> You can handle the impedance matching by using 1/4 wave
> sections of 75 ohm
> coax between the receiver input and the T. The 1/4 wave 75
> ohm section
> steps the 50 ohm receiver input impedance up to 100 at the
> other end, two
> of those in parallel at the T gets you back to 50 to match
> the feedline.
> Any number of receivers other than 'powers of 2' is more
> complicated. This
> does nothing for the loss of course.
>
> Roger Grady K9OPO
The math works in the case of the tee being at the antenna connection, but
the ASCII drawing that accompanied the previous email put the tees on the
backs of the receivers, sort of like old-style thin Ethernet 10base2, except
instead of having terminations at the ends of the backbone, one end is
connected to the last receiver, and the other end is connected to the
antenna. Matching goes out the window with this design.
The problem with even doing the tees with odd quarterwave 75 ohm
transformers is that you have virtually no isolation between receivers. If
your receivers have tight front ends, unless all of the receivers are very
close in frequency, you may end up with more than 3 dB of theoretical loss
due to phase cancellation back at the tee. If you're really pinching
pennies, Kevin's recommendation of using 75 ohm CATV/MATV splitters is
better since they provide port-to-port isolation, and the losses due to the
impedance mismatch (50 versus 75) are insignificant (theoretically approx.
0.4 dB + normal dividing loss).
--- Jeff
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