Dip on the Blue Shift aside of 21cm Peak

[cgd]

Could anyone explain the significant dip be low baseline on the blue shift side of the peak? I am using SDR# with the IF Average plugin.  Background was taken on cold sky.  I am seeing this on all my data acquisitions. Thanks.

 

[Eduard Mol]

That’s probably hydrogen in your cold sky background. Because we’re inside the Milky Way galaxy, there is not really a place in the sky without any hydrogen, just areas with more or less. 

You could try shifting the frequency of your background spectrum by a few MHz (frequency switching). It won’t change the bandpass shape too much but atleast there won’t be that dip overlapping with the hydrogen line you’re trying to measure.

Op wo 26 nov 2025 om 22:11 schreef 'cgd' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

Could anyone explain the significant dip be low baseline on the blue shift side of the peak? I am using SDR# with the IF Average plugin.  Background was taken on cold sky.  I am seeing this on all my data acquisitions. Thanks.

 

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[cgd]

Thanks Eduard! I'll try your frequency shift suggestion for background calibration. I wouldn't have thought of that.

[Alex P]

Yes, that is due to having H line data in the background correction file.

Two other ideas :

1) point the antenna at the ground ( But   this may also capture RFI from nearby electronics,  and create narrow dips in the spectrum )

2) use a 50 ohm terminator on the input to the LNA during the background correction acquisition. ( 100% effective )

Alex

 

[Alex P]

More Info

https://github.com/AP-HLine-3D/HLine3D

Regards,

Alex Pettit

[b alex pettit jr]

[cgd]

Thanks Alex! I've got a 50ohm terminator that came as a cal standard with my NanoVNA.

[b alex pettit jr]

Yup, that will work .. for a while until you loose them in the grass   . 

FYI, in case you are not aware, SMA connectors come in two flavors, 

Normal 

and 

RP = Reverse Polarity =  these don't  work with normal connectors,  but are difficult to tell apart .

[b alex pettit jr]

Let me rephrase that :

Normal and RP can be sometimes be accidentally intermixed, but that creates a discontinuity in the signal path   :)

[cgd]

Just realized I can't access the LNA input. LNA's and filters are internal in a non accessible housing on my feed assembly. Best I can do is to terminate my SDR.

[cgd]

Also Eduard's suggestion seems to have worked. No dip in my baseline on my latest acquisition.

[b alex pettit jr]

If you have the most recent version of IF_avg, you can save the Baseline Correction File for repeated use.

https://github.com/AP-HLine-3D/HLine3D/blob/main/21cm_Step2A_Software_SDR%23_Setup_01_13Jul25.pdf

HLine3D/21cm_Step2A_Software_SDR#_Setup_01_13Jul25.pdf at main · AP-HLin... Beginners Low Cost 21cm Radio Telescope Project. Contribute to AP-HLine-3D/HLine3D development by creating an ac...

[cgd]

Thanks for links Alex!! I just downloaded latest IF_Average.

[Eduard Mol]

Note that with a 50 ohm endpoint you will introduce noise at the level of the ambient temperature- which for a good system will be much more than from tge LNA and spillover (typically 100K). So if you’re using a 50ohm endpoint to capture darks you’re effectively degrading your signal to noise ratio. For strong HI signals towards the Galactic plane this may not be noticeable, but for anything weak I would avoid this method. 

Op do 27 nov 2025 om 02:34 schreef 'cgd' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

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[cgd]

Eduard, thanks for the heads up on the downside to using a 50  ohm termination for background calibration.  I used your frequency shift suggestion for background calibration and it worked very well!

[b alex pettit jr]

Hi All,

There are two correction processes being referenced

=================================================================

1)  Electronics Gain Linearization 

=============================

IF_avg Background Correction is  typically used to normalize the 'M' Gain Curve filter shape,

and  is a Spectral Gain Correction technique.  Its result is Divided into the Data.

Using the 50 ohm load works as does using a conductive foam cover.

    ( you just need a  'uniform'  temp noise source )

==========================================================================

2) "Dark Frame" Sky Normalization

==============================

The Dark Frames are a measure of the  Sky/Environment Background Noise Level and are Subtracted from the Data.

=========================================================================

Each may be applicable..

Regards,

Alex P

==========================================================================

On Thursday, November 27, 2025 at 01:33:03 AM EST, Eduard Mol <eddiem...@gmail.com> wrote:

Note that with a 50 ohm endpoint you will introduce noise at the level of the ambient temperature- 

which for a good system will be much more than from the LNA and spillover (typically 100K). 

[b alex pettit jr]

And, for Galactic HLine data, whatever technique creates results that look like this  is the Correct Technique :)

[b alex pettit jr]

For single frame plotting, try the excel file attached 

AND, for scan sets, try HLine3D  

Alex