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Grounding Antennas

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6078499

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Hey guys,

I read some stuff about basic radio designs, and i know some electronic theory pretty good. Somebody i asked once told me that making RF waves in a circuit coupled to a grounded antenna will produce a potential between the antenna and the ground. This potential is then caught by the receiver, and inducted into the circuitry. This makes sense, but is it a necessary condition for an antenna to be grounded? I can think of things such as the wireless car lock button which seem to get the signal to the car without being attached to the ground. Can someone please help clarify this little aspect of radio circuits?

Thanks a lot!
 
You can't ground an antenna or the signal would go directly to ground. The antenna has to have a ground reference (ususally this is... the ground =)) but no antenna anywhere is directly earthed, or it couldn't function, you may have misinterpreted the person you talked to.

In something like a free floating antenna ground doesn't actually have to be connected to the earth, for mobile antennas you'll find that inside the circuit there is a ground plane beneath or near the antenna, this ground plane is relative to the antenna and has nothing to do with the earth itself, although actual free air radio communication can heavily depend on the earth itself for how the signals transmit even this isn't required, at higher frequency (such as wifi and microwave) ground plane propogation pretty much doesn't exist and the RF energy exists completly in freespace. For very low frequencies such as the ground the earth itself is actually pretty important because the signals travel along it.
 
You can't ground an antenna or the signal would go directly to ground. The antenna has to have a ground reference (ususally this is... the ground =)) but no antenna anywhere is directly earthed, or it couldn't function,...

No! I have several antennas that are directly grounded; they are called shunt-fed antennas.

Even the 5/8 wavelength whip on my car is grounded to the car body (even though, this type uses a tapped coil, [auto-transformer])
 
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...Somebody i asked once told me that making RF waves in a circuit coupled to a grounded antenna will produce a potential between the antenna and the ground. This potential is then caught by the receiver, and inducted into the circuitry. This makes sense, but is it a necessary condition for an antenna to be grounded? I can think of things such as the wireless car lock button which seem to get the signal to the car without being attached to the ground. Can someone please help clarify this little aspect of radio circuits?

Actually, an antenna can be grounded. The RF signal induces a current that flows along the antenna and into the ground plane under it. One way to couple the signal out of a grounded antenna is to place a current transformer at its base. The antenna current can pass through the transformer primary, and the voltage developed across the transformer secondary can be passed to the receiver using a coax transmission line.

A resonant antenna is a dipole, meaning that it nominally one-half wavelength (λ/2) long, and the signal is coupled from the center of the dipole where the current in the antenna is the highest. A lot of cheap remote control stuff uses a non-resonant antenna which is much shorter than λ/2, usually less than λ/4. The ground traces on the PCB are sometimes used as the "ground" for the antenna, at greatly reduced performance compared to what could be achieved with a proper λ/2 antenna.
 
Read up on it a little, interesting Mike, never realized antennas could function like that, not sure if that's what the poster was talking about but great information regardless =)

There's no surprise they use short antennas for cheap toys; at 27Mhz (a common frequency for RC toys in the US) a λ/2 dipole would be 18 feet long!

I'm curious about the 5/8ths whip on your car that's grounded, I have a current model year car right now and they just don't (and probably never will again) have whips like that, I'm not entirely sure how they function even with what you've said. I'm gathering that there is an inductor at the base of the 5/8ths whip you're talking about that, you said it was tapped like an auto transformer, I'm just trying to understand where the signal comes from and I can't quiet wrap my brain around it, I think it's still on topic if not please respond in private I'd like to understand that type of antenna construction better. I've heard the terms base loaded and top fed antenna before in relation to that type of construction but never found anyone that could explain it to me properly for me to understand.

Practical RF theory is a pet peeve of mine because there is a lack of concise information. The bulk majority of the sites I've found that talk about antenna design assume a certain experience and intuitive understanding which I've never been able to grasp especially with the terminology, it's always felt to me the people that know about this kind of stuff are in some secret closed club =)
 
'Grounded' is really very often the completely wrong term - the basic aerial is usually a dipole, all gains are referenced to that, it can be either horizontal or vertical, and it's not 'grounded' in any way.

For a car, or a portable, a whip aerial is commonly used - this is basically half a dipole, the other 'half' is a virtual reflection in the 'ground plane' - in the case of a car, the ground plane is the metal body.

Such a whip is 1/4 wavelength, as is each half of a dipole - and that figure happily matches coaxial cable at 50 or 75 ohms (with minor adjustments for either).

In the case of a 5/8 whip the matching is done by the loading coil, which is often situated at the bottom of the whip - this is required because 5/8 doesn't match 50/75 ohm coax.
 
Ok, yeah RF is definitely a pet peeve for me as well lol, because every time you think you got something, 10 more things pop up that leave you feeling dumb.

Alright, so if i got this correctly, there are 2 types of antennas in the sense that 1 type is actually stuck in the earth as a ground, and the other type uses some kind of large metallic body as it's "ground". I can understand the type for the earth, because the transmitter and receiver would be connected to each other in the earth, and the other terminal would be the EM waves in the air.

I don't understand though how 2 objects can be connected to each other with only 1 radiating terminal (antenna), and each be hooked up to their own metallic bodies?
 
I would like Mike's further information on the grounded 5/8th wavelength aerial he's talking about my understanding was always what Nigel just said, the aerial is not grounded it's the antenna input, but the car is used as the ground plane, for the frequency ranges common AM and FM transmition encounters ground transmission is the primary mode, with FM starting to lean towards freespace transmission.

All modern high frequency antennas (basically 800mhz and up) rely on freespace primarily because there is no ground transmition at those frequencies. 2.4ghz is almost completly freespace, although bluetooth and wifi can take advantage of nearfield effects within short ranges, the earth itself will eat the ground signal.

You have to understand something, radiating electro magnetic fields are their own complete energy source in freespace transmition. ground and nearfield effects not withstanding, basically the objects are NOT connected, they're just using the earth as a whole as a transmition media in the case of ground transmition in AM and low FM frequencies (couple hundred Mhz max) the earth itself becomes the equivalent of the shield in a transmission line. Nearfield basically means magnetic coupling, nearfield is practical around 1mhz or so more commonly bellow. Gigahertz and microwave require waveguides or line of site freespace transmition and require not local ground plane although they are effected by objects near them the signal itself only requires vacant space to exist. The Electric and magnetic fields kind of tumble through freespace alternating phase along the way (E and M fields are generally 90 degrees out of phase in a plane I think)
 
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'Grounded' is really very often the completely wrong term - the basic aerial is usually a dipole, all gains are referenced to that, it can be either horizontal or vertical, and it's not 'grounded' in any way.

But in a ferrite balun (connected near of dipole), there are 4 small coils. Among 4 small coils (inside the balun), RF has been grounded through the one coil. What is it?
 
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OK my turn to jump in here

some antennas even dipoles CAN BE physically grounded. it provides great lightning protection

take a folded dipole as just one example....


View attachment 64544

the point as noted directly opposite the feedpoint is electrically neutral. This point is often physically grounded via the metal boom and then the mast

The bottom end of my 144/430/1296MHz colinear antenna is also physically grounded in 2 ways.
one via the mast to ground, the other via the coax to the radio chassis to ground (EARTH ground --- not floating anywhere)

Dave
 
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Grrrr Willen

you shouldnt reopen 2 yr old threads, you now have me responding to comments made ages ago :(

Dave
 
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