#16
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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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John Last edited by vts1134; 02-12-2014 at 10:19 AM. |
#17
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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. |
#18
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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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John |
#19
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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. |
#20
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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:
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John |
Audiokarma |
#21
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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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#22
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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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John |
#23
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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? |
#24
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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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John |
#25
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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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Audiokarma |
#26
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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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John |
#27
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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
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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? |
#29
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Quote:
Quote:
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John |
#30
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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. |
Audiokarma |
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