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Discussion Starter · #1 ·
Short & Sweet:


STB = Mits SRHD5

Diplexer = CM40002


HD signal strength without diplexer = 65 - 72

WITH DIPLEXER = 0


I have tried multiple diplexers so I know that's not the problem.


When I run the OTA signal thru diplexer with out hooking in the satellite cable I get same results - no signal. Can't be interfence from LNB.


1 channel (ABC) doesn't change in strength nearly as much. Low power stations disappear.


Thanks
 

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You have 2 diplexers, right? One by the input of the signals (close to the dish) and one by the HT equipment?
 

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Discussion Starter · #3 ·
Quote:
Originally posted by TWilly
You have 2 diplexers, right? One by the input of the signals (close to the dish) and one by the HT equipment?
Actually, no. The STB splits the combined signal internally.
 

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Quote:
Originally posted by bob_jr



Actually, no. The STB splits the combined signal internally.


If you have marginal signals, you are better off using an external diplexer on the Mits and feed the diplexer outputs into the separate ANT and SAT inputs of the receiver. I know.
 

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But aside from all of that, how are your analog signals? With a decent spectrum analyzer you could tell in an instant, as to whether you have signal strenghth, multipath or interference issues. A decent diplexer shouldnt have more than a db or so of insertion loss, but if your signals are already at the threshold - then that will be enough to push you off the digital threshold. A preamp would help here - but then you run into the question of how to get power to the preamp (most diplexers dont pass power on the OTA only port), usually be having a separate run of coax - which may defeat your purpose.


Back to the first question, if analog stations are typically snowy - then you have signal strength issues and your coax losses + your diplexer losses are doing you in. If you've got a lot of ghosting on analog stations in the same direction as your DTV stations - then you likely have multipath problems, which are harder to deal with so directly, though most receivers handle a stong, noise free signal with multipath better than a weaker one.


Other than all of that? I dont know the actual characteristics of the CM diplexers you are using. I've used the RCA variety that they sell at RatShack, and I've swept them on a spectrum analyzer. They seem pretty good. One thing about satellite LNB's is that they make pretty good UHF noise sources, good enough that I use one with a couple of inline preamps in lieu of a tracking generator for doing filter sweeps! If your diplexers are passing too much LNB noise through the Sat port - then that would wipe out your OTA. You could try putting a DC block on the line from the outside Diplexer to the dish which will thus keep your RF path the same, but then the LNBs wont get power. This would only work as a test, since without power to the LNBs you wont get any satellite signal. You would however have a better idea of where the problem lies.
 

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Discussion Starter · #6 ·
Thanks for the input. A few answers to your questions/ statements:

Quote:
Originally posted by glgorman
A decent diplexer shouldnt have more than a db or so of insertion loss
1.5db insertion loss with CM4001IFD & 4012IFD

Quote:
but if your signals are already at the threshold - then that will be enough to push you off the digital threshold.
I get one station consistently pinned at 100% without diplexer that gets knocked down 30 pts when diplexing


Quote:
A preamp would help here - but then you run into the question of how to get power to the preamp (most diplexers dont pass power on the OTA only port)
CM4012 has a DC power pass to the VHF/UHF input. Any suggestions on a quality preamplifier for OTA signals?


Quote:
If you've got a lot of ghosting on analog stations
I do. Some snow on other stations, but the analog stations that are also provoding digital signals are typically ghosting.


Quote:
One thing about satellite LNB's is that they make pretty good UHF noise sources
Wouldn't leaving the satellite connection off the diplexer and getting the same digital signal loss eliminate any possibility of LNB noise thru the sat port?



Thanks for your help.
 

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Can you hook up two antennas pointed in different directions, one to the diplexed input, and one to the rf input without cross interference, or does the receiver combine both together internally?
 

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I am having a bit of trouble following all of this. When you say ABC, do you mean DTV ABC or normal analog ABC?


As for LNB being great noise sources for UHF, it is soooo true, but there is a solution. If you look up ChannelMaster (for instance) diplexers, you will find two types - - the normal two-way-splitter size one and a larger one, about 2 1/2" x 4". The difference is that the larger one will block the UHF noise coming out of LNBs, so these should be used to combine SAT and ANT signals.

I recently had no success getting OTA DTV channels with a diplexed signal. SOmeone else had installed the diplexers, and I noticed that the separating diplexer was one of those larger ones. I took it upstairs and put it in as a combiner, and suddenly all the OTA DTV channels worked.


This could be your problem -- I also had great OTS DTV with no diplexers in the circuit.
 

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It doesn't matter if you have the diplexer connected to the LNB, the STB outputs a voltage to power the LNB's. So, this voltage is interfering with your antenna. You need what is called a "DC Block" on the side of the diplexer that is going to your antenna. My diplexer came with one.


Hope this helps.
 

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Quote:
Originally posted by bob_jr
Thanks for the input. A few answers to your questions/ statements:



Wouldn't leaving the satellite connection off the diplexer and getting the same digital signal loss eliminate any possibility of LNB noise thru the sat port?



Thanks for your help.


Probably. Leaving the LNB disconnected would definately keep LNB noise out of the system. I would have put in a DC block for the test only out of general paranoia, in that when you leave the LNB disonnnected, you also leave a unterminated port on your diplexer, which can have other side effects. The ideal would be to replace the LNB with a 75 ohm terminator, just on good princples, but that would actaully be bad becasue 75 ohm terminators dont get along will with the 18 volts DC from the STB.


Call me picky, but I've known certain preamps that oscillate when they dont see the right source or load terminations, for example: the 20db cigar tube style UHF preamp from RatShack is extremely tempermental and will oscillate depending on the source termination.


While I'm on that subject and moving on to your question about preamps, the RatShack 10db 50Mhz-2200Mhz inline amp of the same flavor doesnt seem to have that problem, costs a lot less than a really good Winegard or Channel Master, and doesnt have so much gain that you would be likely to get 0% out because of overload. You can also power it inline with the 18 volts that your diplexer passes - provided that your diplexer isnt the problem.
 

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Discussion Starter · #11 ·
Looks like I'm down to just the amplifier option. All other avenues have led to a dead end.


Another question - How can I tell if the Mits STB can handle an amplifier without an overload. All I need now is to damage the STB. I have about a 50 foot run of cable between the antenna and the receiver. Most of what I've read requires 100' runs before recommending an amp.


Would a passive preamp be an option with a diplexer that allows for a DC power pass to the VHF/UHF input or should I use a 110V power supply?


Thanks again!
 

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What are you using for an antenna? Your 4002 blocks DC voltage to the antenna. Might your antenna already have a preamp in it that is starved for voltage?


I can't imagine DC voltage interfering with an antenna that has no preamp, though that antenna may short out the DC power supply, but if that were happening, then your satellite would go out at the same time, since there would be no LNB power.


Amplifying the sat signal by 10dB just after it leaves the LNB will not cause a problem. Its LNB output level is about -30dBm to -35dBm, and most tiny inline amplifiers can develop an output level of at least -20dBm with a 16 transponder load without developing excessive third order intermodulation distortion (-40dBc), but unless it is a split-band inline amplifier (rare and relatively expensive), the off air signals may hog the amplifier capacity, especially if there are strong local VHF or FM radio signals present.
 

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bob_jr


You might be able to use a preamp if you're not too close to high powered locals.


I installed a preamp/diplexer combo for a friend of mine that didn't want to install a seperate run of coax for OTA,and it worked great.The Sat system was a Dish 500 feeding two receivers.I used a CM 0265 DSB preamp,which gets it's power from the receiver,and a CM 4032IFD diplexer.


I don't know if this would work with your direct-tv Mits box or not.If someone can respond to this question,I would appreciate a reply because I have a job coming up that has a D-tv oval dish and 4X4 multiswitch setup.


Greg B
 
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