Topband: Hybrid Coupler

[Tom Rauch]

> I have not built the specific 90 degree hybrid in ON4UN's book but I have
> built several splitters and phase shift networks using discreet components
> with excellent results. 

This may be unpopular, but I never use hybrid couplers. 

When an antenna is phased, you typically need equal currents in 
the elements. A hybrid coupler or splitter does nothing to 
guarantee that. It nether forces equal voltage (which is turned to 
equal current 1/4 wl away in a transmission line or 1/4 long 
distance away in an antenna) nor equal current. A hybrid or splitter 
actually only works when properly terminated, and that is the case 
where you don't need it (except to shift phase).

I do all my phase shift in transmission lines that are matched. No 
resistors in the system, and no lumped components in high current 
areas.

I believe Roy Lewallen wrote some very good stuff on this topic in 
ARRL publications.

Keep in mind if you are current feeding an array of identical 
elements in a four square or two-element antenna, the antenna 
needs equal currents (not equal power) in each element for 
maximum null. If you are voltage feeding resonant elements, they 
require equal voltages in each element to produce the deepest null.

If you have a 1/4 wl transmission line, the requirement at the 
element inverts at the far end of that line. If you have a mismatched 
transmission line other than a multiple of 1/4 wl long, phase shift is 
not equal to the electrical length of the line.

I can't think of any exceptions, or any cases where equal power is 
required.


73, Tom W8JI
w8ji@contesting.com



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