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Old 12-Aug-2017, 12:23 PM   #21
ADTech
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Quote:
At first I was puzzled by ADTech not suggesting the Juice preamp which is made by the company he works for.
Well, just didn't get around to it yesterday. I'd actually almost suggest the PA18 for its better noise performance but the calculated signal power of KMOS would cause a violation of a rule I made for our sales staff: If there's any "green" on the plot, do not use the PA18. That would either eliminate it completely or, at least, for the north-pointing 91XG unless KMOS was notched out. By that time, things are quickly getting out of control in terms of complexity plus I didn't look at post 2020 channel assignments. FWIW, the power supply and inserter shipped with the Juice is more than capable to supply two of them simultaneously. The amp usually pulls 120-140 mA at 12 volts and the supply is rated at 400 mA output.

Quote:
There seems to be a difference of opinion between ADTech and Tower Guy about the potential success of combining 4 antennas in this manner. With so many factors involved, I would just have to try it.
I don't think so. He pointed out a factor I hadn't considered. I happen to ALWAYS view these installations as experiments with no expectation as to the outcome. Sometimes they work out okay, sometimes they fail miserably. Usually, they work so-so with some wierd issues due to the problems inherent with combining signals in an uncontrolled manner.
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Old 12-Aug-2017, 2:21 PM   #22
rabbit73
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Quote:
FWIW, the power supply and inserter shipped with the Juice is more than capable to supply two of them simultaneously. The amp usually pulls 120-140 mA at 12 volts and the supply is rated at 400 mA output.
Thank you for the clarification.
Quote:
I happen to ALWAYS view these installations as experiments with no expectation as to the outcome.
A good way to view them.
Quote:
He pointed out a factor I hadn't considered.
I couldn't get my head wrapped around what he said:
Quote:
Originally Posted by Tower Guy View Post
There is still a sensitivity problem if a splitter used as a combiner after both preamps. It occurs because any front end noise contributed by the preamp connected to the mis-aimed antenna adds to the noise of the preamp of the marginal signal.
Is he talking about "noise" that is from the internal NF of each preamp, or he is talking about other "noise" that is added to the NFs of the preamps from the mis-aimed antenna?

I am inclined to believe Tower Guy because he has extensive RF experience and knowledge, but I would have to demonstrate it to myself to see what would happen and I don't think I have the necessary equipment.

If one antenna and preamp was receiving channel A with an SNR of 18 dB, and the second antenna (aimed in a different direction) and preamp was receiving channel B also with an SNR of 18 dB, would the SNRs of both signals then be 15 dB after combining because of the NFs adding together, or would the result be difficult to predict because of the contribution of other "noise" from the antenna not properly aimed at the desired channel?
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Last edited by rabbit73; 12-Aug-2017 at 6:02 PM.
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Old 12-Aug-2017, 5:58 PM   #23
Tower Guy
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Quote:
Originally Posted by rabbit73 View Post
If one antenna and preamp was receiving channel A with an SNR of 18 dB, and the second antenna (aimed in a different direction) and preamp was receiving channel B also with an SNR of 18 dB, would the SNRs of both signals then be 15 dB after combining because of the NFs adding together.
That paragraph is exactly correct.
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Old 12-Aug-2017, 6:13 PM   #24
rabbit73
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Thank you for the clarification.

I think I understand how the notch filter (or attenuator) on the output of the antenna not aimed at the desired channel will help, but is there any other way of combining that would retain the SNRs of the antennas not combined for the OP other than single channel amps as in a headend?

Is combining after the power inserters using two coax lines, equivalent to combining after the preamps using just one coax line as far as results are concerned?
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Last edited by rabbit73; 12-Aug-2017 at 8:20 PM.
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Old 13-Aug-2017, 8:33 PM   #25
rabbit73
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Combining the Ouput of Two Preamps can Reduce Weak Signal Sensitivity

I assembled what equipment I had on hand to investigate what happens when the output of two preamps are combined with a splitter in reverse.

Equipment used:

Blonder Tongue HAVM-1UA Agile Modulator - 470-806 MHz, output ~ +30dBmV
Sadelco DisplayMax 800 Signal Level Meter (SLM)
Sadelco 719E Signal Level Meter (SLM)
2 Channel Master 7777HD Preamps
RCA TVPRAMP1R Preamp to increase sensitivity of SLMs
2-way splitter used as a combiner
3 20dB fixed attenuators
1 10dB fixed attenuator
1 6 dB fixed attenuator

For a test signal, I used the video carrier of analog channel 20 from the modulator.

The 6 dB attenuator was used to adjust the sensitivity of the SLM to keep the readings centered on one range of the SLM to avoid switching to another range. The noise floor of the measurement system was well below the noise from the 7777HD, so the NF of the RCA preamp could be ignored.



I made the first measurements with the DisplayMax 800 SLM, on the right in the photo. The modulator signal is quite stable within 0.1 dB, and the SLM has 0.1 dB resolution. However, I had a problem measuring the random noise from the CM7777HD because the SLM digital display would follow the rapid changes, making it difficult to read. I switched to the 719E SLM (on the left) and its slower response was able to average the noise reading for me. The noise was measured on a nearby channel not used by the modulator.





Code:
SNR of CM7777HD #1 

Modulator > atten > CM7777HD > RCA Amp > atten > SLM
  CH 20     70dB    #1, 16dB    23dB      6dB
                  |          |         |          |      
SIGNAL        -38.4dBmV  -22.4dBmV  +0.6dBmV   -5.4dBmV
              -87.2dBm   -71.2dBm  -48.2dBm   -54.2dBm

NOISE                    -41.4dBmV -18.4dBmV  -24.4dBmV
                         -90.2dBm  -67.2dBm   -73.2dBm

SNR = 24.4 - 5.4 = 19 dB
I then added a combiner and the second CM7777HD. I do have two modulators, but not enough attenuators to feed a weak signal to the second 7777HD. This shows the affect of the noise from the second CM7777HD on the SNR of the CH 20 signal:

Code:
SNR of CM7777HD #1 combined with CM7777HD #2

Modulator > atten > CM7777HD >
 CH 20      70dB    #1, 16dB  \
                             combiner > RCA Amp > atten > SLM
                              /          23dB      6dB     
                    CM7777HD >                             |
                    #2, 16dB              SIGNAL        -9.5dBmV 
                                                       -58.3dBm 
  
                                          NOISE        -26.8dBmV
                                                       -75.6dBm 

SNR = 26.8 - 9.5 = 17.3 dB

Loss of sensitivity from combining = 19.0 - 17.3 = 1.7 dB
CONCLUSION:

My equipment isn't lab grade, but I have enough confidence in my measurements to conclude that combining the output of two preamps with a splitter in reverse causes a loss of sensitivity that can have a critical negative affect on marginal signals near the digital cliff.
Attached Images
File Type: jpg DisplayMaxImg1.jpg (92.7 KB, 81 views)
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Last edited by rabbit73; 13-Aug-2017 at 11:04 PM.
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