Circular polarization box for dual yagi

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venom6733

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I'm building a set of yagi's like in figure 3 on this site. Cross-Yagi

In figure 4 of this site is a box for making the antenna right hand/left hand circular polarization. The 3 coax's in the box have a formula under them for length...does anybody know how to read and figure out the length of coax's needed in the box??? The center freq of the yagi's are 1.702 ghz ( not MHz) it's for receiving HRPT transmissions from NOAA and Meteor n2 satellites....I'll try to post a pic of the box on this page if it'll let me

Thanks for any help
 

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Ubbe

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You have to check the data for the cable you choose to use for its transmission delay. Standard 50 ohm RG58 have 0.66 and you then take 300 divided by 1702 and divide the sum by 4 and multiply with 0.66 to get the answer in meters. That would end up as 0.029 meters and equals to 1.15 inches.

Its difficult to make cables with the exact lenghts with those high SHF frequencies. Making that cable nearer to 1.4 or 0.85 inches will make the signal arriving at a phase that totally cancels out the signal.

/Ubbe
 

venom6733

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Thanks....that sucks cause I was looking forward to building it but if I can't get RHCP out of it...it'll do no good but thanks for showing me how the math works
 

venom6733

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Is there another coax that could be used in place of the 50ohm that would make it possible?? And also is transmission delay the same as velocity factor when it comes to coax??
 
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RFI-EMI-GUY

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I think all you need to do is reverse the cables to attain the opposite polarization. The article you reference shows both polarity options, You could also build a helical antenna for RHCP.
 

prcguy

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There are several ways to make circular polarization and having two identical antennas and combining them at the proper distance and feeding them 90deg out of phase is one way. The diagram shown is a combination RF divider and 90deg delay line and these can also be made using 75ohm coax and not paralleling some coax as shown in the diagram.

The problem is at 1.7GHz it would be nearly impossible to build without some expensive test equipment to verify a 90deg phase shift and if its not real close, you wasted your time building it.

Another way to make CP is to use a 90deg hybrid coupler, which is a 50 ohm device and you simply use exact equal length 50 ohm cables from the coupler to the individual antennas. Otherwise there is no calculating or cutting cable lengths, Here is one example of a 90deg hybrid couple that is surplus and cheap to cover the OPs 1.7GHz frequencies. N3164-90 ARRA, Couplers, Hybrid, N-Type 90° | eBay

These couplers are usually labeled with a common port, an ISO port where you would put a small 50 ohm load and a 0deg and a 90deg port. The coax to your radio would go on the common port and you would use two cables of the exact same length (and I mean exact) from the 0 and 90deg ports to your antennas. Swapping the 0 and 90 ports will change you from RHCP to LHCP and its hard to know which port goes to which antenna without seeing the antenna.
prcguy
 

Ubbe

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Also remember that if one dipole are 1/8 inch nearer the satellite than the other dipole in the x-yagi it will make the dipoles 90 degree out of phase and you have to make the coax for the front dipole 1/8 inch longer times the velocity factor, with a RG58 coax that's 1/14 of an inch longer. (but there's no way you could use RG58 at that frequency)

I don't know how much you will loose by not using RCP or LCP with a standard yagi, but all the exact measurements will probably not make it wortwhile to build a x-yagi and phase box for 1.7GHz and it's much easier to use a good amplifier with a standard yagi without having to worry about group delays and phase problems. Actually, you could have a relay at the antenna and switch between the vertical and horizontal dipole in a x-yagi antenna, or build two seperate yagis and have one verically and the other horisontally positioned and switch between them with a relay without having to worry about cable lenghts, just use a standard sat amplifier and standard LNB switch that will switch antennas if the voltage goes over 15v.

/Ubbe
 

RFI-EMI-GUY

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At that frequency, I would build a helical antenna. I have done this (1/8 inch copper tubing wrapped on PVC pipe) and it was a fair Inmarsat antenna. The design is forgiving. The drawback is that the helical antenna isn't frequency selective and in the normal mode for a helical, it can pick up a lot of powerful out of band signals. So your preamp will need some bandpass which eats into noise figure.


https://en.wikipedia.org/wiki/Helical_antenna

John Kraus is the expert on these and if you get a copy of his book, you can replicate a design easily. Also there are some on line calculators.
 

prcguy

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1/4 wavelength at 1.7GHz is about 1.6 inches, so that is how far one dipole would have to be in front of the other to cause a 90deg phase shift. 1/8 inch would not cause much of a problem.
prcguy

Also remember that if one dipole are 1/8 inch nearer the satellite than the other dipole in the x-yagi it will make the dipoles 90 degree out of phase and you have to make the coax for the front dipole 1/8 inch longer times the velocity factor, with a RG58 coax that's 1/14 of an inch longer. (but there's no way you could use RG58 at that frequency)

I don't know how much you will loose by not using RCP or LCP with a standard yagi, but all the exact measurements will probably not make it wortwhile to build a x-yagi and phase box for 1.7GHz and it's much easier to use a good amplifier with a standard yagi without having to worry about group delays and phase problems. Actually, you could have a relay at the antenna and switch between the vertical and horizontal dipole in a x-yagi antenna, or build two seperate yagis and have one verically and the other horisontally positioned and switch between them with a relay without having to worry about cable lenghts, just use a standard sat amplifier and standard LNB switch that will switch antennas if the voltage goes over 15v.

/Ubbe
 

venom6733

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Well I planned to make 2 Yagi's .…one horizontal one vertical and using belden 75 ohm cable
 
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