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Homemade horn antenna for C/Ku-band satellite ?

 
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Vorg
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PostPosted: Thu Mar 18, 2010 6:21 am    Post subject: Homemade horn antenna for C/Ku-band satellite ? Reply with quote

This may seem like an crazy project, but I would like to build a horn antenna to pick up ku and c-band satellite using an lnb like the DMX741U. I know the thing would be long, but bud's are typically 8-10' in diameter and as I understand it, horns need a smaller inlet then the diameter for the same gain dish. I have found 2 examples of people building horns for ku:
http://www.satcrazy.com/horn_antenna.php
http://www.satellites.co.uk/forums/picture-members-setups-general-satellite-installations/125121-my-collection-homemade-antennas-ku.html

I would like to include a right angle entry like the old radio telescopes. This would allow the rotation to be along the length of the horn reducing the area it needs to cover the satellites and reducing both the effective rotational mass and wind loading stress on the motor.

I want to make it out of styrofoam and aluminum foil or metalized paint to keep it light, again to reduce load on the motor, and cheap. Also, I read that styrofoam is the same as air to microwaves.

I have done a lot of searching for information, but most ends up in complex math formulas with symbols I don't know. But based on what I have read, conical would be better then pyramidal. Though pyramidal would be a little easyer to build, a lathe is simple with foam. A way to rotate it and a hot wire/knife.

Corrugated horns seem to be the popular choice having higher efficiency though I don't know if that apply only to transmitters. According to "22a-UPNA_AG_2_A4.pdf" a "Gaussian profiled
corrugated" horn is supposed to be among the best. It also says:
Quote:
In Spring 2002, researchers from the antenna group at UPNA discovered a new way to reduce length, improve radiation properties and facilitate manufacturing of corrugated horns. They used the combination of two technologies, horizontal corrugations for the throat of the horn and vertical corrugations for the flare region.
I would like to keep the length down to the 8-10' range. Laughing.

The pdf "antennas__mcgraw-hill__2nd_ed__1988__-_john_d_kraus.pdf" had something about adding a rolled back lip also helped, but that might not apply when combined with a reflector to give it a right angle view.

I might mount a second smaller ku only horn with it's length at right angles, direct view, not right angle like the main horn, to the main for ku only for circular polarized sats. Right now though they are mainly pay tv only, but a new fta service may be starting up: http://www.freedbs.org/.
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bigSteve
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Joined: 14 Mar 2009
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PostPosted: Sat Mar 20, 2010 9:36 pm    Post subject: Reply with quote

I will attempt to respond to some of your comments based on my knowledge of antennas, in a somewhat random manner.

Yes, styrofoam is equivalent to air as far as RF propagation goes. You'll just need to seal it somehow for weather protection.

The design of a horn antenna isn't particularly hard. You start with some basic design out of an antenna book like kraus or balanis, and reproduce it. The issue is, to verify it is working you'll need a network analyzer to measure return loss (s11, or vswr). The feed position will likely need tweaked a little here and there to get it to work.

The problem is a good network analyzer costs at least 10k. This is why antennas cost so much - high barrier to entry.


In regards to the antenna research you quoted, 99% of all antenna research is nonsense-papers written to justify funding and to create phd's. And I say this as someone with a phd in antenna engineering.

In regards to efficiency, there is no difference in horn efficiency whether it is corrugated or not. Efficiency is the ratio of power radiated to power delivered to the antenna.

The length of your horn won't effect the efficiency. But as it gets longer, the aperture (or horn opening) will get larger, and you will have a more directive (more directional) radiation pattern. This leads to a higher gain for your antenna - which means more power is radiated in a narrower beamwidth or direction you are trying to send power.

If you have no resources, you'll have a tough time troubleshooting. But you could give it a shot, follow some antenna design in a book, and then try to tweak things as you go.

I'm not sure exactly what you mean by "right angle entry".
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Vorg
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PostPosted: Sun Mar 21, 2010 12:38 am    Post subject: Reply with quote

Thank you for answering. It's nice to get a response from sone one who knows the subject.

Quote:
which means more power is radiated in a narrower beamwidth or direction you are trying to send power
Most of the pages on horns is talking about transmitting. This is for receive only. As for the testing, I figured it would be a lot of trial and error finding the correct placement for the lnb. But it would be a waste of time if the horn design wasn't in the ball park. That's why the questions.

Quote:
I'm not sure exactly what you mean by "right angle entry".
A little big, but its what gave me the idea:
http://en.wikipedia.org/wiki/File:Horn_Antenna-in_Holmdel,_New_Jersey.jpeg

What about pyramidal (would use square aperture) vs conical? Square would give more aperture area and easyer to build, but round seems more popular now plus it would have to match a round wave guide in the lnb.

Does square only work with linear or will it work with circular as well. Most stuff coming down is linear but once in awhile there are circular feeds and never know whats coming with all the new satellites being sent up.
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bigSteve
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PostPosted: Wed Mar 24, 2010 9:27 pm    Post subject: Reply with quote

The one nice thing about antennas is that they transmit and receive in an identical manner, so any discussion on receive efficiency automatically apply to transmit efficiency and vice versa.

As for the horn cross section, I would personally go with the square aperture because it would probably be easier to build. But the circular aperture would work as well. As for feeding it, both can be fed with a small monopole in the center that excites the horn. The waveguide method would work, but again you would need to excite the waveguide as well, which would probably be a monopole in the waveguide.

What do you mean by circular feeds?
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Vorg
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PostPosted: Thu Mar 25, 2010 5:18 am    Post subject: Reply with quote

Satellite signals can be ether linear (Horizontal/Vertical) or circular (Right/Left)

What do you mean by fed with a monopole?

What I am think is A partial parabola at the large end to get the signal in to the horn from a right angle. LNBF goes at the small end to receive.

Couple more pics of what I'm thinking of:
http://long-lines.net/places-routes/Garden_City/010221B-17A.jpg
http://www.afcsat.com/SatelliteConicalHorn.html

However, that last one is C-band and is saying 39-43db. I going by what c-band dishes are, it needs to be 38-40db. If it's has to be that big, then it's not an option.

Been talking to a person who built some ku band here: http://www.edaboard.com/viewtopic.php?sid=c20cc399fa93e7d637a00b7a31dbda84&p=1252676#1252676

I'm a bit confused about something. If Styrofoam is like air to RF, then what's this about the dielectric property? Styrene is 2.4 and air is 1. They use teflon disc to change circular polarities to linear and it's rating is 2.
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bigSteve
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PostPosted: Fri Mar 26, 2010 12:53 am    Post subject: Reply with quote

Circular polarization is actually two orthogonal linear polarizations that are 90 degrees out of phase. So if you're antenna is linearly polarized, it will still receive a circularly polarized signal (left or right handed), but there will be an additional 3 dB (50%) of loss (polarization mismatch loss).

The dielectric constant of styrofoam at RF/microwave frequencies is about 1.01. That is, it is virtually identical to air/free space.

Using a dielectric (teflon) to change polarization from circular to linear doesn't make sense to me. I've never heard of a dielectric creating circular polarization from linear polarization.
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Vorg
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PostPosted: Fri Mar 26, 2010 6:58 am    Post subject: Reply with quote

I found out its solid Styrene that is 2.4, When it's in expanded foam form, its less.

Quote:
Using a dielectric (teflon) to change polarization from circular to linear doesn't make sense to me. I've never heard of a dielectric creating circular polarization from linear polarization.

It's a common way to make a linear lnbf work with circular polarity signals.

http://lea.hamradio.si/~s57uuu/emeconf/eme06.htm
http://www.wsidigital.com/dmx741u-cku-lnbf-for-c-band-dish.htm
Quote:
The DMX741U quad polar single LNBF can be used with both C band and Ku band, linear polarity (H/V) or circular polarity (RC/LC). Simply insert the small dielectric plate that comes with your LNBF to convert it from a linear LNBF to a circular LNBF.


http://www.galaxy-marketing.com/Dielectric-Plate-for-C-band-LNBF-&-Circular-Reception.htm
http://www.satelliteguys.us/archive/t-198359.html
http://www.freepatentsonline.com/4613836.pdf
http://forum.ecoustics.com/bbs/messages/2/440218.html

The disc is stuck in the lnb at a 45 to the normal linear polarities. signal is reduced 3db. So it's not as good as using an lnb designed for circular.
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