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Why do we ground radios? As far as I know an antenna is an open circuit...

So we just need to put and filter to remove noise and we are ready. However in most schematics I see the antenna is grounded. Why is that????

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    \$\begingroup\$ If you see a antenna as a no more than an "open circuit", then a capacitor is no more than an "open circuit" too. \$\endgroup\$
    – andre314
    Oct 9, 2020 at 0:16
  • \$\begingroup\$ You are asking some interesting things, but the rapid pace at which you are posting questions and the fact that most are better covered in traditional references rather than on a QA site suggests you may want to change your approach a bit, and start places like wikipedia, with search engines, etc and save questions for what is still unclear. \$\endgroup\$ Oct 9, 2020 at 0:30
  • \$\begingroup\$ Read some of this. It may help a little. \$\endgroup\$
    – jonk
    Oct 9, 2020 at 4:47
  • \$\begingroup\$ You're wanting the site to be a free personal tutorial service and on-line technical encyclopaedia. There is mountains of text already written on this subject on the internet and you find your answers there first. Here, people will help you take the next step - if your questions show you've done as much as you possibly could. Which this doesn't, I'm afraid. Please edit your question and greatly improve it. Show your own work and your own findings in considerable detail. The better the quality of your questions, the better the quality of the answers you will attract. \$\endgroup\$
    – TonyM
    Oct 10, 2020 at 20:08

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Often we don't actually ground them, but the ground symbol is just a design convention. If we had to ground them, then we couldn't have radios in aircraft or spacecraft or make portable hand held ones.

The conceptually simplest radio antenna is a dipole, which consists of two wires running in opposite directions, operated in opposition to each other. Often these are horizontally oriented, but they can also be vertically oriented. You might have seen a variation of one on an old FM high-fi receiver, or at least a setup with two telescoping antennas that open out into a sort of "vee", or a whole series of dipoles of varying length splitting off a common beam in a yagi-type TV antenna.

It turns out though that if you take a dipole and split it in the middle with a big conductive plane, then you can remove one half and drive the plane instead, and the missing half will be "mirrored" by the plane.

So we can take a vertical antenna and set it above a big conducting plane - eg, the ground (possibly reinforced by a spider of wires to improve conductivity) and use that instead of trying to stack one tower on top of another with an insulator in between. This is actually commonly done, for broadcast and ham radio type installations.

But especially at high frequencies, we can also use a single antenna half above a sheet or chunk of metal instead of the ground - for example, a car, or a cookie sheet, or even the body of a handheld radio. Think of it as a counter-balance; the term counterpoise is literally used.

So sometimes we ground a radio only in the convention that there's a side of the antenna system (or radio circuit, or even non-radio circuit) that is conceptually ground.

And sometimes we literally put a stake in the ground, and all that half of our antenna.

And in the realm of "serious" installations of radio and many other types of gear, there are also other roles of ground. For example, it's not uncommon for the housings or radio equipment to be grounded independently of an antenna system which may or may not involve actual ground. Then there may be a safety ground through the power mains (which would typically go to a ground rod somewhere near the service entry), the grounds of lightning protection systems, etc...

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  • \$\begingroup\$ When we amplify the signal with an opamp don't we need a reference voltage to compare? \$\endgroup\$
    – Se1fie
    Oct 9, 2020 at 0:25
  • \$\begingroup\$ We wouldn't typically quite connect an op-amp to an antenna, but in concept, yes you could use the two halves of a dipole antenna to feed a differential amplifier like that. Or you could use the one half and a big ground plane like the actual ground, or an approximate ground plane like a car or cookie sheet or your body... \$\endgroup\$ Oct 9, 2020 at 0:28
  • \$\begingroup\$ Interestingly enough the last active mono-pole antenna that I used for radiated emissions compliance testing between 10MHz and 30MHz had a 400MHz low noise Analog Devices Op Amp as the active element. \$\endgroup\$
    – user4574
    Oct 9, 2020 at 1:10
  • \$\begingroup\$ It's amazing / all the ways / they're using op-amps / now-a-days Now that you mention it I once fooled with so some plans I found for a kludge of an HF rig that used... an LM386 (which of course is neither an op-amp nor an "RF" part) \$\endgroup\$ Oct 9, 2020 at 1:22
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I think that the ground in the schematic is just a voltage reference point.

Here is a schematic diagram of a handheld flashlight (torch).

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Maybe it has something to do with the amplification of the signal in amplifier (op amp) if you use an opamp then we need 2 inputs? \$\endgroup\$
    – Se1fie
    Oct 9, 2020 at 0:24
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Not all radios need to be grounded. The cell modem in your mobile phone is not grounded. The radio transceivers in a satellite are not grounded.

Radios receive electromagnetic waves that pass by the antenna. At the antenna the intensity of the electric and magnetic fields are changing with time. But changing with respect to what? That's why you ground the radio. You need some reference point that your are comparing the signal to.

Remember that voltage is not a measurement at a point. It is a measurement between two points. In this case, one point is the antenna and the other point is the ground.

You could of course have a two part antenna (like a dipole) and compare the signal on one part of the antenna to the other part of the antenna. In that case you are making a measurement which is not relative to ground.

Another example would be an active mono-pole antenna (like the kind used for radiated emissions compliance testing). These antennas have a rod extending upward from the center of a portable metal "ground plane". In this case the antenna is measuring relative to a square of metal which sits below it.

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A mains-operated radio would be earthed to enable ground-fault tripping that would render it safe from electrocution hazards. Likewise, an antenna too could be earthed to provide a safe path for static charges, accumulated on it, to ground.

At low frequencies and good ground conductivity, a grounded ¼ λ antenna would be used without an extensive ground radial system.

enter image description here

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