"Jerry Stuckle" wrote in message
...

Jeff is correct. Your error is believing the combiner has 3.5db loss.

When splitting a signal, you do have about a 3.5db loss per output,
because the signal is halved plus a bit of additional loss. However, when
combining the signals, the signal is NOT halved, so you don't have the 3db
loss there. You only have about 0.5db loss (more or less, depending on
the quality of the combiner and other factors).

Let's take an example. Since a combiner is just a splitter turned around,
we'll start with the splitter end.

Let's feed 2mw to the input of the splitter. This means each output gets
1mw (3db loss) (we could use voltage also, but since power is E^2/R, it's
not so straightforward).

So now each leg has 1mw on it.

Now let's turn the splitter around and make it a combiner and feed two
signals, 1mw ea., same frequency, to the inputs to the combiner. Since
this is a totally passive device, the effects are reversible. If the
signals are 180 degrees out of phase, of course the output is 0. However,
if the two signals are in phase with each other, the putout is 2mw.

Note there is no 3db loss in the combiner. But of course this assumed a
"perfect" combiner, with no losses. In reality, the combiner will have a
bit of loss (typically 0.5db as noted above), so the output from the
splitter will be slightly less than 1mw and the output from the combiner
will be slightly less than 2mw.

Does this help clarify things?

And yes, phasing harnesses work the same way. The can be either splitters
or combiners, depending on how they are used. The advantage is they have
less loss; the disadvantage, as noted, is they have a much narrower
effective bandwidth.


What I am having trouble with is the 'perfect' combiner.
The one by Wineguard specs 3.5 db loss and the MiniCircuits I have specs at
3 db plus slightly more depending on frequency. I had forgotten that I
built one years ago out of the ARRL Handbook. They give it a spec of 6 db
of loss per port. The one I built has that not counting minor errors and
loss. Just checked it out.

My problem is where are you going to find a combiner for a broad frequency
that does not have any large (say over 1 db ) of loss ? Are the ones for
the TV frequencies built differant ?

For the splitters, I see the 3 db because the signal is going to two places
(3 db equals half power as we all know). But then the problem I am having
is the extra 3 db that is lossed in the combiner instead of just half of a
db or so.

Has anyone actually put one on accurate test equipment to see about the loss
like I have been trying to do ?

I understand phasing harnesses for antennas. They are almost loseless. Only
a few feet of coax worth. I have used them on antennas before. They are
not usually very broad banded unless the antennas are broad banded and made
so the impedance is not the nominal 50 ohms. That is for comercial 4 or 8
dipole arays for VHF/UHF. Lots of 'tricks' used to do that.




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