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Old 2-Aug-2016, 12:51 AM   #1
mikelessard
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300 OHM vs Coax w/ Combiner

I want to combine an Antennas Direct 91XG and an deep fringe VHF-Hi for a fringe area installation. Would it be better to use the the 300 OHM to connect the two antennas and then the matching transformer on the bottom one, or use a combiner with 2 transformers and all coax? The first sounds more appealing to me because it would eliminate the insertion loss? Is 3' a good separation between the two?

Appreciate some of you experts weighing in on this. Thanks!
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Old 2-Aug-2016, 1:33 AM   #2
rabbit73
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Quote:
Would it be better to use the 300 OHM to connect the two antennas and then the matching transformer on the bottom one
No

Each antenna should be ready for 75 ohm coax, and then they should be combined with a UVSJ.
Quote:
The first sounds more appealing to me because it would eliminate the insertion loss?
What insertion loss? A UVSJ is a very low loss device. More importantly, it keeps the UHF and VHF signals separate.

A UHF antenna can pick up some VHF signals in addition to the UHF signals. A VHF antenna can pick up some UHF signals in addition to VHF signals.

You only want UHF signals from the UHF antenna, and only VHF signals from the VHF antenna.

With your 300 ohm method, you will have the same VHF signals from each antenna arrive at the combining point. They will interfere with each other if they do not arrive in phase. So also with the UHF signals.

It's almost as bad as trying to combine two UHF antennas aimed in different directions; it often doesn't work.

Why don't you try your method to satisfy your own curiosity?
Quote:
Is 3' a good separation between the two?
yes
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Last edited by rabbit73; 2-Aug-2016 at 1:56 AM.
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Old 2-Aug-2016, 1:42 AM   #3
mikelessard
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I'm assuming a UVSJ is a combiner (splitter in reverse). Would my Denny's HD Stacker perform better if I modified the connection that way?
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Old 2-Aug-2016, 2:07 AM   #4
rabbit73
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A UVSJ is NOT a splitter in reverse.

Do not use a splitter in reverse to combine a UHF antenna with a VHF antenna.

A UVSJ LOOKS like a splitter, but it is very different inside.

A UVSJ is also called a UHF VHF Separator Joiner. It contains a high pass filter that allows the UHF signals to pass but blocks the VHF signals, and a low pass filter that allows the VHF signals to pass but blocks the UHF signals. The two filters are connected together at the common port.
Quote:
Would my Denny's HD Stacker perform better if I modified the connection that way?
I doubt it, but I'm not certain how he does the combining in his antenna. I doubt he would choose an inferior way to combine in his antenna.

It looks like he is using the shorting stub method to combine the two antennas, which eliminates the need for a UVSJ. It is also used in the RCA ANT751.



RCA ANT751

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Last edited by rabbit73; 2-Aug-2016 at 2:25 AM.
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Old 2-Aug-2016, 2:16 AM   #5
mikelessard
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OK - that makes sense. Are you familiar with the HD Stacker? The VHF and UHF are connected with a 3' piece of 300 OHM twin-lead. Would it be worthwhile to re-wire it with the combiner and all coax? The UHF reception is kind of disappointing. It is only marginally better than the Antennacraft HBU33 it replaced which had a UHF antenna about half the size. I even took a hit on one of my channels.
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Old 2-Aug-2016, 2:50 AM   #6
rabbit73
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I just looked at the manual for the Stacker, and it looks like he is using the stub method instead of a UVSJ. It is a popular method that is used in many combo antennas, but Winegard uses a combiner PC board that is similar to a UVSJ.



The stub method is simple and inexpensive. It shorts out any UHF signals coming in on the VHF antenna. The downside is that you need several sets of stubs of different lengths to cover the UHF band.

I suggest you try that modification to see if it improves UHF reception of your marginal channel, but keep in mind that OTA signals are constantly changing in strength.
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Last edited by rabbit73; 2-Aug-2016 at 3:13 AM.
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Old 2-Aug-2016, 1:40 PM   #7
rickbb
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The 300 ohm or splitter in reverse methods are for, combining 2 identical antennas. (Or rather trying to, it rarely works, but enough to keep people trying.) Not when combining UHF and VHF antennas.

A simple USVJ and ample separation between the 2 antennas is the best method.
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