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Excellent, Clive.

Keep us up to date with how you get on.

Neil

You will need to watch out for the joints getting corroded where there are currents flowing across them. This can seriously affect the waveguide loss.

Looks about the same size as a J band one I built at uni.
Of course at J band even though simular size much bigger in wave lengths (J band is 15 to 30 mm ) and from memory Large end was 800 mm..again from memory output was 15 mm widest so area ratio 2800 gain of around 28dB measured mid band..
Learn to weld brass for this..using beautiful Oxy hydrogen torch..

Edited By jason udall on 03/12/2018 19:06:45

Dave,

You may be interested in this discussion on another forum:

stargazerslounge.com/topic/319651-anyone-doing-hydrogen-line-observing/?tab=comments#comment-3496121

An interesting project would be to see if you can measure the doppler shifts of the galactic arms.

@Joe, I think you need huge receivers to detect the faint polarisation modes of the cosmic microwave background.

Neil

That is interesting specially considering the problems I'm having at the moment at the other end of the spectrum using a wire antenna consisting of 15m of wire in my loft and 12m out the window across my back garden, about 8m high. Despite the antenna being carp 200mW of WSPR on 3570kHz was received 2700km away in the Canary Islands, and by k9AN in Illinois on 7040kHz at 6300km. I'm trying to analyse the data to see if I can calculate the height of the reflecting layers in the ionosphere.

Much less success receiving. This is my new toy, a KiwiSDR, which can see from 10kHz to 30MHz in one go.

The waterfall display shows heavy interference and no obvious stations. On the left the Long Wave is full of hash from switch mode power supplies and energy saving lights. I think the Medium Wave is hidden under ADSL internet noise radiated from telephone lines. Shortwave up to about 18MHz looks to be awash with VDSL noise also off phone lines. The bright horizontal lines slashing across the screen are caused by my Ethernet over Powerline equipment.

Partly my own fault – I bought a house that's almost surrounded by pole strung telephone and power cables.

Lot to be said for pointing a beam into the sky – I might be able to hear something! As to detecting doppler shift from the arms of a galaxy, I've no idea how big the effect would be or how long it would take to measure it? I'd need to make a long yagi and a motorised equatorial mount to keep it on target. The radio and data analysis part I think is fairly straightforward. Ten years ago technology would have been special: today it's almost too easy.

Thanks for the link – another fascinating distraction!

Dave

This episode is described in one of John Gribbin's excellent books – I recommend them.

I believe that the functional heart of the so-called ' Holmden Horn' is actually a segment of a paraboloid. That's the base of the 'scoop' on the extreme right-hand end of the structure. The focus is somewhere just inside the hut.

Modern satellite TV antennae have the same basic layout but don't have the enclosing pyramid (or the hut! ).

As I'm not an antenna guru, it isn't clear to me whether the Holmden pyramid is part of the feed to the paraboloid or a purely mechanical structure to hold the paraboloidal segment and the feed in the correct spacial relationship.

Best regards,

Swarf, Mostly!

Edited By Swarf, Mostly! on 04/12/2018 12:53:22

Interesting observation. I'd assumed that the horn provided most of the gain and the scoop was just to make the antenna easier to aim – rather than lift a hefty horn skyward, it only has to be rotated inside its giant ring bearing. Looking at the scoop it does look partly parabolic. This picture of the horn as parked to keep the rain out makes it look a lot flatter, ie perhaps the scoop is only a reflector.

My assumption that the horn does most of the work isn't supported by this statement from the Physics Forums site:

'This type of antenna … consists of a flaring metal horn with a curved reflecting surface mounted in its mouth, at a 45° angle to the long axis of the horn. The reflector is a segment of a parabolic reflector, so the antenna is really a parabolic antenna which is fed off-axis. … It is extremely broad-band, has calculable aperture efficiency, and the walls of the horn shield it from radiation coming from angles outside the main beam axis. The back and side lobes are therefore so minimal that scarcely any thermal energy is received from the ground. The antenna has a gain of about 43.3 dBi and a beamwidth of about 1.5° at 2.39 GHz and an aperture efficiency of 76%.'

Score so far – Swarf Mostly: 1 Duffer: Nil.

Dave

Dave,

Please accept that my post was not intended to be adversarial! You drew our attention to an interesting bit of hardware for which I thank you. My post was offered to broaden the discussion, nothing more.

If I remember correctly, the USA end of the Telstar trans-atlantic link used the same type of antenna as the Holmdel one, if not that actual one. (Perhaps it is time I took a Gribbin refresher! ) I believe that the UK end, at Goonhilly Down, used an alt/az mounted complete paraboloid, in a dome. It was an 'if at first you don't succeed' situation – the two teams had different conventions for the direction of polarisation. But it was 'alright on the [second] night' !!!

Best regards,

Swarf, Mostly!

P.S.: I apologise to the original poster for causing topic-creep!!  I think that horn is a splendid job.  How on earth did you hold the first two pieces in position while you fitted the fasteners?

S,M!

Edited By Swarf, Mostly! on 04/12/2018 15:08:14