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Please look at "https://en.wikipedia.org/wiki/Radio_noise"
. It is a simple discussion. Then explain to me why you insist on NOT using an attenuator at MW as a matter of course? If you had a SMALL antenna like one of those telescoping whips the low noise figure of the HF+ would be really good for you. But with an antenna such as you describe in an environment with a "typical" level of man made noise a 30 dB attenuator would do you no ill and probably a 40 dB attenuator would do you no ill. Look at Wikipiddle er Wikipedia for a perhaps over-simplified discussion of noise figure to see how it works.
Now, knowing the input impedance of the HF+ input may help you optimize a filter to reduce signals in the MW broadcast band when you are working elsewhere. But that would probably be for one tuning setting. The input impedance may vary with the frequency to which HF+ is tuned.
My purpose in trying to drive this home is to help you optimize your time and efforts to best effect. I am trying to boost your game not deride you. Too many years in a very male oriented occupation has left me a tad abrasive. Please forgive.
On 20191017 10:23:46, Wes Stewart via Groups.Io wrote:
On Thu, Oct 17, 2019 at 07:45 AM, prog wrote:
Let's be more pragmatic. What is the real world performance using an antenna?
Thank you for asking. Let me preface the following by explaining the environment. The antenna is a 55 foot vertical over ground radials, For the results that follow, the antenna is connected to the Airspy through a Telonic Industries Model TG-950 stepped attenuator. Although not a traceable device I have verified its performance with my DG8SAQ VNWA. I have several plots, all taken during the same session, which are screen grabs of the SDR# program. The model, serial number, firmware and software versions are all obvious. This unit is as received a few days ago.
I'm not sure I can intersperse comments between the attachments so I will summarize them here. The image "Airspy_1_40db" is of a local MW station with the inline attenuator set to 40dB. The image "Airspy_2_20db" is the same as before except the attenuation has been reduced to 20 dB. It was pointless to go to 0 dB. Images "Airspy_3_40dB" and "Airspy_4_20db" are similar but of a different station.
The next set, "Airspy_5_20db", "Airspy_6_10db" and "Airspy_7_0db" show station WWV being received on 10 MHz with 20, 10 and 0 dB attenuation respectively. Note that with 0 dB attenuation, the Airspy is essentially useless. The next set, "Airspy_8_10db" and "Airspy_9_0db" show station WWV being received on 15 MHz with 10 and 0 dB attenuation respectively. Same situation. Now you might understand my interest in knowing the performance of the input filtering because obviously it isn't up to the task.
But it gets worse. Image "Airspy_10_Spurious_of_88_1_0db" shows an FM broadcast signal, fully readable so I could identify it, that doesn't exist. Image "Airspy_11_Spurious_of_88_1_20db" shows the same spectrum with 20 dB of attenuation inline. The non-existent station disappears.