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"LTE 868 MHz Antenna" Why does 868 MHz stand for? Are antennas specified for a certain frequency or is it necessary for something like input type? For example, I want to transmit a 40 MHz RF signal. Can I use an 868 MHz or 2.4 GHz antenna to transmit? Or am I supposed to make an antenna specified for that frequency band? What might happen if I use an 868 MHz antenna on a 2.4 GHz compatible device like a WiFi router? Lack of performance? Not working?

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    \$\begingroup\$ To clarify, 868 MHz is the (general) specification. MHz is megahertz, or million hertz. 1 hertz is one times per second, so an 868 MHz antenna is intended to operate with radio waves that have a frequency of 868 million cycles per second. \$\endgroup\$ Jun 20, 2022 at 8:22
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    \$\begingroup\$ A quick Google tells me that LTE generally seems to avoid 868 Mhz. Europe has it as an unlicensed IoT band. Japan looks to be a historical exception (they used it before LTE for older phones and are re-assigning it to LTE). This makes me wonder who came up with the "LTE 868 MHz" description \$\endgroup\$
    – MSalters
    Jun 20, 2022 at 13:42
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    \$\begingroup\$ Generally, yes. For 2.4GHz a straight wire/whip antenna needs to be around 1/4 wavelength. Since we know the frequency is 2.4GHz and since radio waves are just invisible light we can calculate the wavelength of light at 2.4GHz as speed of light/(2.4 GHz) -- type that into google and we get around 12.5cm. 1/4 of that is 3.1cm or roughly 1.2 inches. For 868MHz you can do the same calculations and get 8.6cm/3.4 inches for a quarter-wave antenna which is more than double the length of a 2.4GHz antenna \$\endgroup\$
    – slebetman
    Jun 21, 2022 at 7:12

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It's not so much that antennas are specified at certain frequencies (they usually are), but that they tend to work best over certain frequency ranges, determined by their physical shape and the materials they're made of. But -- it's complicated. People can spend entire engineering careers doing nothing but designing antennas.

"LTE 868 MHz Antenna" Why does 868 MHz stand for? Are antennas specified for a certain frequency or is it necessary for something like input type?

"Specified" means someone is promising something. A given antenna will tend to actually work for some range of frequencies.

To work well, an antenna must work over the range of frequencies that the equipment it's connected to must work (i.e., if you're only going to transmit Morse code at only 11MHz, then you can have a really narrow band antenna -- but if you're going to receive any signal between 50MHz and 200MHz without changing the antenna dimensions, or if you have a signal that actually spans that range, then you need a very broad band antenna).

For example, I want to transmit a 40mhz RF signal. Can I use an 868MHz or 2.4GHz antenna to transmit?

Well, you can, in much the same way that you can use a knitting needle or a wheel off of a car for the same task. But it probably won't work out well.

Or am I supposed to make an antenna specified for that frequency band?

"Supposed" and "specified" presumes that there's some Physics Police out there who will arrest you or hand you demerits for getting it wrong. I have no clue what people expect you to do -- but a given antenna will work best at a given frequency, and if you're designing a system then you generally need to design the antenna to match, or obtain one that's going to (usually by design) work the way you wish it to.

What might happen if I use an 868 MHz antenna on a 2.4GHz compatible device like a wifi router? Lack of performance? Not working?

In the case of a consumer-grade WiFi router, probably degraded performance, perhaps to the point of not working unless you're inches away. You probably wouldn't damage it if it has external antenna connections, because a responsibly-designed consumer device will embrace the possibility that someone is doing something dumb with it.

In the case of a professional device that's intended for 2.4GHz and that includes a transmitter (or a really crappy consumer device) the mismatch between the antenna and the transmitter may damage the transmitter, the receiver section (if there) and possibly whatever circuit switches between receive and transmit. The difference here is that a professional device presupposes a professional doing the installation. Most modern RF devices will have some sort of protection, but the more "pro" it gets the more "pro" you are expected to be.

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    \$\begingroup\$ It is good to note that an antenna can be engineered to work acceptably for more than one frequency band. Antennas for modern multiband cell modems or multi-band wifi interfaces are an example. \$\endgroup\$
    – fraxinus
    Jun 20, 2022 at 13:29
  • \$\begingroup\$ There's a tendency for antennas designed to transmit at a particular frequency to be moderately effective at higher frequencies, but very ineffective at lower frequencies. For receiving, the situation is different. An electrostatic probe feeding a high impedance receiver can be a perfectly good receiving antenna at frequencies far below any frequency for which it is effective at transmitting. \$\endgroup\$
    – John Doty
    Jun 20, 2022 at 14:04
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    \$\begingroup\$ "presumes that there's some Physics Police out there who will arrest you or hand you demerits for getting it wrong" -- well, there is the Federal Communications Commission! \$\endgroup\$
    – nanoman
    Jun 20, 2022 at 17:23
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    \$\begingroup\$ Just a note: For a whip antenna (and probably others), the length of the antenna should be a multiple of 1/4 or 1/2 of the wavelength. It doesn't have to be exactly this though ... you can electronically "lengthen" and "shorten" the antenna by adjusting the impedance (capacitance and inductance) of the interface between the antenna and the radio. This is what happens when you turn the dial on an AM radio, for example. Electrically, the antenna will look "ideal" although it really isn't. And not all frequencies will work as well as others - but RF levels vary a lot anyway. \$\endgroup\$
    – Steve
    Jun 20, 2022 at 23:26
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    \$\begingroup\$ My nephew asked why their wifi router has those really stubby antennas sticking out the back "how do they work? they're too small" ... my answer was that they are exactly the right length for the job they are doing. Because engineers designed them so. He replied, "well then I want to be an engineer", which warmed my heart... he finished with ".... so I can use my conputer while driving the train" \$\endgroup\$
    – CGCampbell
    Jun 21, 2022 at 14:09
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Antennas are tuned for particular frequency bands. They won't be damaged by using other frequencies (in theory the transmitter could be damaged by a mismatched antenna, but most commercial transmitters are protected against mismatched (or missing) antennas.

If you use outside the designed frequency band your performance will deteriorate -- likely very significantly (e.g. 10, or 100x worse).

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    \$\begingroup\$ outside the designed frequency band your performance will deteriorate - The antenna can also radiate/receive at unwanted frequencies (harmonics) and in unwanted directions. \$\endgroup\$ Jun 20, 2022 at 10:15
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    \$\begingroup\$ An antenna won't GENERATE harmonics, but conceivably it could be a better transmitter of some harmonica than the fundamental frequency. \$\endgroup\$
    – jp314
    Jun 21, 2022 at 20:58
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Antennas are attuned for one particular frequency, since they need to match the wave length of the radio signal. Wave length can be calculated as c/f where c is the speed of light and f is the frequency.

In you case 299 792 458 m/s / 868*10^6 = 0.345m.

There are various antenna types such as full wave, half wave, quarter wave, which in this case means they have the exact physical length 0.345m/1, 0.345m/2 or 0.345m/4.

This is for the ideal center frequency of the antenna where it sends/receives optimally. From there, antennas can be broad or narrow, as in accepting a wide or narrow span above and below the center frequeny. The longer you stray from center, the worse it will perform.

If you for example have a radio transmitter for 40MHz but an antenna for 868MHz, it will not work at all, as the energy generated from the 40MHz transmitter will just reflect back into the radio electronics instead of getting sent out in the air. Leading to excess energy that has to go somewhere - in the best case the radio circuit can deal with this, in the worst case it will get fried. So apart from not working at all, you might even damage the electronics by using the wrong antenna.

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    \$\begingroup\$ There is nothing in the physics that demands that an antenna be self-resonant. Antennas are not necessarily attuned to a particular frequency. However, it can be somewhat challenging to couple power to a small antenna made of thin wire: self-resonance can be helpful in this case. It's also used to direct the the beam from a Yagi antenna (but a dish can be directive without resonance). \$\endgroup\$
    – John Doty
    Jun 20, 2022 at 14:42
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    \$\begingroup\$ And note that for various complicated reasons, an antenna designed for 80MHz may well work better for a 40MHz signal, than one designed for say 50MHz \$\endgroup\$
    – MikeB
    Jun 21, 2022 at 16:24
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Remember that every technical product is the result of a lot of compromises, or with a better word design decisions.

The engineer designing the antenna will work according to a specification that will outline the priorities for the design. It will include things like: frequency range, amplification, directivity, max ratings for voltage and power, price, size, SVR, weight, materials, temperature range, longevity, ...

This means that a very high amplification very directive antenna with a huge budget (think radio telescope) will look very different from a 5 cent radio transistor antenna.

Trying to use the antenna outside of, say the intended frequency range, then you are on your own. It might work very well, it probably will not be great, but it might be acceptable. The exact design of the antenna might give an experienced person clues to the performance but the proof is in the actual performance.

Transmitting a 40 MHz signal using a 868 MHz antenna will probably result in very poor amplification, meaning that most of the energy will be transformed to heat and very little inte actual radio waves. It might as well show very odd directionality perhaps sending most of then radio energy into only a small lobe. Same effect might happen with a 868 MHz antenna on a 2.4 GHz router -- most of the transmitter radio signal will turn into heat (or perhaps not, depends on the exact design of the antenna). And similarily for the receiving end, the antenna might be quite "deaf" at frequencies outside the intended range, or not. Mty guess is that it will not give a satisfying result.

Additionally, it might hurt the transmitter. Pumping energy inte a circuit and not letting it exit the intended way through the antenna may hurt components and you will see the magic smoke exiting from them. Admittedly, in low power situations and/or consumer electronics the circuits are probably protected so harm may not occur.

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Antennas can be whatever size you like and any frequency you like. At a given frequency, for a monopole antenna, two things depend on the antenna size: input impedance and radiation pattern. Input impedance that is different from your feedline impedance will require some kind of matching network. But all of the power reaching your antenna will be radiated (except the power that is warming the roof of your car). An antenna that has a constant impedance from DC to gamma rays is the holy grail of antenna engineering. Not likely you will ever encounter such a device.

Antennas are generally limited by mechanical considerations. I no longer carry a 66 foot vertical monopole on my car. Also I no longer use a spring at the base of my car antenna due to bad impotence. A California Highway Patrol officer advised me that the maximum permissible height for an antenna was 13 feet above ground. But an antenna that high will not do well around trees and bushes. As said above, engineering is full of compromises.

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