Antenna Amps, Do they help or hurt reception?

FOX TV · 10-29-2009, 06:50 AM

Quote:
Originally Posted by JER
Its been my general feeling for quite a while now that for many people reception problems are usually related more to signal quality than signal strength.

The above post by FoxTV indicates how complicated the signal environment can be and how difficult it can be to resolve these kinds of problems without a truck load of test equipment.

Probably the simplest way to avoid a lot of these problems is to get the antenna outside and to avoid amplifiers.

Putting the antenna outside helps reduce the impact of household noise sources. It also dramatically increases signal level and reduces the effect of indoor multi-path.

While amplifiers increase signal level, they always add something (e.g. noise, harmonic distortion, and inter-modulation distortion) that's not actually on the air. A cheap amp can easily saturate on a strong in-band or even out-of-band signal and generate a wide spectrum of "crud" that makes it impossible to detect anything with the receiver. By starting off without an amp, you can make a better determination of what your true signal situation is like. Then, if you're missing some weak stations you can add the amp and see if it really helped.

JER, I agree with your thoughts on amplifiers. I think they are overused to the point of causing problems instead of solving them. An amplifier is the last thing I would recommend to try and solve reception issues. This is based on my last 3 or so months doing field strength tests in our market that has very mountainous terrain. I have received signals from ours, and other transmitters in this rough reception terrain at 75 miles out, and still had plenty of signal head room for good reception using a Clear Stream C2 or in some cases a C4 at only 20 feet elevation.

I have seen this same scenario at many different locations in our rigorous signal testing project. I use a Rhode & Swartz FSH-3 TV Analyzer for RF signal analysis, along with a Sencore DTU 236 transport stream analyzer to look for echo strength and amplitude, and for echo timing to see how far out on the time line the echos are in highly problematic areas. It seems that most of the multi path issues are from reflective objects that are relatively close to the receive location instead of miles out as in analog.

I have not seen one location at that distance that if enough signal strength was there that I could not receive the signal, and have not really seen a situation where an amplifier would solve any reception problems. We have even seen situations on the Rhode & Swartz Analyzer where the signal was almost near the SNR of the tuner that we were using, but the gain of the C2 would still allow reception of weaker signals WITHOUT an amplifier.

You have also hit on a point that I have noticed on my Sencor TS analyzer that the BER (Bit Error Rate), EVM (Error Vector Magnitude), and SNR (Signal To Noise Ratio) changes drastically when an antenna is not aimed optimally. There is also a great variance in the BER, EVM, and SNR of various signals between different stations in the same market, and this can affect viewers reception of signals due to the signal quality between different stations in a given market.

The EVM and SNR are factors that determine the quality and receivability of a signal, especially out in the fringe areas of any given DTV signal. The viewer has no control over these aspects of the signal, and Broadcast Engineers are the ones responsible for ensuring that there is a quality signal being broadcast from their transmitter site.

If a viewer is having problems receiving a certain station, but all or most other stations can be received well, and the predicted signal levels are close to each other in strength, they may be fighting a loosing battle if the EVM and SNR of the transmitted signal is out of spec, or near the lower limits of legality in regards to FCC minimum specs. This is where the truckload of test equipment is needed in order to evaluate a Broadcast DTV signal for quality, which can effect reception of that signal.

Simply saying that an amplifier will solve these problems is not accurate in the real world. I can envision a day when spectrum analyzers for DTV could be built into receivers, and the terminology changed in order for the consumer to gauge the quality of the received signal themselves. I think this improvement would greatly enhance OTA viewing for the average viewer.

Test gear can show a lot of problems with a signal that are not known to the viewer and cannot be evaluated by any other method. Low signal IS NOT ALWAYS THE CAUSE OF RECEPTION PROBLEMS, as can be seen on test gear. I have seen situations where the RF signal levels were very strong, but multi path echos which could have been caused by the high signal levels in the first place would still kill quality reception of signals in highly reflective signal areas causing breakup and pixelazation of the signals. (RF signal levels and signal quality are measured on a completely different devices on a separate antennas so they can be evaluated simultaneously)

In closing, I do not recommend amplifiers at all, but they may have a use in the far field areas that do truly have a weak or low signal issue, but those situations are not all that common in our market, and evaluating different brands of antenna amplifiers with test gear is not on my agenda any time soon due to cost and time restraints.

JER · 10-29-2009, 04:55 PM

I believe its fairly well known that amplifiers can overload on strong FM signals. The ensuing harmonics and intermod products can easily wipe out reception in high VHF television and higher bands. For those that must use the amp, a FM trap placed ahead of the amplifier can help reduce the overload and improve TV reception. The problem nowadays is that FM stations are only one of many possible strong signals that may enter the amplifier and cause overload and distortion.

I have a hunch (no data at this time) that at least some reception problems being observed may be traceable to cell phone towers. The lower cell bands are easily passed by most TV antennas. Since the cell towers are often very close to homes its easy to envision them saturating a cheap TV preamp since most of them have pass bands extending out to about 1 GHz.

Has anyone conclusively observed such a situation? If so, I'd be interested to hear about it.

IDRick · 10-29-2009, 06:30 PM

Quote:

Originally Posted by FOX TV

In closing, I do not recommend amplifiers at all, but they may have a use in the far field areas that do truly have a weak or low signal issue, but those situations are not all that common in our market, and evaluating different brands of antenna amplifiers with test gear is not on my agenda any time soon due to cost and time restraints.

Interesting, your post from the other day suggested that you supported some amplification to overcome large distribution loses. Why the change of heart? In my case, I lose one station due to distribution losses not poor reception. What do you suggest in my situation? See: Pre-Amplifiers, Usage & Reviews

FOX TV · 10-30-2009, 05:52 AM

Quote:

Originally Posted by IDRick

Interesting, your post from the other day suggested that you supported some amplification to overcome large distribution loses. Why the change of heart? In my case, I lose one station due to distribution losses not poor reception. What do you suggest in my situation? See: Pre-Amplifiers, Usage & Reviews

Antenna amplifiers and distribution amps are not the same animal. The only type of amplifier I would ever recommend would be a distribution amp to feed multiple sets, or to get the signal up enough to satisfy a multitude of splitters in multiple set installations. Possibly an antenna amp may help in fringe areas as long as there are no other strong sources of RF energy that are also amplified and can saturate the front end of a receiver while being amplified along with the DTV signal.

Distribution amps get installed after the signal is decoded in order to amplify the demodulated DTV signal for distribution to multiple sets, and have no relation to amplifying the 8-VSB digital data stream that makes up the DTV broadcast signal.

A lot of people are trying to overcome signal issues who's quality may be in question by amplifying the signal, but a digital signal is not like analog where reception quality was simply an issue of more brute force power, either at the transmitter or at the receive antenna.

I have not heard any mention of signal quality except from JER, and if you look at different signals in any given market, you will see that signal quality can vary greatly among different broadcasters in the same market, and all of the high gain antennas and amplifiers you can buy and and erect will never overcome this problem if that is what is actually causing reception issues in the first place.

Signal quality refers to the signals parameters themselves, and not RF signal levels or power levels. The only way to gauge the quality of a signal is with a transport stream analyzer, and they are fairly expensive items that most people cannot afford, or will not use enough to justify its costs.

In regards to your channel 47 issues, since you are using a homebrew antenna, you could possibly replace the driven elements, or half of the driven elements with a physical size that favors the upper end of the UHF band. I am assuming that these channel numbers are the actual RF channels and not virtual channel numbers.

Since you have plenty of signal on the other channels, you could possibly afford to change the antenna to favor 47 without to much loss on the rest of the band.

EscapeVelocity · 10-30-2009, 07:30 AM

Quote:

I have a hunch (no data at this time) that at least some reception problems being observed may be traceable to cell phone towers. The lower cell bands are easily passed by most TV antennas.

I just hit upon this the other day, in my ponderings and ruminations as well.

I wonder if a new product would be successful, a low pass filter just above UHF Ch. 51. (or 55 for MediaFlo broadcasts).

Perhaps bandpass filters should be more commonly used, in general?

IDRick · 11-02-2009, 08:19 PM

Hi Fox TV,

Sorry, didn't see your reply earlier... I do my testing with an Apex 502 converter box that provides signal strength and signal quality readings. An example graph of my testing with attenuators is attached. Note signal strength has a strong linear relationship with margin to dropout indicating that the Apex is measuring signal strength rather than some other measure. Also note, signal quality was 100% until the margin decreases down to 5 dB or less. A good picture was observed until margin to dropout fell to 2 dB or below. However, recordings on my HTPC have many video/audio errors when signal quality is less than 100%. For that reason, I need to achieve a margin to dropout of least 10 dB for all my channels.

I did try a 4-bay with shorter spacing between bays and shorter elements (supposed tuned higher than my original DIY). Margin to dropout was only 1 to 2 dB higher with the new antenna, not enough to solve the problem.

Adjusting antenna aim to favor ch 47 has merit. I have attached a graph that shows the effect of aiming off axis on margin to dropout. Aiming directly at ch 47 tower saves 4 dB for that station but whacks ABC, PBS, and CBS by 14 dB. That's too large of a whack, given that my distribution losses equal 20 dB and I need to maintain at least a 10 dB margin for each channel.

IMO, a pre-amp or distribution amp is the better choice.

Best,

Rick

DTVuser2009 · 11-03-2009, 01:01 PM

All i know is i need two in-line (those cheap ones you buy at K-mart, signal amp) to pick up half of what i pick up. it doesn't hurt the normal 15 or less i get any time, but without those two i'd have zero access to channels such as WPSD-DT. WAZE-DT (still watching George Lopez on that one

), WSIL-DT and of course my new favorite, WBKO-DT

FOX TV · 11-05-2009, 06:41 AM

Quote:

Originally Posted by DTVuser2009

All i know is i need two in-line (those cheap ones you buy at K-mart, signal amp) to pick up half of what i pick up. it doesn't hurt the normal 15 or less i get any time, but without those two i'd have zero access to channels such as WPSD-DT. WAZE-DT (still watching George Lopez on that one

), WSIL-DT and of course my new favorite, WBKO-DT

No one can say that there are no situations where an amplifier will not work, but if you live anywhere near a source of RF energy that is capable of saturating a DTV receiver, that Rf energy is also amplified along with the desired signal as well. Most DTV receivers were not built with FM harmonic filtering like the old tuners in almost all of the analog sets were. That was a requirement of FCC part 15 acceptance of the tuners if they were submitted for testing under part 15 at all, and not all were submitted for this approval process.

A lot of reception issues also come down to how much power a transmitter is running, and the assigned radiation pattern of their antenna. There are many scenarios that affect reception that the general public is not aware of. Not all stations are running the same power levels as all of the others in a given market. That could be due to limiting factors at the transmitter site, or for financial reasons, or for interference reasons. A lot of broadcasters were assigned directional antennas that have deep nulls in their signal pattern, and that could have been done to protect another channel such as an analog signal that is no longer on the air, or to protect an adjacent channel etc.

There are still financial and technical repercussions of the DTV transition that are not all worked out as of now for a lot of stations. A lot of items such as transmitter power, antenna patterns and many other limiting factors that are a repercussions of the FCC having to accommodate digital and analog channels at the same time that will take years, and many millions of dollars to fix.

At my station, we were assigned a directional antenna at one of or two transmitter sites in order to protect an analog channel that now is no longer on the air. In order to gain that coverage area back, we are now facing the task of raising our tower another 50 feet, and buying another $150,000 antenna, and scrapping the directional antenna that is only 3 years old that cost $100,000 dollars and it will be headed to the scrap yard in the near future. This is in addition to the current top mounted analog antenna that is also destined for the scrap yard.

The transmitter sites of many broadcasters are not optimized at the current time. There are many other factors involved in TV broadcasting that can effect reception that the general public will never know about, and it will take many years and many dollars to fix. I think that TV broadcasters should be eligible for a bail out too, or at least given tax breaks for the forced change over that many of them could not afford in the first place. The Government only said that you had to do DTV, and the Broadcasters were forced to do so, or lose their license to broadcast, with no financial incentives or tax considerations given.

Just another example of the heavy handed tactics of Government, when some business are considered "To BIG to Fail" and they get to rob the taxpayers of their hard earned dollars while some smaller broadcasters had to file for bankruptcy or mortgage their stations to the max to comply with the DTV transition. The DTV transition is not complete for a lot of stations, and there are many of these scenarios taking place even now.

Piggie · 11-05-2009, 07:08 AM

Once the new 700 MHz cell towers come on line and FloTV already has, all of that RF is designed into all the equipment everyone is using.

There are filters that stop everything above 700 MHz but they are about $25 plus shipping, or say about $35 by the time it's all said and done.

We have only begun to see interference from cell towers.

But if Congress and FCC gives them UHF TV, the interference problem will go away...

JER · 11-05-2009, 11:04 AM

There will definitely be times when an amplifier will help reception. The problem is that the average Joe has no idea when to use one or what kind to buy. The fact that there are a lot of bad ones on the market with incomplete specifications doesn't help.

I think the biggest problem is that there are a lot of cheap indoor antennas with amps that have too much gain (I've seen as much as 55 dB advertised) and that have terrible linearity and overload characteristics. Note that roughly 80% of the TV antenna market is for indoor antennas.

This problem is two fold. First it gives people the notion that they can receive TV with an indoor antenna even though the DTV system was developed assuming outdoor antennas. Second, it leads people to believe that its essential to have an amplifier with an antenna. Because of this, its nearly impossible to sell an indoor antenna nowadays that does not include an amplifier. Consumers incorrectly assume that the amplifier somehow magically makes up for the antenna being small and indoors. If this forum did nothing else but dispel this myth then we would be doing a great service to the public.

Now, lets talk about how and when an amp may help.

Please correct me if I'm wrong, but I believe the average DTV receiver has a noise figure of around 7 to 10 dB or so depending on band. Given this, its reasonable to expect that a well designed amplifier with a noise figure lower than that of the receiver will help reception by improving the signal to noise ratio.

For example if you have an amp with a noise figure of 2 dB and the receiver has a noise figure of 7 dB then you will likely see an increase in noise margin of roughly 5 dB (actually a bit less depending on gain, cable and mismatch losses). In SOME circumstances adding the amplifier is just as good as getting an antenna with 5 dB more gain. IN SOME CIRCUMSTANCES!

The problem is that the amplifier and receiver both have limited dynamic range. This fact automatically sets you up for problems.

If you buy a cheap amp (most likely scenario) it will likely overload on something (either in or out of band). When its overloaded, it will generate harmonic and Intermodulation signals that are not on the air. These distortion signals can easily ruin reception on some or all channels. Note that distortion phenomena aren't captured in the noise figure number! Noise figure only looks at random noise added by the amplifier when its not in an overload or even strong signal situation.

If you buy decent amp with good linearity and overload resistance (high OIP3 and P1dB) you will find that the signals out of the amp are clean but now possibly too strong for the receiver. If the receiver overloads you're back in the same situation as above with the cheap amp. You might see reception improved on some channels and ruined on others depending on how the distortion products pile up. A variable attenuator, trap or equaliser after the amp may help at this point but it will be a trial and error type of thing unless you have a truck load of test equipment.

So, the basic recommendation should be to start off with no amp and see what you get. If you get everything then you're done. If not then you can try an amp and see what happens. The amp you want may depend on what problem you're trying to solve.

If you're deep fringe rural with nothing but weak signals (either in or out of band) then buy one with low noise figure (lower the better) and enough gain to overcome your losses.

If you're in urban or suburban area where signals are strong, but you have lots of cables and splitters you want something different. Here, you should opt for an amp with good linearity and overload as measured by OIP3 and P1dB (higher the better on both). The noise figure will only need to be a modest 3 to 5 dB or so. This will ensure that you have clean signals with bit of a bump in sensitivity from the amp to help noise margin on the very weakest stations.

Again, the basic rule is to use outdoor antennas and to only use amps if and when you find problems that can't be solved by other means.


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