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Parabolic Dish Questions
I have a Directv dish 35 inches wide x 20 inches tall 4inches deep.
I calculated the focus as F=D*2 / 16d where D =diameter and d=depth F=18 inches Ok so far. How do you find beamwidth? I think the 35" dimensiom would have a tighter pattern than the 20 ", but how do you calculate, Second, The feed antenna, There can be different feeds. A simple monopole, dipole, biquad, patch, yagi, helix. How does the feed antenna affect the gain of the dish, if at all? Yea, it my be difficult to light the yagi or helix, so if you like throw those out. Thank You, Mike |
Parabolic Dish Questions
On Thu, 26 Jun 2008 07:35:31 -0500, "amdx" wrote:
I have a Directv dish 35 inches wide x 20 inches tall 4inches deep. I calculated the focus as F=D*2 / 16d where D =diameter and d=depth F=18 inches Ok so far. How do you find beamwidth? I think the 35" dimensiom would have a tighter pattern than the 20 ", but how do you calculate, Second, The feed antenna, There can be different feeds. A simple monopole, dipole, biquad, patch, yagi, helix. How does the feed antenna affect the gain of the dish, if at all? Yea, it my be difficult to light the yagi or helix, so if you like throw those out. Thank You, Mike You have a neat gadget here! If you place a lamp (flashlight?) at the focus you can measure the beam on a wall wth a tape measure. Put a microphone at the focus and you can hear things you would not believe... Old ARRL antenna books, and authors like John Krause will be needed to answer all of your questions. John Ferrell W8CCW Beware of the dopeler effect (pronounced dope-ler). That's where bad ideas seem good if they come at you fast enough. |
Parabolic Dish Questions
"John Ferrell" wrote in message ... On Thu, 26 Jun 2008 07:35:31 -0500, "amdx" wrote: I have a Directv dish 35 inches wide x 20 inches tall 4inches deep. I calculated the focus as F=D*2 / 16d where D =diameter and d=depth F=18 inches Ok so far. How do you find beamwidth? I think the 35" dimensiom would have a tighter pattern than the 20 ", but how do you calculate, Second, The feed antenna, There can be different feeds. A simple monopole, dipole, biquad, patch, yagi, helix. How does the feed antenna affect the gain of the dish, if at all? Yea, it my be difficult to light the yagi or helix, so if you like throw those out. Thank You, Mike You have a neat gadget here! If you place a lamp (flashlight?) at the focus you can measure the beam on a wall wth a tape measure. It grey, won't reflect light very well. Basically I wanted to know how the beamwidth changed with the larger width dimension vs the smaller height dimension Put a microphone at the focus and you can hear things you would not believe... I already experienced the reflection. I had a radio playing on my right side, I was moving the dish and got an increase in volume in my left ear. I moved the dish back to play around with the effect. Neat! Old ARRL antenna books, and authors like John Krause will be needed to answer all of your questions. John Ferrell W8CCW Beware of the dopeler effect (pronounced dope-ler). That's where bad ideas seem good if they come at you fast enough. |
Parabolic Dish Questions
"amdx" wrote in message ... I have a Directv dish 35 inches wide x 20 inches tall 4inches deep. I calculated the focus as F=D*2 / 16d where D =diameter and d=depth F=18 inches Ok so far. How do you find beamwidth? I think the 35" dimensiom would have a tighter pattern than the 20 ", but how do you calculate, Second, The feed antenna, There can be different feeds. A simple monopole, dipole, biquad, patch, yagi, helix. How does the feed antenna affect the gain of the dish, if at all? Yea, it my be difficult to light the yagi or helix, so if you like throw those out. Thank You, Mike The DirecTV dishes are not prime focus, bu rather offset. If it came with the original horn mount, use it. You can use: http://www.w1ghz.cx/software/hdl_3b4.zip to find the focal length,. Your formula assumes prime focus. The gain of the dish is not a function of the feed, but the system gain is because one must take into account surface illumination, feed efficiency etc. The subject of feeds for dishes is complex if you really want to optimize G/T etc. Paul W1GHZ has a teriffic on-line text about parabolic surfaces and feeds: http://www.w1ghz.org/antbook/preface.htm Dale W4OP |
Parabolic Dish Questions
On 26 jun, 14:35, "amdx" wrote:
I have a Directv dish 35 inches wide x 20 inches tall 4inches deep. I calculated the focus as F=D*2 / 16d where D =diameter and d=depth F=18 inches Ok so far. How do you find beamwidth? I think the 35" dimensiom would have a tighter pattern than the 20 ", but how do you calculate, Second, The feed antenna, There can be different feeds. A simple monopole, dipole, biquad, patch, yagi, helix. How does the feed antenna affect the gain of the dish, if at all? Yea, it my be difficult to light the yagi or helix, so if you like throw those out. Thank You, Mike Hello Mike, In case of uniform illumination, gain is maximum, side lobes are maximum also and beam width is about 60*lambda/diameter (degrees). You will certainly not reach this, for several reasons. The beam width will be more, it can be twice as high (depending on the illuminator). The maximum gain that you can obtain with a certain aperture antenna is about 4*pi*A/lambda^2. In reality it will be less: When you want uniform illumination, you will have "spill over" (part of the radiated power will not reach the dish). To get most power onto the dish, you must accept that the edges of the dish do receive less power flux density. In addition, the phase center of your illuminator will change versus off-main beam direction. This causes phase differences so a non-planar field. Also polarization may change for off-beam directions. Other things are deviation from optimal parabolic shape, non correctly positioned illuminator, etc. This will all reduce maximum gain. Hope this helps a bit. Best regards, Wim PA3DJS www.tetech.nl |
Parabolic Dish Questions
"amdx" wrote in message
... I have a Directv dish 35 inches wide x 20 inches tall 4inches deep. I calculated the focus as F=D*2 / 16d where D =diameter and d=depth F=18 inches Ok so far. How do you find beamwidth? I think the 35" dimensiom would have a tighter pattern than the 20 ", but how do you calculate, Thank You, Mike The DirecTV dishes are not prime focus, but rather offset. If it came with the original horn mount, use it. Yes, it is an offset feed and it did come with the mount and feed (LNA?) Actually it came with two LNAs, but they didn't match each other. I don't remember what junk pile this came from and don't have any info about it's working condition. I did measure the distance from the front of feed to the dish center and it is 18" as calculated, however as I said I don't know if this worked as I got it. You can use: http://www.w1ghz.cx/software/hdl_3b4.zip to find the focal length,. Your formula assumes prime focus. This link doesn't look right. The gain of the dish is not a function of the feed, but the system gain is because one must take into account surface illumination, feed efficiency etc. So I could use a biquad or patch as the feed antenna without much difference? The subject of feeds for dishes is complex if you really want to optimize G/T etc. Paul W1GHZ has a teriffic on-line text about parabolic surfaces and feeds: http://www.w1ghz.org/antbook/preface.htm Dale W4OP I'll look at the w1ghz link. I was suprised this dish is made of plastic. Does the EM wave reflect off of the plastic or does the plastic have a coating? Thanks, Mike |
Parabolic Dish Questions
"Dale Parfitt" wrote in message news:7xR8k.35$wR.10@trnddc07... "amdx" wrote in message ... I have a Directv dish 35 inches wide x 20 inches tall 4inches deep. I calculated the focus as F=D*2 / 16d where D =diameter and d=depth F=18 inches Ok so far. How do you find beamwidth? I think the 35" dimension would have a tighter pattern than the 20 ", but how do you calculate, Second, The feed antenna, There can be different feeds. A simple monopole, dipole, biquad, patch, yagi, helix. How does the feed antenna affect the gain of the dish, if at all? Yea, it my be difficult to light the yagi or helix, so if you like throw those out. Thank You, Mike The DirecTV dishes are not prime focus, bu rather offset. If it came with the original horn mount, use it. Hey I just found the dish I have it's at http://www.solidsignal.com/satellite...enna_types.asp Last dish on the right side of the page, it's called the International Satellite Dish. International 36". I note from the picture the feeds don't match, when I got the dish the position of the two feeds were reversed. What can you glean fron this? Thanks, Mike |
Parabolic Dish Questions
amdx wrote:
"amdx" wrote in message ... I have a Directv dish 35 inches wide x 20 inches tall 4inches deep. ===================================== I was suprised this dish is made of plastic. Does the EM wave reflect off of the plastic or does the plastic have a coating? ============= It is possibly an aluminium foil reflector sandwiched between 2 layers of plastic. I have a 1 metre dia sat dish with the aluminium foil sandwiched between 2 layers of fibre glass/epoxy. Frank GM0CSZ / KN6WH |
Parabolic Dish Questions
It grey, won't reflect light very well. Basically I wanted to know how the beamwidth changed with the larger width dimension vs the smaller height dimension Tape aluminum foil to it tobetter reflect light... John Ferrell W8CCW Beware of the dopeler effect (pronounced dope-ler). That's where bad ideas seem good if they come at you fast enough. |
Parabolic Dish Questions
"Wimpie" wrote in message ... On 26 jun, 14:35, "amdx" wrote: I have a Directv dish 35 inches wide x 20 inches tall 4inches deep. I calculated the focus as F=D*2 / 16d where D =diameter and d=depth F=18 inches Ok so far. How do you find beamwidth? I think the 35" dimensiom would have a tighter pattern than the 20 ", but how do you calculate, Second, The feed antenna, There can be different feeds. A simple monopole, dipole, biquad, patch, yagi, helix. How does the feed antenna affect the gain of the dish, if at all? Yea, it my be difficult to light the yagi or helix, so if you like throw those out. Thank You, Mike Hello Mike, In case of uniform illumination, gain is maximum, side lobes are maximum also and beam width is about 60*lambda/diameter (degrees). My dish is 35" wide x 20"tall So if Lambda=4.89" Then 60 x 4.89 / 35 = 8.38* and 60 x 4.89 / 20 = 14.67* Then my beamwidth will be taller than it is wide. You will certainly not reach this, for several reasons. The beam width will be more, it can be twice as high (depending on the illuminator). Yes. it's always something. The maximum gain that you can obtain with a certain aperture antenna is about 4*pi*A/lambda^2. In reality it will be less: I was going to attempt this but I don't know how to calculate the area (A) of my non circular dish. Any help there? Thanks, Mike |
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