Re: [SARA] modifying Ka band LNB for 22 GHz water maser reception

[eddiem...@gmail.com]



Hi Wende,

Thank you for your suggestions.

Excuse me for this very late response, last few months I was very busy finishing my Ba-thesis. Now that is all done I have started working again on this amateur radio astronomy project.

The LNB was about 60 euros, so not terribly expensive but still enough to be a bit careful with modifications.

Reconstructing a part of the microwave circuit will be near impossible for me, I do not have the equipment and expertise for that. 

In the meantime I have done some experiments. First of all, I tried to bypass the filter by placing a copper foil stripline on top of it (this was suggested by Michiel). However, while we were able to detect a 22.2 GHz test signal is was still very weak. I then tried to shorten the hairpins, this seemed to work somewhat better. This modification cannot be reversed but I had already given up on trying to keep every modification reversible. We tried to detect W49 (one of the strongest water masers) two times with the modified LNB but both attempts were unsuccessful.

Finally I removed one of the filters completely and replaced it with a copper stripline. With this modification the 22.2 GHz test signal came through a lot stronger but still we failed to detect W49.

Best regards, 

Eduard

Op 28 mei 2021 om 00:30 heeft Captain Anne Flint <bera...@gmail.com> het volgende geschreven:

Hi Eduard, 

I would suggest you reconstruct that part of the circuit and play around with it (the filter assembly may be proprietary, but the circuit is not) to see what happens when one cuts traces and/or installs jumper wires. From the photo, I’m assuming you’d need a lower-temperature surface mount soldering iron and a delicate touch on the original assembly. You’re worried that you couldn’t return the filter to its original state and I tend to agree. Things can go really wrong. 

What about Dr. Reeve? he has a lot of circuits online. 

How much did the original assembly cost? Got any older equipment sitting around that could be sacrificed? 

Feet First, 

Wende 

On May 27, 2021, at 3:18 AM, Eduard Mol <eddiem...@gmail.com> wrote:

Hi all,

After the successful detection of methanol masers at 12.2 GHz I was wondering if it would be possible to detect maser lines at other frequencies with my 1 metre dish. Water masers at 22.235 GHz seemed to be a good candidate, because these can be quite strong (several thousand Jy to >10K Jy). Water masers are also variable, which makes them an interesting target for regular observations.

About a year ago I asked in a post on this forum if there are any useful LNBs for this frequency. Unfortunately the inverto Ka band LNBs Wolfgang recommended seemed to be no longer available, so I ordered a triax twin Ka band LNB. It has an LO frequency of 21.2 GHz, and it is intended for receiving at 19.7- 20.2 GHz. It may be possible to receive 22.235 GHz which would be downconverted to 22235-21200=1035 MHz, well within the frequency range of my SDR. Unfortunately, there are two hairpin filters inside the LNB which almost entirely blocks 22.2 GHz. I therefore want to remove/ replace or adjust the filter so I can (hopefully) receive 22 GHz. However, I am far from an expert at this so I am not sure what to do next.

Would it be possible to "re-tune" the filter to 22 GHz?

If not, what could I do to bypass it? Michiel told me that I could just place an isolated copper stripline over the filter, but I am worried that it may cause some unforeseen problems/ losses. It also looks like there is not enough space on the PCB to place a bypass stripline around the filter without removing the filter itself (which is a modification I can not undo)

Best regards,

Eduard

 

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[Captain Anne Flint]

Hi Eduard, 

Sounds like you are making good progress - one failure and two successes. I used to work with circuit design groups, so it demystified the process for me. Although the purchased circuit assembly is intellectual property, the filter circuit (for example) is in the public domain, often associated with integrated circuits in data books describing typical IC applications. Then someone searches through the data books for parts and applications. Data books used to be provided for free, since parts manufacturers wanted people to use their parts. I suppose now they are online? but I haven’t looked. 

Now it’s time to build and test a prototype, so you have to worry about issues like ground loops and impedance. That copper foil strip bypass perhaps created an extra circuit … element???  but it’s a choice  of priorities to just move on. The circuit designer stops and figures that out; the radio astronomer moves on. 

I have a book called Noise Reduction Techniques in Electronic Systems by Henry W. Ott that I cadged off a retiring engineer, but it was published in 1976, so I’m sure that current technology has moved on!  

Since you are looking at such tiny signals, and building just one (or three) prototypes, you can afford to spend money on more accurate capacitors, maybe 20 cents instead of 3 cents each, and other parts. If you contact a component sales representative, they might actually provide you with a tighter-spec integrated circuit for experimental purposes. The markup for tight-spec components is exorbitant; they can afford to farm out a few for testing. Now that I think about it, you could probably replace parts on the existing board, but at some point you’d do something irrevocable - you’d sneeze and vaporize it with your phaser - I always like to keep a functioning original around. If your prototype gets destroyed, it’s not such a loss. 

Another benefit in building your own is the quality of soldering. If you can solder reasonably well, or can find such a person, it will make a huge difference in RF compared to what you can get from a commercial manufacturer. 

After all this, if you test your prototype and it responds the same or better than the original, why not? If it’s worse, it’s probably just some little error. 

I can understand why a person might not choose to spend their time this way. I’ve spent half my life doing things I don’t enjoy doing, often for financial reasons. I just believe in learning by doing. The person who builds a crystal radio has one level of understanding before he begins, and another after he finishes. Anyway, you are already doing it, in a way. 

Wende 

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