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Carl Reade

Anyone doing hydrogen line observing?

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Hi putting together a radio telescope, I have the following,

A 45 element circular polarised Yagi for 1.4 GHz 20db

A filtered LNA 30db nf 0.5db

A satellite line amp

1.4Ghz filter

A bias tee

A SDR dongle

Any help or tips appreciated.

Carl

 

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A 45-element Yagi...? Are you sure about that? Can you send a pic?


I've never seen Hydrogen-line observations conducted with a Yagi antenna (only dishes), but if beamwidth calculations come out well (the smaller the beamwidth, the higher the sensitivity/gain), you can try doing a drift scan, with the milky way being the target. The milky way is very active in the 21 cm wavelength (~1420MHz) band, so if the antenna is good enough, you should pick up something.

I've been conducting several observations with a 20m parabolic dish antenna, so I can probably answer several questions you might have about target intensity etc.

 

- Coto

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Oh interesting. For the same cost, can Yagi antennas be more sensitive/efficient than parabolic dishes?

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4 hours ago, Coto said:

Oh interesting. For the same cost, can Yagi antennas be more sensitive/efficient than parabolic dishes?

Not seen small dishes used at this wavelength but theory says a 1m dish would match a 45 ele long yagi for gain.  I am not sure how dishes behave as the diameter approaches the wavelength though. The feed horn would presumably block a significant proportion of the aperture ?

Robin

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Hi folks thanks for your interest and Corp will definitely have questions Im sure. The antenna came from the USA made by Directive systems some years ago. I had a go at this a long time ago but was never successful but technology has come a long way so thought I would have another go. Just waiting on the LNA from G8FEK which shouldd be here in the next day or two. Here's a pic of the antenna it's a long beast😀IMG_20180820_192256.thumb.jpg.8349797c735e0e7aeaa57dad2153bc37.jpg

Carl

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Posted (edited)
52 minutes ago, robin_astro said:

I am not sure how dishes behave as the diameter approaches the wavelength though.

Generally, the wavelength should smaller than the diameter of the dish. From there, the gain of the dish (the amount of collected & reflected photons) is the same across all wavelengths. However, due to various minor architectural imperfections the dish might present (definitely will), fewer short-wavelength electromagnetic waves end up right at the focal point and into the feed, so in reality, the gain decreases as wavelength decreases (and frequency increases).

52 minutes ago, robin_astro said:

The feed horn would presumably block a significant proportion of the aperture ?

I'm not sure if you mean the size of the feed would block radio waves from hitting the dish. To avoid that, telescopes are often equipped with a convex/concave secondary reflector (see "cassegrain" and "gregorian" accordingly), or use an off-axis/offset feed.

If your concern is wether both short and long-wavelength waves can be detected by the feed, radio telescopes like the Green Bank Telescope use multiple feeds (8 in the case of the GBT) which are able to detect electromagnetic radiation at both short and long wavelengths.

 

EDIT: @Carl Reade Oh that's a Yagi I've never seen before..! With a 45-element Yagi, your beamwidth should be very small, so hopefully you'll be able to detect some hydrogen emissions.

Which bias tee have you got? I'm looking to get or make one for my own dish telescope.

 

- Coto

Edited by Coto

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Great info and questions guys @Coto ipicked up this one on Amazon. It's well within Freq range.IMG_20180820_195144.thumb.jpg.dcfa05c348448bcbc5843e27439d147a.jpg

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Posted (edited)
50 minutes ago, Coto said:

Generally, the wavelength should smaller than the diameter of the dish. 

Absolutely ! A dish the size of the wavelength will have very low gain. 

Yagis and dishes have their advantages and disadvantages and it is  around this wavelength where the crossover between using dishes and Yagis is found. Typical dish size  at 21cm are at least 2m  though and have much higher gain than any single Yagi so there is not much practical work to directly compare.  I  expect if you Google you will find an amateur radio operators who have used a small sat dish on 23cm 

BTW my knowledge on the subject is from the hands on radio astronomy course at Jodrel Bank, (sadly no longer run) and my  days as a radio amateur (Any amateurs doing moonbounce (EME) at 23cm would be very good source of practical information if you are building a setup for H line work.)   

Cheers

Robin

Edited by robin_astro
typo

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@Carl Reade Let us know if you have any progress or updates on this. Excited to see what this can do!

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12 hours ago, Coto said:

@Carl Reade Let us know if you have any progress or updates on this. Excited to see what this can do!

Hi well I have a filtered 30db 0.5nf lnb then two sat line amps both 20db followed by another filter and so far cannot claim success.

I'm suspecting the antenna is not good enough however I have seen people use less and get results.

The work continues.

Carl

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Have you tried generating electromagnetic noise (with a transceiver or transmitting SDR)? This should point out wether the problem lies in the sensitivity of the Antenna, or if it’s completely deaf (backend problems with amps or cabling).

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6 hours ago, Coto said:

Have you tried generating electromagnetic noise (with a transceiver or transmitting SDR)? This should point out wether the problem lies in the sensitivity of the Antenna, or if it’s completely deaf (backend problems with amps or cabling).

Amps and cables fine. Cannot tx on that band. The antenna feed is a short so also difficult to test. Think I will desolder the antenna feed to test continuity between it and the connector. I may build a horn type as well.

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1 hour ago, Carl Reade said:

Amps and cables fine. Cannot tx on that band.

You mean you don’t have a transmitter at that frequency, or are there legal concerns? You wouldn’t need to transmit more than a few seconds with tiny wattage, so I wouldn’t consider it a big problem.

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Hi Carl,

Can you pick up the increased noise from the sun ? That  will give you a benchmark signal to gauge your sensitivity from. Amateurs doing moonbounce (EME) on 23cm for example use it to optimise their receivers 

Robin

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48 minutes ago, robin_astro said:

Hi Carl,

Can you pick up the increased noise from the sun ? That  will give you a benchmark signal to gauge your sensitivity from. Amateurs doing moonbounce (EME) on 23cm for example use it to optimise their receivers 

Robin

Hi Robin no noise from the sun when pointing. I think that is what I will look at first and start looking at eme setups. It's quite a tech challenge.

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One thing worth doing to the Sdr dongle is cooling. The chips and regulator on the board get extremely hot that you can barely touch.

Six or seven large interference spikes have also gone leading to a smooth spectrum.

 

IMG_20180902_175132.jpg

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18 minutes ago, robin_astro said:

Very clear,  We really do live in spiral galaxy

Congratulations !

 

Robin

Thanks Robin indeed we do. It's quite a challenge but doable on a budget and a bit of patient learning. I will put a post on my setup and a really useful method of processing the data from a site I found 😀

Carl

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Well done Carl! Did you do a drift scan? If so, how did you point the antenna? Do you know its beamwidth and bandwidth or frequency range? You could do a spectral observation (observing a wide range of frequencies), which can tell you if you’re observing hydrogen emissions or synchrotron radiation.

But I don’t see how that can tell us about the galaxy we live in?

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3 hours ago, Coto said:

You could do a spectral observation (observing a wide range of frequencies), which can tell you if you’re observing hydrogen emissions or synchrotron radiation.

Hi Coto,

I think you need to reread your course notes ;) .  This shows the spectrum of neutral hydrogen at 21cm, Doppler shifted by different amounts in different spiral arms. You cannot measure these Doppler shifts in continuum spectra like synchrotron or thermal emission.  

Robin

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