[rabbit73]

Originally Posted by verder

I'm going to show my ignorance here because I don't fully understand what you were pointing out with your last post. You say "you see that -22 dBm for WDAY before adding the antenna gain is getting close to -8 dBm". I don't understand (intuitively) how these are close. Given that dBm is a logarithmic scale, aren't these relatively far apart?

I owe you an apology. I have been dumping a lot of technical information on you without knowing your background, but you seemed to be interested in learning more.

I should have shown the math to support my statement.

I'm at a loss (ha!) at how to correlate from the antenna gain to the levels you point out from the ATSC requirements. The 4Max has these specs: "UHF 11 dBi / VHF 2.5 dBi". If WDAY (ideally) has a signal power of -22dBm, then how does the antenna gain affect what is ultimately fed into the tuner? It's not as simple as adding the two values is it? 11dBi + (-22dBm) = -11dBm? That's getting somewhat close (3dB), but I think I'm horribly conflating things at this point.

It is pretty much that simple. I'm used to using the TVFool reports where Noise Margin (NM) 0 dB is for a dipole antenna. The strength of a signal is shown in two ways on a TVFool report: NM or Signal Power. You can add antenna gain to either one. See this FAQ:
https://www.tvfool.com/?option=com_c...ask=view&id=57

Antenna gain is the only quantity that should ever be ADDed to the NM value. Most antennas will specify their gain in dBd or simply dB, and this is the value that should be used. If an antenna's gain is specified in dBi units, then you need to subtract 2.15 in order to get the equivalent value in dBd units.

More recently, the TVFool reports have become less accurate because they use a database with many errors to generate reports. Trip (Trip in VA on the AVS Forum), who is the webmaster for rabbitears.info and now works for the FCC, decided to create the Signal Search Map to generate reports. Trip asked me to be a beta tester, so I was able to make a few comments that I hoped were useful.

The rabbitears reports are now our first choice and I assume I can also add the antenna gain to the dBm value or the Signal Margin value.



Antenna gain 11 dBi - 2.15 = 8.85 dBd (in reference to a dipole)

WDAY -22 dBm + 8.85 dBd = -13.15 dBm, which is weaker than -8 dBm

I don't quite understand what the multi-signal overload text is stating. Is it saying the more strong signals you have, the more you need to derate these strong signals, and the recommended amounts they need to be derated at? 8 strong signals = derate by 8dB, because otherwise the tuner is overloaded?

The ATSC Guideline says that a tuner should be able to tolerate a signal as strong as -5 dBm. Where there are multiple signals, the tuner should be able to tolerate them if they are no stronger than -8 dBm, which is weaker because we are in negative values.

The derating chart after that is not part of the ATSC Guidelines. It came from holl_ands who is a member of the AVS and DHC forums, is an RF engineer, and does antenna computer modeling. The derating chart says that the signals must be even weaker if you have more of them.

Does all of this come together to indicate that maybe it would have been better if I had gone with the ClearStream 2Max instead? It has the specs: "UHF 8.7 dBi / VHF 2.6 dBi". All of this of course assumes that the signals are indeed too strong for the Tablo.

The CS 2MAX does have less gain than the CS 4MAX, so there is less chance of overload if that really is a problem with the Tablo. The advantage of the CS 2MAX for you is its wider beamwidth because your two directions are not 180 degrees apart. I have heard reports that the TiVo and HDHR don't tolerate strong signals as well as the average TV tuner, but I haven't made any tests to confirm that.

I have made overload tests on tuners. The first one I did was from ignorance. I connected two preamps in series because I thought more gain is better; I didn't get any signals at all.

The more informed tests that I made later showed that the strongest signals created IMD (Intermodulation Distortion) products called spurious signals (spurs) that wiped out the weakest signals. As I increased the gain even more, only the strongest signals were left, and finally even they were gone. I was working in the range of -5 to +5 dBm. I didn't want to go any higher because it might damage my tuner.

THREE TYPES OF OVERLOAD

There are three types of preamp or tuner overload, in order of increasing signal strength:

1. The strong signals almost cause enough intermodulation distortion (IMD) to interfere with the reception of weak desired signals, but the spurious signals are at or below the noise floor of the weak signals. This is the point that holl_ands uses in his preamp charts to obtain max SFDR (Spurious Free Dynamic Range). No damage will happen.

As the strongest signals continue to increase in strength, more of the weaker signals are damaged until you reach:

2. The strong signals cause overload to the preamp or tuner that makes it impossible to receive any signals. No damage will happen. The strongest signals are still there, but they can't be decoded because the IMD products have damaged them so that they contain more errors (high BER....bit error ratio/rate) than can be corrected by the FEC (forward error correction).

3. The signals are so strong that the input transistor is toast. You are not likely to encounter OTA signals that strong, unless you live next door to a high power transmitter and you have your high gain antenna aimed at the transmitter's antenna.

As a general rule, tuners can tolerate stronger signals than preamps before overload. The difference in strength is approx. equal to the preamp gain.