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Phasing of 2 yagis for satellite work



  • Elmer

I have two 70 cm yagi antennas on one boom. One yagi is vertically polarized and the other is horizontally polarized. They each have a 50 ohm BNC cable connection. How do I make a phasing harness for circular polarization and a 50 ohm coaxal feed?

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To create circular polarization, you would need to combine the signals from the two antennas (one vertically polarized and the other horizontally polarized) in such a way that there is a 90 degree phase difference between them. This can be achieved by creating a phasing harness. 

Here's a simplified step-by-step guide to create a phasing harness for circular polarization:

1. **Materials**: You'll need two equal lengths of 50 ohm coaxial cable for the harness. The length of each piece of cable should be a quarter-wavelength (λ/4) at the frequency of interest. You'll also need a three-way splitter with BNC connectors, a 50 ohm termination resistor, and a BNC connector for the feedline.

2. **Calculation**: Calculate the quarter-wavelength of the cable. The formula is λ/4 = c/(4f), where c is the speed of light (approximately 3*10^8 m/s) and f is the frequency in Hz. Note that the actual length will be slightly shorter due to the velocity factor of the coaxial cable, which is typically around 0.66 for RG-58 type coaxial cable.

3. **Cutting the Cable**: Cut two pieces of 50 ohm coaxial cable to the quarter-wavelength length calculated in the previous step.

4. **Connection**: Connect one end of each coaxial cable to the three-way splitter. Connect the antennas to the other ends of the cables. You may need BNC adapters to ensure a secure connection.

5. **Termination**: Connect the 50 ohm termination resistor to the third output of the splitter. This will help to ensure the impedance remains at 50 ohms.

6. **Feedline**: Connect the feedline to the input of the splitter.

Remember that the cable length is critical for creating the 90 degree phase difference necessary for circular polarization. Also, the antennas should be placed at right angles to each other (one vertical and one horizontal) and ideally should be identical to each other in design and construction.

As a final note, it's important to mention that while this setup can generate circular polarization, it may not be the most efficient way to do so, especially for high-power applications. If you are operating a high-power transmitter, you may want to consider a more robust setup or consult with a radio engineer or technician to ensure you are not damaging your equipment or causing interference.

This is a simplified version of the process. For a detailed and professional setup, it is recommended to consult with an antenna engineer or a similar expert in the field.


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