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An outside source is broadcasting an RF signal at 406 MHZ. I want to know if it's possible to receive this signal with one antenna, and then simply send the signal to another antenna that sends it right back out. I want this to be a simple circuit where the receiving antenna only recieves and the transmitting antenna only transmits. The transmitting antenna can have a slightly different frequency but it would need to be as close to 406 MHZ as possible. The intention is to reflect a signal right back at it's source, in as simple of way as possible. Will this work?

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  • $\begingroup$ Are you considering a completely passive setup? If so, then the returned signal cannot be at a different carrier frequency. $\endgroup$ – Juancho Oct 4 '18 at 14:57
  • $\begingroup$ Yes, I want it completely passive. I would like it to be the exact same frequency. Similar to a corner reflector, but using antennas instead of shape to reflect the signal. $\endgroup$ – Brad Oct 4 '18 at 15:01
  • $\begingroup$ Welcome to Ham StackExchange, Brad, and thanks for the additional details. Please, explain your remark, "using antennas instead of shape to reflect the signal." Since all antennas have a "shape" and that shape influences their behavior, perhaps you have in mind some constraints. $\endgroup$ – Brian K1LI Oct 4 '18 at 15:04
  • $\begingroup$ Yes, I realize the antennas would essentially have a shape. But let's say I want to stand outside and hold a device to reflect an RF signal back at it's source. If I used an actual corner reflector, it would be a large physical object at that frequency. It would reflect the exact same signal but I would like to do it with something that is more like a circuit board. As small and simple as possible. $\endgroup$ – Brad Oct 4 '18 at 15:08
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Yes, it's possible. Simply connecting two antennas with a feedline will do this.

If you want to reflect the signal right back at the source, you can more simply just omit the second antenna, and terminate the feedpoint of the first antenna in a short or leave it open. Since the short or open can't accept any power, any power received by the antenna must eventually be re-radiated by it, or lost in the resistance of the antenna, etc. Chaff works on this principle.

I can't imagine any practical application for this, however. Sometimes people construct passive reflectors this way to get a line-of-sight signal around an obstruction. But reflecting the signal directly back at the source seems rather pointless, since the field strength at the source is already strong. The minuscule power reflected back at the source is insignificant compared with the power already at the transmitter. It's like trying to burn out the sun with a mirror.

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    $\begingroup$ Thank you for the informative answer. I've designed circuits but I've never worked with radio waves. I understand the basics and it seemed like a possible solution, but I figured I should ask the experts before I wasted too much time on it. The goal is basically a cross between RFID and radar. You have a signal being transmitted in a short pulse and then it's going to listen for a return signal. I want a circuit about the size of an RFID tag to pick up that signal and send it back. I don't want to send information. Just detect if the circuit is in range. $\endgroup$ – Brad Oct 4 '18 at 17:56
  • $\begingroup$ @Brad you might look at how chaff works, as it performs a similar function, albeit with a different intent. $\endgroup$ – Phil Frost - W8II Oct 5 '18 at 9:05
  • $\begingroup$ Passive reflectors were actually used in real situations, but rarely and only by "big guns". Looses are huge and you have to use very high power transmitters and high gain antennas to get decent or better said usable results. $\endgroup$ – Pedja YT9TP Oct 10 '18 at 22:39
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It would help if we knew the end result you're trying to achieve. As the one commenter said, the power reflected back to the source will be much lower than the radiated power produced by the source.

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