Saturday, December 5, 2015

DIY 137 MHz WX SAT V-dipole antenna

This is a short version, the final of the ARTICLE describing how to build the Weather satellite 137 MHz V-dipole antenna. Complete text article will be published soon in the PDF format that will include background and a bit of theory how I came to idea to design this simple to build antenna. I decide to build the horizontal V-dipole antenna and to use all advantages this antenna offer.




Horizontal V-dipole

Ones we know how the antenna should look like just a simple dipole antenna calculations should be applied. Using the simple formula for the ½ wavelength dipole we get the following:

L(m) = 147 / F(MHz)

L(m) = 147 / 137.5

L = 1.068 m = 106.8cm

Each leg = 53.4 cm

Important: The length of the each leg should include the connecting wire's length up to the coaxial connector or coax. Keep this length as short as possible but it will be difficult to stay bellow 1.5 cm.


For a dipole legs I did use the 1/8“ (3.25mm) aluminum rods. Do not use a ferromagnetic materials due to increased losses caused by the skin effect. The center of the dipole is made of Choc block terminals where the aluminum rods are secured on one side and the coax from the other side of the terminal. The center hole on the Choc block is used to secure the  Choc block to the /mast bracket so the antenna can be easily mounted on the mast/pole. Run the 50 or 75 ohms coaxial cable to your receiving equipment. Bend the dipole legs to create a 120 degrees angle and point the antenna to the North-South direction. You are ready for the NOAA WX sat reception.

Results

My location is not a perfect but still very good for the satellite reception. I have a clear horizon to the North but hills and a high voltage power lines to the South limiting my south elevation to 7 degrees. The antenna is fixed on the roof antenna pole, 8 meters above the ground.

 
First test made was including the barefoot setup, usually used by the beginners looking to have fun and quickly receive the weather sat pictures. Starting from the antenna (V-dipole) there was 2 mtrs of RG-213 cable / N connector followed by N/N adapter and 12 mtrs of H-2000 cable. Another N/N adapter followed by 10 mtrs of RG-214 cable with another N/PL259 adapter. Finally, there was 1mtr of flexible but lossy RG-58 cable and BNC/SMA adapter to match the RTL.SDR  dongle connector. All together, there was 25 mtrs of the mixed quality cables and 4 coaxial adapters. On the dongle side there was also SMA/SMA DC block used to isolate the DC from the dongle bias-t.


I was really pleased with the received picture quality despite the mentioned limitations. There is a noise in the picture present close to the horizon due to obstacles in the South and a weaker signals on the North. These are really remote parts of the World and the air masses from that areas are not affecting the weather where I am living. I found out that the Signal to noise ratio (S/n) required for the noise free picture is close to 22dB. Bellow that, the picture start to be noisy, resulting a black strips across the received picture.
First test conclusion, a simple DVB-T dongle and a 20 meters of the SAT-TV grade coaxial cable using the horizontal V-dipole can deliver quality and good enough picture. Here you can see the reception using a barefoot setup.


Second test was made using the advanced setup. V-dipole antenna and an FM-notch filter with the LNA4ALL with a Bias-t and ESD protection modification on the antenna. The same coaxial length and the RTL.SDR v.2 dongle. WXtoIMG was setup for 0 to 0 degrees elevation reception to get the complete pass and evaluate the location regarding the blind elevation spots. Performance and received picture was much better, mainly towards the North direction where the picture was received down to 0 degrees elevation. Towards the South, the problem with the hill and the power line is still present but less visible than using a barefoot setup. Another advantage is that the signal to noise is higher and the degradation in the picture caused by strong airplane signals in 137Mhz band is not visible as the S/n is still high, over 22dB to meet the requirements for the noise free reception. The top-notch setup should include the 137Mhz steep band-pass filter and the LNA on the antenna.


Presented V-dipole horizontal plane antenna is a simple DIY project that will deliver good results in receiving the 137 MHz weather satellite transmissions. All performed test delivered very good and excellent pictures despite using a home depot cheap materials and hand tools. This can be a beginner antenna project for all RTL dongle users where the success is guarantied.
 

6 comments:

  1. Awesome what is the software setup?

    My kids will love this

    Love to know

    ReplyDelete
  2. Hello Paul, used SW:

    SDR#
    WXtoIMG
    Virtual Audio Cable

    Adam

    ReplyDelete
  3. Adam, sure beats the QFH antenna and the 3db loss is nothing especially with your L A in line to boost the signal.

    ReplyDelete
  4. Adam, thanks for these instructions. I've been using a discone and RTL-SDR.com v3, which gets good enough pictures, but the SNR hovers around 23-24, and there are many fuzzy lines. With your design the SNR is 28-30, and I get super-clear pictures.
    Thank you!
    -Gabe

    ReplyDelete
    Replies
    1. Hello Gabe, thanks for the update. Glad to hear that you have improved your WX sat reception. Saying that the simplest things are the best is true in this case.

      Delete
  5. Any update on new working keys for wxtoimg? The ones listed on its homepage are no longer valid after mid February.

    ReplyDelete