Very weak singnal from NOAA with QFH anthenna

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estario
Posts: 8
Joined: Sun Nov 25, 2018 5:43 am

Very weak singnal from NOAA with QFH anthenna

Post by estario » Sun Nov 25, 2018 6:04 am

Hi,
I have been trying for some time to get images from the NOAA sattelites using this: My setup is as following:
  • A Laptop with Obritron , SDR sharp and WHToimg.
  • A USB SDR - R820T2 based
  • An FM Band Filter
  • A QFH antenna with 3d printed parts.
For the antenna I used several online guides, but mainly the one from the link above. I used the calculator at jcoppen's website.
The wiring was a bit tricky to understand, but what I basically did was the setup shown here:
Image
Yesterday I went to my village outside of town to test my setup once again. I wanted to be somewhere with a clear view of the sky and as few signals as possible. Yet again I got just a bit of signal only when the satellite was passing exactly above my head.
Image
Image
This is my antenna.
Image
Image
Image
What am I doing wrong?

rtlsdrblog
Site Admin
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Joined: Mon Nov 19, 2012 11:54 pm

Re: Very weak singnal from NOAA with QFH anthenna

Post by rtlsdrblog » Sun Nov 25, 2018 10:47 am

Make sure that those stray shield copper fibres aren't shorting anything out.

Do you get decent reception of broadcast FM with the antenna? Checking that should indicate if there is a major problem, or just a tuning issue. Check for continuity on your connectors to antenna too.

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Evap1
Posts: 26
Joined: Sun Nov 25, 2018 2:54 pm

Re: Very weak singnal from NOAA with QFH anthenna

Post by Evap1 » Sun Nov 25, 2018 3:08 pm

Hi Estario,
The outer connections are not being made.
This antenna uses the braid as the receiving element.

Please look at the JCoppen pictures again.

The Inners & Outers should be shorted.
The signal flows on the Outer of the Coax.
The way you have it connected you are shielding the incoming satellite signal.
Please rewire and try again.
Evap1

estario
Posts: 8
Joined: Sun Nov 25, 2018 5:43 am

Re: Very weak singnal from NOAA with QFH anthenna

Post by estario » Sun Nov 25, 2018 9:33 pm

I was wondering about the shielding effect of the braid, but I thought I am not able to understand the mechanism. Now it all comes into place. So I should connect the braid of the long and short loop on one and to the braid and to the middle of the coax that goes into the dongle.
I will re-solder and retry in the next few days.
FM reception without the filter is perfect, but those signals are quite strong.

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Evap1
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Re: Very weak singnal from NOAA with QFH anthenna

Post by Evap1 » Sun Nov 25, 2018 10:54 pm

Hi again Estario,
Just think of your Coax being a solid element. Signal flowing on the outside.
You short the inner to the outer at each end of the loop then follow your Feed-line to
Loop diagram as you have it above.

You don't really need to connect the Loop inners but it makes the connections more stable and rigid.
Make sure you have no short or open circuits on your feed-line connections Antenna end + Plug end.
If you look at JCoppen's pictures it tells you to remove the dielectric from each end of your Loop
Coax &solder the inner and outer together.
Good Luck.
Nice 3D printing by the way!!
Regards,
Evap1

estario
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Joined: Sun Nov 25, 2018 5:43 am

Re: Very weak singnal from NOAA with QFH anthenna

Post by estario » Mon Nov 26, 2018 7:56 am

Hi,
Why am I using Coax if I only need the braid to receive the signal? My coax is with very thin braid and I don't think it is the best option. Is it not better if I use copper or aluminum rods or wire straight away? Which one is better from the 2 metals?
Should I use RG-58 cable instead of normal coax?
There are many free models on Thingiverse. Life nowadays is easy, if you have a 3d printer.

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Evap1
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Re: Very weak singnal from NOAA with QFH anthenna

Post by Evap1 » Mon Nov 26, 2018 11:25 am

Hi Estario,
Convenience is the word.
Coax is in plentiful supply, easy to work with and resistant to the elements,
the outer sheath is proven against UV rays water ingress etc.
If you did use bare copper you would have protect it.
Aluminium is not straightforward connection-wise to your Coax feed-line.

Satellite Coax cable with copper shield and copper braid is quite good for this project.
Also use it for the feed-line as it is low-loss up to 1 GHz.
Look for CT100 or equivalent in your area.
Evap1

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Evap1
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Re: Very weak singnal from NOAA with QFH anthenna

Post by Evap1 » Tue Nov 27, 2018 1:42 am

Hi Estario,
Found a Picture for you.
You should be getting a signal similar or better than this when your setup is correct.
Evap1
noaa-18e.jpg
noaa-18e.jpg (480.3 KiB) Viewed 3636 times

Aussie Susan
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Joined: Sun Jul 31, 2016 1:55 am

Re: Very weak singnal from NOAA with QFH anthenna

Post by Aussie Susan » Wed Nov 28, 2018 1:24 am

In the past I made a QFH according to the instructions at http://www.g4ilo.com/qfh.html and it worked well.
I mention this here as it includes a diagram and description of how to connect the wires at the top.
There are also lots of other example on the Internet of how to construct (and wire) a QFH antenna.
Susan

estario
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Joined: Sun Nov 25, 2018 5:43 am

Re: Very weak singnal from NOAA with QFH anthenna

Post by estario » Thu Nov 29, 2018 2:00 pm

Ok. I made a new antenna according to this 3d design. I wanted more space to work, because the previous central pipe was too small. It was definetely working better for signtals and I could pick up NOAA 18 for more than 15 minutes, but the SNR was between 12-14 at best. The image was again mostly noise. Today I will upload new pictures of my setup.
I was not able to understand the instruction from the provided link - http://www.g4ilo.com/qfh.html. Don't you simply solder the braid of the long and short loop to the feed coax cable's core and braid according to this schema?
Image
Do I need to solder anywhere else? I see very crappy setups working much better than mine. This is my third attempt for a QFH. I am starting to think about building a Double Cross Antenna such as this one. https://www.thingiverse.com/thing:2963713

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