Originally Posted by Prototype3a
In other news, Ch9 WSWP is still very unpredictable for performance. I haven't really messed with the setup any as I intend to get an RTLSDR and then see what I can learn from that. As such, can multipath be "seen" with an RTLSDR?
There are two types of multipath, static and dynamic. I will cover static first.
Notches in a scan of the channel might indicate a multipath problem, sometimes not. It depends upon the shape of the signal when the RTL-SDR scanner software crosses that channel.
Channel 13 in my avatar to the left was multipath with an indoor antenna. The signal was strong enough, but the image was frozen; too many errors. This is the TV screen that was at the same time:
I moved the antenna to the center of the bedroom in a high traffic area (of course); the reception was excellent:
It is also possible to have a flat scan but the tuner can't decode the signal. When Trip in VA was living in Chattanooga, he had an unusual multipath problem:
Originally Posted by Trip in VA
What's strange is that the multipath here doesn't really show on the spectrum analyzer I have. The signals all look flat, they just don't decode. And I've moved my outdoor antenna all over the deck in every possible position.
I did a side-by-side on WSB and WDSI because WSB decodes and WDSI does not.
Trip's solution was to put the antenna in a trash can which narrowed the acceptance angle of the antenna and rejected out-of-phase reflections.
Originally Posted by Trip in VA
I bought a metal trash can from ACE Hardware on my way home this evening and in my bedroom, I stuck my Silver Sensor inside it aimed in the direction of the transmitters. Voilá, 100% signal quality on WFLI, high 80's on WDSI, WTCI and WYHB-LD. The Silver Sensor is too small for the VHFs but it's a start!
A possible explanation for a flat scan when there is nultipath is given by Dr. Bendov in his paper DTV Coverage and Service Prediction, Measurement and Performance Indices
VIII. SNR AND “FIELD STRENGTH” MEASUREMENT VIA SPECTRUM INTEGRATION
Defining the Signal as the total received power and the Noise as AWGN (Additive White Gaussian Noise) leads to the conclusion that the SNR at the input to the receiver increases with increased multipath.
In urban and indoor situations, there may not even be a main signal, only reflections, some of which are of equal magnitude.
If all multipath signals are part of the signal power, then the SNR margin may not be an indicative figure of merit of reception robustness. In any case, even accurate measurement of the total received power may not be trivial.
The integrated signal power is not just the Desired Signal power. It includes, Man-made, Galactic, and thermal noises and residual transmitter generated in-band noise. It also includes some but not necessarily all multipath signals. For example, pairs of identical and asymmetric echoes, one of positive amplitude and positive delay relative to the main signal and one of negative amplitude and negative delay relative to the main signal, will cause only a second-order distortion of the displayed power spectrum. They will create group delay. Thus, in a multipath channel, a pair of such echoes would measure high SNR when using the spectrum integration technique whereas in reality, the true SNR would be much lower.
There may be other combinations of echoes that would yield essentially flat spectrum display.