Advice wanted - Considering preamplifier change in Rochester, NY 14617 - AVS Forum
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post #1 of 14 Old 02-19-2014, 10:18 AM - Thread Starter
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Right now, I have an attic mounted Winegard HD7694P on a rotor and a Chanel Master Spartan 3 mast-mounted preamplifier. I recently purchased a HDHomeRun Dual HDHR4-2US due to the fact that it is said to have the latest in ATSC tuner chips.

 

With the software that came with the HDHomeRun, I can manually set channels and get signal strength, quality, and symbol quality readings. On several UHF band channels at least 19 and 33, I get signals that are what might be marginal, yet not enough to get an ATSC feed from them.

 

The Spartan 3's gain figures are VHF 16 dB, and UHF 23 dB, and noise figures are VHF 3.0 dB, and UHF 2.2 db. I specifically chose that amp because at the time, it had the lowest noise figures that I could find.

 

I have now come across these preamplifiers from Research Communications LTD that claim to have extremely low noise figures of 0.4 dB. The gain on both VHF and UHF is 23 dB.

 

Assuming there is an apples to apples comparison of the noise figures (gain being identical to that of the Spartan 3 on UHF at least), if I were to switch to one of the Research Communications preamps, should I expect to see a noticiable difference - such as now getting those just below threshold UHF stations I mentioned? Will the lower noise figures of the Research Communications preamps make a difference?

 

TV Fool info

 

Thanks.

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post #2 of 14 Old 02-19-2014, 10:29 AM
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Not likely. The antenna noise power needs to be added to the preamp noise power to get system noise power. The antenna noise power is probably higher than your preamp if it is meeting spec so the improvement will be very small.
John
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post #3 of 14 Old 02-19-2014, 12:11 PM - Thread Starter
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Thanks for the info.

 

Given that the HDHomeRun is a networked device, I can probably eliminate about 45 feet of RG6. That may help a bit.

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post #4 of 14 Old 02-20-2014, 04:19 AM
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The Research pre-amp looks like it was made in someone's garage. I would not waste time with it.

Any way you can post your tv fool results?

Also what kind of results do you get if you just go directly from the antenna to the HDHomerun with no amp, just a short length of coax?
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post #5 of 14 Old 02-20-2014, 09:00 AM - Thread Starter
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Quote:
Originally Posted by mikepier View Post

The Research pre-amp looks like it was made in someone's garage. I would not waste time with it.

Any way you can post your tv fool results?

Also what kind of results do you get if you just go directly from the antenna to the HDHomerun with no amp, just a short length of coax?

I am not so sure that it bothers me that the Research pre-amps look like they are made in someone's garage. There are RF ic's out there that do a great job. Places like Channel Master and Winegard are shooting for the "general" consumer market at a much lower cost. The Research amps do not look like they are aimed at the same target market. A "little guy" with the right equipment can sometimes do a better job than the "big guys" since the big guys tend to have limitations imposed by management that the little guy might not have - IMHO.


I thought that I posted the TV Fool results in an edit to my original post. If these are not the correct TV Fool results, please point me in the right direction.

 

Without running a network cable into my attic along with power for the HDHomeRun, I am not really in a position to run a short piece of coax. And since the attic is not finished, the environment is just not suitable for something like the HDHomeRun especially in the summer.

 

Last night, I removed a splitter I have in the line. (I'm moving away from DishNetwork to an HTPC / Internet solution - therefore, I am setting up and experimenting at this point.) I had the splitter driving an OTA tuner in the Dish receiver (a 612 and a model that is known to have OTA issues), and the OTA tuner in my PDP. With the antenna pointing in the "wrong direction," I got much better signals on several stations with the HDHomeRun. (The wife was sleeping, and the rotor is right over her bedroom.) I plan on experimenting in the near future to see if I might receive those stations by aiming the antenna in the right direction.

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post #6 of 14 Old 02-20-2014, 03:36 PM
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In some reviews, the Research Communications preamps performed better than the original Channel Master Titan 7777. Which was a very good reviewed preamp. But you may want to consider a Kitztech 200 preamp. It has a very low noise figure of .04db also, and can pull in weak stations the 7777 will not. Cheaper and easier to order; made in the USA.
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post #7 of 14 Old 02-20-2014, 07:37 PM
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Based on that TVfool report, you DO NOT need an amp. If you do use an amp, you are probably overdriving your signals. Try removing the amp completely out of the mix. Don't just unplug it, it will not work, you need to bypass it completely.

Also you mention you pointed the antenna the "wrong way" and got better signals. Take a look at your antenna:



The right side of the picture is the front of the antenna. It should be pointed to the South where the towers are.
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post #8 of 14 Old 03-28-2014, 08:49 AM - Thread Starter
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Thanks for all the previous replies.

 

First, let me say that I've been working with TV antennas for a long time, so I do know what end of the antenna gets pointed towards the station.

 

I finally had a chance to revisit the amplifier last night after making a few other changes in my setup. I moved the HD Home Run tuners to the basement which got rid of about 45' of RG-6QS. There is probably about 45' of cable between the antenna and the tuners. I also dropped Dish Network and got rid of our 612 DVR. This DVR apparently has known problems with overloading on its ATSC tuner.

 

I bypassed the amp last night, and I thought that there was an improvement in with less instances of dropout on the stations that normally exhibit droput which are all UHF (real channel). We have two high VHF stations, 10 and 13, and they are not problematic at all.

 

After I removed the amp, I could see that the signal strength of our two strongest local UHF (real channel 45 and 16) stations dropped from 100 % (measured using the HD Home Run GUI) to about 75% but signal quality seemed to remain more stable. Unfortunately, I do not have more advanced signal analysis equipment, so I am unable to provide a quantitative measurement other than that.

 

So, I decided to run with the setup and see how it performs. It started raining in our area last night and was still raining this morning. I checked out one of the UHF stations, real channel 16, and the dropout was probably about as bad as it has ever been when I had the amplifier operational. One consistent pattern to the dropout issues is that it is significantly worse when there is a lot of wind and/or precipitation.

 

One other thing is that my house is located in somewhat of a valley which I am imagining complicates things more.

 

At this point, I am willing to bet that the problem is due more to multipath interference rather than too much or too little signal, and as such, I doubt that a lower noise amplifier will cure the issue. Correct?

 

It also seems logical to me that when I pointed the antenna in a direction other than at the stations, the signal seemed to improve - I figure that I was picking up perhaps the strongest reflected signal.

 

At this point, I figure that there are three things that I could do:

 

1. Buy a bigger, more directional antenna

2. Get the antenna out of the attic and as high up as possible.

3. Both 1 and 2.

 

Which of 1 and 2 are more likely to make the biggest improvement? I would assume that it is 2.

 

Any thoughts?

 

Thanks again.

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post #9 of 14 Old 03-28-2014, 01:49 PM
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Quote:
Originally Posted by wiyosaya View Post

2. Get the antenna out of the attic and as high up as possible.

At 5.5 miles from the transmitters you don't need a preamp. If you don't overload the preamp you'll most likely overload the tuner by using one.

With your signals if you're having dropout problems you most likely have multipath. Indoor and attic antennas are famous for this even with strong signals. I'm a strong advocate of outdoor antennas.


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post #10 of 14 Old 03-28-2014, 02:20 PM
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Originally Posted by ctdish View Post

Not likely. The antenna noise power needs to be added to the preamp noise power to get system noise power. The antenna noise power is probably higher than your preamp if it is meeting spec so the improvement will be very small.
John

What do you mean by "antenna noise power?" kTB? Or something else?

The only number that really counts is System Noise Figure, which is the preamp noise figure plus 2nd stage noise contribution. It doesn't matter what the noise level at the antenna terminals is. The lower the system noise figure the less noise is added. Of course high noise at the antenna terminals increases the signal strength required to decode the ATSC signal. This does not negate the function of a preamp. Lower noise figure is better.

There are a couple of reasons why people don't see any difference when going to one of these very low noise preamps. In many cases weak signal is not the problem. It's multipath. Even when weak signal is the problem, going from 2.2 dB to 0.4 dB noise figure isn't enough to make much of an improvement over the long run. Weak signals are usually distant signals and a noise improvement of 1.8 db isn't enough to overcome the natural fluctuations in those signals due to changes in atmospheric conditions. That improvement might take a 50% station and raise it to a 70% station but that's still not good enough to be reliable. A weak signal near the digital cliff needs a 10 dB signal-to-noise boost for reliable watching and recording. That's exceedingly difficult to obtain if you already have an outdoor antenna and a preamp.


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post #11 of 14 Old 03-28-2014, 05:43 PM
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The system noise temperature is:

Ts = Ta + (Lr - 1)Tl + LrTr

Ta = antenna temperature (K)
Lr = receiving feed-line loss (ratio)
Tl = physical temperature of feed-line (290K)
Tr = receiver noise temperature (K)

If the antenna temperature is high for some reason (power line noise for example), the receiver noise temperature gets swamped out.

Let's say the antenna temperature is 5000K and ignore feed line losses. The noise floor for a 0.4 dB NF receiver is 5000K + 28K = 5028K or -93.80 dBm (in 6 MHz). The noise floor for a 4.0 dB NF receiver is 5000K + 438K = 5438K or 93.46 dBm. Only a 0.34 dB difference.

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post #12 of 14 Old 03-28-2014, 07:05 PM
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Hi Ron,

Of course you are right but I was not thinking about an antenna with a 5000K noise temperature. smile.gif It is helpful to look at the extremes at times. This is like a cascaded noise figure calculation except instead of a low noise preamp followed by lossy coax/other components/noisier tuner which impacts the overall noise figure, this is a very noisy antenna followed by not much noise at all. What follows the antenna has almost no impact on the system noise.

I still disagree with John that this is a very likely scenario on UHF or even high VHF. Low VHF is another story. I think the reasons I gave above for not noticing much difference in preamps are much more likely than a high noise environment at UHF.

There is another factor that I never see discussed. When I was receiving Low Earth Orbit NOAA weather satellites at 1700 MHz with a 0.5 dB NF preamp and a 24 dB gain dish, you could see Earth noise when the dish elevation was less than 5 degrees. I'm not sure if this is an issue with the lower gain antennas used for UHF TV.

For anyone still reading this and wondering how temperature in Kelvin gets converted to dBm to determine the noise floor:

Noise Floor (dBm) = 10 Log (kTB) + 30

k = 1.38E-23
T = Temperature in Kelvin
B = Bandwidth in Hz
30 converts from dBw to dBm (dB referenced to 1 watt to dB referenced to 1 milliwatt)

Chuck


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post #13 of 14 Old 03-29-2014, 09:55 AM
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Quote:
Originally Posted by dr1394 View Post

The system noise temperature is:

Ts = Ta + (Lr - 1)Tl + LrTr

Ta = antenna temperature (K)
Lr = receiving feed-line loss (ratio)
Tl = physical temperature of feed-line (290K)
Tr = receiver noise temperature (K)

If the antenna temperature is high for some reason (power line noise for example), the receiver noise temperature gets swamped out.

Let's say the antenna temperature is 5000K and ignore feed line losses. The noise floor for a 0.4 dB NF receiver is 5000K + 28K = 5028K or -93.80 dBm (in 6 MHz). The noise floor for a 4.0 dB NF receiver is 5000K + 438K = 5438K or 93.46 dBm. Only a 0.34 dB difference.

Ron


I was playing with some numbers and was a little confused at first until I realized that the conversion from noise figure in dB to Tr is referenced to 290K. Ta is also typically 290K. I think the equation would be more descriptive if a term Te was added for Noise Temperature of Environmental Sources to separate those sources from the antenna temperature.

Chuck


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post #14 of 14 Old 03-29-2014, 03:14 PM
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The Ts = Ta + (Lr - 1)Tl + LrTr equation is more useful for satellite or moon-bounce links where the antenna is pointed at cold sky and Ta is less than 290K. When Ta = 290K (UHF antenna pointed at the horizon), then the equation can be simplified to Noise Floor (dBm) = -174 + 10log(b) + line loss dB + receiver Noise Figure dB.

Ron

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