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Home Amateur Radio N2QG d.i.y. Combined L-Band (23 cm) Helical Feed and Ku 0.4 dB LNBF for 120 cm Dish
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d.i.y. Combined L-Band (23 cm) Helical Feed and Ku 0.4 dB LNBF for 120 cm Dish

Helical feed for L-Band dish for amateur space communications. N2QG David Prutchi PhD www.prutchi.com www.diyPhysics.com

The high-gain antenna on my space communications array is based on a Fortec Star 120 cm dish.  This is a high-quality, large dish designed for motorized FTA reception of analog, digital, and high-definition programs on Ku band satellites.  I built a feed that combines a KU band horn for free-to-air (FTA) satellite reception with a helical feed for transmission and reception in the amateur L-band (23 cm, 1,296 MHz).

Being a peace-loving atheist, I am not interested in the religious and radicalized political junk that is most prominent over FTA.  Instead, I use the Ku-band transmissions from geosynchronous FTA satellites as known-Az/El beacons to calibrate the aim of my array.

Helical feed for L-Band dish for amateur space communications. N2QG David Prutchi PhD www.prutchi.com www.diyPhysics.com

The Fortec Star 120 dish is small enough to be handled by a Yaesu G5500 Az/El rotor, yet provides 21.7dB gain in the L-band, and 27.7 dB gain in the S-band.  In addition, it provides enough gain for the Ku LNBF that good FTA satellite reception is achieved despite the polarization mismatch that results from Az/El aiming without a dedicated geosynchronous horizon-to-horizon motor.  The following table shows the specs for the Fortec Star 120 cm dish:

Specification

Value

REFLECTOR

Type Offset
Offset Angle 24.62 0
Short Axis 120 cm (47″)
Long Axis 132 cm (52″)
Aperture Efficiency 75% min.
C – Band Gain @ 4.0 GHz 32.78 dB
KU – Band Gain @12.5 GHz 43.32 dB
F/D Ratio 0.6
Focus Length 720 mm  (28.3″)
Material Galvanized Steel
Finish Polyester Powder Coating
Color Grey / Cool Grey
MOUNTING
Mounting Type Ground, Pole & Wall Mount
Adjustment Type AZ / EL Mount
Elevation Angle Range 25° – 77°; 17° – 90°
Azimuth 0° – 360°
Material Steel
LNBF Arm Length 755mm (29 3/4″)
LNBF Arm Support Struts Length 901mm (35 1/2″)
Finish Polyester Powder Coating
Color Grey / Cool Grey
Pole Diameter Acceptable 45 – 75 mm  (1.75″ – 3″)
Net Weight 31 lbs reflector and brackets without floor mount
ENVIRONMENT
Operational Winds 25 m / sec
Survival Winds 50 m / sec
Ambient Temperature -40°C ~ +60°C
Relative Humidity 0 ~ 100 %

The Fortec Star 120 comes with a 40 mm LNBF plastic mount.  I purchased an Invacom SNH 031 LNBF to mount on the dish.  This horn is among the best LNBFs on the market, boasting an incredibly low noise figure of 0.3 dB as shown in its table of specifications:

 Helical feed for L-Band dish for amateur space communications. N2QG David Prutchi PhD www.prutchi.com www.diyPhysics.com

Input Frequency
10.7 – 12.75 GHz
Output Frequency
950 – 2150 MHz
LO Freq
9750 / 10600
22KHz
ON (required)
Noise Figure
0.3dB typical
Gain
50 – 60dB
Interface
40mm clamp
Ideal dish F/D
0.6

Helical feed for L-Band dish for amateur space communications. N2QG David Prutchi PhD www.prutchi.com www.diyPhysics.com

I fabricated an aluminum plate that serves as both the L-band’s helix reflector, as well as a mount for the L-band helix feed that fits over the neck of the Invacom SNH 031 LNBF:

Helical feed for L-Band dish for amateur space communications. N2QG David Prutchi PhD www.prutchi.com www.diyPhysics.com

Helical feed for L-Band dish for amateur space communications. N2QG David Prutchi PhD www.prutchi.com www.diyPhysics.com

Helical feed for L-Band dish for amateur space communications. N2QG David Prutchi PhD www.prutchi.com www.diyPhysics.com

A Delrin (acetal) stand and a panel-mount female N-type connector mount directly to the plate.  In turn, the plate is attached to the LNBF mount with a stainless-steel machine screw:

Helical feed for L-Band dish for amateur space communications. N2QG David Prutchi PhD www.prutchi.com www.diyPhysics.com

Helical feed for L-Band dish for amateur space communications. N2QG David Prutchi PhD www.prutchi.com www.diyPhysics.com

I made the helix out of semi-rigid coax cable (the “shield” serves as the conductor).  Feedpoint impedance of and air-wound helix antenna in the axial mode is 137 Ohm, so matching is required to enable use of standard 50 Ohm coax cable. I did the impedance matching with a quarter-wave stripline above the reflector’s ground plane as shown in the pictures below:

Helical feed for L-Band dish for amateur space communications. N2QG David Prutchi PhD www.prutchi.com www.diyPhysics.com

Helical feed for L-Band dish for amateur space communications. N2QG David Prutchi PhD www.prutchi.com www.diyPhysics.com

The  first quater turn (from the feed point) of the helix carries the matching stub at a distance of 5 mm above the groundplane (the helix reflector).  The stub is a 58 mm piece of 0.5 mm thick copper sheet, 12.5 mm wide.  I measured the SWR of my helix feed at 1,296 MHz to be below 1:2 over a bandwidth of 700 MHz.

 

Please visit www.diyPhysics.com and www.prutchi.com for other cutting-edge d.i.y. projects, and remember to check out our new d.i.y. Quantum Physics book:

 

 
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2 Comments  comments 

2 Responses

  1. [...] so many people these days David Prutchi has a space communications array in his back yard. His modification of his FTA dish allows it to be used to receive Ku-band transmissions from geosynchronous FTA satellites as [...]

  2. [...] uses the dish by using the FTA satellites to calibrate the alignment, then repositioning it to receive L-Band radio transmissions with his own add-on hardware. In the image above it’s the spiral of wire attached to the dish’s [...]

  3. Pete

    1. I was just curious if you thought about using a combined C/KU Band LNBF to receive other FTA services as well.
    2. Also, I was thinking of using a DirecTV dish to do similar in the VHF/UHF range since I don’t have a 1.2gig radio. Have you done any testing in this area?

    thanks

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