Originally Posted by RudyG
Now I studied the plot that you attached and noticed that KNBC is broadcast at measly 296kW so I though perhaps that is why it comes in so weakly, however, KLCS is transmitted at an unbelievable 26kW and yet it comes in fine. On top of that KCOP is blasted at the highest kilowatt rating in the list 835 and yet I have a lot of trouble locking onto that signal. What could the explanation be for such erratic reception behavior?
Most likely, it's because of multipath. One of the ironies of OTA reception is that sometime stronger signals makes things harder to receive. When you are close to a strong transmitter, your house and everything around it are swimming in strong signals. This actually increases the number and strength of multipath reflections that might reach your antenna.
It's a little like the way you should avoid turning on your high-beams while driving on a foggy night. You can sort-of see ok with low-beams, but switching to high-beams causes increased light scatter and actually makes things harder to see.
Since multipath varies with frequency, the hot and cold spots may be at different locations for the various channels. You might get a "weak" channel to become "strong" and vice versa by moving the antenna around or pointing it in a different direction. Keep in mind that the signal strength meter on your receiver is probably not really a measure of true signal "strength", but rather a measure of signal "quality". All the signals in your area are pretty strong, but some of them might be prone to worse multipath interference than others. If you are still having a hard time fighting multipath, then it's probably time to consider moving the antenna higher and possibly to the attic or roof.
The reason for the variation between broadcasters is mostly because of the choice of directional antennas they are using (there's no point pumping a lot of energy into the Angeles National Forest and the mountains behind Mt. Wilson). Most of the broadcasters are transmitting with similar total power levels, but due to the antenna selection and exact orientation of the main lobe of the antenna pattern, you'll see slightly different power levels being sent in your particular direction.
Even through 296 kW sounds like a lot of power compared to 26 kW, it's only about a 10.5 dB difference (in power, everything is on a 10*log10 scale), which is why you see the ~10 dB difference in the Rx_dBm column. The total spread of all the channels is not that much (~20 dB), so I don't think the variations in transmit power you're seeing will actually matter much. The fact that all the channels are quite strong (above -70 dBm) means that the small differences in channel power will matter even less.
My second question about the chart is what is Rx_dBm?
That is the predicted field strength of the signal at a hypothetical point in space (your receive antenna's coordinates). Knowing the transmit power of each broadcaster, their antenna pattern, and their frequency, we can compute the estimated signal loss over the terrain and through the air to reach you. The results are converted to dBm units, which is something you might read directly off a spectrum analyzer when looking at these signals.
Since these estimates are for a virtual point in space, the actual signal reaching your TV tuner depends on everything in your "RF chain", including your antenna's gain, any amplifier gains, cable losses, splitter losses, etc. The plots will tell you what's "in the air", and it's up to you to fill in the blanks for everything between "the air" and your receiver.
Based on my own experiences and the feedback from others who have used these simulations, I would generally categorize channels as follows:
- Channels above about -70 dBm are very strong and there's a good chance they can be picked up with an indoor antenna (although indoors is the most vulnerable location for multipath interference)
- Channels between about -70 and -100 dBm are where signals are getting weaker and the antenna will probably need to move to a better location (attic or outdoor) in order to receive it.
- Channels below about -100 dBm are getting difficult to receive and are best served by a rooftop or mast mounted antenna.
- With high gain antennas (e.g., XG91, DAT-75, CM 4228) and a good installation (i.e., appropriate pre-amps, quality cables, etc.), it should be possible to receive channels down to about -110 dBm, give or take.
Of course, these are very rough estimates and every situation is different, so multipath, co-channel interference, adjacent channel interference, overload constraints, and a slew of other factors must be considered in order to understand the challenges facing any specific setup. The simulation software, the plots, and the tables are merely tools to help us make more informed decisions, but are by no means meant to be taken as gospel.
A question about this antenna and my location. According to the chart I'm less than 30 miles from the transmission source and this antenna is very much a long range antenna. Is that going to be a problem?
I don't consider the Silver Sensor to be a long range antenna. It is directional, but the actual gain of the antenna is not that high. The antenna pattern plots are scaled to the highest gain point of the antenna, but do not actually show the total gain of the device. The Silver Sensor has a maximum gain of approximately 7 dBi, whereas an antenna like the CM 4228 has a maximum gain of approximately 16 dBi, which is more typical of what people think of as "long range" antennas.
In either case, I don't think there's enough gain in the antenna to cause any problems for you. The signals are indeed strong, so I would recommend that you DO NOT install any amplifiers in your RF chain. An amplifier could have overloading problems since it has to simultaneously amplify so many strong channels in the spectrum. Your tuner, on the other hand, should be fine with these power levels, since it only looks at one channel at a time, and any one channel's power is not high enough to cause any problems. Even a high gain antenna would not pull in enough signal to be a problem for your tuner.