Troubleshooting my Nooelec dish

[JERRY TAYLOR]

After a few months of operating fine, my system has suddenly lost the ability to receive the hydrogen line.  I have a Nooelec paragrid dish with the standard dipole antenna, Nooelec H1 LNA, KMR 240 coax and an RTL SDR V3 running SDR# with the IF average plugin.  I triple checked all the connections, the LNA and software and everything seems OK except that there appears to be a short in the antenna.  With my ohm meter I read 2 ohms between the center conductor and shield on the coax. This doesn't seem right. Is this some sort of impedance matching? I have attached a couple of photos.  Can anyone tell me if this is correct?

Jerry Taylor

[Scott Elliott]

Jerry,

You are doing the right thing with a divide and conquer strategy.  You have visually confirmed the antenna active elements are not touching or broken.  Perhaps, weathering and water could have caused some uncleaned flux to detune the antenna.  You could try some rubbing alcohol and a q-tip to clean away any remaining flux.  Leftover flux looks like semi-clear caked on honey.  Try and keep any flux from migrating back into the coax as you clean.  Keep the coax pointed down to keep dirty fluids away from the coax.  As a side note, the plastics probably do add some small capacitance and may affect tuning.  Again, clean any gunk from the plastics, inside and out with soap and water.  Always test cleaning solvents in a safe area (even water and isopropyl alcohol) before going in whole-hog.  

-scott

On Fri, Apr 4, 2025 at 8:33 AM JERRY TAYLOR <gfaj...@gmail.com> wrote:

After a few months of operating fine, my system has suddenly lost the ability to receive the hydrogen line.  I have a Nooelec paragrid dish with the standard dipole antenna, Nooelec H1 LNA, KMR 240 coax and an RTL SDR V3 running SDR# with the IF average plugin.  I triple checked all the connections, the LNA and software and everything seems OK except that there appears to be a short in the antenna.  With my ohm meter I read 2 ohms between the center conductor and shield on the coax. This doesn't seem right. Is this some sort of impedance matching? I have attached a couple of photos.  Can anyone tell me if this is correct?

Jerry Taylor

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[Captain Anne Flint]

Hi Jerry, 

Here’s my 2 cents' worth (or my 1 cent's worth, like the man said): 

0. Think about the last thing you did, or the next-to-the last thing you did, before you noticed the failure. IF you did something to cause this, that’s most likely when you did it. 

1. From the photographs, your solder looks good. 

2. I don’t believe this is an impedance matching problem. For several reasons, but from what little I know about impedance matching, I don’t think it would manifest as a sudden, catastrophic failure. 

3. Scott’s right on - if your little guy is outside and pointed straight up, water can leak down into the coax. If there is extreme weather, it speeds up the entropic process. Coax can hold a surprising amount of water, and corrosion can start at a weak spot and crawl along the surrounding cable. Flux has corrosive chemicals, but so does water. 

4. Hook up your ohmmeter and wiggle every connection (not just the ones you created). 

5. Start replacing parts with ‘known good’ parts, i. e. the antenna as an assembly, the coax, the LNA, the SDR etc.; this might take a while but you will be glad you have them next time. Maybe an assembly just died. 

6. When you find it and fix it, write it up and post it online. I am looking forward to The Answer. 

Regards, 

Wende 

[andrew....@googlemail.com]

Speaking as a novice, can you dismantle everything and check each component, including the waveguide, coaxial cable, filters, with a NanoVNA? That is what I did, and found a really surprising cause for my dish not functioning!

I realise you may have done this already…..but just in case you haven’t!

Other thing is that sometimes SDRs can fail – do you have spare receiver or alternative setup you can plug everything into.

I guess if that little lot doesn’t work then, if you can, take it all to another location and see if it works there.

Alex pointed out to me that SDR# is really useful for quick testing as gives you an immediate result.

Andy

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[Captain Anne Flint]

I meant to send this to Jerry as part of the troubleshooting discussion. 

Now I must troubleshoot also: was that Google, or was it me? Wende 

Begin forwarded message:

From: Captain Anne Flint <bera...@gmail.com>

Subject: Re: [SARA] Troubleshooting my Nooelec dish

Date: April 5, 2025 at 5:14:09 PM CDT

To: 'Andrew Thornett' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

[Captain Anne Flint]

Andy, 

Yes, you can troubleshoot that way. 

Accepted methods also include dividing the circuit chain in half, in quarters and so forth (you have to know what signals are supposed to look like at that point) to localize the problem. A long, long time ago, when I was a young person and circuits were simpler, the manufacturer would provide that information. A person could create that kind of information for equipment that is working with analytic equipment, component data etc. but I don’t think many people would actually do that. 

Another method is to log problems and fixes. This works well if there is enough data. Since we are hobbyists, there is no way to enforce reporting; also, people are reluctant to report due to the fear of being classed as technically inept, or admitting to doing something really dumb. Technical expertise is a really big issue in the engineering world, as is comprehension in the sciences. 

Of course one can ask if anyone on the SARA forum has seen something like this before. It’s just that antenna systems are highly individualized. The sealed dongle assemblies are supposed to be replaced instead of troubleshot. I myself am not very good at opening sealed assemblies without destroying them, but some golden people do have the knack. 

And if you can open them, you can troubleshoot and/or find more information about the circuits within on the SARA forum. A lot of folks are really knowledgeable. I fixed a cheap import once by reflowing the ground plane solder. The people who build these things work under conditions too horrible to contemplate, so the quality of their work suffers. You might find yourself challenged by surface-mount components. Money vs. time vs. fine motor skills. 

Sorry so wordy, 

Wende 

[Alex P]

The photo of the antenna shows a type of Guanella Transmission Line  Balun.

The Coax SHOULD measure 0 Ohms with a DC ohmmeter

[JERRY TAYLOR]

Thanks Alex. So am I correct that it is for impedance matching and for broadband operation of the antenna?

Jerry

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[b alex pettit jr]

It is to match the Unbalanced Load of the Coax to the Balanced ( Symmetrical ) Dipole.

The shape looks trivial, but I do not understand how it works .. 1/4wl lengths of transmission line are the rated

Z on one end ( 50 ohms or 75 ohms ) and very high on the other ( circa 2500 ) ohms. ( RF impedance not DC )

The term " Balun " is so misused.. it Should define  the above and a method to prevent outer shield radiation,

but impedance matching voltage transformers are also lumped into the name with quite different functionality.

Anyhow 0 DC ohms is proper

Alex

[JERRY TAYLOR]

If I could impose on you a bit more....I am having trouble understanding what is going on with this Nooelec paragrid antenna.  I remeasured the resistance and it is now 0.1-0.2 ohms.  When I put it on my NanoVNA the SWR is sky high at 1420 mhz.  See the attached graphs.  A couple of questions:

The antenna is supposed to have a bandwidth of 1400-1700 mhz.  I guess it "technically" does since there are 3 dips in the SWR.  (See the attached graphs.)  However at 1420 mhz, the red marker, the SWR is off the chart.  I assume this is why my system stopped working.  Could this be because of moisture that got into the coax or dipole?

The specs call for 1400-1700 mhz bandwidth.  Is it typical to have just a few dips in the SWR at the specified bandwidth?

Is it possible to modify the dipole so that minimum SWR is at 1420 mhz?

I am also working on a 21 cm loop feed that I plan on installing using 3/8" stainless threaded rod run through the boom on the antenna.  Will this be superior to the dipole and will the stainless road affect the performance of the loop feed?

Thanks for your assistance!

Jerry

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[b alex pettit jr]

I built the LoopFeed before I found the WiFi dish so did not do much with it ... AND, it was the 1.7GHz version

However, there is not much that can go wrong with it ...  I'd look elsewhere ..

Do you get ~ 5 dB difference on SDR# between having the SBird  LNA Input  Open and w/ a 50 Ohm Load ?

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

[JERRY TAYLOR]

Thanks for the tip.  I got 0db difference with the LNA (Nooelec H1) open and with the 50 ohm load.  I tried an older Nooelec H1 LNA and got a little over 4 db difference.  My problem wasn't the antenna at all.  It was a bad LNA.  I made the assumption that the LNA was good because I saw a significant rise in the noise floor when I turned it on.

Thanks!

Jerry

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[Marcus D. Leech]

On 06/04/2025 12:46, JERRY TAYLOR wrote:

Thanks for the tip.  I got 0db difference with the LNA (Nooelec H1) open and with the 50 ohm load.  I tried an older Nooelec H1 LNA and got a little over 4 db difference.  My problem wasn't the antenna at all.  It was a bad LNA.  I made the assumption that the LNA was good because I saw a significant rise in the noise floor when I turned it on.

Thanks!

Jerry

The fact that it produces a rise when powered-up, indicates that the LNA chip, per-se, is probably fine, but that the input circuit
  probably has an open or short in it.

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[andrew....@googlemail.com]

Thanks Wende

 

From: sara...@googlegroups.com <sara...@googlegroups.com> On Behalf Of Captain Anne Flint
Sent: 06 April 2025 01:15
To: 'Andrew Thornett' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: Re: [SARA] Troubleshooting my Nooelec dish

 

Andy, 

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