On 1/4/2014 9:23 PM, Ralph Mowery wrote:
"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.
A perfect combiner (like anything else "perfect") doesn't exist. But it
is a very common (and handy) way of specifying how things work. It's
used all over the place in EE degree programs, for instance.
So you start with the perfect item, then add losses, phase shifts, etc.
as they occur to get a "real" part.
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 ?
There are good combiners and bad combiners. The commercial grade ones
we use typically have 1db loss from 50Mhz to 2Ghz. Note that these
are basically splitters which are reversed to form combiners, when
necessary.
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.
In a good quality combiner, there is no extra 3db of loss.
Has anyone actually put one on accurate test equipment to see about the loss
like I have been trying to do ?
I haven't actually measured it myself, but I do use commercial grade
splitters/combiners (not as much any more because a lot of video has
gone digital). Typical loss as a combiner is around 0.5 - 0.7 db from
50Mhz to 2Ghz.
But you also won't find these at Radio Shack or Best Buy.
And there are testing labs out there who do test these things; if any of
the ratings were off, the manufacture would quickly lose credibility in
commercial circles.
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.
Phasing harnesses are just another form of splitter/combiner. One way
they combine; turn them around and they split. That's why they work for
both transmitting and receiving.
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