Generally speaking you do not need to get too aggressive about creating a perfect conjugate match for designing an antenna for a wideband receiver.
From the point of view of the receiver, the antenna will only truly be non-reactive (purely resistive) at one frequency anyway and will stray off of that as you deviate from the center. Also, as the antenna is placed in various locations with different nearby metallic or otherwise conductive objects its impedance will change anyway. As it and the feedline ages the impedance may change. As gunk accumulates on the antenna itself the impedance may change. As the receiver's components age its input load impedance may change. All of this means, essentially, just try and hit that 50 Ohm resistive point with your desired center frequency and let it go at that. Especially if you're not going to do any transmitting it isn't worth getting too tangled up in nitty gritty conjugate match details.
The 50 ohm point is a common target to hit for communications equipment using unbalanced coaxial lines; it was decided on long ago as it was a decent balance between loss characteristics versus frequency and power handling (for transmit uses) and because it provided a good common design goal for equipment manufacturers. The television receiver industry centered on 75 ohms as that impedance for coaxial transmission lines yielded slightly better loss versus frequency characteristics than using 50 ohm lines and the need for power handling was not important (receive only or low level signals as in cable systems).
A receiver designer (like all designers) needs design goals to shoot for. One of those is the input impedance desired by the system. You look at what the expected feedline impedance is and usually go from there. As the other poster mentioned, the transistor characteristics are important when designing the input RF amp block of the receiver chain but that is the job of the designer - to build a front end which can effectively make use of an ideal 50 ohm resistive source impedance across the frequency range of the receiver. That designer is the one who has to wrestle with the conjugate matching across a certain frequency spread. Everything in engineering is a compromise. Things vary and change over time (and of course, fail). Antenna and feedline designers also use that 50 ohm resistive impedance as a design goal. Again, stuff varies and guarantees of exact nature are not expected, certainly not across a multi-octave frequency range!
That's why you see specs for antennas which say things like "50 ohms NOMINAL" for the feedpoint impedance. "Nominally" they are "50 ohms" which is to say, the antenna was designed for use in a 50 ohm system for best performance and will yield that approximate impedance at the center point of its frequency range with no significant reactive components; outside of that all bets are off. Performance wise, within the designed limits you won't experience enough degradation on the edges of the spec'ed frequency range to be an issue, especially for receive only use; don't worry about that reactive component that much.
Most "nominally 50 ohm" receivers won't hardly notice a problem with looking at a 75 ohm source and vice-versa. In fact, if you look at a wideband receiver's input with a calibrated network analyzer across the entire range of its coverage you probably won't see a nice resistive 50 ohm flat line (or "spread" on a Smith Chart, etc.)! About the only time you would see that in common practice is if the front end had a resistive attenuator on its input - that's actually one reason in test scenarios it is common to stick attenuators on inputs and outputs of things being tested since the test equipment is calibrated for 50 ohms! The main deal here is what was the receiver designed to best work with in terms of expected source impedance - usually 50 to 75 ohms. If you're in that range or close to it and you can keep the reactive component down from completely nuts across the range you are most interested in then you're essentially good to go. Don't sweat it further.
-Mike
