If a diode or directional coupler is fitted at the outer ends of the elements of a dipole, would this stop current reflected from the ends and so result in no standing wave on the antenna ? It can't be otherwise someone would have already thought of this. Why wouldn't this work ?
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1$\begingroup$ What are you going to connect the other end of these devices on the ends to? $\endgroup$– Mike WatersCommented Sep 2, 2021 at 11:33
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1$\begingroup$ What are you trying to accomplish? What's your definition of "work"? $\endgroup$– Phil Frost - W8IICommented Sep 2, 2021 at 13:58
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1$\begingroup$ Think this through... Even if it worked as you propose, you are effectively just shortening the antenna since the reflection would be from where the diode appears to "terminate" the flow. $\endgroup$– David Hoelzer ♦Commented Sep 2, 2021 at 18:24
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2 Answers
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Any change in impedance at the end of a wire, different from the rest of a transmission-line-like wire, will reflect some current.
An impedance matched resistor at the ends of a dipole won't reflect a current wave, but will be highly lossy, which for an dipole antenna, is usually considered not working well. One usually wants EM radiation, not thermal radiation.
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2$\begingroup$ Agree with this, but there are many antennas that are resistor terminated. Just because an antenna is inefficient doesn't mean it doesn't work and isn't used. Sometimes broad band with a low SWR is more important than efficient. The B&W folded dipole is one example of this. $\endgroup$ Commented Sep 2, 2021 at 0:54
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2$\begingroup$ Rhombic antenna is another popular resistor terminated antenna. $\endgroup$ Commented Sep 2, 2021 at 4:39
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1$\begingroup$ Resistors have two terminals but there's only one wire at the end of a dipole. How would you propose to connect the resistor? $\endgroup$ Commented Sep 2, 2021 at 13:57
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1$\begingroup$ I suppose there could be folded dipoles with a resistor termination, but an ordinary resonant folded dipole has a short at the tips. A resistive termination would in many cases be undesirable because it would reduce antenna efficiency. $\endgroup$ Commented Sep 3, 2021 at 12:46
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2$\begingroup$ @user10489 The question does not mention a folded dipole. $\endgroup$ Commented Sep 3, 2021 at 13:02
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There are a number of problems with this idea.
Firstly, diodes have two terminals. The end of a dipole is just one wire. How would you even connect a diode at the end?
Also, the current at the very end of the dipole is zero. The defining property of a diode is it rectifies current, but there's no current at the end, so exactly what would the diode do? (Assuming you could even place a diode at the end, which you can't, because at a minimum the diode has a lead which acts as a little extension of the dipole beyond the diode.)
And then, rectifying your signal will introduce terrible harmonic distortion, if somehow you manage it. This is pretty undesirable in most antennas.
Directional couplers have similar problems to diodes in that it's unclear how you'd even connect them at the end of a dipole. Worse, a directional coupler has eight terminals (four ports with two terminals each) but you still only have one thing to connect.
Finally directional couplers don't separate waves going one way from the other way, which is I think what you are hoping would happen.
If you wanted an antenna with no standing waves, there is however a way to do it: make it infinitely long. In an infinitely long dipole the wave would just keep traveling down the unending wire until it had lost all its energy to radiation, and there would be no standing wave. Usefully, it would also have infinite bandwidth.
Of course infinite length is a pretty tall order, however for some microwave antennas we can settle for "long enough". For one practical application of the idea, see the infinite balun.
answered Sep 3, 2021 at 22:24