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pipnina

Horn antennas / feed horns, how do they convert EM to signals?

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I've been trying to understand some antenna-related theory as my friend and I are going to attempt to use some software defined radios for different astro / spacey projects (like downloading from weather sats, detecting the milky way's 21cm hydrogen etc). So far I am aware of a few types of antenna and the difficulty of mapping the hydrogen line to an image in 'good' quality.

I do have a question or two, however. First off, as the title says, how do radio telescopes that utilize horns convert the waves captured by the horn into electrical signals? Diagrams that I have seen refer to them being used to funnel waves into a waveguide, and not any elements required to efficiently convert that concentrated wavefront into useful electrical signals.

The other question is to do with dipoles: Diagrams typically show a dipole as two opposed bits of metal that don't meet in the middle but bend down into some receiver marked only as " ~~R~~ " or such. It doesn't mention how those two separate electrical signals finally combine. How do they?

 

Thanks :)

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57 minutes ago, pipnina said:

I do have a question or two, however. First off, as the title says, how do radio telescopes that utilize horns convert the waves captured by the horn into electrical signals? Diagrams that I have seen refer to them being used to funnel waves into a waveguide, and not any elements required to efficiently convert that concentrated wavefront into useful electrical signals.

The other question is to do with dipoles: Diagrams typically show a dipole as two opposed bits of metal that don't meet in the middle but bend down into some receiver marked only as " ~~R~~ " or such. It doesn't mention how those two separate electrical signals finally combine. How do they?

 

Thanks :)

A feed horn will have a probe, a short length of wire or something like it that sits in the wave guide.
The wire is connected to the centre of a socket like an N or SMA connector and this transitions to coax to
feed your receiver.

This link shows how, dish feed.

Dipoles don't actually bend down at the centre.
There are various ways to feed a dipole, most often coax is used, centre to one side braid to the other and then on to the receiver.

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12 minutes ago, wxsatuser said:

A feed horn will have a probe, a short length of wire or something like it that sits in the wave guide.
The wire is connected to the centre of a socket like an N or SMA connector and this transitions to coax to
feed your receiver.

This link shows how, dish feed.

Dipoles don't actually bend down at the centre.
There are various ways to feed a dipole, most often coax is used, centre to one side braid to the other and then on to the receiver.

I see! Is there a relationship between the scale of the probe vs the wavelength being observed? In the example the probes seem rather small compared to the wavelength they are receiving and the diameter of the guide.

Now that you've mentioned the braid/core coax fed dipole, I went and found a pdf which uses that as an example in a figure.

image.png.784fed2142dd460ac24a948e88c1e385.png

Why is it that dipoles are often fed in this way?  Does the signal through the shielding interfere constructively with the signal in the core somehow?

 

Thank you for the info :)

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The length of the antenna is directly related to the wavelength one wishes to receive/transmit, usually some fraction of the wavelength. A quarter wave is fairly typical but it depends on the type.  The basic point is that the antenna needs to resonate at the desired frequency.

Using a quarter wave dipole, if you superimpose a wave on the dipole, one end will be at the point where the wave crosses the horizontal (zero) axis, and the other end will be at the peak of the wave. The EM wave induces a voltage differential between the two ends, and by splitting the dipole in the middle and connecting each side to the opposite sides of a circuit (via the coax), a current will flow through it which can then be measured.

At the receiver, one then measures the voltage and turns it in to sound, pictures or data depending on the application. How that is done will depend on the signal - AM, FM, etc. For SDR it's relatively simple, the basic premise is that one samples the incoming signal using an analog to digital converter which turns the measured current to numbers. You sample at at least twice rate of the highest frequency you wish to measure (Nyquist). You then use the FFT process in software to extract the different frequencies in the samples.

That's the simple version as I understand it, but it's a big subject and lots to learn.

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20 minutes ago, pipnina said:

I see! Is there a relationship between the scale of the probe vs the wavelength being observed? In the example the probes seem rather small compared to the wavelength they are receiving and the diameter of the guide.

Now that you've mentioned the braid/core coax fed dipole, I went and found a pdf which uses that as an example in a figure.

image.png.784fed2142dd460ac24a948e88c1e385.png

Why is it that dipoles are often fed in this way?  Does the signal through the shielding interfere constructively with the signal in the core somehow?

 

Thank you for the info :)

One thing to remember is the dipole is balanced but the coax is not.

Because of the imbalance currents will flow on the braid which is not desirable.
When using coax a balun would be included at the feed point to stop the braid radiating especially in a transmitting station.

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A dipole is normally 1/2 wavelength from end to end, though 3/2 etc also works.  An analogy is a vibrating string fixed at each end. This will resonate at a particular wavelength of sound and the vibration will be at a maximum in the centre.  In a  similar way an antenna resonates at a particular wavelength of EM waves, generating an alternating current, in the case of the dipole at the feed at the centre. The rest (horn, dish, parasitic elements etc) are there to concentrate the EM waves at the dipole.

Robin

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Quarter waves generally work very well as a ground plane, that's a quarter vertical with 4 elevated radials.

Here is my 14MHz quarter wave GP vertical, next to the obsy on 7foot pipe.
Length is approx 16feet but using just one elevated radial, wire going off to left.
White box at the feed is a 1to1 choke balun, just to make sure the coax does'nt radiate.

groundplane.jpg.a959dfcd378e6c772aa4f2baaf197b6c.jpg

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2 hours ago, IanL said:

The length of the antenna is directly related to the wavelength one wishes to receive/transmit, usually some fraction of the wavelength. A quarter wave is fairly typical but it depends on the type.  The basic point is that the antenna needs to resonate at the desired frequency.

Using a quarter wave dipole, if you superimpose a wave on the dipole, one end will be at the point where the wave crosses the horizontal (zero) axis, and the other end will be at the peak of the wave. The EM wave induces a voltage differential between the two ends, and by splitting the dipole in the middle and connecting each side to the opposite sides of a circuit (via the coax), a current will flow through it which can then be measured.

At the receiver, one then measures the voltage and turns it in to sound, pictures or data depending on the application. How that is done will depend on the signal - AM, FM, etc. For SDR it's relatively simple, the basic premise is that one samples the incoming signal using an analog to digital converter which turns the measured current to numbers. You sample at at least twice rate of the highest frequency you wish to measure (Nyquist). You then use the FFT process in software to extract the different frequencies in the samples.

That's the simple version as I understand it, but it's a big subject and lots to learn.

2 hours ago, robin_astro said:

A dipole is normally 1/2 wavelength from end to end, though 3/2 etc also works.  An analogy is a vibrating string fixed at each end. This will resonate at a particular wavelength of sound and the vibration will be at a maximum in the centre.  In a  similar way an antenna resonates at a particular wavelength of EM waves, generating an alternating current, in the case of the dipole at the feed at the centre. The rest (horn, dish, parasitic elements etc) are there to concentrate the EM waves at the dipole.

Robin

1 hour ago, wxsatuser said:

Quarter waves generally work very well as a ground plane, that's a quarter vertical with 4 elevated radials.

Here is my 14MHz quarter wave GP vertical, next to the obsy on 7foot pipe.
Length is approx 16feet but using just one elevated radial, wire going off to left.
White box at the feed is a 1to1 choke balun, just to make sure the coax does'nt radiate.

 

Thank you all for the help!

If I read through this tutorial series, do you think it would provide a good enough grounding for me to bootstrap from?

 

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If you're just getting started, look for the meteor scatter posts in this subforum. Yoy can build the "Sky at Night" Yagi antenna and get a cheap SDR dongle for about 50 or 60 quid all in assuming you've got a PC or laptop you can run the software on.

It's a simple and satisfying project to dip your toes, you don't need anything more than a small hacksaw, tape measure, Stanley knife and screwdriver by way of tools to build it. You can then look to try other projects using the SDR like weather sat data.

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50 minutes ago, IanL said:

If you're just getting started, look for the meteor scatter posts in this subforum. Yoy can build the "Sky at Night" Yagi antenna and get a cheap SDR dongle for about 50 or 60 quid all in assuming you've got a PC or laptop you can run the software on.

It's a simple and satisfying project to dip your toes, you don't need anything more than a small hacksaw, tape measure, Stanley knife and screwdriver by way of tools to build it. You can then look to try other projects using the SDR like weather sat data.

My friend and I both bought nooelec SDR dongles for about £24 (to dip our toes). Mine burned itself out after about half an hour of AM listening and had to be returned and his runs hot so they clearly aren't of the best quality... But if we get along with this we will probably invest in much better kit.

I'll check out the subforum, cheers again :)

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They definitely run quite hot, that is normal hence the aluminium casing. It would be simple enough to add an old computer heatsink and fan if a concern.

I have two that have run 24/7 for a large part of this year including the epically hot summer without any ill effects so would be happy to buy another if needed.

 

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58 minutes ago, IanL said:

They definitely run quite hot, that is normal hence the aluminium casing. It would be simple enough to add an old computer heatsink and fan if a concern

 

Reportedly a useful mod to do in this case for example

Robin

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10 hours ago, Carl Reade said:

I take it part of your question is how to amplify the signal to a useable level?

Carl

Well, I suppose the general area of converting EM in the air into voltage in a wire efficiently.

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Hello... if you superimpose a wave on the dipole, one end will be at the point where the wave crosses the horizontal (zero) axis, and the other end will be at the peak of the wave. The EM wave induces a voltage differential between the two ends, and by splitting the dipole in the middle and connecting each side to the opposite sides of a circuit (via the coax), a current will flow through it which can then be measured.

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Posted (edited)

How can the absorption of electromagnetic energy by a conductive metal (antenna) be explained if electromagnetic radiation is interpreted as packets of energy (photons)? How can the interaction of photons with an antenna create an alternating electrical current?

Edited by Coto

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