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Comment by FredPret | Hacker News Reader

I'm getting into a new hobby: amateur radio astronomy.

From what I understand you need an dish with d > 1m to pick up hydrogen line emission well.

I'd be interested to know if I can just build a wide wire helix like this instead of investing in a big dish. It'd have to look nice and be weather-proof to be wife-approved though.

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To get sensitivity you actually need collection area.

Fair point. I’m trying all kinds of backflips to avoid buying a huge dish

I have a similar interest. One way to avoid the large antennas is to have multiple separated small antennas - like the VLA [1] - using an SDR receiver like Kraken [2]

[1] https://www.vla.nrao.edu/ [2] https://www.crowdsupply.com/krakenrf/krakensdr

That is so cool. I’d call mine the Very Small Array.

I was always under the impression that it’s super hard to pull off interferometery due to precise positioning and timing requirements, but looks like the Kraken multi-antenna you linked has that figured out in a simple way.

I never done sth like this but you can lock the clocks of each of the samplers I guess. Simplest solution might be using a single source clock and distributing it to each board with equal length of cables. You can calibrate the lenght of the cable precisely if you have access to a pulse generator and a scope. Though, alternative would be shifting the sample times in post-processing the data by searching for a high correlation. I believe both are used in practice.

There are other SDR receivers that have external clock inputs. The Kraken does that for you because it is a single unit. Of course there is likely a limit to how long the coax to the antennas can be. Just a guess - for an amateur rig I would think 30 to 60 ft antenna separation would be impressive resolution. Certainly better than buying a 60 ft dish;)

One can always use clock repraters or a reclocking with a new PLL in the middle if the cable length gets too long and caused ISI (inter-symbol-interference). I don't think 30 ft requires this. At 60ft you might need it though it depends on the clock frequency. If the syncronization is done with low frequency clock (10 - 25 MHz or so) you don't need any of that. 60ft cable won't have too strong of an attenuation at those frequencies. GHz rate clock would be a problem.

can we not now use fiberoptic cables to provide a common timestamp signal well above the shannon limit of the emmission lines that would need to be reassembled

Another interesting possibility is to use GPS for a clock. [1] Apparently 1 microsecond accuracy. Maybe useful if you want to make the distance really far. Someone has an article [2] Would be cool if you could pool the resources of multiple amateur rigs over a great distance in ad hoc antenna arrays.

[1] https://digitaldisplay.com/network-clock/master-clock/gps-vs....

[2]https://destevez.net/2022/03/timing-sdr-recordings-with-gps/

Yeah GPS is a good tool to sychronize over very long distances. Locally locked clocks might still be better for an array not far away from each other if done properly. GPS might be easier to implement for amateurs though.

Imagine every amateur radio astronomer pointing their dishes at the same thing. If they all upload their data with accurate enough GPS position & timing, it could probably be used.

We just need a standard file format that incorporates the thing being observed and some GPS time sync info (or other time sync source). Anything else? I'm game.

I think this would be amazing to ttack movements but the combined images would be extremely noisy wouldn't it?

Short answer is: no. The long answer is: Speed of light on glass is not so different than electromagnetic signal speed in copper, approximately 0.66C. Considering clock sources output electrical signals, using a coax cable instead of fibre optic has several advantages: simpler signal chain means lower delay, and avoiding highly noisy optical-electrical conversion means less jitter.

Fun fact: RF signals in air travel faster than light in glass fibre (0.8C instead of 0.66C).

A belated Thank You! for your excelent reply.

Seen https://www.crowdsupply.com/krakenrf/discovery-dish ?

people will throw them away or pay you to take them away

I think I need a dish larger than a TV one though

Old tv dishes go up to about ten feet.

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