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  • Question
  • Updated 5 years ago
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I'm listening to VLF beacons and find plenty of them however there are hundreds of birdies from local BC stations totally dominating the spectrum. My inclination would be to enable ATTN. When I open the ANT button and slide RF Gain to the right, ATTN shows up at the top of the screen. I assume this means attenuation has been inserted however not only do the BC birdies vanish but so do the VLF beacons. The band becomes effectively DEAD. 
So what is the cure? Should I buy a third party filter and forget about ATTN? 
I am still not clear on what RF Gain does. Flex needs to label this control correctly in SSDR because it really doesn't make sense. 
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Steve N4LQ

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Posted 5 years ago

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Steve - N5AC, VP Engineering / CTO

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Official Response
The FLEX-6300 is one of the first direct sampling radios that allows you to listen to multiple bands at the same time with only a single physical "receiver" (we call it a signal capture unit or SCU).  In other words, you can place multiple receivers, called slice receivers, into a single physical receiver, called an SCU.  In the old paradigm radio where there is only a single physical receiver that can listen to one frequency, the decision you as an operator make regarding preamplification or attenuation applies only to that one receiver.  With the FLEX-6000 Signature Series radios, you could have one receiver listening on 6m and another on 160m all using the same SCU.  This capability is new to amateur radio and the old preamp/attn concept is not really the best anymore.

The atmospheric noise around the Earth increases as you decrease in frequency and so less noise figure is required lower in frequency.  As a result, you generally want gain and low noise figure at high frequencies (such as 6m) and you prefer attenuation at lower frequencies (like 160m).  Since you can now listen to both high and low frequencies at the same time, you are likely to want both at the same time.  The traditional concept of an attenuator and a preamp won't help you since you have to put one or the other on.  

As a result, we designed a hybrid that provides gain at high frequencies and attenuation at low frequencies with a crossover at about the 20m band.  For each panadapter in the FLEX-6300, the indicator will provide an idea for what is happening at your current band -- amplification or attenuation.  If you have both a 6m and a 160 panadapter open at the same time, you'll see that you get both gain and attenuation at the same time.  This hybrid circuit provides what you will need to take the most advantage of the FLEX-6300's capabilities.  It will provide significant attenuation in the VLF range and may be more than desired for some operations.  It is designed as a "95%" device -- the goal is to meet the operational needs of most hams.  For VLF, it may not be just what the doctor ordered and you may need to use an external device to get exactly the gain or attenuation you need.