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Are transmit antennas the same as receive ones? I am making a extended Wi-Fi comms CCTV camera.

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    $\begingroup$ Hi Miner! I opened up a private window, and typed in your exact title into a search engine, and got a lot of good results! While we definitely appreciate questions from all levels of radio understanding, we must also assume that you do your fair bit of research before asking. $\endgroup$ – Marcus Müller May 19 at 13:30
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A transmit antenna for a frequency should work well for receiving that frequency, but the reverse may not be true.

Generally, while an antenna meant for transmitting works much better when resonant on that frequency (e.g., a half wavelength for a dipole, a full wavelength for a loop), receive antennas are not necessarily resonant. Although in most cases a resonant antenna may allow greater receive sensitivity, it isn't strictly necessary. When receiving, signal to noise ratio is typically more important than sensitivity, and a more sensitive antenna might just receive more noise.

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In short, yes. The nature of an antenna is that it is naturally resonant on one frequency. Now, that doesn't mean that you can't use an antenna if it isn't exactly at its resonant length (most in fact are not, but that is a different topic). Literally entire books are dedicated to that subject. However, for the question you asked, the only difference between a receive and transmit antenna is what is connected to the other end.

Do you have a receiver connected to it, like your car radio? It's a receive antenna.

Do you have a handheld transceiver like a UV-5R connected? It's a transmit antenna when you're talking and a receive antenna when you're not.

To use your Wifi question, take a look at your wireless router and think about what's going on. Sometimes you are sending data to the router, and sometimes you are getting data from it. The same antenna can be used to transmit or receive.

If your question is, can I send and receive at the same time on the same antenna? Well, that is dependent on the transceiver you're using, not the antenna.

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    $\begingroup$ Few antennas are a wavelength long. A 1 wavelength dipole, for example, has a very high feedpoint impedance which would make it very difficult to match with ordinary feedlines. $\endgroup$ – Phil Frost - W8II May 20 at 15:36
  • $\begingroup$ @PhilFrost-W8II I intentionally didn't discuss wavelength too much because it was irrelevant to the question he asked, but I added in a clarification anyway. Thank you for the feedback. $\endgroup$ – SandPiper May 20 at 23:50
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    $\begingroup$ Active antennas are often unidirectional - the active components don't like to go backwards. $\endgroup$ – user3486184 May 21 at 6:00
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    $\begingroup$ I think it still reads as if you are implying "resonant" means "1 wavelength long". But that's very much not true: resonant antennas are approximately half a wavelength (dipoles) or a quarter wavelength (monopoles). $\endgroup$ – Phil Frost - W8II May 22 at 14:52
  • $\begingroup$ @PhilFrost-W8II I reread it again, and you are right that is how it was sounding, and not what I intended to imply. I straight up removed that reference to wavelength and just replaced it with "resonant length". $\endgroup$ – SandPiper May 26 at 12:11
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Because of the general nature of your question and the likelihood that future questioners might find their way here, I wish to provide a general answer.

It is possible that a transmit antenna and a receive antenna may serve quite different purposes in a radio communication system.

A transmit antenna may have the primary purpose of producing the strongest possible signal at a distant receive antenna within constraints of cost, space, technology, etc. Conversely, the performance of a receiving system may be optimized by using an antenna whose primary purpose is to reject sources of unwanted noise and interfering signals.

This situation is commonly encountered on the longer-wavelength amateur bands, where an omnidirectional quarter-wave monopole could approach 40 meters in height with a radial system that occupies most of an acre of land. Such a monopole may stretch the financial- and real estate budget, making directional arrays of such antennas impossible. However, arrays of much smaller antennas can be used to increase the signal-to-noise ratio of a receiving system by rejecting atmospheric noise in undesired directions. The EWE, K9AY Loop, Shared Apex Loop Array and Waller Flag are examples.

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  • $\begingroup$ Don't forget about the Beverage. Although it is usually used for receiving MW and HF signals, I have successfully used it for transmitting. Even though it has ~10 dB loss, at over 800 miles I get better signal reports on it than on my dipole. $\endgroup$ – Mike Waters Jun 3 at 20:36
  • $\begingroup$ @MikeWaters Good point, Mike. How much real estate does a beverage occupy? $\endgroup$ – Brian K1LI Jun 4 at 0:24
  • $\begingroup$ Beverages are generally about one wavelength long at the lowest band (580' for 160m). And since they are directional, orientation matters. $\endgroup$ – Mike Waters Jun 4 at 14:12
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From what I know there are two main differences between transmit and receive antennas:

  1. Receive antenna may but doesn't have to be resonant (and thus effective) on any particular frequency. For instance, consider very wideband antennas, designed for monitoring, that may have SWR ~3 or greater. Or very small loop antennas designed for radio fox hunting. The small weight and directivity are more important for these antennas than effective transmission.
  2. Receive antennas may not be designed to transmit on high power.
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    $\begingroup$ Antennas don't need to be resonant to be effective either for receiving or transmitting. $\endgroup$ – rclocher3 May 22 at 13:49
  • $\begingroup$ @rclocher3 Exactly. All that has to be done is to tune out the reactance or match the impedance with an unun or balun of the correct Z ratio. $\endgroup$ – Mike Waters May 22 at 14:28
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There may be current carrying and legal requirements for transmit antennas (minimum 16 gauge wire size, insulators, separation distance from human bodies and heads, etc.) that do not apply to receive only antennas (invisible 30 gauge woven into your shirt , etc.)

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The keyword here is tolerance. For receiving antenna you don't really have to have a match or low SWR with the receiving system unless for a high Q antenna like a mag loop. Tho if you have a very great unmatch you won't receive well too. I have received 70cm on a 40m dipole, it probably was received on the coax but it did receive well.

Now if you try to transmit 70cm on a 40m dipole the mismatch well thru the roof, and almost all your power input will be reflected.

That is why there are antennas for receive only. Active antenna uses an amplifier for the received signal and you will damage it if you transmit on it. Some receive antenna just either don't have a good match for trasmitting or not build for power handleing or both.

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