Originally Posted by kenglish
In would be interested in knowing how they measure their overload specs. Many of the domestic ones rate overload (IM3) at, something like, "3 channels" or "5 channels".
In some areas, like SLC, you may have well over two dozen channels on the air in the UHF Band (analog full-power, analog low-power, analog translators, digital full-power, digital low-power, and even digital translators....plus, those new non-broadcast users that are starting to show up).
As you add channels, you de-rate the spec by almost 3 dB every time you double the number of stations. So, if rated at, say "+47 dBmV at 3 channels", that becomes +44 at 6 channels, +41 at 12 channels, ..........
Hey Ken, the essence of your conclusion is right, but the math isn't. Everytime that you double the number of channels, the instaneous voltage doubles, not the instaneous power. When the voltage doubles, the power goes up by a factor of four. Hence the overload resistance gets worse by 6 db whenever you double the number of stations.
The same thing happens when you increase the number of stations in a common TV or FM transmit antenna, the limiting factor becomes the peak power, not the average power, in the feedline.
See the following for the basics:http://www.dielectric.com/broadcast/...issionLine.pdf
"When combining channels into one transmission line both average and peak ratings must be evaluated. In general, for carriers of equal power, the peak power rises as the square of the number of carriers."
If you do the math, don't forget that all DTV carriers have a peak to average ratio of about 6 db.
For this reason, I predict that DTV reception will get much better on February 19, 2009, the day that analog stations are turned off.