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.
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.
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
. 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
'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.