VLF and ANT SETTING?

Steve N4LQ

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. 

SteveM

Steve,

Check the AM broadcast spectrum. If you see a station at 0dBm I think you'll need an external filter. Are you using a vertical antenna? They are prone to groundwaves from nearby towers. I bought a brickwall filter from Doug Williams (http://dlwc.com/). His are very high quality and worth every penny, but only handle ~200W if you need to transmit through the filter.

Andrew VK5CV

Steve,

Which radio, what antenna? I recently had a similar problem with a fractured solder joint in a coax connector causing intermodulation. The 6500 is a fantastic LF radio here.

Andrew.

 

SteveM

Sorry no one is addressing the ATTN issue. I'm not sure about the 6300, it has a different preselector than the 6500. I don't think the band should go dead - it doesn't on the 6500. Sounds like a helpdesk issue. Maybe Tim will chime in.

SteveM

Just checked the specs on the 6300, its preamp is 0-20dB. Sorry, no attenuation on it.

I just realized you're doing VLF work. I guess a brickwall filter would only hinder. Maybe a coax stub on the offending signal(s).

Stu Phillips - K6TU

If you are moving the RF Gain slider on the antenna panel to the right, you are ADDING gain and I believe the ATTN on the pan adaptor is telling you that the ADC is being overloaded and you need ATTN!

With my 6700, I don't have any RF overload issues even living in the SF Bay Area.  My hottest BC band station is -8 dBm and there are many more at -10 dBm.  So even with my 160m vertical, I don't have any VLF issues and don't normally have to add in any attenuation.

Which model of radio are you using and what is the antenna?
Stu K6TU

Ken - NM9P

IIRC...If he has a 6300, and is on 160 and below (or maybe even below 80?)  then the Preamp's response curve is actually an attenuator, even if it is raised to maximum.  But not so on the 6500/6700.

SteveM

Ken,

Where did you find out about this? I can't see it from the datasheet specs. Is there an extended version somewhere?

Ken - NM9P

I remember either reading it on a post here, or from a presentation at Dayton.  I don't remember for sure.  It had to do with the simplification of the preamp/attenuator control and the normal needs in most situations.  That is about all the details I remember at the moment.

Ken - NM9P

Check this link...
https://community.flexradio.com/flexradio/topics/since-v1-3-i-am-now-see-attn-on-the-screen-just-lik...

SteveM

Hmm. So the way I read that thread is, if Steve turns on his preamp by sliding the control to the right, then he will get attenuation in the low bands (say, 40m and lower), and amplification in the upper bands (say, 15m and up), but nothing is for sure until he checks the indicator in each of the bands. It's strange that the spec for the 6300 preamp is written as 0-20dB (see above). No wonder Steve is confused about the control.

SteveM

So, thanks to Ken I think we understand what's going on here. When the preamp is enabled, attenuation is turned on for the frequencies of interest. That attenuation, I assume, slopes downward toward 0Hz, and so your VLF signals are severely attenuated (i.e., gone). Since the 6300 is spec'ed from 30kHz - 54MHz, it's not an issue.

I guess you'll need to do your own customized attenuation for your VLF work.

Dave KD5FX

I agree with SteveM, my 6300 does the same thing, turn on the 'preamp' ALL the VLF signals are gone! 

Steve-N5AC

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.

James Whiteway

I had a similar issue with my 6300. I was tuning below the BC band listening to airport beacons, when all at once, the panafall display went nuts. The pan part turned into a constantly moving (right to left) sawtooth wave and the slice flag could not be moved. Freq numbers and the pan's background grid just kept scrolling right to left and the freq counting down. I got out of it by changing bands. Then all was fine. I figured I had hit the SCU's lower limit without realizing it and SmartSDR didn't have a preset in it to prevent going out of range like that and the ADC became unstable. Haven't tried that since. James

James Whiteway

Just tried again, and at first, when I selected Gen from the Band selection panel, it started doing the same as before. So, I switched to 160 meters and started tuning down from there. Eventually I got below the broadcast band and at first, the ATT indicator was showing on the screen. But, very low audio and signal levels. Then, I turned ATT off from the ANT selection, and the broadcast band came alive. Tuned below the BC band and started seeing, and hearing, the beacons. Using AirNav's website, I was able to ID several stations. A couple were several hundred miles from me and only running 25 watts! So, VLF DX'ing works, but, really iffy. I found one station only ID'ing as AM(not the three digit code normally sent) . I'm not so sure that one was a beacon.
Fun, fun!
james
 

Steve N4LQ

$195 for a BCB filter! Must be a good one then....