Its hard to know what they mean, but you DO know FOR SURE that noise figure of a receiver used for HF communication does not matter much due to the high level of man-made and atmospheric noise. Unless the noise figure is greater than about 20dB below 30MHz you won't even hear receiver noise compared to signals because external noise is much higher than that. (Unless you can do noise canceling.)
Of all receiver performance factors noise figure does have about the greatest wiggle room at HF. However, that does not mean it is not important.
Man-made and atmospheric noise varies a great deal dependant on location. Some areas are more quiet than others. They both vary with time, at times a specific area might be 20 dB (or more) more quiet than at other times. The difference between Galactic noise and man-made noise is very small when at a quiet site, on the order of a couple of dB. And Galactic noise falls off sharply between 3 and 4 MHz. So that at a quiet site (when atmospheric noise is low) the man-made noise is dominant from around 4 MHz down, and Galactic noise is dominant from 4 MHz up. At a noisy site, say near a business district, man made is dominant throughout the entire HF spectrum, well into the VHF and UHF, with Galactic noise never being a factor. And when atmospheric noise is high both average man-made and Galactic noise is undetectable.
With that in mind you have to plan receivers for the quiet site, low atmospheric noise, scenario. If you don’t do so the user in a noisy site is blissfully unaware, while the user in a quiet site is ticked off.
A quiet rural area might have “natural” noise levels (combination of all noise sources external to the radio and antenna) around –15 to –21 dBuV/m (converted to 9 kHz BW) from 3 MHz and up.
Now, if we convert all of this to bandwidths used for communications, vs BC, something like 3 kHz and we consider the performance limitations while attempting to meet a 10dB SINAD, we end up needing a noise floor of around –126 dBm to ensure we are always externally limited, or -161 dB/Hz. Naturally, this will be more important towards 30 MHz than at 5 MHz. Of course, this could always be argued up a bit based on the site variations, maybe pushing up to –158 dB/Hz, or –123 dBm noise floor. But my measured antenna port natural noise floor (3 kHz BW) here at 7 MHz is –125 dBm, right this minute, at times it is better, at times worse. I want a receiver that takes advantage of the best conditions.
This means a noise figure of about 13 dB would be required at a really quiet rural location. The acceptable noise figure will, naturally, go up as you get into more noisy locations. Many users would never have a problem with a 20 dB noise figure, others definitely would.
[This is why whip antennas aren't much worse than long wires at HF.]
Aren’t much worse in what way? While a short built in whip does receive less natural noise it also receives less desired signal. I have, in many, many instances seen a signal go from unreadable on the whip to 20 over S9 on a long wire. Same radio, same location, same noise limit, just hooked to the long wire or not.
That is typically one of the first things I like to show new users with portables who might be getting exasperated. I have them bring the radio over to the house and we hook it to an antenna, showing them it is not necessarily their radio to blame for their frustration.
Of course, this can often show other limitations in the portable, such as dynamic range issues, filter issues, and front end overload.
Another problem with comparisons like the youtube ones is you never get exactly the same propagation when you compare two radios tuned to the same signal. The noise is different, the fading is different, etc.
Yes, for a comparison to be really valid it needs to be done on the same antenna at the same time, preferably with a proper splitter network. Even if that is not the way the radio is going to be used it is a better comparison of performance limitations. Of course, for portables it is also useful to compare them on their built in antennas, particularly if that is the way they are going to be used. It might be possible to have a well designed radio that is superior on an external antenna than a less well designed one, but if you are never going to use an external antenna and the better design is deaf on it’s whip then that will not do you much good.
Yep, way many variables. And that does make it interesting.
T!
But yes, the agc on the 750 is the major issue. To make matters worse, when I did some research on it, all you can come up with is a block-diagram, and the rest of the circuitry seems "proprietary" I believe. So no chance of looking at the schematic to see if it could be properly modified.
