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Originally Posted by ghz24
All you uber-gurus out there.
If any one can show how to calculate this effective increase in noise factor for any given swr It would be greatly appreciated
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I don't know any way to calculate it. My approach would be to introduce a high SWR line between the antenna and the tuner; like a 75 ohm line (with choke balun) between a 300 ohm antenna and the tuner for a 4:1 SWR. Then I would measure the signal dropout point, with an attenuator and a signal level meter, and note its difference in dB from the 1:1 condition, which should indicate the amount of deterioration of NF. The EVM theory says that this deterioration is more than what would be accounted for by the increased line loss because of a higher SWR.
holl_ands has a few things to say about EVM:
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High...and even moderate SWR can distort the digital ATSC waveform, as signals mimic multipath bouncing up and down the coax downlead (Preamp's not so much).
EVM (Error Vector Magnitude) is a measurement of the amount of distortion:
http://www.digitalhome.ca/forum/show...&postcount=716
Technically speaking, the digital decision "eye pattern" becomes very noisy, as the 8 VSB amplitude levels become very indistinct, since the SWR reflections are mis-aligned in time to the primary signal. Dr. O Bendov describes this as so many dB of Noise Figure degradation, which is somewhat of a misnomer....
Bear in mind that, in a perfect world, ATSC needs more than 15 dB SNR if the Noise is truly Random, which corresponds to an SWR of about 1.4.
Fortunately the "multipath" bouncing up and down the coax isn't Random, and the Tuner isn't affected until seeing the TWICE reflected signal (down, up and back down), which is about 7.5 dB each way or SWR of about 2.5.
So ATSC actually tolerates a significantly higher SWR....I try for under 2.7.
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In the link to post 716 by holl_ands, he seems to think there is some justification for calling an antenna an HD antenna if it has a low SWR:
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So indeed there IS a difference between Analog & DTV antennas....
DTV Antennas also need very good SWR performance.....
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