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(6300) 6M preamp == 160m attenuator?

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N7CXI
N7CXI Member ✭✭
edited June 2019 in SmartSDR for Windows
I was working 160m tonight and decided to open a slice on 6m, to see if I could hear any auroral activity on my 160m antennal. 

I noticed that when I created the "B" slice and changed to 6m, my 160m signals dropped. I confirmed this by turning off the preamp in the 6m slice - the 160m signals in slice "A" came back up to normal strength.

I think I understand what it's doing, but is there a way around it? I'm going to guess not, since we are after all talking about one set of RX hardware here.

Answers

  • Al K0VM
    Al K0VM Member ✭✭✭
    edited June 2019
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    Jim,
      One thing that happens when you have only one slice open is that there is a band pass filer inline before the SCU. When you open a slice on a different band, that band pass filter is switched out and the SCU is operated in "WIDE" mode ( note the "WIDE" indicator in the upper right of the panadapter ).  A second thing that happens when you turn on a preamp is that the gain in WIDE mode is intentionally not flat across the spectrum.. There will be more gain at 6 m where it is likely needed and less gain at 160 where it is likely almost never needed and could lead to intermod on strong 160 m signals.  

    AL, K0VM
  • W5XZ - dan
    W5XZ - dan Member ✭✭
    edited December 2016
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    Al, I thought the 6300 lacks BPF's; is this wrong?  And, I bet FRS chose coupling caps for the preamp that roll off the gain at lower freq's, maybe?  73, w5xz, dan

  • KY6LA_Howard
    KY6LA_Howard Member ✭✭✭
    edited January 2017
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    The 6300 preamp low frequency attenuation was already discussed by FRS staff in another thread Search for it
  • N7CXI
    N7CXI Member ✭✭
    edited August 2018
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    No doubt, but searching for it using most combinations of those words didn't find it.

    The point of my original post is that operating 160m *and* monitoring 10 or 6m for weak signal openings in a different slice appears to be impossible because of design choice. There may indeed be more use cases where it does work in a beneficial way, this is one case where it doesn't.

    --jim


  • KY6LA_Howard
    KY6LA_Howard Member ✭✭✭
    edited January 2017
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    Here is one https://community.flexradio.com/flexradio/topics/when_do_i_need_to_use_the_flex_6000_rf_preamp
  • N7CXI
    N7CXI Member ✭✭
    edited August 2018
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    Thanks. I did see that one, but I'm not sure how it's relevant to my posting.
    I agree *most* HF work doesn't need a preamp with a decent (read 6000-series) receiver.
    10 and 6M weak-signal work do.

    That message doesn't address low-frequency attenuation either. I'd swear I saw that somewhere while
    digging through messages, but can't find it now. I don't suppose it matters - it works like it works.

    --jim



  • KY6LA_Howard
    KY6LA_Howard Member ✭✭✭
    edited January 2017
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    I can't find it either but it was a posting by Gerald (?) but it was official that the 6300 deliberately attenuated 160"when turned on preamp The search facility on the community sucks. You can only search titles and not content or authors
  • KY6LA_Howard
    KY6LA_Howard Member ✭✭✭
    edited January 2017
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    Here is another official reply https://community.flexradio.com/flexradio/topics/since-v1-3-i-am-now-see-attn-on-the-screen-just-like-pre
  • N7CXI
    N7CXI Member ✭✭
    edited August 2018
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    Got it, thanks for looking that up.

    I understand *why* it works that way, and the design trade-off. Whether it's a desirable feature or not I suppose is a matter of opinion and circumstance.  :-)

    --jim
  • KY6LA_Howard
    KY6LA_Howard Member ✭✭✭
    edited August 2018
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    Realistically. Unless ur living on another planet, all modern radios have too much gain at the lower ends of the spectrum The base level of atmospheric noise in even the quietest rural situation is -98 dBm on 160m The MDS of the radio is -120 dBm. So u actually need attenuation or your just amplifying noise.
  • N7CXI
    N7CXI Member ✭✭
    edited August 2016
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    Yes, in general. If you're using a dedicated 160m receiving antenna, not so much.
    Many 160m ops use things like beverages, small loops etc to improve the SNR. Most of those have a significant loss as compared to a Marconi, but a far better SNR. So subtract 20-30 dB for that and there you are - right at the edge of usable.

    I said I understand why it works that way. I was just pointing out that there are use cases where it isn't desirable. I think the 6300 is a great value, but after having owned one a few weeks I'm thinking I might want to upgrade to a 6700 once I see that the software adds a few necessary features - for *my* particular needs/wants. I don't claim that my use cases are typical or should be used to sway design decisions... ;-)

    --jim

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