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Old 17-Aug-2013, 9:10 PM   #59
Pete Higgins
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Join Date: Apr 2013
Location: California, 58 miles @112 degrees from Mt. Wilson
Posts: 83
Antenna or Spatial Diversity

tripelo,
Quote:
Interesting: The depths of frequency selective fading, seen with the single Yagi, imply that gains in signal strength alone would not be sufficient to overcome the effects of the deep fades. Clearly, a 25 dB selective fade could not be mitigated by a 3 dB increase in signal strength (in reality, stack gain is likely less than 3 dB).
From the little bit of reading Iíve done on the subject, VHF is more prone to rapidly-changing multipath conditions than UHF although the new demodulators with long equalization spans have dramatically reduced multipath effect, both static and dynamic for 8-VSB reception.

None the less, your early sample 2 data showed ~25 dB of what appears to be frequency selective fading evident at the upper portion of the 6 MHz channel 11 spectrum. Assuming your test setup was undisturbed between samples, I think the apparent loss of signal has to be attributed to some mechanism external to your test setup. In my experience frequency selective fading (when neither the transmitter nor the receiver is moving) is generally due to destructive interference from multi-path causing cancellation of certain frequencies at the antenna. For high VHF & UHF, which are usually received via a LOS mechanism, this is typically from reflections off the ground and nearby objects.

For WHAS-11, using 984í (300m) Tx antenna height and guessing your tower @ ~65í I get a line-of-sight path of roughly <50 miles. I reasoned that when there isn't a line-of-sight path between transmitter and receiver the signal has to be both diffracted and reflected along multiple paths before finally reaching your array. I expect some signal arrives via tropospheric scattering due to changes in temperature, humidity and barometric pressure causing slight changes in the refractive index although Iím not sure that would in and of itself explain short term selective fading.

In any case, each of these arrival mechanisms introduces signal anomalies that can destructively interfere with one another at the aperture of a receiving antenna. With a single antenna or two closely spaced antennas, I would expect little difference in coherence of the arriving signal(s) yielding a theoretical overall 3dB stronger signal, but, exhibiting the same overall characteristics (frequency response). By coherently combining two separate apertures spaced ~1.5 wavelengths apart, however, I would expect slightly lower overall gain reflecting a summation of all the different paths coherently combining at the antennas. The phase differences causing frequency selective fading at one antenna would in effect be mitigated by the arrival phase at the other antenna.

From WikipediA:
Antenna diversity, also known as space diversity, is any one of several wireless diversity schemes that uses two or more antennas to improve the quality and reliability of a wireless link. Often, especially in urban and indoor environments, there is no clear line-of-sight (LOS) between transmitter and receiver. Instead the signal is reflected along multiple paths before finally being received. Each of these bounces can introduce phase shifts, time delays, attenuations, and distortions that can destructively interfere with one another at the aperture of the receiving antenna.

Spatial diversity employs multiple antennas, usually with the same characteristics, that are physically separated from one another. Depending upon the expected incidence of the incoming signal, sometimes a space on the order of a wavelength is sufficient. Other times much larger distances are neededÖ

I guess my thinking is that if your frequency selective fading data is the result of multipath signals arriving out of phase at a single antennas aperture, combining two antennas narrows the aperture (beam width) eliminating some portion of the fade contributor. Additionally, with 1.5 wavelengths spacing, you have signal arrival from different paths at each antenna. As one antenna receives frequency selective out of phase signals (a fade) there is a good probability that the other antenna will receive in-phase signals, mitigating the depth of the fade. If that mitigation is sufficient to keep you above the detection threshold the equalizer can do its job, further minimizing the fade effect on reception.

It will be interesting to see what you postulate in your next installment.

Last edited by Pete Higgins; 18-Aug-2013 at 5:07 PM. Reason: Summarize my thoughts (in blue)
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