Another happy customer

Discussion of large prime focus satellite dishes used for C-Band signal reception.
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tek2000
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Another happy customer

Post by tek2000 » Mon Apr 09, 2018 5:23 am

All,

A customer sent us this pic of his new dish installation. This is our 10' Mesh Antenna with pyramid base pole mast. I understand he is on the west coast and just locked his first satellite with the 4K channels on 101W. If I recall correctly, I think he said he used 30 bags of cement to build the pad.

Well done and enjoy your new dish!!

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Re: Another happy customer

Post by tek2000 » Mon Apr 09, 2018 5:30 am

If anyone else would like to share their installation pictures, please don't hesitate to do so. As you know, we have many different types of pole styles and antennas for practically any type of installation and seeing a picture of some of these installations will help newbies decide what is the best approach for them.
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Re: Another happy customer

Post by Arion » Mon Apr 09, 2018 7:09 am

30 bags of cement....wah!!! Fine job there. I only used 7 bags on my 10' dish from you with a pole mount. Wasn't sure how it was going to hold up during a U.P. Michigan winter but it did just fine as long as I would brush the snow off the dish after every storm otherwise it would sag too much. Still have to see about frost heaving when we finally get our snow melt but so far so good.
10' Tek C-band dish, 1.2 meter KU band dish, ASC1 mover, Octagon/Zgemma/Icecrypt dual band STB's.

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Re: Another happy customer

Post by fatso » Tue Apr 10, 2018 12:28 am

For sure the OP did an excellent job. It looks like he's got some trees behind the dish which will dampen high wind gusts and allow the dish to be operational at higher wind speeds. If I remember correctly, the accepted rule of thumb for pad constructions back in the day was

Dish Radius x Dish Radius x 2(Dish Pole Diameter)

In the above example

Dish Radius = 5ft
Dish Pole Diameter = 4.5 inches


So I would construct a pad that is at least 5ft x 5ft x 9 inches deep
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Re: Another happy customer

Post by belter-one » Wed Aug 01, 2018 5:22 pm

Hello,

We replaced our C band only LNB with a C/Ku combo LNB, and expected to see some signal loss. However, since the C/Ku LNB is longer that the C band only LNB, I was able to find a position further away from the dish centre that works far better. Our NASA and Reelz SNRs (Galaxy 13) went from ~ 9.4 and 10.2 to 14.2 and 12.0 respectively. Awesome! We are now able to tune in a lot more programming on all of the arc we can access, which is circa 139W to 107.3W. The rest of the arc is hosed by trees, so far. My question is, when trying to access Ku channels off of this LNB, can I assume that a good LNB location for C-Band signal, will also work optimally for Ku signals? I know that there is a lot of wisdom and experience on these boards, so any information on this will be greatly appreciated.

Enjoying this C/Ku LNB from tek2000 immensely,

belter-one

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Re: Another happy customer

Post by tek2000 » Wed Aug 01, 2018 7:35 pm

belter-one,

Glad you like the dish!!

What you discovered while experimenting with the location of your feed is that the optimum performance of the antenna occurs when the phase center of the feed coincides with the focal point of the reflector. Let me explain.

Plane waves incident on a parabolic reflector focus at a single point which is known as the focal point. Conversely, spherical waves emanating from the focal point are reflected by the parabolic reflector and form plane waves. Antennas work exactly the same whether they are operated in TX (transmit) or RX (receive) mode. Sometimes it is useful to think of an antenna (or feed) operating in TX mode, while at other times it is more convenient to think in RX mode.

The phase center of a feed is defined as that point within the feed structure from which (in phase) spherical waves emanate when the feed is excited. What all this means is that the power collected by the little probe in RX mode within the feed will be maximum when the phase center of the feed is coincident with the focal point of the reflector.

If the phase center and focal point are not coincident and we could observe the radiation of the antenna in TX mode, we would find that the antenna does not emit plane waves, but waves with a slightly distorted pattern. But if this same antenna was operated in RX mode, for maximum power reception, it would also expect incoming distorted waves. Since the satellites in geostationary orbit emit waves that become perfect plane waves when they reach our antennas, energy would would be lost (actually, some of the energy gets reflected).

Every feed has a different phase center. This is why people get confused and report different focal lengths for our antennas. In fact, the focal length is a constant for the antenna, but the phase center changes with different feeds. At a certain Dish Network sponsored forum, the geniuses at that place ran across the same problem and concluded that we were publishing inaccurate focal lengths on account of our inability to use a tape measure. :utterlyconfused

People need to understand this concept and understand that if they change feeds, they will have to experiment to get the phase center to coincide with the focal point for optimum efficiency. This is why all our new antennas have several pre-drilled placement holes for the feed arms to help people find the perfect spot for the feed.

While we are on this topic, another way of thinking about these matters is in terms of wave transmission, reflection and absorption. When a travelling wave moves from one medium to another, some energy will be transmitted and some will be reflected. You all know what happens when a wave hits a metal wall - nearly 100% of the wave energy gets reflected. If this same wave hits a brick wall, well, maybe 10% gets transmitted, 10% gets reflected and the rest is absorbed by the bricks. It is no different when a wave encounters a parabolic reflector, a feedhorn or a waveguide or the probe within the waveguide. Some energy will be transmitted, some will be reflected and some will be absorbed. To maximize energy transmission at the medium boundary, you have to ensure that the wave fronts leaving the one medium are exactly identical (or as close as possible) to the wave fronts that can exist in the second medium. Any mismatch will result in power loss. This is the correct way of thinking about these matters if you wish to design something that works properly. The rest is geometry and arithmetic subject to the medium boundary conditions...

That being said, if any of you want to understand the purpose of loading the feed with a scalar ring, you can read about it here:

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

Our TVRO :bow: 'ancestors' discovered, or rather re-discovered (actually, it was radio astronomers who discovered this in the 1950s) that adding a ring to the throat of the feed altered the radiation pattern of the feed and made reception more efficient. Why? Depending on where and how you place the ring, the radiation pattern broadens somewhat and produces better illumination across the parabolic reflector and thus 'better' plane waves are emitted when the antenna is operated in TX mode. This means more energy will be transmitted to the feed and less will be reflected when operated in RX mode. That is all there is to it and everything else you read on the other forums about the purpose of scalar rings is mostly hot air.

The point of all this is to get people thinking correctly about these matters and convince them to experiment with their feeds until they are absolutely sure they got their antenna properly peaked. So read and re-read this post until you understand it. :bigsmile

phase_center.png
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Re: Another happy customer

Post by fatso » Thu Aug 02, 2018 11:30 pm

belter-one wrote:
Wed Aug 01, 2018 5:22 pm
My question is, when trying to access Ku channels off of this LNB, can I assume that a good LNB location for C-Band signal, will also work optimally for Ku signals?
belter-one
Not necessarily. Getting both c and ku with one LNB will always be a compromise. If you find the sweet spot for c band, you will find your ku will be subpar. The opposite is also true. Best solution is to have c and ku LNBs ride shotgun beside each other so you can adjust each independently. Other solution is to separate bands and use a small dish for ku.
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Re: Another happy customer

Post by belter-one » Fri Aug 03, 2018 7:38 pm

Thank you all,

For this useful information. Got a lot of reading, experimentation and DIYing in my future.

Enjoying the Clarke Belt,

Belter-one

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