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Discussion Starter · #1 ·
I've already ruled out using a preamplifier. The signals are too strong where I live. But would a distribution amplifier help me without causing overload? I want to split the signal to three TVs.
 

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I've already ruled out using a preamplifier. The signals are too strong where I live. But would a distribution amplifier help me without causing overload? I want to split the signal to three TVs.
It would depend on how long the cables are. A 3-way splitter is about $5 at Home Depot so with your strong signals it may be worthwhile to just try it and see. All splitters have some signal loss. On a three way splitter one port will have about 4 dB loss and the other two ports will have 8 dB loss each. If your splitter is marked, connect the 4 dB port to the longest run of coax going to a TV.

If you determine you do need a distribution amp, I can recommend the Channel Master distribution amps. If there is any possibility you may add a 4th TV at some point, you could go with the 4 port amp and cap the unused port with a terminating resistor until you are ready to use it.
 

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To prevent Overloading the Distribution Amp or the attached TV's (weak signals swamped by IMD Noise), insert a Variable RF Attenuator on its Input (or try 3.5-4+ dB Loss through one...or two...spare RF Splitters):
http://www.solidsignal.com/pview.as..._campaign=PJ_AFF&utm_medium=AFF&utm_source=PJ
You only need a small amount of attenuation, since 3rd Order Intermod Distortion Noise is reduced by 3 dB for every 1 dB of Loss, hence 3.5 dB Loss results in 10.5+ dB IMD Noise reduction....and TWO RF Splitters in series results in 21+ dB IMD Noise reduction.
 

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Discussion Starter · #4 ·
To prevent Overloading the Distribution Amp or the attached TV's (weak signals swamped by IMD Noise), insert a Variable RF Attenuator on its Input (or try 3.5-4+ dB Loss through one...or two...spare RF Splitters):
http://www.solidsignal.com/pview.as..._campaign=PJ_AFF&utm_medium=AFF&utm_source=PJ
You only need a small amount of attenuation, since 3rd Order Intermod Distortion Noise is reduced by 3 dB for every 1 dB of Loss, hence 3.5 dB Loss results in 10.5+ dB IMD Noise reduction....and TWO RF Splitters in series results in 21+ dB IMD Noise reduction.
So basically, a distribution amp can overload just like any other preamp. What's the difference between these anyways? I know that part of the preamp mounts near the antenna, but besides that, what is the result that each performs?

Could I use a variable attenuator with a preamp in order to avoid overload? Would that benefit me compared to not using a preamp at all?
 

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Channel Master distribution amps are much more difficult to overload than any pre-amp I have found. I have used them in 2 locations with very strong signals & had great success both times.

I see you have one very strong signal, so a small attenuator may be needed, but I would try w/o one first. I highly recommend the Channel Master 3414.

A TV Fool report of my strongest location is attached for reference.

http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d2c1500243955ab
 

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You have very strong signals, you likely don't need any amp anyway. Pick up a three-ports splitter at Home Depot and try the system with it installed vs. an F81 coupler (test each TV, one at a time).
 

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So basically, a distribution amp can overload just like any other preamp. What's the difference between these anyways? I know that part of the preamp mounts near the antenna, but besides that, what is the result that each performs?
An antenna preamp is designed for only a couple dozen or so channels, while a CATV distribution amp such as the Channel Master one referred to (more commonly called a Drop Amp) is designed for 77 analog channels plus a bunch of QAM (digital) channels.

Drop amps want to see a signal level of ~+5-10dBmV or so and have a gain of 15dB, while preamps can accept much lower input levels.

Some of the levels on your chart are plenty strong enough to feed 3 or 4 TV sets, while the majority definitely need amplification. WOIO si the last one that needs no amplification.

If you'll be looking in the 325-350 degree direction, you'll definitely want to use a high gain preamp, such as the Channel Master CM-7777. The signal level on some of those channels is ~-15-20dBmV.
 

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If you'll be looking in the 325-350 degree direction, you'll definitely want to use a high gain preamp, such as the Channel Master CM-7777.
Huh????

The dynamic range of the 7777 would likley be overwhelmed regardless of antenna aim. NO preamp will help the OP & will only create issues with the nearby searing RF levels.

It would be like putting a 500 HP engine in a Toyota Corolla w/o upgrading the drivetrain.(ouch)
 

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The TVFool report he posted has the channels in that direction coming in with power levels in the -40 to -69dBm range. The first channel listed from that direction comes in with a power level of -40dBm, which, according to both the conversion table and the conversion calculator I used, equates to 8.8dBmV (which would be fine all by itself). The next channel listed comes in at 2.3dBmV (which, by itself, would call for a distribution amp), and levels get lower as we continue down that list. The last one in that direction comes in at a whopping -69.2dBm, which equates to -20.3dBmV.

Yes, there are a 4 channels that would cause grief with a preamp. Most of those channels are, however, going to need a 500HP engine. The best thing to do would be to just connect the antenna to the TVs with only a splitter and go from there. Those CM-7777s will take an input level of +15dBmV, while the CM-7778 will handle up to 34dBmV. (The first channel on the report comes in at 41.1dBmV)(It will be a little lower because of the orientation of the antenna)

If both of those tools I used are incorrect, I do apologize for spreading misinformation.
 

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Differences between Mast Mounted Preamps and Distribution Amplifiers:

Preamp design is (hopefully) designed to be more robust for survival OUTDOORS, with very high and very low Temperatures, Rain, Ice and Static Electricity buildup.

Unfortunately, manufacturers are no longer providing much in the way of ANY useful Specs re "Overload" Desensitization for OTA Applications....some don't even specify the Noise Figure. So there is NO WAY to know whether a particular Preamp or Distribution Amp is better or worse than another from the so-called "Specs". I've discussed my distain for trying to extrapolate the 3rd Order IMD Noise levels that are hopefully close to the Thermal Noise Floor (e.g. -106 dBm) when the Amp is operated for Maximum Spurious Free Dynamic Range (see my posts re "SFDR")....from the FICTIOUS EXTRAPOLATION that is the 3rd Order Intercept Point (IP3) (e.g. +10 to +30 dBm)...the crossing point of the Input vs Output Curve and the Input vs 3rd Order IMD Noise Curve, when I KNOW that ACTUAL Curves can deviate quite a bit from perfect straight lines:
http://www.fiber-span.com/Application_Note_Spurious%20Free%20Dynamic%20Range.pdf
http://www.avsforum.com/forum/25-hdtv-technical/798265-how-build-uhf-antenna-134.html#post18729254

Distribution Amplifiers typically include an RF Splitter and MOST of them Passively Pass the 5-42 MHz Band [some countries are different] in the REVERSE Direction for use with CATV systems (although you can also get them with Gain in the Reverse Direction). Because they have to pass over a HUNDRED very strong signals, occupying nearly ALL Channel positions, they usually use a PUSH-PULL Transistor Design to cancel the Even Order Harmonics, rather than the Single-Ended Transistor Design found in the various Preamps I've seen. Surprisingly, this has NOT adversely affected the Noise Figure compared to most available Preamps....in the ballpark of NF = 2.5 to 3.5 dB. Because Distribution Amps are targeted for CATV System Applications, they do NOT specify "Overload" Desensitization in the presence of a small number of strong signals (like old C-M & W-G Preamps were). Their Distortion specs typically measure how much splatter gets into ONE unoccupied Channel position from the dozens of surrounding strong Channels. There are a very different variety of Distortion Tests, NONE of which can be used to determine "Overload" characteristics for OTA Applications:
http://www.keysight.com/upload/cmc_upload/All/E206CATVAB6.pdf?&cc=US&lc=eng
http://literature.cdn.keysight.com/litweb/pdf/5965-9434E.pdf
http://www.cablelabs.com/wp-content...or-Nonlinear-Distortion-in-Cable-Networks.pdf
http://people.seas.harvard.edu/~jones/cscie129/nu_lectures/lecture13/CATV/CATV.html
 

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On preamps versus distribution amps....

Preamps are designed to be mounted outdoors while distribution amps are expected to be used indoors.

Preamps normally have some filtering to reject out of band signals although this is minimal in most preamps I have measured. Tin Lee preamps have the best filtering I have seen. Distribution amps have no filtering. They are broadband devices. The Kitztech preamp is an example of a preamp with no filtering but marketed primarily as a preamp.

Both preamps and distribution amps can perform the other's function. There's no minimum signal requirements or anything like that. That's why Kitztech says you can also use their amp as a distribution amp.

The easiest way to look at maximum signal handling is to measure the 1 dB Gain Compression Point (P1dB). This is the power output where the amplifier is no longer linear and the output is 1 dB lower than expected, i.e. the gain is reduced by 1 dB. Bad things start to happen when you get this much output. Another standard way to measure the amplifier performance is to perform a two tone 3rd order intermod test. This requires two signal generators and a spectrum analyzer. You usually don't find this much equipment outside of test labs.

I measured P1dB on the CM3410. It is +16 dBm. This is right in the range of most preamps. I've seen as low as +12 dBm and as high as +20 dBm. My 30 dB gain Tin Lee measured +20 dBm. You don't want higher than that because you run the risk of blowing out the front end of the TV.

In the real world you have to look at the sum of all the signals the amplifier is seeing to determine if the amp is being overdriven (better word than overloaded). This is called Total Signal Power. Just because you have no ONE signal strong enough to drive the amp into gain compression, doesn't mean that all the signals taken together aren't overdriving it. The easiest way to measure this is to put a broadband power meter on the amplifier output and that will read the sum of all the signals.

A low noise figure distribution amp like the CM3410 can be used as a preamp if you're sure that it will not be overdriven by out of the TV band signals and you can keep water out of the connectors. It might be an excellent choice for an attic mounted antenna where 15 dB gain is enough.

The idea that distribution amps require some output signal level has nothing to do with the amplifier itself. It's a cable TV standard for adequate signals to meet some SNR spec.

Edit: I see holl_ands beat me to it. :)

On 3rd order intercept point (IP3).....

It's a fictitious number in that the amplifier cannot output that level (P1dB kicks in long before you reach IP3) but it is useful because if you know IP3 you can calculate 3rd order intermods for any pair of equal tones and thus get some idea of the maximum signal inputs before interference from intermods will become a problem.
 

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Discussion Starter · #12 ·
A lot of this terminology is confusing to me. I'm not an expert at this. So let me see if I understand something...

If I boil all this down, can I put an FM trap AND a variable attenuator before the distribution amp? Won't that filter out most third order interference before it hits the distribution amp? According to what I understand, attenuating just a little bit will help significantly reduce third order interference compared to attenuating the main signals. So if I do that, by the time the coax hits the amp, the main signals (the ones I really care about) will be boosted above the losses before the amp, and the third order interference will be lowered. Does that make sense?

From what I was reading, FM stations may be more the problem than TV stations. That's why I was thinking of adding the FM trap in with the variable attenuator.
 

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I didn't mention Input Filtering because I don't think it makes much difference wrt Preamp vs DistroAmp.

Preamp Filter should attenuate below Ch2 (54 MHz) and above Ch69 (700 MHz) for a VHF/UHF Amplifier with a SINGLE Broadband Transistor (e.g HDP-269 and probably ALL In-Line Amplifiers).

DistroAmp for CATV should attenuate below Ch2 (54 MHz) and above 1002 MHz....making it more susceptible to high level signals above the TV Bands....mostly Cell Phone 4G/LTE towers within a couple miles....but beyond that distance it's not really a problem due to sufficient frequency separation. [Until a Spectrum Auction results in a nearby Tower using some of the interspersed Ch38-51 frequency band]:
http://imageevent.com/holl_ands/files/emitodtv
And ALSO TACAN/DME navigation beacons, usually found at Airports, transmitting HIGH POWER on a freq as low as 960 MHz.

SEPARATE VHF & UHF Amplifier Preamps (e.g. C-M Titan/Spartan and W-G AP-xxxx) appear to have VHF (Ch2-13) and UHF (Ch14-69) Input Filters, so they should greatly reduce strong signals from 216 MHz to 470 MHz (220/440 MHz Ham Radio & mostly low power aircraft transmissions for the few seconds they transmit while passing within LOS of YOUR house). I don't see enough complexity to ALSO filter out the Gaps between Ch4/5 and 108-174 MHz (High Power VOR & ILS Navigation Aides, mostly near Airports and 110 MHz Ham Radio that MIGHT be a neighbor)....and of course they don't provide ANY attenuation for 4G/LTE above Ch51:
http://imageevent.com/holl_ands/files/ota

Since NONE of them attenuate nearby 4G/LTE Cell Phone Towers (the REAL EMI Threat....after the FM Band), I just don't see much difference wrt Input Filtering unless you are within a couple miles of a Cell Tower or several miles from an Airport.

Only SOME of this can be CURED by inserting a GOOD 4G/LTE Low Pass Filter....careful, some LTE Filters are designed for EUROPE, where their LTE Band starts at 800 MHz, vice our 700 MHz. I posted details for 4G/LTE Frequency Bands and Power Levels here:
http://www.digitalhome.ca/forum/showthread.php?p=1389931
I also discussed effectiveness (and lack thereof) for various LTE Low Pass Filters:
http://www.digitalhome.ca/forum/showthread.php?p=1388395
But that was several years ago....be sure to search for newer, better Filters....
 

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A lot of this terminology is confusing to me. I'm not an expert at this. So let me see if I understand something...

If I boil all this down, can I put an FM trap AND a variable attenuator before the distribution amp? Won't that filter out most third order interference before it hits the distribution amp? According to what I understand, attenuating just a little bit will help significantly reduce third order interference compared to attenuating the main signals. So if I do that, by the time the coax hits the amp, the main signals (the ones I really care about) will be boosted above the losses before the amp, and the third order interference will be lowered. Does that make sense?

From what I was reading, FM stations may be more the problem than TV stations. That's why I was thinking of adding the FM trap in with the variable attenuator.
Using GoogleEarth (with TV & FM Icons downloaded from TVFool), I determined your LAT/LOG (to nearest 0.1-mile) and entered into www.fmfool.com. You have FIVE Very Strong (higher than -20 dBm) signal level predictions, coming from the same locations as nearby TV Stations (1 to 2-miles) North and East of you. So YES, you definitely NEED a Full Band FM Filter. Antennas-Direct or MCM-Electronics Filter would be much better than the one from Radio-Shack, which is a compromise to PASS Ch7, which you do NOT need.

Mast Mounted Preamp should be mounted on the TOP of the Mast, so unless you're in the Attic, a Variable RF Attenuator isn't very practical since it isn't designed for Outdoor use....but you could use it to determine HOW MUCH Attenuation you needed and then buy FIXED RF Attenuators that provide the desired amount of attenuation and are intended for Outdoor use.

Moving the Mast Mounted Preamp INDOORS is a viable alternative if you KNOW that you're going to need some Input Attenuation for that particular Preamp....the additional 30-50-ft Coax Cable Length simply becomes part of the total Input Attenuation.

However, for BEST performance, a low-noise LOW-GAIN Preamp or DistroAmp would usually work out better, esp. if it did NOT require any Input Attenuation. Which is why we keep saying DON'T USE HIGH GAIN CM-7777 except in a RURAL location, well away from TV and FM stations.

I realize that all of these technical details are confusing.....so I'm going to try to keep it simple. We think that you have sufficient signal strength to drive all THREE TV's, using a PASSIVE 3-Way Splitter.....and if not, add a Low-Gain Distribution Amp (could still use 3-Way Splitter) with an (Optional) Variable RF Attenuator [or try inserting 1 or 2 RF Splitters] and tweak for best performance.

If you prefer, you could also use a Preamp (RCA TVPAMP1 appears to have the highest user PROVEN Overload) and insert attenuation on it's input until you achieve the best results....
 

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There's something we're skipping over again when worrying about overload from the stations TV Fool shows as extremely strong. TV Fool does not take into account the vertical radiation pattern of the broadcast antennas. This is a major factor when you're very close to the transmitters. You have to locate the original applications the stations filed with the FCC to obtain the construction permit to find the vertical pattern. They are on-line.

Let's look at WVPX at 1.6 miles away. From the FCC database the antenna is 925 feet above the ground. I'm going to assume that you're at the same elevation as the antenna tower. At 1.6 miles from the tower you look up at a 6° angle. Searching through WVPX FCC applications I located the original request for a construction permit which include pattern data for their antenna. Attached is the elevation pattern for their antenna. I added a red line at 6° below the horizontal. This shows the radiation pattern to be about 9% of the main lobe. This translates to 21 dB* less power than what you'd see in the main lobe. Since TV Fool doesn't take into account the reduction in ERP due to the elevation angle, the Noise Margin it shows is not 72 dB but 51 dB. I assume your antenna is pointed at 350° so you can safely add another 10 dB reduction and probably more from the receive antenna pattern. Now you're down to around 40 dB Noise Margin on WVPX. This is much more reasonable.

I also analyzed WDLI. It shows a 19 dB reduction due to elevation pattern so the real Noise Margin is about 64 dB but that's a lot better than 83 dB plus there will be receive antenna rejection. WEAO comes in at -14 dB lower or a Noise Margin of 61 dB. WEAO won't benefit much from antenna rejection since it's only 25° off from your other stations.

To answer your original question, you should be okay using a typical distribution amp with 15 dB gain with your antenna pointed at 350°. You may still have issues though if you point that 91XG at the strongest stations.

If you try an FM trap let us k ow if it makes any difference.


*Reduction dB = 10Log(.09^2)
 

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Offhand, I don't recall whether TVFool included the Elevation Pattern, incl. Beam Tilt Angle. Can you cite where stan.s.lee said it was NOT included in the calculations??? Or perhaps Longley-Rice model didn't include it because FCC figured it was "good enough" to ONLY do calculations at the Radio Horizon.....which would overestimate signals for many viewers surrounding 5000-ft HAAT Mt Wilson in L.A.

Fol. Article may be of interest, it describes Power Curves for a variety of Beam Tilt Angles:
http://www.eriinc.com/files/18/18e2193f-2290-451d-b7a6-3ed813271957.pdf
 

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Discussion Starter · #17 ·
Here is my update on what I ended up doing... I went to the local Home Depot and purchased the only distribution amplifier they sell in their stores. I opened the package and read the instructions which gives the specifications as 4.5 dB amplification, 5.5 dB noise figure. I was kind of surprised by the lousy specs, but I tried it out anyways. It was only $20. It has four outputs. I guess the whole point is that it's supposed to split the signal into four outputs without causing any reduction in the signal, and that's exactly what it seems to be doing for me. It certainly didn't cause me any overload, which was a primary concern for me. I didn't notice any reduction in signal whatsoever. In fact, the TV upstairs with the longest cable length now picks up one more channel that it couldn't receive before... WYTV, which has a noise margin of -17.8 according to TV Fool. I'm beginning to wonder if I should go with a Channel Master distribution amplifier. I've heard the specs are better, but I certainly can't complain about this one, as it seems to be working for me.

Edit: I also want to point out that I'm using an MCM Electronics FM trap before the distribution amplifier.
 

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Offhand, I don't recall whether TVFool included the Elevation Pattern, incl. Beam Tilt Angle. Can you cite where stan.s.lee said it was NOT included in the calculations??? Or perhaps Longley-Rice model didn't include it because FCC figured it was "good enough" to ONLY do calculations at the Radio Horizon.....which would overestimate signals for many viewers surrounding 5000-ft HAAT Mt Wilson in L.A.

Fol. Article may be of interest, it describes Power Curves for a variety of Beam Tilt Angles:
http://www.eriinc.com/files/18/18e2193f-2290-451d-b7a6-3ed813271957.pdf

I cannot cite him saying this but it's pretty obvious that it doesn't. First off there's no table of elevation pattern values that I've ever seen in any of the FCC data like there is for azimuth patterns. Where would TV Fool get it from? The only place I've ever found it is in the applications for construction permits and it's always a graph.

About a year ago I visited an AVS member in San Francisco who lives 3/4 miles from Sutro Tower. He is 18.5° below the horizontal plane of the antennas. TV Fool predicted Noise Margins of >+80 dB for those stations. I expected to see tremendous signals on my spectrum analyzer. Instead I saw signals no stronger than I receive here 54 miles and 2 edges from my local transmitters. That was a real eye opener. Extrapolating the elevation pattern as shown in the attached image indicates he's only seeing about 1% of the signal which means about 100 watts for a 1MW station. I've attached a couple of spectrum analyzer traces from the stations. The non-flatness is caused by the gain of the antenna varying greatly with frequency when very far off the main lobe.

For comparison, the last image is an average signal I see here at 54 miles and over 2 edges.
 

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You probably won't do any better with the Channel Master, but if you are like me you'll get it anyway. . . .

If so, I would get the 4 output 3414 model. Some say that the single output model(3410) combined with a passive splitter will give the same result, but I still prefer the multiple outlet amp.
 

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I guess the whole point is that it's supposed to split the signal into four outputs without causing any reduction in the signal, and that's exactly what it seems to be doing for me. It certainly didn't cause me any overload, which was a primary concern for me. I didn't notice any reduction in signal whatsoever.

You got it! That's all it's supposed to do. You don't need or want a lot of gain in DA. I have mine set up so it has 2 dB net gain. A lower noise figure would only have an impact on the very weakest most marginal stations. You'll see the CM recommended here the most because the specs are well known and they've been tried and tested. The inexpensive hardware store ones are generally unknown. There are other DAs that appear to be exactly the same as the CMs.
 
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