Balun measurements

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

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  1. G3TXQ

    G3TXQ Ham Member QRZ Page

    Brendan,

    The choking impedance of a balun can vary from something which is almost purely Reactive in the case of a "coiled coax" design, through to something which ispredominantly Resistive in the case of one wound on a lossy ferrite core.

    The CM path into which you insert the balun will exhibit a complex impedance - in some situations it might be Inductive, in others it might be Capacitive.

    So the effectiveness of a balun in one situation may be very different from its effectiveness in another. Measuring how much the CM current drops in a single test environment could be very misleading, unless that test environment is your operational antenna system.

    There's an example on my website of how a Reactive Balun can actually increase CM current in certain environments; look at the "Why reactive chokes are undesirable" section here:

    http://www.karinya.net/g3txq/chokes/

    Yes, you could simply measure the improvement in CM current on your final system. But I would advocate instead designing a Balun to have high Resistive choking impedance over the frequencies of interest.

    Steve G3TXQ
     
  2. VK1OD

    VK1OD Ham Member QRZ Page

    Brendan,

    I have seen a vast array of methods of trying to qualify a balun in, or for use in an antenna system.

    Some, if not most, are abject failures that show a lack of understanding of transmission lines, common mode and differential mode components of current, baluns, antenna systems, and instruments they are using.

    As you have seen, some are dismissive of modelling, but it is a means of obtaining an insight into system behaviour, insight that can inform experiment design and help to explain experimental results. You make your own choice about which tools you use.

    Amongst the many methods of trying to describe the effectiveness of a balun is a measure of the reduction in common mode current. If the purpose of the balun includes reducing common mode current, that seems a good measure, doesn't it?

    So, if someone says their balun reduces common mode current to 10% or lower, or by 20dB or more, does that mean it will always reduce common mode current at every location, in any antenna system, balun at any location, at any frequency, with any radiator?

    Of course it doesn't. It isn't that simple.

    Lets say you measure common mode current at a single point, and it is at or near zero, is that good? Well, you could be measuring it at a common mode current minimum which is also a voltage maximum or point of high electric field gradient... not a good situation at the tx end of your feedline for instance. This single point test is not conclusive.

    If I asked you to calculate using simply ohms law, the effect of an arbitrary complex impedance located at an arbitrary location in a dipole leg, you wouldn't try. Yet people do it for a balun which is a very similar problem in being some approximately lumped complex impedance at some point in the common mode current path, where that current distribution is typically a standing wave, and where the common mode conductor is coupled to the nominal radiator in the system, ground, etc.

    The fact is that changing the balun location on the feedline can significantly change its effect. If your analysis method does not allow that, then it is not an appropriate method.

    Owen
     
  3. VK2FXXX

    VK2FXXX Ham Member QRZ Page

    G`day again guys.

    Steve I have looked at your chart several times ,however it has been a long time since I read the article.
    I re -read it again last night and I realized I had not been thinking about the reactance of the common mode currents. I have been thinking in DC terms!
    Some other things fell into place as I read the article,and I now feel I have a deeper understanding of how a balun works as part of the antenna system.
    Owen
    Owen ,I will use any and all tools at my disposal to understand something.
    This includes models,internet forums,books ,experiments etc.

    Yes this is why I proposed to have a transmission line at least a 1/4 wl long within easy reach, so I can slide the meter along and "see"the current on the tx line ,like a Leichter (spelling) line.

    Moving the balun along the line would prove more difficult.I understand that the measurements I make with the test tx line would only be valid for that particular system.I may be able to do a test with an operational antenna once I put it back up. (#$@*&^cheap rope).

    One more Q ,has anybody tested a ferrite rod type balun( am broadcast type) I made one a while ago and was using it with my 80/40m dipoles.I had a little tvi, but moving the coax away from the tv antenna helped.

    Thanks to all posters ,I have learnt a heap from this discussion ,I hope other readers have as well.

    Brendan
     
  4. W5DXP

    W5DXP Ham Member QRZ Page

    Yes, as you imply, common-mode RF signals obey the wave reflection model. When the common-mode wave encounters the common-mode choke at the tuner output, what may happen is a reflection of the common-mode energy back toward the antenna. That reflection certainly reduces the RF-in-the-shack but does little to reduce common-mode standing waves on the transmission line between the choke and the antenna. The first step in minimizing common-mode waves on the balanced transmission line is to have it feeding a balanced (center-fed) antenna system. (The Carolina Windom has turned the OCF common-mode problem into a feature.)
     
  5. VK2FXXX

    VK2FXXX Ham Member QRZ Page

    Cecil I hope you`re not poking Owen?
    He bites! :]
     
  6. W5DXP

    W5DXP Ham Member QRZ Page

    One can bark on a newsgroup but I don't see how anyone could actually be bitten (especially from thousands of miles away).:)

    I'm sure Owen understands the implications of a voltage maximum occurring at a current minimum common-mode point. That can only occur when common-mode standing waves are present. Standing waves can only exist when a component forward traveling wave and a component reverse traveling wave exist. Those component waves must necessarily obey the laws of physics.

    A funny thing happened when QRZ cut off all posting during the software changeover. For a few minutes, some of us thought our posting privileges had been suspended due to misbehavior. That seems to have resulted in a kinder, gentler newsgroup.
     
  7. VK1OD

    VK1OD Ham Member QRZ Page

    Quite.

    In the general sense, a common mode choke may be located anywhere on the feedline, indeed there could be more than one.

    The feedline on both sides of each common mode choke is coupled to the other system conductors by their shared fields. The source of common mode energy is not solely by conduction from the feedpoint.

    In the case of baluns that are physically small wrt wavelength, it is a reasonable approximation that the current into one terminal is equal (in magnitude and phase) to the current out of the other terminal.

    So, whilst a balun might give rise to reflection of traveling common mode waves, it is wrong to think that means the common mode (standing wave) current on one side is reduced to near zero on the other. This misunderstanding pervades a lot of the discussion of common mode chokes.

    From the article Baluns in antenna systems...

    [​IMG]

    Above is Fig 3 (Model 3), a diagram of a half wave dipole, fed very slightly off centre, a half wave above ground, and the feedline is grounded at ground level. The green line is the magnitude of current in the dipole and common mode feedline conductor.

    There is quite modest common mode current on the feedline. I say modest because the current moment (ie the integral of current * length) is small compared to that of the dipole.

    [​IMG]

    Above is Fig 6 (Model 6) which is the same configuration with a balun added about a third the way up the feedline. Note that it has considerably increased the current moment due to the feedline, the current immediately adjacent to the balun is equal on both sides, and that the common mode current minimum is not exactly at the balun location, in fact there is a local current maximum.

    The example was designed to show some interesting effects that are not implied by common discussion of how baluns work.

    If you were to measure the common mode current at the shack (ground in this case), it would be very low in both cases, less than 10% of the dipole current maximum. Measurement at a single point does not reveal what is happening, does it.

    Measurement with an inappropriate instrument provides even less information. The MFJ-835 in an interesting case, promoted by MFJ thus:

    They are correct when they say "your feedline is balanced if the currents are equal", but correct only when "equal" means equal in both magnitude and phase. The instrument appears to measure only magnitude, and if that is the case, is quite inappropriate as equal magnitude alone does not prove balance.

    Owen
     
  8. W5DXP

    W5DXP Ham Member QRZ Page

    Of course, the choking impedance is only a bump in the common-mode impedance path. It is usually not high enough to cause a complete reflection. (Ten meg-ohms of choking impedance would be nice).:)

    An interesting condition exists when the resistance of the common-mode choke is close to the Z0 of the outside braid of the coax, e.g. a few hundred ohms.

    If one will sit down and sketch out the phasors involved, one will discover that, when common-mode is present, the only condition that will not unbalance the two measured magnitudes is when the common-mode current is orthogonal to the differential currents. Since the odds are that the common-mode current will not be orthogonal to the differential currents, the MFJ device may be more useful that it appears at first. You are correct that equal magnitudes does not prove balance but it seems somewhat improbable for the random difference in phase between the common-mode signals and the differential-mode signals to be exactly 90 degrees.

    What we can say is that if the properly calibrated MFJ device indicates imbalance, that imbalance indeed does exist and that although the conditions that cause a false positive indication of balance can occur, that occurrence has a relatively low probability.

    Here is an example of the most probable condition of the common mode current not being orthogonal to the differential currents. Please note that the Im1 and Im2 meter amplitude readings are not equal.
     

    Attached Files:

  9. K1BQT

    K1BQT Ham Member QRZ Page

    Owen, I get your point that measuring common-mode suppression over a short 50-Ohm path in the lab isn't a legitimate yardstick for how it might perform under all circumstances on every antenna, and I never implied that it would. For me, it is simply a convenient comparative test to plot and evaluate the choking performance of one balun vs. another across a given frequency span. And, if we're talking about isolating an antenna element from the influence of its feedline via the conduction path, I can't imagine installing a balun anywhere but at the matched-in feedpoint. I realize energy may also be coupled inductively to the shield, depending on how the feedline is routed. I also realize this is a tremendously complex topic. However, at some point, don't we need to wax pragmatically and get the job done, warts and all? If we took every conceivable factor into account, how could we possibly ever design a product for general usage?
     
  10. KM1H

    KM1H Ham Member QRZ Page

    Some engineers have a hard time getting a project from the lab and into production. In the real world perfection is often not required as long as it meets the design specification they were tasked with.

    Carl
     
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