digidish selfsat forum -das

In the last years, i have found that some antennas with less gain (or size) can equal to a offset dish with bigger gain/size...why does this happens?


I give three examples:

". At just 43cm, this powerful dish offers the performance of an offset 60cm dish.
Gain (dB): 33.0"

Digidish 45cm
"The carefully calculated parabolic shape ensures that the reception performance of the DigiDish 45, which has a diameter of only 45 cm, is better than that of many 60 cm dishes - in spite of the much smaller size."

Selfsat.. 33dBi equal′s to a 60cms parabolic dish.

Answer: device technology for front-end receivers

I remember in the early 80's commercial satellite TV dishes were half the size of a single family home rooftop, and they had to be accurately aligned to the "bird in the sky" in order to decently receive a signal. In the mid-90's, HEMFET technology took off and a higher signal-to-noise ratio (SNR) could be received at the dish.

One of the major developments in HEMFET technology was the placement of the transistors conducting channel a few hundreds of nanometers below the air-semiconductor interface. The reduction of surface scattering is essentially what allowed for higher SNRs. Therefore, less signal energy needed to be collected and as a result, dishes could be made smaller.

I assume that the same is true today. As device engineers make transistors with higher SNRs, component engineers can make better front-end receivers, and systems engineers can reduce the size of their dishes.

i know very little in satellite communications...so you think that the answer is in the receiver′s?

my confusion started when i build my own antenna and i only needed a antenna with 20cm diameter to receive a 54dBW satellite..i compared the result′s with my 30cms offset dish using the same receiver and the result′s were pretty the same. In my university i simulated the antenna and it was a 25dBi antenna.

So the principle that some antennas need less gain/diameter to equal a offset parabolic dish exist′s..but i thought that the answer was in the antennas, maybe some antennas have a caractheristic that allows to perform as equal has bigger offset dish′s..

Part of the variability is the feed horn illumination loss. The ideal feed would have the same gain out to the edge of the dish and zero at larger angles. Another source of error is the dish shape not being exactly right. This wastes some gain as sidelobes.