I now have some observation experience between the two detection systems.
On an HT with direct conversion, the sensitivity is very high on the unit I have at .1 uv according to specs for the unit. That' well up there with the best.
Observations:
It hears a lot of power line leakage, a lot of trash being from other sources, and systems as I drive around in mobile.
In my area it can sense high powers above the 2m band in business and mobile phone and paging systems.
Case in point; F =152.720. High power was overloading the HT.
To solve the issue, I built a trap for that frequency to the tune of more than -55dB down and solved the problem. The trap was tuned on a VNA then installed in series with the HT to antenna.
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The two take aways are the sensitivity and the single conversion.
Both are very good with the absence of interference but can be a problem in that presence.
It seems that more selectivity can be accomplished after the IF is converter to with more filtering but is a function of the radio application, design and what can be tolerated in operation.
I'm not condemning single conversion but understanding what disadvantages there may be in certain circumstances.
I use an HT and small 35 watt Linear amplifier as a mobile radio and love the operation otherwise.
Radio hears better than it gets out for mobile; the Tx audio is first class with a hand mic.
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In an HF radio, I would expect more filtering before and after the mixer and the best mixer design to eliminate harmonic mixing results my HT does not have in a small package and price point.
I was lucky to get away with the HT so easy.
The out of band interference is from stations that are allowed to run a lot of power if needed for their service.
Not good for 2m in some areas when you live close by.