[MKANET]

I just bought a 2 piece +12db amp from Radio Shack for 100ft of coxial cable running from the aerial to the receiver. I dont have experience with a two piece amp...supposedly it's much better than a one piece amp. One piece is closest to the aerial, the other piece with power is closest to the receiver. I guess one pushes, while the other one pulls the signal...not sure.


Anyway, I want to know if adding an extra 12db would hurt or help 100ft of coaxial cable? It's not that easy to tell as analog reception.


Thanks for you help in advance...


------------------

-Michael

[email protected]

 

[ScottMac]

It's going to depend alot on how strong the signal is when it hits the pre-amp, local noise, etc.


You don't say what flavor of coax you have (Rg6 or RG59), but assuming a reasonable signal level at the input, you have more amplification than loss (generic RG/6 loss is about 6.5 db/100 ft versus RG/59's 8.5db /100 ft -both at 400Mhz..about a 60% improvement), which is (usually) a good thing.


The unit near the antenna is the pre-amp, the unit nearer the TV is just for power injection to operate the pre-amp (and block the DC to your set).


Make sure you have good connectors, and that the exterior connections are well weather-proofed.


Good Luck.


Scott

 

[dipper]

Got a similar question....

Most members here are using the CM777x pre-amp... you guys may be able

to answer this: [thinking of getting one]


If you are running a single RG-6/RG-59 cable into the house for a dish

and an OTA antenna, where do you mount the power unit of the pre-amp?

I saw a warning about spliter being use from CM that could short the

power unit or something like that.


Thanks for the help in advance...

 

[Bob Smith]

The power unit MUST be placed between the UHF preamp and the point at which it connects to the diplexer. That means the power unit must be placed outside near the antenna.


The Satellite receiver sends DC to power the LNB (2 different DC levels to determine the polarization), and in the case of 2 LNBs, a 20KHz tone to select which LNB is used. The power unit for the UHF preamp also sends DC power up the coax to power the UHF preamp. If you try to do this at the satellite receiver, in the best case it won't work, in the worst case it might damage the preamp, the LNB, the satellite receiver, or the power supply unit. There is no good solution to this one, either run a separate cable, or try to find a spot protected from the weather near your UHF antenna to put the power supply unit.


If technically inclined, you might find a way of robbing the 13 to 18 V power from the LNB feed to power the UHF preamp, but it would require the RF design of bias-tees, and other things to make it happen, and it would be quite a kluge.


Bob Smith

 

[dipper]

Thanks Bob. I was afraid that was the case.

I am okay with Bay Area OTA reception with my current setup. [In North SJ]

Got most of the UHF signal pretty clean... Just wish I could get cleaner

signal for KRON... I get about 65-75 signal lock and my DST3000 sometimes

breaks up [not often]... while other time it is very clean.


Currently has 3016 with crappy RS Indoor amp.

 

[dipper]

Thanks Bob for trying to help me.


I got a terk diplexer on the roof. It came with the DC blocker that

you were describing... so that is already done. I know the DC blocker

is pretty good... I accidently plugged the blocker and the Sat Dish

connection... the receiver could not pick up any [0] bird signal..


I know I could have gotten a larger antenna, but my house is so small

that it will make my roof look ulgy.


I might drop by into Fry's and try their 30xx indoor amp and see if

it will help.


OR


I could raise another 5ft off the roof.... but I think it might not

help... since I am already on the foothills in N. SJ.


Again thanks for the help.

 

[Bob Smith]

Dipper, If your feedline to the antenna isn't that long, you might try getting an indoor amp with a lower noise figure. I think the RS amps have about a 6 dB noise figure, Channel Master amps have about a 2 dB noise figure. The noise figure of your receiver is probably about 10 dB, Use the best RG-6 you can find, minimize the length, and try a better preamp, you might be able to get an additional 4 dB in signal to noise ratio, which may make reception more stable.


Bob Smith

 

[Bob Smith]

Just realized in your situation you can't run a better feedline, that was your point. You might be losing more than you think with your shared feedline/diplexer combination.


An issue with a diplexer is the fact that out of band noise from your LNB may be contributing noise to the UHF signal. The bandsplit filters in the standard diplexer doesn't really do a great job of separating the UHF/LNB output. Try removing the connection to your DSS receiver and see if the signal gets stronger. If it does, you could order better diplexers that do a good job of separating the UHF/LNB outputs.


Another thing you can do is to get a higher gain antenna. The one you are using is a GREAT antenna, but if you compare gains with other antennas, you might find one with higher gain.


Also check with some of the after market DSS equipment providers, there is a chance they might have come up with a diplexer that has the functionality I mentioned in an earlier post, that is, to rob power from the LNB feed to power a preamp. I haven't seen one, but a little research on your part might help.


Good luck,


Bob Smith

 

[BruceL]

Bob - found your comments on pre-amp & power supply interesting. I have had Sat dish with 2 lnb's and ota ant wired the following way with no real problems. One sat line direct to sat rec. 1. Other sat line diplexed outdoors with ota line then reverse diplexed indoors with sat line to sat rec 2 (dtc 100) & ota line wired to power source & then into 2 way splitter. Split one goes to dtc 100 ant A input, split 2 goes to 3 way splitter for VCR, 2nd tv, & fm. I have had no problems with sat reception and receive 4 of the 5 digital channels in the DC area well. I do not get nbc (digital 48-48)with any consistency. I am using a Winegard VHF / UHF yagi with a Winegard 8700 (VHF / UHF pre amp).


I am thinking of going with a UHF only ant and running a direct line to dtc 100 ant a input. Would install with rotor in hopes to get Balto channels (approx 45 to 50 miles).


Any comments apprectiated - Bruce

 

[Bob Smith]

Bruce, You might be in for a surprise. If you REMOVE the preamp from the antenna, I'll bet your missing stations come in as well as better reception from the other stations.


You are receiveing a signal from an UNPOWERED preamp! Hard to believe, but a disconnected preamp will pass an attenuated signal. I haven't measured it, but I was getting marginal reception one night, and discovered someone had disconnected the preamp power supply. I was still getting a signal, but with lots of dropouts.


Try an experiment. Disconnect the power source for the OTA antanna at the wall, and see if there is any difference in reception. I'll bet there isn't. The diplexer passes only signals above a few KHz to the high end of the UHF band to/from the OTA output/input of the diplexer. It can't pass DC, the DC and low AC portions are reserved for LNB control from the satellite receiver.


Bob Smith

 

[BruceL]

Bob - thanks for the suggestions - I will give it a try sometime this week and report back.


Bruce

 

[dipper]

Bruce,


I have put in a backorder for a CM7777 last Saturday with

my local CM dealer.....


So you will have to let me know if everything worked out for

you without a dead premap.... If it all works out for you,

then I would not have to wire a second set of cable into the house.


Thanks.

 

[BruceL]

Bob & Dipper - disconnected power source from ant input line, pre-amp still at antenna - no change in signal levels for digital channels.


Next I eliminated the diplexer from the antenna line completely and reconnected the power source - so I should now have a powered pre-amp at the antenna. No significant change except that I did get higher readings on Ch 48 - it would still fluctuate a lot with breakups at about the 50 to 60 level. However I could get readings up into the 80's occassionally.


Any other suggestions - should I just remove the pre-amp and power source and run direct line from antenna to DTC 100? I am sure that if I ever want to get Balto digitals I would need some type of amp and probably a different antenna with rotor.


All suggestions greatly appreciated - Bruce

 

[Bob Smith]

Sounds to me like you should just eliminate the preamp from the system. It had a slight improvement on one channel, but not enough to cause a difference. The reason for a preamp is to enhance the signal to noise ratio. The indication of the "signal" meter on the DTC-100 is SNR, not absolute power, that is why the increase in gain had no effect.


It does verify that you can't send power through the OTA port of the diplexer.