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How to calculate signal loss when using a combiner?

mark22
03-25-2005, 08:42 AM
1-Please need help to figure out how to calculate the loss when using a 2-way combiner (diplexer). Assuming that I have to leads that I want to combine; OTA signal 14 dB and satellite signal 24 dB; the loss would be -4 dB. From which number I subtract 4, from 14 or from 24?
2-Also, it is absolutely necessary to have the signals to be combined of the same level (or very close; how close?) or they can be like in my example (14, 24)?
3-If they have to have the same level, how I do that?

Thank you

Mark

Tony R
04-01-2005, 06:04 PM
Mark, are you planning to combine the VHF/UHF OTA signal with the output of an LNB? If so, why would you do this?

mark22
04-04-2005, 06:13 AM
To get both signals on 1 coax inside the house.

rbinck
04-04-2005, 07:37 AM
I don't know where the 14 and 24 dB came from, but at a Spaun seminar a couple of weeks ago they use a figure of -35 dB for the satellite signal out of the LNBs. From that the other component losses are added to the -35 dB figure with the result needing to be -55dB or below. If it is higher, amplification is required.

In any case the Spaun diplexers have a terr loss of 1.5 dB and a satellite signal loss of 2 dB. The terr frequency range is 5 to 862 MHz and the satellite frequency range is 950 to 2200 MHz. Note the higher the frequency the higher the loss.

If you are using a multiswitch the amount of loss will depend on the particular multiswitch. A powered multiswitch will amplify the signals to reduce the loss. The Spaun SMS series active multiswitches have a 12 - 14 dB terr through loss, but a 4 - 0 dB satellite through loss.

Then there is the RG6. Belden 1152A RG6 for example has a loss of 6.6 dB per 100 feet at 700 MHz and 8.2 dB at 1000 MHz. They recommend using 8 dB as the loss figure. See http://bwccat.belden.com/ecat/jsp/Index.jsp?&P1=undefined&P2=undefined&P3=undefined&P4=undefined&P5=undefined&P6=undefined and then search for RG6 for tables.

Assuming the 14 abd 24 figures were the multiswitch losses, then you would add all of the losses together to get your overall loss.

total loss = (lead in cable)+(mxswitch or diplexer)+(distribution cable)+(diplexer)

mark22
04-04-2005, 08:29 AM
14 and 24 are not losses but the signal strength (assumed) from the OTA and LNB.

Tony R
04-04-2005, 05:40 PM
Is it not possible to leave the OTA antenna feed on it's own dedicated coax? Adding a $35 UHF/VHF line amplifier at the antenna may take care of the terrestrial signal level at the receiver.

rbinck
04-04-2005, 09:41 PM
Like I said, at the Spaun seminar where we were going over distribution systems design, they use a figure of -35 dB out of the LNBs. Where did you get the 14 and 24 dB? I mean, it was written on what?

It is doubtful it would be signal strength for the OTA as they would have no way of knowing what your antenna signal strength would be. Satellite is different because the distance from the bird is relatively the same for a given spot beam area. What Spaun is saying is figure there will be 35 dB loss through the atmos essentially. They further said keep it no more than 55 dB total loss, or 20 dB in the distribution system.

mark22
04-05-2005, 06:13 AM
I picked up those level from the net. In this case I really don't understand. First the LNB signal level is a negative? In my understanding was that with good position you get from LNB around 24 dBv! Secondly what would be the signal strenght out from OTA two story high with no obstruction around? Also a negative? Puzzled

rbinck
04-06-2005, 11:23 AM
If you could supply a link, I'll try to make sense out of it. In any case I've given you the figures to use for the distribution system losses, which is all that you can control anyway.

mark22
04-08-2005, 07:51 PM
http://www.hometech.com/learn/video1.html

I had more links but I can't find them anymore.

rbinck
04-09-2005, 08:52 AM
I know it probably does not mean much to you that the original question started with 14 dB and 24 dB when you really meant 14 dBv and 24 dBv, but that changes the question entirely. By dropping the v which indicates volts, you are not talking about a loss, but a starting signal level.

From the link you supplied:
Cable TV companies are supposed to deliver around 15 dB of signal strength at the side of the house, but I've seen this range from below 0 to well over 25 dB. An antenna can deliver a wide range of signal strengths depending on the strength and distance of the stations.

The Spaun seminar was based on what they can control, specifically losses, because they have no way of knowing what the signal strength will be without a meter. If we use the figure of 8-15 dBv at the wall plate mentioned in the link, then starting with 14 dBv (I still don't know where that came from) then your distribution system can have between 14-8=6 dB and 14-15=-1 dB loss for the OTA signals. Obviously to get the 15 dBv at the wall plate you will need an amplifier.