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Old 23-Aug-2012, 4:33 PM   #9
thom
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Join Date: Aug 2012
Posts: 67
Thanks again for the info; I appreciate the feedback. You've hit upon the one thing I consistently find divergent information about. I completely understand that a theoretically perfect splitter will cut the signal strength in half because that's exactly what it's supposed to do.

However, when used as a combiner instead of a splitter, weird things start happening with (typically) out of phase signals getting from antenna A getting pushed back up the wire of antenna B and transmitted out of antenna B and vice versa. If the downleads to the splitter are of identical length, the signals will be in phase and you should (other things being as they should be) get that 3db gain less any efficiency penalties.

from: http://www.hdtvprimer.com/ANTENNAS/ganging.html
But you ask, “How can the power at the load equal the total input power if some of the power was reflected backwards?” The answer is that superposition applies to currents as well as voltages. The phase of the reflected currents is such that they subtract instead of add. The reflected currents will cancel each other out completely.

"The doubling of the output power is equivalent to a 3 dB increase in the signal. If the combiner is 90% efficient then a 2.5 dB gain is seen. Note the dichotomy:
· If the antennas point in different directions, there is a 3.5 dB loss at the combiner.
· If the antennas point in the same direction, there is a 2.5 dB gain at the combiner.
This is a 6 dB swing. 3 dB of this is just the adding of the second antenna, but the other 3 dB is from the combiner becoming a much more effective device."
The most compelling thing I've seen so far is this test of combiners:
http://www.antennahacks.com/CombinerComparison.htm
It seems that if you manage to do everything right, you can eek out a 2.5 db gain.

So, it sounds like it is possible to get added gain, just not necessarily easy without a spectrum analyzer or a DisplayMax 800.
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