Offset-fed Antennas & LNBF Combinations.

The dish design of choice for most digital DTH systems is called an offset-fed antenna The manufacturer uses a smaller subsection of the same parabolic curve used to produce prime focus antennas, but with a major axis in the north/south direction, and a smaller minor axis in the east /west direction.

With the offset-fed design, the feed horn is no longer positioned at the front and center of the reflector but rather offset to the bottom of the dish. If the section does not include the center of the dish then none of the incoming signal is blocked by the antenna feed or its support structure.

The offset fed antenna design offers several distinct advantages over its prime focus counterparts. There is no feed horn blockage, an important consideration when the antenna aperture is less than one meter in diameter.

The major difference between Prime focus and offset dishes is that offset angle at which the feed horn tilts up toward the reflector on an offset dish is such that if the feed looks over the antenna's rim it will see the cold sky. The prime focus dish however would receive any spill over from the ground generating more noise than the offset dish .Due to these advantages; the offset-fed antenna can achieve higher efficiency levels than prime focus antennas can generally attain.

When using an offset antenna one must realize that the shape of the feed horn is a very important aspect of the dish design. The efficiency of the dish can be severely compromised if the feed horn section of the LNBF is not matched to the dish. This is especially true of the oval dishes because of their shape and the fact that the F/D ratio of the dish is different horizontally than it is vertically.

The feed horn must be optimized for the F/D ratio of the dish otherwise it either won't fully illuminate the entire dish or will "see" around the edges of the dish and pickup terrestrial interference. With an oval dish like the F/D ratio is different in one plane than it is in the other. Therefore you need a feed horn designed the same way.

Otherwise you'll either only see a smaller circular portion of the dish or you'll pickup background noise from around the edges of the dish on its minor axis. The actual performance of such dishes can differ considerably. Those who try to use improperly designed feed horns or LNBF’s are probably only illuminating a small portion of the dish and the result is they get the performance of a smaller dish as the F/D ratio of most LNBFs is closer to that of the minor axis of these dishes

The F/D ratio of most  offset  dishes is around .6 so, LNBF F/D has to  be close enough to matching dish F/D that the system won't exhibit a lack of performance due to a miss match between the LNBF F/D and the actually required Dish F/D

JX Satellite Offset Dish Specification Table

 The two LNBF’s illustrated sold by JX Satellite both have an F/D of .6

 Specification: for Gen Cen GKF-2101(right) and Gospell GKF-0110 0
           Both have an F/D ratio of .6

Input frequency range: 10.70-12.75GHz
Output Frequency Range: 950-2150MHz
F/D Ratio: 0.6
Noise figure: 0.5dB (Typ. )
Conversion gain: 55dB (Typ. )
Input VSWR: 2.5: 1
Output VSWR: 2.0: 1 (max. )
Output connector: F-female connector
Output impedance: 75 ohms (Typ. )
Cross polar isolation: 20dB (min. )
Image rejection: 45dB
Phase noise:
-50dBc @ 1KHz
-75dBc @ 10KHz
-95dBc @ 100KHz
Voltage: V: 11.5-14V. H: 16-19V
Operation Temperature: -40 to +70 degreee (17.56in)

Universal Single Ku Band LNB (STP05622)

Product Description
Model: STP05622
Input Frequency Range: 10.70~12.75GHz
10.70~11.70GHz (9.75GHz)
11.70~12.75GHz (10.6/10.75GHz: 22K)
F/D Ratio: 0.3
Image Rejection:  40dB (Min.)
Input Reflector Type: Offset
Cross Pol. Isolation: 20dB (Typ. )
Conversion Gain: 55dB(TYP)@25OC
50dB(min)
Gain Flatness: 4.5dB(Over Band)
About 1dB/54MHz
Noise Temperature: 0.3dB(TYP) 1.2dB(max)
L. O. Frequency: 10.60GHz/10.75 9.75GHz
L. O. Frequency Stability:  about 1.0MHz@25C
About3.0MHz@~40C~+60C
L. O. Phase Noise: -50dBc/1KHz
-75dBc/10KHz
-95dBc/100KHz
Output Frequency Range: 950~1950MHz(9.75GHz)
950~2150MHz(10.6GHz)
1100~2050MHz(10.75GHz)
Output Power(at 1dB gain compression): 0dB(Min. )
Output Return loss: 7.5dB
Output Connector:  75Ω F-Type Female
Polarity Switching Voltage: H: 16~18V
V: 11~14V
D. C. Current Consumption:  120mA (TYP) 150mA (Max. )
Operating Temperature: -40º C~+60º C

•  F/D Ratio .59

However as can be seen the Model: STP05622 has an F/D of .3 which means that its F/D is not matched to any of the dishes that JX satellite sells.

Is your system performing like it should? Does the Dish F/D match that of your LNBF? For optimum results choose only specified tested Dish / LNBF combinations from JX Satellite.

Home Dishes C Band LNB