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  #31  
Old 02-28-2015, 07:22 PM
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Thanks!!

I recapped the other mod, and replaced the bigger rectifier diodes, it's noticeably better, and able to program it for UHF frequencies now, but it still has issues with reception, it's still weak, not as weak as it was before but still much weaker than my second mod.. I did order up the three transistors, and the smaller rectifier diodes, I'll try replacing those tomorrow to see if it makes a difference..

With the bowtie I'm able to get reception about a block radius with an area where it's completely dead.. I thought it would go a bit farther with UHF..
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Last edited by tvcollector; 02-28-2015 at 07:29 PM.
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  #32  
Old 03-01-2015, 11:23 AM
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Originally Posted by tvcollector View Post
Thanks!!

I recapped the other mod, and replaced the bigger rectifier diodes, it's noticeably better, and able to program it for UHF frequencies now, but it still has issues with reception, it's still weak, not as weak as it was before but still much weaker than my second mod.. I did order up the three transistors, and the smaller rectifier diodes, I'll try replacing those tomorrow to see if it makes a difference..

With the bowtie I'm able to get reception about a block radius with an area where it's completely dead.. I thought it would go a bit farther with UHF..
About a block with the unit on the second floor is what channel 7 gives me....I always figured VHF (especially low band) would give better range since the lower the frequency the less a TV signal behaves like light/FM radio (line of sight transmission), and the more it behaves like AM/SW signal (sky waves and skip)...I hear tales of people with just a few trees between them and the towers loosing DTV signal when the wind blows the trees, so if there is something like a brick or metal wall, vent ducts in the walls, a fence, or something of that nature between your transmitter and and your receiver in your UHF dead zone odds are it is the cause of your dead zone.

Two other things to consider: All antennas are directional to some degree (google radiation pattern of antenna in question, in your case UHF bowtie dipole, to see it), and dipoles have the weakest reception at an angle at which the transmit antenna is pointing at you*. Also the RF power of the BT seems to vary from channel to channel (which is not too surprising given is is hard to make wide band tuned circuits and reflected wave traps completely flat over the whole TV spectrum).

*Another side effect of directionality is what IIRC is called polarity mismatch....If the TX and RX dipoles are oriented to face 90 degrees different from each other then minimal signal transfer for the other given conditions will be minimal.

I took an antennas elective last year, and despite it being the first time the class was offered and the professor diving way to deep and spending way too much time on the maths of infinitesimal dipoles I did take some useful stuff away from it.
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  #33  
Old 03-01-2015, 07:58 PM
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Walking with the TV, it's mostly in and out, static and picture after about 2 houses down. I tried it with VHF and 7 seems to be the channel that goes the farthest.. Channel 6 Seems to work better closer, but a radio station interferes..

I opened the other modulator and realized that one of the 10mf 50volt caps I took out during the week, I forgot to replace, so I was actually operating the modulator without a cap that is needed.. I replaced it, no change in performance, and I also changed out the rest of the diodes and the 3 transistors, and it's still very weak.. I'll switch the mods and use the same channel and antenna and the TVs will come in crystal clear with the new one, and an unwatchable pic with the one that has problems..
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  #34  
Old 03-01-2015, 09:37 PM
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Originally Posted by tvcollector View Post
Walking with the TV, it's mostly in and out, static and picture after about 2 houses down. I tried it with VHF and 7 seems to be the channel that goes the farthest.. Channel 6 Seems to work better closer, but a radio station interferes..

I opened the other modulator and realized that one of the 10mf 50volt caps I took out during the week, I forgot to replace, so I was actually operating the modulator without a cap that is needed.. I replaced it, no change in performance, and I also changed out the rest of the diodes and the 3 transistors, and it's still very weak.. I'll switch the mods and use the same channel and antenna and the TVs will come in crystal clear with the new one, and an unwatchable pic with the one that has problems..
You might just have a dead one. You gotta remember these modulators are extremely high hour devices. Some might be dead.

I got mine from some seller down in Texas. It works good. I'm pretty happy with it. All my video devices are plugged in to it. I love wireless video
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  #35  
Old 03-02-2015, 12:09 AM
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One thing to be aware of is reflection damage. It is a phenomenon caused by impedance mismatch. If you connect a 50 ohm antenna to a transmitter and coax system designed for 75 ohms there is an impedance mismatch. Impedance mismatch on a transmission line (75 ohm coax is a good example of transmission line) has varying effects depending on how long the line is relative to the wavelength of the transmitters signal (google Smith Chart to learn more about this and it's uses). If it is at one fraction of a wavelength the transmitter will see an open circuit as the device connected to it's antenna terminals (which may or may not be good, can't remember), if it is at another fraction of a wavelength the transmitter sees a short circuit (because all the output power is reflected back into the transmitter) which means no power is transmitted and the transmitter is in danger, and at still another fraction it sees the 50 ohm antenna as a 75 ohm load (which is ideal).

In the early days of SS transmitters the short condition (all power reflected back) would usually destroy the output transistors....More recently reflection traps were devised to protect the outputs, but they are not infallible, and if damaged can block/attenuate forward signal flow, or allow enough reflected power to reach the outputs to cause damage. Your units traps or outputs could be damaged.
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  #36  
Old 03-05-2015, 03:24 AM
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Here's my setup.. I just got another one that's a 450A model that only does the lower end VHF.. I got a good deal on the 450 model and took a chance with an as is auction, and it also works great.. I've got it the broadcast channel 10 which I can go down a few houses with my Sony Watchman and it still having a viewable pic.. I don't even have antennas on the TVs in the house other than the coax cable hookup and they come in clear.. I'm broadcasting on a VHF and a UHF channel between the two.. I may go as far as putting an antenna outside with a transformer to see if I can get several blocks of a watchable signal.. I found an old computer power station last Saturday at a local thrift for $3.. So that's good for shutting things off.. TV below it has slight hum bars and the one in the living room does too, but the other sets don't.. I think it's because they are plugged into a power strip, or near the cable hookups..

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Last edited by tvcollector; 03-05-2015 at 03:32 AM.
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  #37  
Old 03-05-2015, 11:54 AM
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One thing I do is power my mods through a line voltage isolation transformer. Sometimes the power supply and grounding of the BT mods does not like to play nice with the PS and jack grounding of video sources creating hum bars....
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