"Ian Jackson" wrote in message
Strange as it may seem, if you use (for example) a TV 2-way '3dB'
splitter to combine two identical in-phase signals, you DON'T lose 3dB.
Apart from the unavoidable slight inherent losses of the two transformers
the circuit uses (a total of around 0.5dB at low VHF, increasing to 1dB at
high UHF), the splitter is lossless. Ignoring the transformer loss, the
3dB loss occurs simply because the power at each output port is half of
that at the input. You haven't actually lost anything.
If the splitter is now turned around to become a combiner, it doesn't
suddenly become more lossy. If you again ignore the transformer losses,
the two identical in-phase signals you feed into the 'output' ports are
added, and the result is a signal 3dB higher.
I don't have a TV splitter/combiner to play with. I do have a MiniCircuits
combiner I have been playing with.
http://www.minicircuits.com/pdfs/ZFSC-2-2.pdf
I am not sure what is in the combiner but it must be transformers of some
type as the resistance of the ports are near zero ohms.
Their specs is for a 3 db loss and then an aditional loss of about .2 to
1.2 depending on the frequecy. Are you sure that is not the case where you
are saying you do not loose the 3 db ? That is the ratings is for the extra
..5 or so not counting the already 3 db of loss.
Maybe you can tell me if I am playing with the wrong type of combiner.. Here
are some results of my tests. using a HP 8924C service monitor and another
signal generator.
With one input port of the combiner having a signal and the other port
terminated with a 50 ohm load (the nominal impedance of all devices) there
is a 3 db loss (small values not being included) not the low values you
mention to the output port. When I hook up the other signal generator, I
get from almost a total of 0 db to 6 db of loss. I asume the spectrum
analizer going from 0 to 6 db is the phasing of the two generators.
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