Since all antennas require a ground, reference or otherwise, there are two poles for their operation. For a mag mount, the car body is the reference ground. A vertical cannot operate without a ground or reference ground. In reality, a vertical "monopole" uses capacitive coupling when it is mounted on an insulator. Monopoles use the "mirror image" provided by the ground to operate. As a clarification, imagine a satellite's antenna. The case, or housing becomes the ground, again a "reference" ground. In cases where the case is not used, the RF-negative portion of a circuit board becomes the reference. Since two things are necessary to radiate a signal, doesn't that make a dipole?
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2$\begingroup$ Hi Rich! I edited your post to put it in mixed-case, rather than all caps, to make it easier to read. I don't mind doing that at all if mixed case is difficult or impossible for you, and lots of others wouldn't mind either. But on the other hand, if you have no trouble doing mixed case, then that's what we'd prefer. I see your other post was also all caps, so I'll assume that you have a reason. Also, let me offer a belated welcome to the group! $\endgroup$– rclocher3Commented May 25, 2021 at 18:13
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$\begingroup$ A vertical without a ground reference can operate. Consider a half-wave end-fed in free space, with no feed line, but instead a ground isolated miniature transmitter and super high ratio transformer at one end. But some would call that a vertical dipole. $\endgroup$– hotpaw2Commented May 25, 2021 at 20:11
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1$\begingroup$ If every antenna is a dipole, what name would you give to a linear long-wire center-fed antenna, so you could meaningfully talk about the properties of different forms of antennas? $\endgroup$– Pete NU9WCommented May 26, 2021 at 12:11
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$\begingroup$ An antenna without a feed doesn't radiate because no power can get into it. An "isolated miniature transmitter" connects to the antenna with a feed line, and that feed line includes two wires, one of which can be considered to be a "ground". In this case, the ground wire would go into the impedance matching section. An earth ground, however, is not necessary for any half wave antenna or any other non-monopole antenna. $\endgroup$– user10489Commented Jun 2, 2021 at 14:47
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3 Answers
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No.
A dipole antenna is so named because the electric field around it is an electric dipole.
Derived from work by Geek3, CC BY-SA 4.0, via Wikimedia Commons
This is intuitively obvious: a dipole consists of two wires. At any instant, one has a positive potential and the other has a negative potential, except for the instant where the polarity flips and the potential is zero everywhere.
You will notice that at all points equidistant from the two poles form a plane. The potential at this plane is zero. This is true because these points are balanced between the positive and negative potentials of each pole of the antenna.
A monopole antenna has just one wire, and a ground plane. The ground plane is both physically a conductive plane (or something made to approximate it, like radials), and the imaginary plane just described which is all the points that would be equidistant between the two poles a monopole would have if you considered it to have a imaged second pole.
So while a monopole does have two parts, one of them is not a pole. It's ground: not because it's in the dirt, and not because it's connected to the coax shield (although either of these things may be true), but because its potential is zero.
answered May 25, 2021 at 20:57
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$\begingroup$ Although the assumption that Earths potential is zero is common, it’s not technically true. The Earths surface is charged with respect to the various layers of the upper atmosphere (resulting in near constant RF noise from lightning), and the entire Earth + ionosphere system has a charge due to the higher escape velocity distribution of free electrons vs. ions. $\endgroup$– hotpaw2Commented May 27, 2021 at 14:58
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1$\begingroup$ @hotpaw2 "earth" isn't in my answer even once. $\endgroup$ Commented May 27, 2021 at 19:27
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$\begingroup$ @Phil Frost, so by saying 'dirt' you're not referring to the 'Earth's dirt?' I think that's what hotpaw2 reads in to your answer (as am I). $\endgroup$– captchaCommented May 28, 2021 at 2:54
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2$\begingroup$ Phil is specifically saying that the monopole ground has nothing to do with the dirt or earth. He is pointing out that the word "ground" has 2 distinct meanings that are sometimes confused for each other. 1) electrical ground: the reference point from which voltages are measured and defined as 0 volts, 2) dirt/earth, the thing we stand on that can sometimes be used as an electrical ground. $\endgroup$– webmarcCommented May 28, 2021 at 15:57
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No.
Aside from magnetic loop antennas, which are kind of the dual to dipoles (remember how a current in a conductor like a dipole causes a magnetic field? It makes no difference whether you hang a dipole into the electric field of an EM wave, or a coil into the magnetic field of the same EM wave.), there's many other types of antennas:
Horn antennas, bowtie antennas, slot antennas, E-field lenses at the end of waveguides, …
Your assumption that you always need to have a second electric branch simply doesn't hold.
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1$\begingroup$ Nice to see you here again Marcus! $\endgroup$ Commented May 27, 2021 at 0:00
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$\begingroup$ How does one get power out of an RF transmitter or into a receiver box without two electrical branches to close the circuit ? $\endgroup$– hotpaw2Commented May 27, 2021 at 1:21
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$\begingroup$ @hotpaw2 that's a good question, but it's often not "in scope" of the antenna itself: I define antenna to be "an (ideally) intentional means to match a transmission line wave impedance to free-space wave impedance, or vice versa", so all it has to do is "capture" the wave and guide it into a waveguide (e.g. a coax cable). The conversion to something that is actually a current can happen way later (or not at all). $\endgroup$ Commented May 27, 2021 at 9:30
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1$\begingroup$ Isn't a bowtie a type of dipole antenna? Doesn't a horn contain a dipole? Usually a waveguide contains a dipole. $\endgroup$ Commented May 30, 2021 at 13:20
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$\begingroup$ No, a bowtie is not a dipole; no, a horn antenna most definitely doesn't contain a dipole (otherwise, it'd not be a horn antenna); no, a waveguide is a waveguide, and how you feed it is a different question! Also, most waveguide feeds are not dipoles. $\endgroup$ Commented May 30, 2021 at 14:42
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This depends on your definition of "dipole", as in actual use (which may or may not conform to anyone's particular dictionary of terms) the meaning of the term is varied.
The EM field generated by an antenna depends on accelerating charge. And conservation of charge always requires that for charge to accelerate to some physical part of the antenna, it has to come from some other part, and vice versa. You can call those two parts the two poles of a dipole. Or maybe you don't like calling it a dipole if the feed point isn't anywhere near the center, or if the majority of the charge doesn't travel in anything like a straight line to a single point. In some cases, one "pole" might be too defuse to recognize as a pole (for instance, an entire planet, as in Earth ground, or other vast astronomic object, such as solar plasma toroidal currents which may or may not have any poles, and may or may not radiate any EM, as in solar storms, etc.)
A magnetic loop antenna, magnetically fed (no electrical feedpoint or blocking series capacitor) might not be a dipole.
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$\begingroup$ TKS HOTPAW2. SINCE E-M RADIATION IS IN A CONSTANT "FLUX", ITS COMPARABLE TO THE ACTION REACTION LAW. PUSH - PULL, ATTRACT - REPEL, IN ORDER TO GT TWO STATES FOR A GIVEN DISPERSION OF ENERGY, YOU NEED TWO OPPOSITE "POLES"TO GENERATE AND RADIATE THAT ENERGY. IGNORE THE TX, I AM ADDRESSING THE ANTENNA ONLY. TO SUCCESSFULLY RADIATE A TWO PIECE COMBINATION THAT ARE 180 DEGRES OUT OF PHASE WITH ONE A ANOTHER, YOU HAVE TO HAVE TWO RADIATING ELEMENTS THAT ARE 180 DEGREES OUT OF PHASE. IN ANTENNA SPEAK, A DIPOLE. JUST MY CONSIDERATE THO HUMBLE OPINION. '73 $\endgroup$ Commented May 30, 2021 at 2:15
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$\begingroup$ I agree KF9F, with you and W8II. To the point Phil, W8II, makes, there must be two potentials that are 180 degrees out of phase from each other. For those who have a hard time imaging what this means, I liken a monopole circuit to residential AC electricity in North America, where with a 120 volt outlet, you have a hot wire which has 120VAC potential, this could be a representation of the energy on the center conductor of a coax. $\endgroup$ Commented Dec 20, 2022 at 12:05
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$\begingroup$ And a neutral wire on an AC outlet which has 0 potential (like a counterpoise or ground). Whereas a 240 VAC household circuit has two wires, each with 120 VAC potential, and 180 degrees out of phase with each other, creating the total potential between them of 240 volts. In a monopole, the counterpoise does not radiate, at best it has reflections; a dipole has two elements driven by voltages opposite to each other. $\endgroup$ Commented Dec 20, 2022 at 12:05