High Efficiency Multimode C-Band Feeds

Discussions about low-noise blocks, waveguides, horns and other components used at the prime focus of your reflector dish.
elko
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High Efficiency Multimode C-Band Feeds

Post by elko » Sun Sep 11, 2016 12:22 am

Has anyone played around with mulitmode feeds?

For those that don't know, our regular scalar ring, LNBs, LNBFs are all designed to capture just one mode (dominant mode or TE11) of RF power, even though there are an infinite number of RF modes in the focal region of our antennas. Fortunately, the TE11 mode contains 84% of the incident RF power. Our antennas discard the other 16% of RF power. :verysad

The next mode with the most power is the TM11 with another 10%. If we designed a feed assembly to extract both TE11 and TM11, we could in theory reach 94% efficiencies :biggrin with our antennas. This source explains the concept

http://www.w1ghz.org/antbook/ch6_5-2.pdf

I don't know how to measure the efficiency of my c band dish but I would guess that my Chaparral scalar ring with ortho feed is not much better than 60%. This is the primary reason most of us with 8' or 10' antennas can't always lock the 16apsk or 32apsk stuff.

I also looks at CSD and CATJ but didn't come across any manufacturer attempting this in the 1980s. I am surprised that Taylor Howard and Steve Birkhill, the two smartest guys of that era didn't look into this. :utterlyconfused

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Re: High Efficiency Multimode C-Band Feeds

Post by Foxcreek » Sun Sep 11, 2016 1:56 pm

elko,

It's neat to see that your on the right track here. You'll notice that
as I've pointed out before the real sources of innovation in this
technology are always "HAMS".

foxcreek :cool2

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Re: High Efficiency Multimode C-Band Feeds

Post by Foxcreek » Sun Sep 11, 2016 2:33 pm

elko,

This is the feed horn that I'm currently using on my 1.2 meter
Patriot.

foxcreek :cool2
IMG_0279_1.JPG
Attachments
IMG_0281.JPG
IMG_0280.JPG

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Re: High Efficiency Multimode C-Band Feeds

Post by elko » Mon Sep 12, 2016 6:51 pm

Thanks for the pic foxcreek! That looks like a corrugated conical horn for ku band but I don't see how it will bring multiple modes in phase. How much does it improve the gain over the standard mickey mouse ku lnbfs?

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Re: High Efficiency Multimode C-Band Feeds

Post by elko » Mon Sep 12, 2016 7:03 pm

By the way guys, you can easily make a waveguide band pass filter in this manner
220px-Waveguide-post-filter[1].jpg
220px-Waveguide-post-filter[1].jpg (6.21 KiB) Viewed 15813 times
Although this is a rectangular guide, the same trick will work with a circular guide. Just drill some holes in the right places and then place this circular guide between your scalar and LNBF. The dimensions for 7.2GHz BPF are like this
Five-resonator triple-post waveguide bandpass filter made by Ferranti. A length of WG15 (WR112) is divided into five coupled resonant cavities, by fences of three posts each. Each of the two end-cavities is separated from its respective port by a single post and a short adjustment screw. The ends of the posts protrude through each broad-wall of the guide, making them clearly visible from the outside. The resonators are each about 30 mm long and have a central tuning screw. All seven screws are secured by jam-nuts and thread-lock and are normally protected by plastic caps (removed for the photo). Centre frequency 7.200GHz Dimensions / mm (averaged and with spurious last digit) Along guide (post-diameter)<---post-centre-to-centre-distance--->(post-diameter): (3.18)<---29.40--->(2.82)<---31.34--->(3.18)<---31.55--->(3.18)<---31.34--->(2.82)<---29.40--->(31.8) Fence pitch across guide: 7.11 Tuning screw diameter: 3.95, insertion ~2 End screw diameter ~2.2, insertion ~2 Guide: WG15-ish (tapers too narrow to too wide)

You can scale the dimensions down to 4GHz and then play with the screw settings to see if you can improve reception on a particular transponder.

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Re: High Efficiency Multimode C-Band Feeds

Post by Foxcreek » Mon Sep 12, 2016 11:03 pm

elko,

You know Steve Birkill is still around. If you want to communicate with him
he's available on Linkedin .

foxcreek :cool2

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Re: High Efficiency Multimode C-Band Feeds

Post by elko » Wed Sep 14, 2016 1:03 am

Foxcreek wrote:elko,

You know Steve Birkill is still around. If you want to communicate with him
he's available on Linkedin .

foxcreek :cool2
Coooool!

I will let him know that a new generation of tvro enthusiasts is taking interest in his work after all these years.

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Re: High Efficiency Multimode C-Band Feeds

Post by Foxcreek » Wed Sep 14, 2016 11:20 am

elko,

And don't be afraid to ask about incorporating more modes in
feed horn design. I'd like to know the answer to this myself.
He is definitely the guy to ask because I know that this has
crossed his mind previously.

foxcreek :cool2

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Re: High Efficiency Multimode C-Band Feeds

Post by tek2000 » Wed Sep 14, 2016 9:40 pm

All this business about feeds comes down to the fact that the focal field generated by a paraboloid reflector miraculously assumes the same mathematical form as the dominant fields supported by a circular waveguide. Were it not for this happy state of affairs, the paraboloid would be useless to us as an efficient reflector.

I won't bore everyone with the mathematical details, but the E and H fields at the focal plane takes the form shown below (eq. 2.12 and 2.13). Now, if you check any microwaves handbook, you'll find that the TE1p and TM1p (where p=1,2,3...) modes supported by a circular waveguide take the same form. If you could add all these modes up (and do it in phase), then your feed would be 100% efficient because all the energy in the focal region would be coupled into your perfect feed.

Unfortunately, the perfect feed is unrealisable because different diameter circular guides are needed. For example, the TE11 mode is supported by a 2" diameter guide whereas the TM11 mode requires close to 4". To combine them, you would have to gradually taper from 4" to 2" and get the fields in phase. Combining more the two modes gets even more complex and hardly worth the trouble.
focal_field_1.jpg
focal_field_2.jpg
Last edited by tek2000 on Wed Sep 14, 2016 11:52 pm, edited 1 time in total.
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Re: High Efficiency Multimode C-Band Feeds

Post by tek2000 » Wed Sep 14, 2016 11:49 pm

As for the history of feed horns, these things were first seriously investigated in the 1940s for use with radar antennas. The most common type (and easiest to fabricate) is the rectangular horn. This was the horn first used by Taylor Howard in the mid 1970s when he constructed his first home-brew TVRO. He actually wrote (and sold) a manual called 'The Howard Terminal Manual' on how to construct these horns. See CSD, April 1980, page 12

http://theoldcatvequipmentmuseum.org/22 ... ressed.pdf

Keep in mind that Howard and Cooper were 'pals', hence the favorable reviews (and marketing) of anything Howard related in those days. At this point, I think Howard started to do some digging at his University library and found some papers published by radio astronomers on how corrugations inside a tapered conical feed horn could improve efficiency even more (i.e. better match for the focal region fields discussed above). It turns out radio waves don't like to transition abruptly between two media. For example, if you don't include any kind of tapered horn at the throat of your circular/rectangular waveguide, those waves will experience edge diffraction and not match the focal region equations mentioned. The astronomers found that 1/4 wavelength corrugations were needed to minimize edge diffraction!

Armed with this information, Howard developed the modern commercial scalar ring for deep TVRO dishes (i.e. f/D <0.35). It is a much simpler feed than a properly tapered corrugated horn. Actually, you can think of it as a conical corrugated horn with the widest possible flare angle (i.e. 90 degrees)!

It was reviewed in CSD, July 1980, page 10

http://theoldcatvequipmentmuseum.org/22 ... ressed.pdf

Okay, so now Howard got almost everything right. He transitioned from rectangular to circular guides which better match the focal region equations and he added those 3 (quarter wavelength) chokes which reduce edge diffraction as the radio waves enter or leave the circular guide. I can understand why he chose a scalar choke feed over a more efficient corrugated horn (easier to construct), but why did he settle on a 6" diameter with only 3 chokes? If you look at those focal region equations, you can see that a larger diameter scalar horn will encompass more of the focal region and thus pass more power to the waveguide. Of course, there is a limit to this, because if you make the scalar ring too wide, it will cast too large a shadow on the dish and all bets are off. Is there an optimum scalar diameter?

Also working on TVRO feeds during the late 1970s was Steve Birkhill. He read the same papers as Howard and designed the same scalar ring as Howard, except it had 7 chokes and was almost 10" in diameter. You can find the details in CSD, February 1980, page 9.

http://theoldcatvequipmentmuseum.org/22 ... ressed.pdf

So which is the better designed feed? Howard or Birkhill? Let's see if anyone knows. :thinking

By the way, if you are using a modern Chaparral feed it is based on Howard's original design. If you are using a cheap Chinese imitation, you might as well take it off and throw it away because the Chinese have butchered the choke dimensions (for economic reasons) to the point where they are no longer a quarter wavelength and won't increase your efficiency beyond what your circular waveguide gives you by itself (though these Chinese feeds should perform well at 6.5GHz). :doh

The reason for this post is to give the layman some insight into TVRO feeds and help take the mystery out of them. It is probably the most critical component of your satellite system, but it is so poorly understood and so poorly designed, that most TVRO antennas in the field are under performing by at least 1.5 - 2.0 dB. In fact, when I searched all the satellite boards on the net for information on this topic, I found one interesting post from an individual who claimed his 7.5' solid dish gave the same performance as his 10' mesh dish, so he simply came to the conclusion that a solid dish will always outperform a mesh one. Little did he realize that the culprit for his underperforming 10' mesh dish was the poorly matched feed.
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