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Hall Of Fame
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Discussion Starter · #1 ·
I installed the HDTV headend in a twin tower highrise in metropolitan Washington DC (Arlington, VA, actually) that has 550 units. The antenna array is on the East tower, the MATV trunkline runs from there down through eighteen floors, into the underground parking garage, 500 feet across to the bottom of the almost-twin, seventeen story West tower, and terminates in the seventeenth floor cable closet.

I recently noticed that I can get a clear shot at the Baltimore TV stations from the West tower, but if I moved the whole headend to the West tower, I'd also have to reverse the taps in 70 cable closets and relocate eight distribution amplifiers, so I most likely will send the signal from a new, West tower antenna to the east roof headend, which means the signal will travel down 200 feet, across 500 feet and up 200 feet. Since there is already an unused 500' RG-6 jumper between the two towers, I'll probably use that, but for the top to bottom lengths I'll use RG-11.

There will be a UHF channel 51 in the system even after the transition, so it will be incurring 700Mhz losses through 400' of RG-11 and 500' of RG-6. I will therefore probably have to boost the west tower UHF signal twice to sustain it before it hits my headend, and then it gets amplified two or even three more times in the distribution system.

I am a little concerned about the intermodulation distortion that might come from amplifying broadcast 8VSB signals five times, especially since, even with the signal level balancing that I do, there will be maybe fifteen dB difference between the strength of the strongest and weakest signals.

What would it take to relay the West tower signal to the East tower through 900 feet of fiber? What are the least expensive converters, and what would a 1000 foot roll of pre-connector-ized fiber cost me?
 

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Old Guys Rule!
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You might check with Graybar Electric in your area. They are a distributor for Panduit products. Check www.panduit.com also.
 

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Hall Of Fame
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Discussion Starter · #5 ·
I just got a price quote from the distributor who is usually my lowest cost provider: $2,700 for the transmitter, receiver, and cable. For $2,700, plus my labor, it would be cheaper to move the headend and turn all the taps around, especially since with fiber, I'd face the stressful situation of being responsible for something that I am not outfitted or experienced to support and maintain.

Wireless relaying is out of the question, since, among other things, there would be six Baltimore digital channels involved, as well as three to six analogs and a Washington DC digital and analog. Basically, I have to furnish the entire VHF high band and UHF spectrums off that remote, auxiliary antenna to my headend.

As a practical matter, it would be most efficient at this point to use one or two low noise, high gain preamps and see what kind of results I get. The problem is, it would be virtually impossible to reliably assess the extent to which intermodulation distortion has corrupted the digital 8VSB signals after they have gone through five amplifiers to the most distant apartments. And I'd look like a hack if I told the customer, "give me $3,000 to try to do it on the cheap" (I also have to design and install the filtering and mixing network) but if it doesn't work, give me another $5,000 for the fiber", which is the least I could change the customer for fiber link and its installation.
 

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You might be successful at using separate band (lo,hi,uhf) antennas and preamps for the Baltimore stations, then use some tiering filters to keep the out-of-band junk out of the other bands, and replace the RG-6 with RG-11.

I'm using about 600 feet of RG-11 for UHF from our roof. With a couple of 8 dB antennas, a 20 dB preamp, and 600' of RG-11, I can just make an 8-way split and still have enough to drive my proc's. So, maybe RG-11 all the way would work.
 

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Godfather
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Neil Derryberry said:
I know less about the 75-ohm stuff than I do the 50-ohm stuff we use for feedlines in amateur radio.. do they make 75-ohm hardline?
Yes, I have about 400' of 3/4" 75 ohm hardline on my UHF TV antenna. It has about 8 db of loss on channel 35. I picked it up surplus from a CATV installer.
 

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Hall Of Fame
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Discussion Starter · #9 ·
The hardline isn't that expensive, but is a pain to install. It normally is sold in 2,400 foot rolls, but sometimes, a "broken roll" can be found with as little as 1,200 feet on it. I think I paid under .40 per foot five or six years ago when I last shopped for it, but with present copper prices and its huge copper center conductor, I wouldn't be surprisd if I had to pay double that price today.

The other thing is, the 500 foot jumper between the two towers is lashed to the plumbing near the ceiling of the underground parking garage, It would be a lot of work slowly paying out the hardline as I moves a ladder five or ten feet at a time to replace the coax jumper with a hardline one.

I'd lose 2.2dB per 100 feet at 700 Mhz with .500 hardline, or about 20dB over 900 feet, so even then, I'd just barely get back to the signal levels I needed with one preamp. This is not intended to minimally drive a residential 8VSB tuner. This will be a source for passive filtering and padding circuits which must have a final, mixed output that allows my Blonder Tongue BIDA 750Mhz, 43dB gain amp to develop full power from that input level, which would be a little over 55 dBmV for analog channels 50 and 56, and about ten dB less than that for the highest digital channels (48, 51, 52, 57 and 59).

This is a good, repeat customer, and I like using them for a reference, so I just gave them a price of $3,100 to move the headend and add and incorporate the Baltimore signals, and another $1,800 to basically flip the distribution system around. That gets me back to no more than three amps in cascade.
 
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