Are transmit antennas the same as receive ones? I am making a extended Wi-Fi comms CCTV camera.
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.
From what I know there are two main differences between transmit and receive antennas:
- 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.
- Receive antennas may not be designed to transmit on high power.
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.
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.
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.