Wednesday, October 18, 2017

The Double Cross Antenna - A NOAA Satellite Receiving Antenna Construction Project



Recently I've been playing with receiving weather images on 137 Mhz from the NOAA satellites.  I'm currently using an SDRPlay RSP2 and initially I received some very good images using a horizontal V-dipole antenna that I had found on the internet.  Other software that I use is a Satellite Image decoding program called WXtoimg or APTDecoder, a Satellite Tracking program called Orbitron, and a virtual cable called VB-Cable to route the audio from my SDRPlay to WXtoimg.

As my interest in this activity grows, I wanted to experiment with building a different receiving antenna to help reduce the fading that I was sometimes experiencing with the V-dipole.  Initially, I wanted to make the QFH antenna and the construction seemed to be a little complicated so I opted to build the DCA, also known as the Double Cross Antenna.

In building this antenna, I wanted to document each step of my construction project so that it would be available to others who would like to give it a try.  There are other construction methods out there however this seemed to work fairly easy for me.  I've saved the documentation this PDF file for download...DCA Antenna Construction Project.

The antenna works very good and I'm really happy with the results.  If you really want to know how this antenna works, you can read the article An Illustrated Guide to Understanding the Double Cross Antenna (DCA) for APT Reception posted on the PY4ZBZ website.

You can view my captured images that the decoder programs upload to their own created webpages stored on my server....

Weather Satellite Images Recorded at Bunker Hill, IL United States

The Daily APT Log @ W9FE



And now....Building the Double Cross Antenna

Double Cross Antenna
A NOAA Downlink Satellite Antenna

Parts List:
QTY     Lowes Item #         Description                                             Total
(2)        #23786                   ½“ x 10’ Copper Tubing Type M          17.64
                                             2 @ 8.82 ea.

(1)        #23967                    ½“ x 5’ Sched 40 PVC Pipe                    1.67

(4)        #23873                     ½“ Sched 40 PVC Tee 401005               1.64             
                                              4 @ .41 ea.

(8)         #23761                   ½“ InCoupling CPVC                              2.00
                                             8 @ .25 ea

(1)        #22695                    ¾” Sched 40 Adapter 4361                         .98

(1)                                        1” x 5’ Sched 40 PVC Pipe

12’                                        RG-58 Coax (with PL-259’s)

Miscellaneous Hardware

(8)                                        6x32x1” screws

(8)                                        6x32 nuts

(24)                                      6x32 washers

(8)                                        Electrical Crimp-on Connectors

(1)                                        PL-259 Barrel Connector

(1)                                        PVC Cement or Gorilla Glue 







  
Construction:

  1. Cut 8 pieces of ½” copper tubing to 18.5” lengths each.  You’ll be able to get 6 pieces from one of the 10’ sections and 2 pieces from the other 10’ section.  It was cheaper to purchase 10’ lengths of tubing than smaller lengths for this project.




  1. Cut 4 pieces of ½” PVC tubing to 10” lengths 


  
  1. Drill 1” hole in center of ½” PVC Tee
  2. Label openings of PVC Tee 1, 2 at 0, 180 deg. and 3, 4 at 90, 270 deg. 




  1. Apply PVC cement or Gorilla Glue to ¾” adapter.  Screw on the adapter to PVC Tee.  This assembly will be the mast attachment at the project completion. 




  1. Cut 1 piece of RG-59 Coax terminated with one PL-259 to approximately 24”
  2. Separate Braid and Center conductor on the free end of PL-259 terminated coax. 

  1. Cut additional 4 pieces of RG-59 Coax to approximately 36” to allow sufficient coax to work with.  The coax pairs will be trimmed down to the proper lengths of 14.25” and 18.5” later in the project. 



  
  1. Separate Braid and Shield on one end of the 4 cut pieces of Coax. 

  1. Label the 4 cut pieces of coax, 1, 2, 3, and 4 near the separated end. 
  1. Make Wiring harness per figure 9 below. 

    • Solder Center Conductors of Coax 1, 3 together
    • Solder Center Conductors of Coax 2, 4 together
    • Solder Shield of Coax 1, 2 together.  Tape cable pair to insulate shields
    • Solder Shield of Coax 3, 4 together.  Tape cable pair to insulate shields.
    • Solder Center conductor of free end of PL-259 cable to 2,4 cable
    • Solder Shield of free end of PL-259 Cable to 1,3 cable
    • Trim excesses as needed, tape entire connection assembly.

  1. Measure and label at 14.25” on Coax cables 1, 2.  Shield and center conductor will be separated up to this point later.
  2. Measure and label at 28.5” on Coax cables 3, 4.  Shield and center conductor will be separated up to this point later. 


  
  1. Feed numbered wiring harness cables through center hole of PVC Tee and through corresponding labeled hole.



  

  1. Insert ½” PVC arms into the center PVC Tee assembly. 



  1. Cement or glue the ½” cpvc incouplings into ends of the ½” PVC Tee’s

  1. Insert and snugly secure ½” copper tubing into the ends of the ½” PVC Tee assembly. 




  1. Drill hole through center of PVC Tee Assembly of sufficient size to allow coax cables to be fed through it. 


  
  1. Drill holes through each end of the copper tubing at the PVC Tee assembly.  Insert a 6x32x1 screw, washers, and nut to secure copper tubing to Tee assembly. Use 1 washer at the screw head and two washers at the nut end. This will be the Dipole coax feedpoints. 

  


  1. Pull the longer coax cables 3, 4 back through the center hub until marks meet up with the end of the pvc arms. 



  
  1. Attach the completed PVC Tee dipole assemblies to arms and feed coax through center hole. 



 
  1. Separate center conductor and shields of each coax cable back to previously marked end length. 


  
  1. Attach electrical connectors to coax ends.  Secure to the dipole element between the double washers and screw nut. After securing the connections, dipoles will be oriented in the following manner when placed on the mast....Dipole 1 (center conductor up).  Dipole 2 (center conductor down), Dipole 3 (Center Conductor up), and Dipole 4 (center conductor down).
 


  

  1. Tape excess coax in center of hub.  Run a length of 50 ohm coax through 5’ PVC mast.  Connect to antenna assembly with a barrel connector. 




  1. Attach Antenna to the 5’ PVC mast.  Secure assembly on a tripod or other mount.
  2. Orient dipole 1 (center conductor up) facing to the North.  Rotate dipoles to approximately 30 degrees as indicated in article.  Dipoles 1, 2 will be offset opposite of each other as will as Dipoles 3 and 4.  
  1. Enjoy the NOAA satellites. 


  

Click to Download the QST Article – February 2008 - Double Cross - A NOAA Satellite Downlink Antenna, by Gerald Martes, KD6JDJ




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