6400 sensitivity discussion

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I've always felt the 6400 reciever was lacking when it came to RX sensitivity, particularly when it comes to pulling out weak signals. Tests I've done with signal generators and the recent Sherwood report confirms this. It's the worst on the list!Using a preamp at +16 on bands above 40m is a must if you're trying to hear a weak signal. The method listed in the article below is irrelevant when it comes to the 6400. https://helpdesk.flexradio.com/hc/en-...
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A.J. AJ2I

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Posted 2 weeks ago

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Paul Leach

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This is disappointing news for 6400 owners. I am currently away from home ( cruise, celebrity edge - in case any fellow hams/flexors on board)
I will check this out later today and read sherwood report..

73’s
Paul
Gw4amz

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Stephen Hawkins NG0G

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What "recent Sherwood report" are you referring to?  My 6400M seems to have no receive sensitivity problems.
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A.J. AJ2I

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6400 non m
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Robert Lonn

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My Flex 6600M is better in many comparisons to my ICOM IC-R8600... Something does not seem right. You should open a ticket with Flex... And that is with the PREAMP set to off or first setting...

Robert
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Phil G4UDU

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Just done a quick test GB3RAL 28.215 MHz is about 75 miles from me on ground wave I can just hear it above the noise - 6400 with preamps on CW mode 50Hz bandwidth with APF - compared to a KX3 set up the same way, same BW  CW etc.   if anything the Flex has just got the edge on the KX3.
The beacon CW ident is almost "subliminal hearing" but it is certainly better on an A/B test than the KX3

I have no complaints.

Phil G4UDU

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A.J. AJ2I

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This is regarding a 6400. Not a 6600,6700 or 6400m. A plain Jane 6400. The numbers are different for each radio. -112dBm on the noise floor. Every other radio on that list is around -124dBm with no preamp. My eagle with no preamp beats the 6400 on weak signal rx. An Icom 706 is -135! And that radio is 20 years old. Flex really dropped the ball on this.
(Edited)
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Phil G4UDU

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You can not compare radios by the manufactures description of pre amp on, off or whatever - they all have different gains and noise figures.  You also have to be very careful over what IF bandwidths you are using and again measure it for proof that what is stated for a filter is actually what you get on the receiver - they can all vary.

THE ONLY way to check it is on a calibrated signal generator and measure the MDS and S/N ratio, then you can compare the readings, when I get a chance I will put my 6400 on the test set and measure it.

For reference I have a K3 a KX3 and an Icom 7100 to compare it with.


Phil G4UDU
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A.J. AJ2I

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I've used a signal generator and so has Sherwood. Conclusion to achieve equivalent RX of MDS vs my eagle is you need to go +16 on the 6400. Eagle does same with no preamp. -112 6400, -124 eagle. The numbers dont lie
(Edited)
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Bill -VA3WTB

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The 6400 keeps up with the very best receivers made. I would start a help desk if you feel that the receiver is not performing well for you. Perhaps Mr Sherwood tested a brocken Flex, he has done that before then published the results.
(Edited)
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KF4HR

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Some people rely way too heavily on Rob's receiver chart. 
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Craig Williams

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My 6400 hears fine. Compared to my other radios. Tested with my Motorola communications test set. Hope you find your problem.
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A.J. AJ2I

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-112 is about what I got also. Bottom line is, you need +16 preamp to achieve the same RX compared to most other radios. That was the point of this post originally. To spread the word that you cant use the above method for determining if you need preamp. You need it period or you wont hear weak signals compared to other radios that can BECAUSE -112dBM is -112dBM is -112dBM. Which means deaf to.weak signals. Find a weak signal on 17 meters or 20 meters etc. Turn off preamp , then do same with a superhet radio and do A/B I bet the superhet will hear it and the flex wont
(Edited)
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A.J. AJ2I

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Thanks Joe. My flex shipped 5/2018. Which was supposedly after the updates were rolled out so it should be fine. I dont think there is anything wrong with the radio and its operating within spec, unfortunately just a poor spec. I think the 6400 is the black sheep of the 6000 series as it seems every other 6000 series is fine.
(Edited)
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Bill -VA3WTB

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Again, never mind the specs. A direct sampling radio receiver will out perform any superhet receiver. The 6400 is not a black sheep in the Flex radio line up and have very good receivers.

Any difference in specs you read can not be heard with the human ear, they are only lab test.

Most comments I have read is that people are very happy with the receiver. Some here have just said they find the 6400 to be just fine to them.

I will repeat. you should start a help desk. something is not right.
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K5ROX

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a general statement like this simply isn't true.
A direct sampling radio receiver will out perform any superhet receiver.
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KF4HR

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"A direct sampling radio receiver will out perform any superhet receiver."

Check the specs on the ICOM IC-7851.
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mikeatthebeach .

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Yes, the IC-7851 is great but check the price !!!!!
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A.J. AJ2I

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I'm not spending $150 in shipping costs to have flex tell me there is nothing wrong with my radio and its operating within spec. I will make a video with a signal generator, proving the flex hears within the published spec. I will then make a video showing the eagle hears signals the flex cannot unless you turn on the preamp to +16 which correlates to the published spec.
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Ken - NM9P, Elmer

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Are you talking about the S-Meter level of the SIGNAL or of the background noise?  or both?
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Bill -VA3WTB

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Ken, 6m is the only band were the ant gain effects both the noise floor and the S meter 
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Ed Stallman

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50.092 CW , filter 100Hz 
Gain @ -8 
S meter = 1.5
DBM-115

50.092 CW , filter 100Hz 
Gain @ 0 
S meter = 1/2 to 3/4
DBM-123

50.092 CW , filter 100Hz 
Gain @ +8
S meter = 1/4
DBM-130

50.092 CW , filter 100Hz 
Gain @ +16
S meter = 0
DBM-135

50.092 CW , filter 100Hz 
Gain @ +24
S meter = 0
DBM-137

Going to +32 , maybe DBM went to --138 , so with preamp at +24 on 6m you scratch 2 coat hangers together and I'll hear you 
Ed N5DG
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Ken - NM9P, Elmer

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On my 6500, I always use the max preamp (+20 dB) for best noise factor on 6 meters.  It is standard operating procedure for any of these rigs.  

As I understand it, the standard test for whether you need the preamp (and how much you need) is to switch between your Dummy load and your antenna.  If the switch to antenna makes the noise floor rise by 8 dB or more, then your receiver noise figure (which is improved by the preamp gain) is low enough so that the signal from the antenna is overcoming the internal noise generated in the receiver.  If not, you need to add preamp.  

On lower bands the preamp is not necessary and their use only reduces the dynamic range by roughly the same amount as the excessive preamp that is being used.  

Needing to use 24 or 32 dB of preamp gain on 6 meters is not a design flaw, it is part of the system gain distribution.

Another crude way I check it, is as I move the preamp from 0 to 10 to 20 dB with the antenna connected, if the noise floor drops appreciably, I needed the gain.  If it doesn't drop more than 5-8 dB, then I may not need the extra notch.

s-meter variations is something I will need to examine.....

Ken - NM9P
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Ed Stallman

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Ken tnx , very good explanation  . I love playing with SNR ! I also play with SNR on 160 and 80m using and 8 circle array BSEF of short active verticals . with this setup , I get to play with preamps at the array and attenuators at the RXin  of the Flex 6600 and I normally add +8 to 16 db of gain using the Flex menu . With  this setup my S-Meter is just bouncing off of zero making an S1 signal easy copy .
I like to say 160 and 6m is the Chicken and all bands in between is the gravy 
Ed N5DG 
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HCampbell WB4IVF

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The particular preamp setting that results in the best receiver noise floor is immaterial.  The available settings are a design decision (such as compensating for bandpass filter loss), and varies by radio.  It’s a trade-off.

BTW, high sensitivity affects dynamic range.  By your reasoning the Collins 75-S3B (on the Sherwood chart) beats them all.  Great rig in its time I’m sure, but looks at its dynamic range!  You need to look at the stats in context. 

But as was mentioned, if you actually suspect low sensitivity, by all means submit a trouble ticket.

Howard


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A.J. AJ2I

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Exactly howard. The best way to get dynamic range is to make a deaf reciever. Different tools for the job. The reason I have more then 1 radio in the shack. Some are better at things then others. But the people that say a flex 6400 will hear as good as a ftdx5000 etc are just plain wrong. The proof is in the testing and the numbers.
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mikeatthebeach .

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Dynamic Range is great with the IC-7851
but you have to pay more for it than a SDR
Rig
73 Mike
(Edited)
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HCampbell WB4IVF

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Deaf receiver?? 141 (ftdx5000) vs. 136 dBm?  That’s down in the noise.  Sorry, couldn’t resist!  (-: 

Howard

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David Okay Patton

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With the preamps engaged, the radio has good sensitivity; if the band noise is so high that the front end gain causes problems, then the full 16dB gain isn't needed. For a location with very low ambient noise, and under ideal band conditions, the 6400, even with +16dB of preamp, may not have sufficient RF gain.  But for most of us, under most conditions, the receiver sensitivity isn't the limiting factor on what we can dig out of the noise.  

A.J.'s initial comment is spot on: if you own a 6400, you likely want to operate the radio with the preamp at +16dB, and only drop the preamp gain under very noisy or large signal conditions.  Otherwise, that really quiet band may have some weak signals you'll never know are there.  

I'm on the fence whether to call this a design flaw, but it is important that 6400 owners know that the optimum preamp settings are different for the 6400 than for most rigs.   With the right preamp settings, this radio is a delight!
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A.J. AJ2I

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Thanks for the confirmation David that I'm not insane .
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David Okay Patton

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Which Tim from Flex confirmed earlier in this thread: "You have to use the preamp.  There is insertion loss from the preselectors and the RF preamp is needed to overcome the insertion loss."

BTW, I'm not saying your not insane, just that you're not wrong!  :^)
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Michael Wheatley

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Why and how is the 6400 different from the 6400M? I hought they were the same radios except for the front panel on the M.
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Rich McCabe

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Yea I was about to ask the same. The front panel (M) should just be a client to the rest of radio.
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A.J. AJ2I

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Then the M is just as deaf without a preamp
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Art C

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Well now that we’re on the subject of Test Lab Numbers.. Yesterday the results were posted and the King Flex 6700 was finally beat by the New kid on the block. The Yaesu FT-101D. Flex 6800 ??
(Edited)
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Bill -VA3WTB

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Amazing considering the 6700 is around 8 years old.
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Greg SP7QJF

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Namely, 6400 and 6600 are worse than 6700 from before 8 years.
Flex goes backwards instead of growing
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Bob Young

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I guess the best solution for A.J. is to vote with his feet and sell his Flex. 
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Bill -VA3WTB

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I would not blame him, who wants a deaf receiver as he says. First time I ever heard that complaint about the radio.
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David Okay Patton

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To say the 6400 is deaf is a real overstatement; however, the preamp setting are much different than for any other HF rig that I've used.  As Tim (Flexradio Customer Experience Manager) pointed out, the preamp is needed because of insertion loss in the preselectors.   

If you have a 6400, you will most likely realize the best receiver performance using 16bB preamp gain as your default setting, and adjust as needed due to band conditions.

I won't argue with anyone that this is an unusual way to configure gain stages. Information on correctly setting the preamp level should be emphasized in the 6400 manual, so that owners don't draw the incorrect conclusion that the 6400 has a poor receiver, when in fact it is actually quite good with correct preamp settings.
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Robert Lonn

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OK, the Yaesu beats out even my IC-R8600 radio, but my 6600M Flex is just better then my IC-R8600 for raw sensitivity and digging out those weak stations.. OK, so a Camaro all specked out is 1 MPH faster then a BMW M6, but we all know that most would prefer the BMW over the Chevy for so many reasons... Think what one would give up, to jump away from FLEX and over to the 101D!!! The list is very long on give up's... Yaesu did some clever engineering building more of a HYBRID radio then a true 100% SDR box... The July release of the 101MD  200 Watt version will have many options as standard features and said to be $6000 !!!  Raw specifications is good to know, and I am glad Bob creates this list... 

As a Vintage Audio File person, I have the TOP 10 Best all Time FM radio tuners.. This is right off the  master FM radio evaluation.. Took me almost 9 years to get all 10, but have about 25 in my collection.... Funny that most of my all time favorites are NOT in the top 10... Not that they dont perform as good as the top 10, but the Looks Of The Tuners and feature set, is what sets them apart...My Favorite is my rare Grundig T-9000 receiver...

Basically I am saying their is So Much More then just the raw specifications of a radio.. But hey,, I am not telling you something you already dont know!!! :-)

Here is a link in case you never visited this site.. These top tuners do sound SO GOOD!!!! Makes digital audio sound DULL!!!

http://www.fmtunerinfo.com/shootouts.html

Robert

Grundig T-9000 ,, Second best for me, Pioneer F-93 with Space Diversity FM reception..




(Edited)
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Doug

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I think his receiver is a snowflake in that it is unique and 1 of a kind ! What I find amazing is I actually have to throttle my 6400M back with the AQCT on all bands through 20M. But I have found out you can't operate a Flex like a standard ham receiver. I really feel sorry for him because he is so unhappy and I don't see anything that will change short of selling  He needs to sell his 6400 and move on. Too many great radio's anymore and unlike most if not all other brands a used Flex will command most of it's original purchase price.  I mean this in all sincerely I wish him the best.
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N5LB - Lionel B

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I did some testing.

28.005Mhz   Slice A on 10m; Slice B on 75m. CW mode 400hz BW  Ant 1 either grounded or connected to 40m dipole E-W at 50 ft. WBN, NB, NR off.  SWR seen by Flex ~ 1.7:1 thru tuner.  Front end filters in Wide mode.  No signal within ~25 khz.

Changing Slice A only;  Preamp is common by design to Slice A and B.

Preamp: 0 dbm
Grounded: -107dbm
Connected -108 dbm ~ 107 dbm

Preamp: +8
Grounded: -116dbm
Connected: -116dbm

Preamp: +16
Grounded: -124dbm
Connected: -121dbm

Preamp: +24
Grounded: -132 dbm
Connected: -124 dbm

Preamp: +32
Grounded: -140 dbm
Connected: -124 dbm

In my location external band noise seems to limit at -124 dbm.  This is a similar situation as frequency is lowered.  Above +24 db on 10m on this day and time, additional gain doesn't seem to be useful. 

The 6400 is my only Flex so for 6600, 6700, I don't know what they would show, though the same setup and documented results would be interesting. 

I ran the same test on my prior Flex 5000 and 6300 but, of course the data was misplaced by the IT staff. 
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A.J. AJ2I

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Thanks for the additional testing lionel. I think I'm going to keep my 6400 and just use +16 all the time. Thanks to all the people who have done additional testing and help in documenting this limitation and workaround confirmation. I'm not unhappy, just disappointed. I'm on the other end of the spectrum. I'm not a contestor, I'd rather have a more sensitive radio then a top of chart dynamic range radio. When the guy in Wallis and Futuna is 4/3 with heavy qsb on 15m, I need that extra sensitivity and lower noise floor. I dont need to worry about someone 2 kcs away. When I bought the radio I expected great performance, not just ok performance. Different tools for different purposes
(Edited)
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N5LB - Lionel B

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I'd like to see the numbers from a 6600 or 6700 using the same test setup.

Using a 40m dipole on 15, especially with the EW orientation, the band is generally dead at all preamp settings, a situation that I'm correcting with a NS dipole, I hope. 

When I get to 80-40-30 a low noise rx antenna might help; my local suburban/rural noise level is above the ITU curves.

For my use the ability to hear weak stations generally seems adequate but I know it depends on the particular activity and the entire station system.  That said, expectations matter, and I can see how it is letting you down in a quiet environment. When I chase spots I do get frustrated at times when others are working the DX and I cannot hear them at all; more a function of the antenna - my working theory anyway. 

Enjoy 15, I'll have to wait for higher SSN's. I checked 10m this morning and the band noise is -118 here and could be from many sources, more or less uniform across the spectrum.  Solar I guess.