current ?

    All understood.

    This is all due to magnetic fields inducing voltages in nearby conductors?
    I see where I went wrong ,and why It can be so difficult to choke some CM currents.
    They are induced differentially after the high impedance choke !? If that makes sense?

    Got to go now
    Thanks again
    Brendan
     
    Last edited: Feb 20, 2011
  9. W8JI

    W8JI Ham Member QRZ Page

    It's better to consider this a problem of two independent systems.

    One is the differential mode system where the feedline conductors, even coax, are in push pull.

    The other is where the feedline behaves as a single conductor, driven by external sources or by antenna fields coupling into the single wire. The feedline in essence becomes a random wire antenna in this mode. The balun's "balancing" job is to make the feedline a poor single conductor mode system without disturbing the dual conductor (push pull) mode.

    The single conductor mode, where the feedline acts like a random length radiator, is the mode that causes all the problems with extra noise pickup and RF in the shack. This is the currents are all common mode impedance related, so anything connected to the feeder (like the radio chassis and eventually the power lines powering the radio and everything connected to the power lines) are all part of the radiating system.

    A high common mode resistance is better than a high common mode reactance (even though the reactance portion is lossless) because the unwanted system is a series system. If the rest of the system causes the common mode impedance to be the opposite reactance sign as the balun, and if the common mode reactance presented by the balun less than twice the common mode reactance of the system at the point where the balun is inserted, common mode current will INCREASE from adding the balun.

    The only impedance that will always, under every condition, decrease common mode current at the balun is the resistive part of the impedance.

    Another but generally smaller effect, unless the feedline is routed poorly, is the one Owen pointed out. There is coupled energy from antenna fields that drives the common mode of the feedline.

    In all of these cases of a give balun common mode impedance.....where the balun is inserted, how long the feedline is on each side of the balun, how the feedline is "grounded", and how the feedline is routed...... all determine common mode current.

    It's a very complex system.

    There are only a few statements we can make:

    1.) A balun with common mode reactance as a large portion of the balun's impedance can make the system WORSE as often as it can make it better.

    2.) A balun with a dominant resistive part to the common mode impedance will always reduce common mode current at the insertion point of the balun.

    3.) We can, if the rest of the system is laid out incorrectly, actually make things worse by adding a balun.

    4.) Grabbing the coax with our hands and looking for an SWR change to determine problems is an idiotic analysis, it is virtually meaningless.

    Sevik, in his book Baluns and UnUn's, made several key mistakes. One was he tested a dipole with a 1/4 wave long feeder to a good ground system and concluded a dipole did not need a balun because the coax was a small diameter compared to the length of the dipole. What he really proved was a 1/4 wave of coax, grounded at the transmitter end to a good ground, makes a damn good balun because common mode impedance at the balanced to unbalanced junction is very high.

    Another was not realizing the importance of terminal voltages, and having voltages be able to freely float to any level needed to minimize common mode currents (ground independent current source). A recent white paper on baluns at an audio group has a similar error.

    73 Tom
     
  10. 4L4AGU

    4L4AGU Ham Member QRZ Page

    Just don't want to open another topic, I have a balun question:

    I have some heavy-duty military 1:1 voltage balun, which is wound using trifilar teflon coated wire on 5 stacked ferrite cores. What if I use two windings in a choke balun way, and leave 3rd winding just not connected? will it work?
     
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