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  #16  
Old 02-12-2014, 10:02 AM
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Can you elaborate a little bit on RF reflection into the final stage? I would like to learn what that is.

BTW I'm using a run of RG6 about 15ft long run from the RF out on the modulator into a matching transformer set on 75ohms to a pair of 16ga wires approximately 43 inches long each. Could a part of my setup be causing RF reflection? I'd love to have an antenna analyzer to fine tune my dipole, but I don't have that in my bag of tricks. Maybe I could find a ham guy here in Pittsburgh to help me out.
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  #17  
Old 02-12-2014, 01:32 PM
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See the section on "feeding a dipole antenna"
http://en.wikipedia.org/wiki/Dipole_antenna

I presume your matching transformer is 75 ohms (your coax side) to 300 ohms (your dipole side)? According to the above you should not be using a 75 to 300 ohm matching transformer unless you use a folded dipole.
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  #18  
Old 02-12-2014, 02:39 PM
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From that article it looks as if I should be using a current balun for my antenna for best results, although it states "Many people have had success in feeding a dipole directly with a coaxial cable" so maybe I'll try coupling my RG6 line directly to my dipole leads and see what I get.
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Old 02-12-2014, 07:24 PM
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Regarding RF reflection back into the transmitter: When using an impedance-matching balun( which is essentially a transmission-line transformer), not all emitted energy gets relayed to the antenna. Some gets sent backward into the final stage. This causes less output power and heats up the rf amp more. That is my understanding of it, anyway.
Remember, these agile modulators, as originally used, fed coax cable distribution networks which were a solid 75-z load.
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Old 02-13-2014, 10:20 AM
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Tried a direct connection from coax to dipole leads, as well as increasing the coax feed length to 70+ feet, both to no avail.

Quote:
Originally Posted by zenith2134 View Post
On one of my B-T AM-60-550 units, I had a rolling bar which was antenna-dependent. upon installation of a proper coax run and dipole antenna, the issue went away 100 percent of the time. Prior to the antenna switch, I was using an F-connector-to-300 ohm balun and I figure that's what caused it.
Can you tell me exactly what antenna setup you have? Maybe there's something small that I'm overlooking.
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Old 02-13-2014, 12:07 PM
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8 feet of RG-6 cable terminated into a Yagi antenna, with the shield mounted right the base element. It is hidden in the attic. The others have simple dipole antennae. I run 3 channels all VHF
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Old 02-13-2014, 04:42 PM
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I'm at my wits end! I've swapped antennas, swapped sources, killed 100% of the power to my entire home except source, modulator, and monitor, I've removed my cable input to the house, I've replaced the 4 main filter capacitors on the modulator power supply. I don't know where to go next other than to hard wire the modulator to all of my vintage sets (no small feat at all!!!!). There has got to be something I'm missing.
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  #23  
Old 02-13-2014, 04:51 PM
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Just so we are all thinking of the same thing, is the following correct?

1) coax output of modulator -> 75/300 ohm transformer -> TV monitor 300 ohm input terminals: picture OK
2) coax output of modulator -> any kind of antenna; some kind of antenna -> TV antenna input: hum bar

Is that right?
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Old 02-13-2014, 05:00 PM
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That is correct, except you can also add coax output of the modulator to coax splitter (main splitter for my house) to coax input on a modern flat panel in the "picture ok" camp.
I've uploaded a video to youtube to show what it looks like on my test monitor https://www.youtube.com/watch?v=bBi1uZ7j0ng.
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Old 02-13-2014, 05:06 PM
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Same hum bar on any other TVs using an antenna? How do you know it's not your TV?
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Old 02-13-2014, 05:32 PM
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Should have also added coax output of modulator -> any kind of antenna; some kind of antenna -> TV antenna input on modern flat panel: hum bar
Same problem on all TVs in the home, vintage and modern alike.
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Old 02-13-2014, 07:46 PM
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Hiya John--sorry to hear of the continued woes. My humbar was different than the one on your video link. Perhaps it is not a transmission issue, but a circuit issue in the B-T. It would be interesting to see whether or not a different B-T unit would have the same fault, in the same room and set-up. Could the interference be external? (doubtful I feel)
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  #28  
Old 02-13-2014, 08:46 PM
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Just had a chance to look at the video - something in a power supply somewhere is oscillating on your channel frequency. probably radiated, but might also be carried on the power line.

1) can you see it in the "snow" when your transmitter is off?
2) if you have a small portable TV with rabbit ears and can see it on that, go snooping around your house for where it is strongest.
3) you said you tried turning things off - how about light dimmers, compact fluorescent lamps, anything on the AC line with a clock display that runs even though the main function is off (like a microwave oven?).

Edit: how about your furnace?
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  #29  
Old 02-14-2014, 05:42 AM
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Quote:
Originally Posted by old_tv_nut View Post
Just had a chance to look at the video - something in a power supply somewhere is oscillating on your channel frequency. probably radiated, but might also be carried on the power line.

1) can you see it in the "snow" when your transmitter is off?
2) if you have a small portable TV with rabbit ears and can see it on that, go snooping around your house for where it is strongest.
3) you said you tried turning things off - how about light dimmers, compact fluorescent lamps, anything on the AC line with a clock display that runs even though the main function is off (like a microwave oven?).

Edit: how about your furnace?
I actually killed power to my entire house at the breaker box with the exception of the dryer's 110 outlet. To this outlet I plugged my monitor, my modulator, and my source. If there is interference from another device it is either battery powered, or at my neighbors house.

Quote:
Originally Posted by zenith2134 View Post
Hiya John--sorry to hear of the continued woes. My humbar was different than the one on your video link. Perhaps it is not a transmission issue, but a circuit issue in the B-T. It would be interesting to see whether or not a different B-T unit would have the same fault, in the same room and set-up. Could the interference be external? (doubtful I feel)
This is the worst I can get the hum bar to look. I adjusted antenna placement in transmitter and receiver and stood between the two to get it to this point for the sake of showing it on video. In my frustration I bought another modulator last night. There happened to be one on eBay for $39 including shipping. We'll see if it exhibits the same symptoms.
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  #30  
Old 02-14-2014, 07:45 AM
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Let me repeat,
1) can you see it in the "snow" when your transmitter is off?
This would prove it's from something else.
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