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Old 6-Dec-2016, 5:37 PM   #1
davodavo
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Join Date: Dec 2016
Posts: 6
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:
  • Am I thinking correctly: is it completely unreasonable for the Weingard 3700 to have this much insertion loss? I've seen articles indicating nothing better can be achieved for combiners. If that were true, why would you ever stack identical antennas?
  • Is there any standard for how much loss the impedance mis-match of my original configuration would have caused? I saw equations indicating 11%, which is <0.1 dB loss
  • Are there any antenna-combining baluns that are known to have very low loss (looking for brand + model number)
  • Should I try to make my own?

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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