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Author Topic: dual mismatched antennas  (Read 1788 times)
Muggle
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« on: September 04, 2012, 01:06:37 AM »

I have a standard ground plane - am considering adding a yagi (using a BNC T-connector) for better coverage of the tower itself. Is the impedance difference a BIG deal ?
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KLGB Feeder
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« Reply #1 on: September 04, 2012, 10:35:15 AM »

You are going to lose a few db with the impedance mismatch.  If your received signals are weak now, they will get weaker with the db loss.  If you have strong signals now, the mismatch shouldn't make much difference.
     If you need a yagi just to receive the tower, you may try just the yagi since the planes in the air will be strong enough to be OK.
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datainmotion
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« Reply #2 on: September 04, 2012, 09:47:55 PM »

I guess I didn't catch what the impedance difference is. Is one antenna 75 ohm and the other 50 ohm?
The BNC-T would definately induce a couple dB of insertion loss...
« Last Edit: September 04, 2012, 09:51:42 PM by datainmotion » Logged

Muggle
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« Reply #3 on: September 05, 2012, 01:03:23 AM »

Thanks...I am building my first yagi using element separation to ensure 50 ohms...if I'm  a little off, I was hoping it wouldn't make that big a deal.

Thanks again.
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KLGB Feeder
dave
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« Reply #4 on: September 05, 2012, 06:45:36 AM »

The effect can be worse than the impedance mismatch.  Depending on the separation distance, the antenna patterns will overlap, and not necessarily in "constructive" ways.  You may get destructive interference in some directions.  So don't expect great results by just "tee-ing" the antennas together.  To get the pattern you want, you'll need to insert a different phase via one of the coaxial lines going to at least one of the antennas (by trying different lengths of coax cable).  It can be time-consuming, and typically you won't have any reasonable way of measuring your results...unless you pay very close attention to signal levels from desired stations, and you have a way of seeing or hearing that signal strength change reliably.

At a minimum, try to use a proper power combiner/splitter, like this one from Mini-Circuits:

http://www.minicircuits.com/pdfs/ZFSC-2-1.pdf

At least with that combiner you know you have the proper isolation between RF ports to start a proper evaluation. 

But all that being said, sometimes you can luck out and get something that was better than what you started with.


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Muggle
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« Reply #5 on: September 05, 2012, 08:23:14 PM »

Hello Dave....thanks for the assist.

The separation of the antennas will be roughly 20 - 25 feet horizontally and the elevation of the ground plane is about 25', the yagi would be at about 12'.

Would that be enough separation ?
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KLGB Feeder
Muggle
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« Reply #6 on: September 05, 2012, 08:36:13 PM »

I forgot to mention in the last post that the ground plane is using 50' of RG58, the yagi will be using 20' of RG58.
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KLGB Feeder
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