Baluns & antenna combiners: insertion loss?
I live in a deep fringe area and have a pair of identical UHF/VHF combo Yagis properly stacked. When I first hooked them up, I didn't have an antenna combining balun, so I just wired them in parallel and then used a standard 300 ohm->75 ohm balun and it worked well enough (i.e., I got a few more channels with the second Yagi than with just one). Of course, there was an impedance mismatch, but it did work.
I then bought a Weingard 3700 antenna-combining balun (2x 300 ohm in, 1x 75 ohm out) and hooked it up; it was only marginally better even though it was "correct." Disappointed, I researched that 3700's insertion loss and the only spec I could find was 3.5 dB. So, let's see: we add the second antenna and get +3 dB, only to lose all of it in the antenna-combining balun. I called up Weingard tech support and they said its insertion loss was 3 dB...still seemed ridiculous. Of course, if what they did was measure the thing backwards (putting a signal into the 75 ohm side and measuring the output on the 300 ohm side), that 3.5 dB loss is exactly what you'd expect: it's acting as a splitter. But I'm using it as a combiner, which is what it was designed for. A good ferrite balun ought to have a loss of a 1 dB or so from 150-800 MHz...and since I need to use a balun anyway, the combining balun should be no worse than a single-antenna balun, right? So my second antenna ought to give me something around 2 dB more signal than a single one, right? I don't have the equipment to measure the insertion loss myself, but maybe somebody in the forum can answer these questions:
Sorry if this is newbie stuff. I spent about an hour researching here and in AVSforum and got confused by the apparent contradictions. Thanks in advance for any info you can provide on this. |
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