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A center fed dipole, with equal arm lengths, is a balanced device.

If it is fed using a gamma match - does it change (the system) to be unbalanced?

Kevin Reid AG6YO
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Jan Hus
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  • Does this answer your question? What is a Gamma match in the context of the driven element of a Yagi antenna? – Marcus Müller Oct 16 '22 at 19:08
  • No. none of the articles in the reference answer my question . "A center fed dipole, with equal arm lengths " cannot be fed using gamma match - only "continuous dipole" can . That is why gamma match is commonly used in Yagy antenna , where active element is CONTINUOUS, not split, and each arm is not isolated from each other. – Jan Hus Oct 17 '22 at 04:25
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    There's no such thing as a dipole that is not split in the middle - it would not be a dipole then. And all yagi constructions I've seen so far have a classical dipole as the driven element. See, for example, the Wikipedia soccer an Yagi-Uda Antennas. You might be confusing the driven element with the directors, which indeed have a point of zero current in the middle and can be built as a continuous rod; but those are not feed from any waveguide. – Marcus Müller Oct 17 '22 at 07:12
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    @MarcusMüller you can have a Yagi where the DE is electrically continuous, and optionally connected to the boom, if you feed it with a gamma... as in the question that you linked. The Arrow is a familiar example. – hobbs - KC2G Oct 17 '22 at 13:06
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    A half wave end fed dipole is also continuous. – user10489 Oct 18 '22 at 22:38
  • @MarcusMüller Did you mean to say waveguide here? I think you meant feedline. :-) – Mike Waters Oct 20 '22 at 14:35

2 Answers2

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A half wave wire has a low impedance point at its midpoint. If you put two half wave wires at 90 degrees with respect to each other so the midpoints come close, but not in electrical contact with each other you can measure the impedance between the midpoints. You will find that it is very low.

Each half wave can be seen as two quarter waves. When you feed two quarter wave rods in phase, the radiation from both will be very similar, but 180 degrees out of phase so the radiation will cancel. A quarter wave will transform the very high impedance at one end (the tip) to a low impedance at the other end. Since there is almost no radiation the impedance at the tip becomes very high and the impedance at the midpoint very low.

The secret of the gamma match is that you use the virtual ground point at the midpoint to connect the screen of your feed cable. It is the same phenomenon we use in ground plane antennas.

Actually half wave rods can be used to place ground points on support structures and cables along them for example when using X-yagis. Such a virtual ground point should typically be parallel to the boom tube, but placement is critical, one wants the virtual ground point to create a high impedance point where the support structure connects to the boom tube typically 3/4 wl from the boom tube (1/4 wl is too close.)

Once the screen is connected to a virtual ground point you can always match the impedance by a suitable LC link. The current on your coax screen will be small. In a yagi where the element length is significantly different from 0.5 wl it is a good idea to connect the midpoint as well as the screen to the boom tube to further bring down the impedance.

sm5bsz
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The system says I need a 50 reputation to add a comment, which I don't have yet so I'll add it as a post. The short answer to the original question is, the original question needs to have some terms defined because there are some contradictions:

  1. Yes, a center fed dipole, with equal arm lengths, is a balanced device (because there are two legs to the dipole, fed by a differential signal).
  2. A gamma match does not turn a 1/2λ dipole into a half wave radiator, since by definition a dipole has two poles, and a gamma match antenna is simply a continuous 1/2λ radiator, and as sm5bsz said, the shield is connected at the center of the radiator where voltage should be essentially 0.
  3. A gamma match arrangement does not turn an unbalanced coax feedline into a balanced feedline, and if current appears on the coax shield, where it shouldn't be, you have a matching problem.
Lou-in-USA
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  • gamma match does not turn a 1/2λ dipole into a half wave radiator, since by definition a dipole has two poles, and a gamma match antenna is simply a continuous 1/2λ radiator, and as sm5bsz said, the shield is connected at the center of the radiator where voltage should be essentially 0. does not turn a 1/2λ dipole into a half wave radiator, What are you saying here ? – Jan Hus Oct 19 '22 at 21:15
  • gamma match does not turn a 1/2λ dipole into a half wave radiator, since by definition a dipole has two poles, and a gamma match antenna is simply a continuous 1/2λ radiator... What are you saying here ? The question was - does gamma match turns the system into unbalanced - perhaps you are correct and using "system" in that case is wrong . Of course assuming that coax is used as feed line makes the discussion ever crazier. Perhaps this forum format is not best to carry on decent discussion. – Jan Hus Oct 19 '22 at 21:26
  • "The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole" The 1/2λ radiator is a dipole regardless of how one feeds it. With a gamma match, over a gap at the center, or at the end (like a GP.) One can even feed over a gap off center using an appropriate balun. – sm5bsz Oct 20 '22 at 22:55
  • OK, just been told this forum is not for discussions - with that said - I would like to invite anybody interested in DISCUSSING if using gamma match turns the ANTENNA SYSTEM to unbalanced system. https://www.facebook.com/groups/588480802627423 – Jan Hus Oct 21 '22 at 16:53
  • OK, just been told this forum is not for discussions - with that said - I would like to invite anybody interested in DISCUSSING if using gamma match turns the ANTENNA SYSTEM to unbalanced system. ( Lets put the dipole to bed and get to "unbalanced versus balanced system" ). https://www.facebook.com/groups/588480802627423 don't let the forum title discourage you , if it gets posted. – Jan Hus Oct 21 '22 at 16:59