[NLRS] Phasing cables for 2m

[Dr. Gerald N. Johnson]

On 9/14/2016 5:09 PM, Kirk P wrote:
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> Good afternoon!
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> VHF/UHF Newbie question (After 39 yrs as ham..HI)
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> As noted in previous emails to group, Im putting up 2 - 2M12 yagis on AZ/EL mount for EME.
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> I have power divider but need phasing lines..I looked at M2 web site and they wanted over $250!
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> What is the secret to making these myself? How "special" are they really. Wont 2 equal lengths of low loss feedline work?
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With the power divider, you only need 50 ohm coax of any length, you 
aren't doing any impedance matching with the runs, like you would with 
75 ohm coax and a plain coax T at the junction so you can use any length 
just so they match in length. As Jon has posted.

Equal lengths off the same roll of coax should work. Make them as close 
to equal length as you can. Measure them side by side and do all the 
cutting and trimming for the connectors so they are the same length. The 
most picky sticklers will use a vector network analyzer to check the 
phase shift of each length and make them match. I think today's coax 
will match pretty good with just a good tape measure measurement or side 
by side measurement. With a wavelength of 2.0805 meters (299.8 / 144.1), 
6.825786 feet, or 81.909 inches the phase shift is about one degree for 
every 0.227 inches in air coax. The phase shift is more rapid with a 
dielectric reducing the velocity of propagation to 82% for foam coax or 
67% for solid polyethylene dielectric coax so one degree phase shift is 
0.152 inches with the solid dielectric.

The problem many years ago was that the density and diameter of the 
center insulation varied along the length of the coax because the pump 
pressure of the insulation extruder varied with time as the piston pump 
moved through strokes. With foam coax that could change not only the 
characteristic impedance but the velocity of propagation and in some 
coaxes actually made for a rapid increase in attenuation when the peak 
densities or diameters were a half wave apart. I think today's coax 
makers have overcome that fault and it was not a big problem at 2 meters 
Though it could affect phasing.

A phase difference in the feeds can shift the combined lobe and if that 
shift is more than about 1/10th of the individual antenna beamwidth it 
can reduce the gain at the peak and keep that peak from being aligned 
with the booms. So match the lengths as close as possible, then check 
for sun or moon noise to see if the main lobe matches the direction of 
the booms. The 2m12 shouldn't be all that picky. I had an M2 5WL up on 
2m and pointing it was a pain. Its beamwidth was narrower than the brake 
notches on my rotor. In the extreme weak signal situation of EME 0.1 dB 
is a intolerable loss.

73, Jerry, K0CQ
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> Please help me understand...
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> KK
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