a gold mine and about an hours drive
will probally jump on that and store it till i move to somewhere more bud friendly
fabricating a cband dish
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a gold mine and about an hours drive
will probally jump on that and store it till i move to somewhere more bud friendly
Judging from the photos, the reflective surface is not a true parabolic. Instead of the focal point being the size of a tennis ball, it's the size of a beach ball and can't get into the feedhorn. Much of the reflective surface is not being used because the microwaves are being reflected someplace other than the focal point, and many of the ones that are at the focal point are out of phase and cancelling eachother out. End result is a giant dish that performs like a six footer.
Harold
Actually, without expensive RF probe and lab equipment you'll not know for sure what position/size of the 'ball', or its shape, or phase shift. I'm not telling you about RF chamber of the size (!). 
One thing worried me and I did ask nelson - parabolic curve precision; if he controlling only radial directions, it wouldn't be enough - flat segments will create unwanted diffusion of incoming signal in chord (90 degree) paths and will reduce efficiency of the big dish.
One thing worried me and I did ask nelson - parabolic curve precision; if he controlling only radial directions, it wouldn't be enough - flat segments will create unwanted diffusion of incoming signal in chord (90 degree) paths and will reduce efficiency of the big dish.
Until the surface is dulled or painted the focal point is visible at night or in a darkened room by shining a floodlight boresighted on the dish.
A mold machined with CNC machinery with a plus or minus 0.001" accuracy only produces an FRP or stamped metal dish rated as 50% efficient by most manufacturers. Some of the Oriental manufacturers are now claiming 75% efficiency with their stamped dishes, but the "old" rule of thumb was 50%.
Bending the struts to any degree of accuracy is hard enough, but welding them together accurately is another problem all together.
Diffusion equals signals out-of-phase that cancel out in-phase signals drastically reducing efficiency or a focal point three feet further out than it should be..
Electrically, the big dish is now really a fraction of it's physical size.
If you wish we go into more deep details - the light's wavelength is WAY out for measure 5 GHz RF focal size. I emphasized the mentioned "ball" characteristics - and would tell you again: you can't get them by using flashlight.
I'm see the issue as reflection in other direction not to focal point, if you circular profile approximated by flat segments in the dissection.
- I'm talking about same effect.
why dont you borrow a birdview dish, pour fiberglass in it, then glue some screen dowm, pur a little more, the viola, then make your own mount
Can you explain why a flashlight or floodlight would not work??? I was going to try and shine my car's headlight on a DN 500 dish and see if holding a black piece of craftpaper in front of the lnb to see if it show a round spot of light as a focal point. I would love to hear your opinion.
Thanks
mikelib
I beg to differ with your opinion, which is not founded by science or engineering, but a parabolic surface is not frequency dependant. It does not know if it's reflecting mircowaves, lightwaves, or soundwaves.....It just reflects what hits it. In its geometry, any "wave" (electromagnetic, sound, or light) reflected travels exactly the same distance to the focal point, arriving in phase with all the other "waves".
OK, we are talking about measuring REAL characteristics of REAL surface. So, the resulting 'ball' or focal spot will depend on interference of real length of waves.
You know, if you ever works with RF chambers as I'm you wouldn't ask the question.
While I'm agree the visual test will be starting point for beginning real tests. Use it, but not deduct from the result you got it works as expect with 5 GHz range.
You know, if you ever works with RF chambers as I'm you wouldn't ask the question.
While I'm agree the visual test will be starting point for beginning real tests. Use it, but not deduct from the result you got it works as expect with 5 GHz range.
Mr. Smith, I was under the same impression as caddata, and I was hoping that you could share more information why this would not work. I am trying to learn as much as I can about satellite parabolic dishes. I am reading W1GHZ antenna book and he does mention using the sun for sun reading. W1GHZ Microwave Antenna Book ONLINE
I will try the sun test for a focal point asap, thanks for sharing the info, I am always trying to learn.
mikelib
In short - different length of waves - different interference while other parameters of a reflector are the same.
Wave length has nothing to do with it because the parabolic curve acts as a mirror, just a reflecting surface. The antenna located at the focal point is resonate to a specific band of the reflected frequencies.
If you will recall physical law of electromagnetic waves, you must accept a fact: with all other equal parameters, different wavelength will create different interference field.
Add to that not ideal surfaces (this is the point of discussion) and you will reach some knowledgeable position.
Add to that not ideal surfaces (this is the point of discussion) and you will reach some knowledgeable position.
Well Guys, at 12:30 pm I was in front of my house with a 18 x 20 inch offset Dtv dish and a 20 x 23 inch offset Dn dish and I point the dishes at the sun and tilted the dish around until I had the surface fully luminated (by eye) and I held a dark blue square peice of paper in front of the LNB and I did not get any noticeable beam of light on the paper.
What I did notice that I took the dishes out of a cold garage and with 5 minutes of pointing the dish at the sun, the dishes got hot. I was touching the front and the back of the reflector and they were hot to touch. I guess solar reflection works.
I'll try the headlight from the car when it gets dark tonight. Interesting thread and dialog on parabolic dish design.
mikelib
What I did notice that I took the dishes out of a cold garage and with 5 minutes of pointing the dish at the sun, the dishes got hot. I was touching the front and the back of the reflector and they were hot to touch. I guess solar reflection works.
I'll try the headlight from the car when it gets dark tonight. Interesting thread and dialog on parabolic dish design.
mikelib
Well, that was predictable - I would clean paint to bare metal, polish it and then return to sun light test.
- I would clean paint to bare metal, polish it and then return to sun light test.
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