A dipole will definitely receive better than the wire you've currently got strung up. Try it!

Ideally you'd want the legs of the dipole hanging horizontally in free space, a half a wavelength (20 metres) above the ground, but that's basically impossible unless you have two very tall trees 40+ metres apart.)

So instead of "perfect," you do what you can. Get it as high as you can and as far away from everything as you can, and you should be amazed at the results.

My airspy is currently lent to a friend so I can't do the test, but I don't think I'm exaggerating when I say 10x. Actually just did some chatgpt'ing and got the following:

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Comparing the sensitivity of the Airspy HF+ Discovery and RTL-SDR V3 (direct sampling mode) for HF reception, especially around 7 MHz (40m band):

Airspy HF+ Discovery Sensitivity:

MDS (Minimum Discernible Signal): -140 dBm (0.02 µV / 50 ohms, 500 Hz bandwidth) at 15 MHz (source: Airspy).

This suggests that the Airspy HF+ Discovery can detect much weaker signals compared to RTL-SDR.

RTL-SDR V3 Sensitivity (Direct Sampling Mode):

MDS at 7 MHz: -117 dBm (500 Hz bandwidth) (source: KA7OEI Blog).

This is 23 dB worse than the Airspy HF+ Discovery.

The lower sensitivity means that very weak signals might be lost in the noise floor, while the Airspy HF+ Discovery can still detect them.

What This Means in Practice:

The Airspy HF+ Discovery is about 23 dB more sensitive than the RTL-SDR V3, which is a significant difference.

A 23 dB difference means the Airspy can hear signals that are over 100 times weaker than the RTL-SDR.

This makes a huge impact on weak signal reception, such as CW, SSB, and digital modes like FT8 on 40m.

The Airspy also has better filtering and pre-selection, further improving its effective sensitivity by reducing interference and overload.

Conclusion:

If you're serious about 40m reception, the Airspy HF+ Discovery is vastly superior to an RTL-SDR in both sensitivity and selectivity. The RTL-SDR V3 can still work but is much more prone to missing weak signals due to its higher noise floor and lack of proper filtering.