Why would you expect a credit card to be discoverable by radio? Tap-to-pay is near-field technology, it's only meant to work within a few centimeters. It might be possible to do, but only with the construction of specialized and expensive equipment. You are better off just phoning and getting a replacement.
I figured if the TX power was high enough, the return power would be higher. Although the card might have a limit to how much it'll return. I can make a helical antenna for high directional gain. It doesn't have to be 50 meters, but even something like 5-10 meters would be good. It's not just a card, but a whole wallet with lots of different stuff in it. Thought it'd be worth a try.
Edit: I was assuming the frequency would be high, like in GHz. Looks like I can't really make a helical antenna since it is HF around 13MHz (i think).
Sorry to hear you lost your whole wallet, I understand that's an enormous hassle. But this is one of those cases where a flashlight and a patient retracing of your movement is probably a better use of your time and effort. Best of luck!
It's not my wallet - my wife's. She can't retrace her steps. I will try to find it the old fashioned way, but it's likely in a field with about 8" grass. Currently it's covered in snow (she lost it before the snow but didn't tell me).
It may be doable, a budget setup would be two half-duplex SDRs (synced with the same clock source between them), paired with diametrically opposed end-fire radiation pattern antennas in which one transceiver acts as Tx sends step-frequency continuous waveforms into the field and Rx collects the reflected signals, baring the snow the surroundings should be homogenous enough to pick up the wallet hopefully!
Initially thought of a different approach than bouncing sfcw as well, but suggesting this because it has a properly documented success after a quick search: http://pe.org.pl/articles/2019/9/7.pdf
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[ 3.1 ms ] story [ 24.4 ms ] threadEdit: I was assuming the frequency would be high, like in GHz. Looks like I can't really make a helical antenna since it is HF around 13MHz (i think).
Best of luck :)