Building A Horn Antenna Radio Scope - Advice Needed

[saac]

Motivated by the recent posts on Hydrogen and Methanol line detection I thought that building a radio telescope would make a nice project for the classroom .  I'm looking to follow the open source design offered by DSPIRA .  In their build the horn is constructed from aluminiumized insulation board.  I was looking to do something similar and in the local B&Q store today I picked up what I thought would be suitable material in the form of a pack of aluminiumized faced underlay for hardwood flooring.  When I got it home though I've found that the reflective facing is actually something similar to mylar. I 've checked it with a multi meter and it's certainly not conductive .  Would I be right in thinking that this material would not be suitable for the horn ?

Jim 

 https://wvurail.org/dspira-lessons/tour/

[andrew s]

I would concur not suitable. Could you use aluminium cooking foil to line it?

Regards Andrew 

[saac]

Thanks Andrew. Using aluminium foil was my next question!  Looking on-line at the cost of aluminium sheet gave me a fright , particularly when delivery costs is added.  I think as you suggest I'll  use the boards that I bought as a backing and try gluing thick cooking foil to line them.  In terms of gluing the foil to the boards, I take it i will need to ensure  that each overlap section of foil would need to have continuity with the one it overlaps with . Hope that makes sense.  So I shouldn't use glue/paste on the underneath side of the overlap. I was planning to use aluminium tape to join the overlaps from the top - that way they will have electrical contact underneath !  Not a great explanation , maybe the diagram would be helpful. 

I bought an SDR usb dongle from AMAZON and have been playing around with it using AirSpy software - it came with a simple telescopic whip antenna so a bit limited but it was cool to pick up some mainstream stations. I'm sitting listening to a very noisy Classic FM through the laptop while next to me is a very capable Denon home entertainment amp with beautiful sound .  Isn't it strange that it's the simpler technology that is often more enjoyable     Anyway, I have a massive learning curve ahead ;  dipoles and wiggly amps and waterfall diagrams  is an area that is black magic to me .  I have an LNA  arriving tomorrow so I may be able to try a mock up by the weekend all being well. 

Jim 

Edited April 8, 2021 by saac

[saac]

Apologies the diagram is pretty poor   

Jim 

 

Edited April 8, 2021 by saac

[Corncrake]

Fascinating forum !

What are the dimensions of this thing ?
I had a quick look at the site you linked but what an awful jumble, and no time to watch videos, they are impossible to 'flick' through like a book or proper website !
It has been a long time since I was involved in mm, and u,wave so beware senior moments but for what it is worth :-
I would go for longitudinal not crosswise (if I am following right ?) that way each segment can be more continuously intimately connected to the ground plane (shield, outer) of the feed back down at the real antenna (a monopole inside the rectangular waveguide is it ?)  instead of floating in the outer regions.

How about constructing the waveguide and first portion of the expanding horn match out of copper-clad pcb and then the rest out of foil or mesh (mesh with holes smaller than 1/10th or 1/20th wave.)

For bigger than usual aly foil, search turkey or catering, I've seen 600mm wide, there may be larger ?

Lots of strip edges just stickied down are liable to introduce lots of noise better to fold over then clamp down with a rigid strip, pop-rivet to backing if it cannot be spot weld, at intervals of less than 1/10wave again.

Speaking of spot welding, how portable do you want? Have you a car/truck breaker nearby, a couple car bonnets soon beaten into shape by your local body shop repair man? At 21cm you can allow a few little bumps !

Edited April 10, 2021 by Corncrake

[saac]

Thanks Corncrake. I had wondered if orientation of the Al foil would matter but couldn't think of any reason.  What you said makes sense so a longitudinal overlay it shall be.   I'll look to use the full width of the aluminium foil off the roll (about 350 mm) as far as I can - the diagram I put up was only a quick outline.  I know what you mean about the website the documents are scattered about, I'll post up the plan from the pdf with dimensions.  As far as portability is concerned I'll be happy so long as I can transport it in the car to school. Thereafter it's a short lift from the classroom to outside.  My thinking is that if I can get a useful result with this first attempt (foam board and tin foil) then I may invest in some aluminium sheet and do it all properly.  If I can use it to show the 21 cm hydrogen line and perhaps an idea of doppler shift then I'll be happy - it would fit nicely into our Higher Physics class work.  To be honest, this is a diversion - the kids have had a really rubbish year, as we all have,  so I'm just looking for something to lighten the load a little - and it will be so cool if we can do it     I also know absolutely nothing of radio propagation/antennas/reception/processing.  Waveguides, side lobes, ground clutter were only words the avionics technicians would use to hoodwink me in my RAF days.  So it's also a nice wee project to challenge me; hopefully I may even learn something.   Here's a photograph of where I am at the moment.  The gallon of oil container is due to become the wave-guide.  I need to decide where to put the oil !   I was supposed to do an oil change on my mgb  last month but put it off due to weather. If I can't find a spare container to decant the oil into then I guess I will have to bite the bullet and just put it in the car   Just waiting for the final delivery from Amazon, coax connector, and then I think that is me.  I would like to think I could test it sometime tomorrow.

Jim 

 

 

[saac]

Here's the pdf showing the dims of the antena and general construction outline. 

Jim 

 

 

DSPIRA_Horn_Assembly.pdf

[Corncrake]

2 hours ago, saac said:

Thanks Corncrake. I had wondered if orientation of the Al foil would matter but couldn't think of any reason.  What you said makes sense so a longitudinal overlay it shall be.  

  in my RAF days.

Ah so ! you will know all about the theorists - then the best practice bods - and fixing summat that woiks  then !

Looking good,

The good thing about big horns instead of dishes is you dont have to worry about the feed side lobes and spillage seeing warm things it shouldnt

Edited April 10, 2021 by Corncrake

[Corncrake]

Oh look !  We dont even need to build a horn or Rx, observe live online !

https://pictortelescope.com/

 

[Macavity]

Before the Pandemic (B.P?) I was doing a few tentative experiments at that sort
of frequency (actually 23cm amateur band) using a wideband Discone antenna.

I did get as far as buying a wideband Noelec Preamp --- That seems to work...
But then work kinda stopped owing to a general concern re. high ladders! lol 🙄

Were I looking at H-line, I'd be thinking (easily available) "Sawbird" Preamp? 🤔
https://www.amazon.co.uk/NooElec-SAWbird-H1-Applications-Frequency/dp/B07XPV9RX2/

At 21cm, there is a need for even MORE (wideband) RF gain in the system?!?
There are a number of ideas that have seemed to work for other people e.g.
https://physicsopenlab.org/2020/07/26/sdr-based-receiver-for-the-21-cm-neutral-hydrogen-line/

I was just experimenting with an SDRUno as a receiver (+ software). As ever
I sense there is real SGL expertise on such things...  which will arrive shortly? 😉

Edited April 10, 2021 by Macavity

[Corncrake]

23 minutes ago, Macavity said:

I sense there is real SGL expertise on such things...  which will arrive shortly?

Thank you ! I can take a hint.

[saac]

My connectors have just arrived together with some Aluminium conductive tape - "smoke on go".   The little black device is the low noise amp and the blue usb dongle is the SDR  decoder that connects to the laptop.   I'll need to solder a short length of copper wire into the coax connector to act as the dipole (not sure if that is the right term) - the detector of the wiggly RF amps !   

 

 

 

 

 

[saac]

5 minutes ago, Corncrake said:

Thank you ! I can take a hint.

Chris , corncake and all , keep the advice coming  - like I said this really is the dark arts to me !  I'm not looking forward to the part where I have to shake the chicken bones and see where they land.  

Jim 

[Macavity]

2 hours ago, saac said:

Chris , corncake and all , keep the advice coming...

Jim - At the risk of embarrassment, you seem a really KEWL teacher to me! 😎
But then a good *practical* project still has the possibility to inspire me...
Who knows? I may yet resurrect this project. So many ideas... so little time? 😛

Aside: I often think about the HUGE role of Teaching, "public engagement" etc.
Sometimes I play a role of comedy straight man... agent provocateur to FLUSH
out experts? 😇 (Post above implied nothing particularly re. anyone here!) 😉
 

Edited April 10, 2021 by Macavity

[saac]

Cheers Chris, I hope I live up to your kind words.  To be honest, I've always thought that you have to be pretty determined to muck up teaching Physics  - I've always thought of it as "the beautiful subject".  At its heart the subject has a very playful nature - that childlike sense of curiosity (let's put a bigger rocket motor in it )  ; for me, teaching Physics is a privilege - I get paid to play    Having said that, I turned 55 the other month so maybe I do need to start weaning myself off the skateboard ; I took out 3 chairs and a group of retort stands just before the Easter break  

Jim 

 

Edited April 10, 2021 by saac

[saac]

Well , I didn't get as far on as I thought I would but that is the horn lined with the aluminium foil. What a nightmare trying to stop it from wrinkling .  I don't know how sensitive it will be to all the discontinuities but it looks ok.  I have some aluminium angle section that I'll use to brace the corners to give it a little structural support and to better hold its shape.  I'll make a start on the wave guide tomorrow. Lol there are foil off cuts and aluminium speed tape all over the kitchen. My darling wife was convinced that I was making a tin foil hat !

Jim 

 

 

 

 

[Ed astro]

Nice project!

Don't worry about all the wrinkles in the foil. As long as they are smaller than about 1/10th of the wavelength they would not be a problem. Since the wavelength is 21 cm a bunch of wrinkles of a few mm or even a cm high will probably have little effect.

I once have made a conical horn antenna from steel mesh with 1cm wide holes. Worked fine for detecting the hydrogen line.

[saac]

Thanks Ed astro, I was hoping that would be the case.

Jim 

 

 

[Corncrake]

6 hours ago, saac said:

My darling wife was convinced that I was making a tin foil hat !

  She's not the only one

Ooops no, I didnt mean to say that, now how do I edit ,, ,

Hehee, but seriously : that looks to be a manageable size, and I have googled/updated me'brain on 21cm+sdr so, following with much interest.

Edited April 11, 2021 by Corncrake

[Corncrake]

13 hours ago, Macavity said:

 (B.P?) I was doing a few tentative experiments at that sort
of frequency (actually 23cm amateur band) using a wideband Discone antenna.

Discone ! I dont think ET has yet discovered that tech nor the power needed to communicate therewith, a Jpole may be more their style ?

Edited April 11, 2021 by Corncrake

[Carl Reade]

Hi here a link to a really useful site which helped me enormously. http://www.parac.eu/

Take a look in the project section  especially project 3 and 9 in the list.

Regards Carl

[saac]

I didn't make as much progress today as I had hoped. I made a trip out (stay local)  to pick up another roll of aluminum tape and got lost in a few other distractions - snowing here today  Anyway, I cleaned out my old oil can (mgb can wait for its oil change until better weather) and cut the top off using tin snips.  As feared, the throat of the horn and the opening of the wave guide don't match in size exactly!  As this will be sitting in a classroom, being moved about, I'm keen to make it as robust as possible. With that in mind, I was toying with the idea of making a flange to attach the wave guide to the horn.  I have a laser cutter which I could use to cut 3mm ply to form the flange - the ply would then be covered in conductive Al tape.    My question is, how critical is it to have the cross sectional area of the transition from the horn to the waveguide constant ie no step changes ?   Hope that makes sense.   I would try to minimise the size of any step change but I thought I better ask before proceeding on blindly. 

Jim 

 

 

 

 

 

Edited April 11, 2021 by saac

[saac]

6 hours ago, Carl Reade said:

Hi here a link to a really useful site which helped me enormously. http://www.parac.eu/

Take a look in the project section  especially project 3 and 9 in the list.

Regards Carl

Carl, many thanks for that link. There is a wealth of information tether which I going through now.  Thanks again . 

Jim 

[Corncrake]

On 11/04/2021 at 19:25, saac said:

 My question is, how critical

Oh sorry, I ve not been paying attention, or maybe waiting for a real expert to show up !!

shows how long Im out of practice, I didnt notice the Xsections were different, which is another doubtful area

Bumps less than 2 cm are best, and gradual not sharp. How much the mismatch between the Xsections causing relections/standing wave is another "length of string"

Guess this one may be the prototype !?

 

[saac]

6 hours ago, Corncrake said:

 

Guess this one may be the prototype !?

 

Yep that was the intention. I invested in the components that I can reuse (filter, cable connectors and SDR dongle) the rest I can remake fairly easily once perfected.   I think I have a solution to the mismatch - find smaller can lol    Guess who is doing a recce of B&Q  - I don't care what the contents are I just want the can  

Jim