SATCOM Antenna-----need help----

[zguy1243]

I Have built a few satcom antennas in the past, all of them being 5 element yagi's. I am trying to acheive better results by building a crossed yagi to achieve circular polarization. I am having some trouble finding out to feed and or phase the two parts of the antenna together(the vertical and the horizontal sections). So far I have basically a vertical polarized yagi and a horizontal polarized yagi built on the same boom. Both the horizontal side and vertical side are the exact same yagi. I am looking for tips on how to feed it or phase it. Is imedance a big issue? I did not use baluns of any sort on my regular 5 element yagi for satcom and I acheved good results just feeding the driven element directly with the coax. Most of the miltary surplus satcom antennas that i see are cross yagi designs, so I assume there is some benefit to this design. I undestand that you loose 3db or better from not having a circular polarized antenna. Any help that I can get would be very appreciated. I will post pics of the antenna when I get it built.

Jody
KG4FVG

 

[K2GOG]

since you built the vertical and horizontal elements on teh same boom , essentially having two of the same antenna and both needing to get fed somehow, you may just want tot make a co-phasing harness. if both antennas are 50 ohms, you can use two 1/4 wave length 75 ohm jumpers to feed it. the jumpers should be as long as the median frequency you intend to monitor (250Mhz-260Mhz?) they both need to be the exact same length or they will be fed out of phase, possibly creating worse reception.

that is a start off the top of my head. I am not getting into much more theory here as I dont have time for a 3 page answer. But that sounds like an interesting project.

 

[prcguy]

You need to delay the signal to one yagi by 90 degrees to create a circular polarized signal and you have to delay the correct one for RHCP, otherwise you’ll have LHCP and no reception from the RHCP downlink satellites. This can be done by feeding one yagi directly and the other is fed from a ¼ wavelength section from the first yagi. Another way to achieve circular pol is to mount one yagi ¼ wavelength behind the other and feed them in phase with an equal length harness. There’s lots of info on this, Google is your friend. See this for a starter: http://www.qsl.net/dk7zb/Cross-Yagi/crossyagi.htm
prcguy

 

[prcguy]

It might be easier to make a 5 turn helix for UHF satcom. All you need is some ¼” copper tubing, PVC pipe fittings and sheet metal or stiff screen for the reflector.
prcguy