In message , Ralph
Mowery writes

"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.


Jeff found a url with the specs for the combiner.
http://www.solidsignal.com/pview.asp...cc-7870-2-way-
tv-antenna-joiner-coupler-(cc7870)

$18.99? They're having a laff! $5 max.

It says 3.5 db of loss. I assume they use the simple resistor network
instead of transformers. If so, then the net results not counting feedline
loss would be a minus .5 db.

A typical TV 2-way equal splitter has 3dB splitting loss (because it
splits the signal in two), plus a little loss in the two ferrite-cored
RF transformers inside. The spec is usually something like 3.5dB at
50MHz, increasing to 4 or 4.5dB at 900MHz.

A resistive 2-way star or delta splitter/combiner has a loss of 6dB (3
due to the splitting, and 3 in the resistors), However, whereas the 3dB
transformer splitter theoretically has infinite isolation between the
outputs (when the input is terminated) - and in practice it is 25 to
40dB - the 6dB resistive splitter/combiner has only 6dB (essentially,
all three ports are interchangeable).

That combiner does not seem to be made to add signals from idinitical
antennas for more gain, but just to let you use one feedling for several
differant antennas such as putting a FM antenna up and a TV antenna up, or a
seperate UHF and VHF antenna up and using one feedline to the receiver.

It is extremely bad practice to use a wideband combiner to connect two
antennas pointing in different directions. For analogue signals, that's
a sure recipe for multipath and ghosting - and it can't be good for
digitals either. The combiner needs to be filtered (eg a diplexer), so
that one antenna provides signals (on the appropriate frequencies) from
one direction, and the other antenna provides signals (on the
appropriate frequencies) from the other direction.

I have one for designed for my ham transceivers,and have measured less
than .5 db of loss, but that is for differant frequency ranges and not to
combind two antennas on the same band.

That sounds like a diplexer (frequency selective), and not a wideband
splitter/combiner. These can indeed be low loss, as they consist of
frequency-selective (or lowpass-highpass) filters connected to a common
port, and there is no need for wideband combining/splitting.

Even if he does get the maximum of 3 db of gain, will that acutally get him
anywhere ? Will that be enough gain for the FM broadcast band to be
noticiable ?

Well, every little helps!


--
Ian