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15 dB distribution amplifier

Posted by bus_driver (My Page) on
Thu, May 13, 10 at 13:22

I use antenna with digital converter box to analog TV for much of my viewing. From some stations, the signal will occasionally break up briefly and it shows on the on-screen signal strength scale that the converter provides. I wish to add another set to the antenna and bought a 15 dB dist. amp. Installed it immediately before the converter as a test before making any other changes. It does not shown any change in the signal strength on-screen scale whether on or off. The power-on diode lights properly. Does this seem correct? What should I expect from this test? How else to test?


Follow-Up Postings:

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RE: 15 dB distribution amplifier

The reason for this because the tuner in the converter box has an RF AGC (automatic gain control) circuit to normalize all RF signals to the same level. So, if it see's weak signal it will amplify it (noise and all) to bring it up and if it see's a strong signal it will bring it down to prevent overloading. While a weak signal may vary on your onscreen signal strength meter, a reduced strong signal will look just the same.

Distribution amplifiers are used for exactly what you want to do - distribute an existing signal (which is both signal & noise) to more than one location and not lose the existing level when splitting it.

For weaker or marginal signals, an antenna pre-amplifier may help by boosting the available signal at the antenna before loss and noise are introduced running through a coax cable which may be fairly long in terms of signal loss. (RG59 has more loss at higher frequencies, RG6 less at higher frequencies, and RG6 quad better in noisy environments.)

Using both a pre-amp at the antenna masthead, (remotely DC powered by a wall-wart transformer that couples into the coax inside) and then a distribution amplifier for splitting is often used in a layouts where signal strength may be marginal.

Also, don't forget the "virtual channel number" and actual broadcast channel have nothing to do with one another.


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RE: 15 dB distribution amplifier

Perhaps I understand. The signal strength scale from left to right shows red (weak), yellow (normal) and blue (good). Some stations go to 100, the extreme right hand in the blue, and stay there. One station is in the yellow and varies from 40 to about 48, but occasionally dips to the red range and drops out momentarily. Adding the distribution amp does not improve the posted 40 to 48 at all. Your post seems to indicate that this is normal and expected.
I do have antenna preamp mounted just inside the attic about 14 feet below the antenna. Accessible for easy service. Would moving it up to the antenna produce much additional gain?


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RE: 15 dB distribution amplifier

I readily admit that I do not have experience with these devices and do not really know what to expect. But in thinking more about it, this is the idea that came to mind. Suppose that one adds a splitter to the end of a coax antenna lead and runs coax from the splitter to connect two converter boxes. The signal strength is then checked at each converter box and the value recorded. Next the distribution amplifier is connected to the single coax from the antenna, the output of the Dist Amp run to the splitter, then to the boxes as before and the signal strength checked and recorded. If the signal is not changed with the addition of the amplifier- higher numbers- then what is the function of the amplifier? If the DA boosts signal strength for two boxes, why not for one?


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RE: 15 dB distribution amplifier

Whew! Long day... covered in splashed concrete and mosquito bites!

In answer to your last posting first, suppose we had an electrical generator that could supply 1200 watts at 120vac and did not have a circuit breaker at the output. We plug in one 10 amp device and everything is fine. We then plug in another 10 amp device and our 120vac is going to drop way down to nothing. But if there was some way we could boost the generator, through some sort of magical amplifier to 2400 watts, everything would once again be fine. You would measure 120vac with either one device or two connected. That's kind of what is happening with a DA - it has the ability to drive several devices with a signal that was only meant for one device.

Your experiment in theory should show some difference but I'm not sure about the design of signal strength meters in your converters. There are stand alone television RF signal strength meters designed to do this but are priced in the test instrument price range. Also DAs should not really be increasing the signal but should have the capacity to supply that signal to more than one load.

The preamp at 12 feet should not be a problem. Ideally, outdoor bullet amplifiers right at the antenna is best but I don't think you are not going to see any difference compare to your current setup.

Thing to remember is that digital signals have to have basically all the information there in order to decode. Analog on the other had simply gets snowy but can still be demodulated. What digital video brings to the table is the ability to transfer larger amounts of data, through compression (ie MPEG), in less bandwidth than analog. If the bandwidth existed to do analog HDTV it would actually blow away digital - but it's a trade off we have to make.


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RE: 15 dB distribution amplifier

I'd suggest connecting the converter box as close to the antenna as possible. If the signal still breaks up adding an amp probably isn't going to help. You may just need to aim the antenna or get one with more gain for the channel that you are having trouble with.


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RE: 15 dB distribution amplifier

I appreciate that some are staying with me trying to help me to a clearer understanding.
The generator analogy is right up my alley. I do know that the generator outputs, within the limits of the overcurrent protector or design output limits, whatever the load demands. For the 1200 watt load, the factor limiting the output to 1200 watts is the combination of resistance and inductive reactance of the load. The load limits the output in this case, not the generator.
Now, the converter box accepts signals that are on the scale at, say, 95 or 100. It operates well at signals on the scale at 70. For the generator analogy to fit perfectly for me, I would have to believe that the distribution amp could supply stronger signal if the converter box would or could accept it. Lets say that Channel A has scale signal of 65. So are we saying that without the amp, it will supply signal to one converter box at 65. If I add the amp the box still gets 65. If I add the amp plus one additional box, do both boxes then get 65? If I add the second box with no amp in the supply coax, do the two boxes each get less than 65? I am having difficulty understanding exactly what the amplifier adds to the signal.


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RE: 15 dB distribution amplifier

I think you got the generator analogy down the way I intended but it's just simply the 2400 watt generator can supply more current at the same voltage - and the voltage level is how the signal level is being measured.

There are several types of amplifiers available for RF.

One, is commonly called a pre-amp and is located as close as possible to the antenna to raise the received signal as much as possible without hopefully inducing too much noise into the overall signal. With that you should be able to measure an increase in signal amplitude.

Two, is an in-line amplifier that may boost the signal by 10-20 dB for sending it down a long feed. Calculated in loss per foot.

Three, is a DA with multiple outputs, which provides through amplification, unity gain. Or basically, the same signal level in and the same signal out to each output. Some do add maybe 3dB gain to each output to make up for signal loss in the then extended coax feeds.

The other use of an inline amplifier, especially in the lower-end, is to boost the signal in anticipation of splitters afterwords. You'll notice a 2-way split usually indicates a 3-4dB drop at each output. So, boost it first and then split it.

The problem with DA type of amplification is that it is only meant to deal with distribution losses and not to improve a already questionable signal.

It is correct to amplify, via a preamp, an antenna feed to get the best possible signal (without amplifying too much noise) and then using a DA to get an even distribution of signal to each receiver.

Ideally, the least of amplification you can get away with is best since you end up amplifying both the signal as well as the noise.


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RE: 15 dB distribution amplifier

bus driver,

A thought that came to mind to me today: is your antenna VHF as well as UHF? The reason I ask is that during the transition to digital, there was a widely held misunderstanding within the consumer electronics industry, and even by antenna manufacturers, that all DTV would be UHF. Many antenna manufacturers were making UHF only antennas. After the transition, many stations that were temporarily broadcasting on UHF returned back to VHF frequency assignments.

So, many people who bought these antennas have found that they can not properly receive stations in the VHF band. Like I said, just something that came to mind...


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RE: 15 dB distribution amplifier

My antennas are full spectrum, VHF/UHF. Both are current items in the Winegard line.


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RE: 15 dB distribution amplifier

Contacted the "manufacturer" of the unit I have, actually made in China, and here is most of the response I received.
"The only real way to evaluate the performance of the XXXX is to connect it to a network analyzer to measure the actual gain in dB, which is how we test them here.

The signal strength meter on your TV although a good tool to tell what
is going on with your setup. However it does not measure the actual
output of an Amplifier. For instance it is possible to overdrive your TV
with an amp that is to strong and you could have your signal strength
drop to near zero due on strong stations. The signal meter on your TV is
a handy tool for measuring your system as a whole (Antenna, cables,
connectors, amp and amp placement in the system).

In your case the amps appear not to be helping your system, it doesn't
mean they don't work, but rather they simply do not add anything with
your setup. It is extremely rare that the amps themselves fail and the
only way to really know if an amp is good or not is to test on a network
analyzer."
Comments?


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RE: 15 dB distribution amplifier

Well, since you did not mention the brand name, I'm assuming it must be an "American brand name", that's really made in China, and probably supported by an Indian call center. The proper term for the test equipment to test the device is an RF spectrum analyzer - not network analyzer. So I guess he didn't read the right script to answer your question.

Still, a distribution amp is used to distribute a known good signal to multiple devices, not to improve antenna reception performance.


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RE: 15 dB distribution amplifier

The discussion has helped me better understand this equipment. I see some inline coax amplifiers offered that do not require an external power source. Sounds like baloney to me. Doesn't fit with the physics I learned 50 years ago.


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RE: 15 dB distribution amplifier

I'll give them the benefit of doubt and say maybe the marketing person writing the ad didn't quite understand - but then again... I've seen worse. They were probably talking about a bullet type inline amplifier, installed at the antenna, that then uses 12vdc that is supplied upstream through the coax using what they sometimes call a "power injector" to power it. The power injector can be installed at the TV end (before a splitter) in the house and sends the DC voltage up through the coax. DC blockers, which are just feed-through F-connectors with a small capacitor inside to block the DC will still pass the signal.

I'm attaching a link to a common Radio Shack one. I've used them in the past and actually have one installed on my own antenna right now but they usually only last a few years - using compression F-connectors and wrapping the connections with electrical tape does seem to help. (There use to be more choices available but then again, things change.) If you look below the picture it shows a link to a pdf copy of the little "manual" that comes with it but the diagrams should give you a better understanding of the hook-up.

Here is a link that might be useful: Antenna pre-amp


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RE: 15 dB distribution amplifier

I forgot to add that you may see inline amplifiers for use with Dish/DirecTV receivers that are powered from the same low voltage that is already being sent up to the LNB in the dish. If somebody had a system that mixes the antenna and satellite signals on the same coax using diplexers, and wanted to amplify just the antenna, they could also power that off the satellite receiver. Sometimes you'll see splitters that indicate one side passes DC and they are used in similar applications. Also, some TV antennas are "active" meaning they may have a built-in preamp or other electronics (ie RV antennas) so they also already have low voltage feeding up the coax.

In the above cases, you would not use a separate power injector.


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RE: 15 dB distribution amplifier

I truly am grateful for the things I learned from the responses. Cable is not available at my address. The local phone coop is gradually installing "Fiber to the Home" and they offer some sort of TV programming but the fiber is not yet in my area. I was going to get Dish network for TV, but just happened to learn that they do not offer the subchannels for the local stations. Like one station is 12.1 with subchannel 12.2. Dish and TW cable apparently offer only the 12.1. Here 12.2 is THISTV and luckily I can get on my antenna a station that is 108 straight-line miles away which has RetroTV on their subchannel. Those offer quite a selection of older programs. Plus I get two different PBS stations with different programming schedules. Who needs a monthly bill?


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RE: 15 dB distribution amplifier

Glad to hear I may have been able to provide some help. My background is in video and broadcast engineering, so I appreciate first hand what goes into over the air television.

I'm not a big fan of fiber over copper for telephone service. During a power outage, you only have very limited backup of phone service. And by the way, cellular towers are not required to have backup generators. Also, if the outage occurs during a natural disaster, and you depend on cable/telco for television, you may not have any source of news or emergency information. I went through the 1989 Loma Prieta earthquake in the Bay Area in California and saw the problems first hand.

I'm now up in the Sierra Nevada Foothills (still in California) and went with Dish for convenience since OTA was a little spotty with too many big old oak trees in the way. However, I recently had to run a new electrical to the well house and also ran coax (and everything else...) in separate conduit in the trench up there. So, I plan on putting up a new antenna on my barn that sits about another 50 feet higher than the house. Then maybe, I can drop Dish and the monthly bill. Enjoy PBS and similar programming as well.

Oh, by the way, Cable has been fighting the "must carry" rule for local stations for a long time - so don't ever expect to see anything more than the .1 channels for DTV.


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RE: 15 dB distribution amplifier

"Still, a distribution amp is used to distribute a known good signal to multiple devices, not to improve antenna reception performance."

That's they key comment in this thread!


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