|Shown in shipping
Tripod top at left, then electrical box, then antenna.
|Another view of antenna in
The bottom of the tripod head can be seen.
|The Tripod has been pulled
of the antenna.
|Antenna (still folded for
mounted on tripod.
|Another view fully
|The two top holes are for
tripod legs in the stowed position.
It's not clear what the center hole is used for? Maybe there's an alternate tripod/mount that uses this hole? Let me know.
|Instead of using
lines to do the phasing needed to generate circular
antenna uses a 90 degree hybrid coupler.
The BNC connector feeds the IN port and a termination is on the ISOlated port. The 0 and -90 degree ports feed adjacent tubes in the antenna structure. This is done by running the coax up the tube and connecting at the front of the antenna where the elements are diagonally connected.
The BNC connector shows a DC short and that's normal since there is a DC path from the IN port to the -90 port which in turn is connected to ground.
|Front of antenna showing
electrical connection between diagonal supports.
Most microwave antennas for satellite communications use a parabolic reflector and a feed. The feed is designed so that it illuminated the reflector with as strong a signal as possible and also so that it does not send any signal that does not intercept the reflector. Any feed spill over will result in wasted transmit power and increased noise when receiving. The noise increase is because the dirt is at ambient temperature but the sky temperature at microwave frequencies is almost zero Kelvin.
For reception the figure of merit is G/T where G is the antenna gain and T in the noise temperature.
When C-band satellite first came out it was the way cable TV head end systems got the programming to feed the cable. So if you put up a medium sized (8 to 20 foot diameter) dish at your house you could watch cable movies for free. That lasted for a number of years and then they added encryption to prevent the free movies. I'd estimate that 99% of those dishes were made with an f/d (Focal Length / diameter) ratio of about 0.45 since that provided the highest gain. But that was a mistake since for the lowest G/T the f/d should be more like 0.3 (i.e. a deeper dish, not as flat as the 0.45 f/d type).
The 10 foot spun aluminum dish I got from the Stanford radio astronomy site in Portola Valley (the other side of 280 from the big dish) had an f/d of 0.3 and outperformed larger dishes that had a 0.45 f/d.Modern Ku-band satellite antennas use an orange peal dish shape with a offset feed. The C-band dishes were round and had a centered feed. The offset feed eliminates the problem of the feed blocking some of the dish area which reduces the gain and adds noise. But you need to take into account the reflection angle. My first Ku-band antenna was improperly installed and I've seen other Ku-band antennas with the same problem. To see it look at the lower antenna mounted on the wall and whose beam hits the metal gutter at:
The Free To Air antenna is round, but it has an offset feed.
[an error occurred while processing this directive] page created 17 Jun 2010.