Today we will improve the 70 cm Yagi antenna built for satellite communications. To reduce flutter and fading we will turn our standard yagi into a circular polarized yagi. To do this a second yagi is added to the boom perpendicular to the first yagi. The dipoles of this antenna must be fed with 90 degrees of phase delay. For this project i choose to mount the second yagi 1/4 wave length away from the first. This will create the phase delay and make construction of the matching easier.
All of the element from the original antenna were used in this project. The boom had to be replaced.
The Dipole for the yagi has been updated. Connections for the dipole have been printed into the mount instead of using a terminal strip. This made it easier to connect the dipole to the feed line. Stainless steel screws are used to hold the element in place and connect the feed line.
The length of the elements for the yagi is the same as the original project.
The pink circles in the picture above notate position of the horizontal elements.
A matching section has to be constructed to match the 50 Ohm feed line. Feeding each yagi with a 1/4 wave piece of 75 Ohm Coax brings their impedance up to 100 Ohms. When connected together this brings the impedance down to 50 Ohms a perfect match for standard feed line.
For this project RG-59 is used in the matching section. Originally I wanted to use RG-6 but it was not as flexible and harder to work with. When cutting your 75 Ohm coax make sure to leave extra for your connections. Cut two 8 inch pieces of RG-59. Strip both ends of the coax and leave 6.5 inches of undisturbed coax in the middle. Make sure both of your 75 Ohm coax cables are cut the exact same length. If they are off, you will have issues tuning and getting the SWR down.
A stand was constructed to make it easier to install the additional elements. This will also come in handy in the field. The stand is held together with friction and can be taken apart.
Bench testing went well for the Circular Polarized Yagi. One of the dipoles had a little corrosion on it at the feed point but once cleaned with a piece of steel wool the SWR showed good. Field testing will be done and updates will be made.
Links
https://km4nmp.com/2020/02/01/portable-70-cm-yagi-antenna-for-satellites/
https://www.amsat.org/articles/n2spi/
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