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#19
January 18th 07, 05:18 PM
posted to rec.radio.shortwave
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Sync detectors and fading
bpnjensen wrote:
wrote: dxAce wrote: But, is it 'sucking it out' or merely propagating it somewhere else other than that particular spot where your antenna is? And that 'somewhere else' might not be very far away, but merely a few wavelengths in distance. dxAce Michigan USA Before satellites carried most of the milcom they used "diversity receivers". Two, or more, receivers tuned to the same frequency but located some distance apart. The logic being that when the singal faded at one location, the other didn't fade at the same time. The more important a comm cicuit the more receivers spread over a wider area. A friend and I played with our receivers feeding phone patches and since we live 30 miles apart it was clear this approach was workable. With signals that experienced deep fades we were able to listen to nearly all of the time. Real (commercial or military) had AGC based voting systems to decided which signal to pass. We ran into issues of our audio phases shifting producing very odd sounding "flanging" effects. I have often thought about trying this with receivers whose antennas are only a few hundred to thosand feet apart. I never have gotten around to it. The military also used freqeuncy diversity, sending the same singal on more then one frequency. Kind of like listening to WWV on 5 10 and 15MHz at the same time. Terry Fascinating. It sounds like a couple of antennae, maybe even on the same property but spaced some modest distance apart, maybe a few hundred feet, and phased into the same radio, might also be a solution to the problem. Anyone try this with a 50-acre lot and a phasing harness? Bruce Jensen Diversity reception has been a well established practice since the early days. Hallicrafters produced a diversity receiver, which was actually two receivers diplexed into a single audio stage, fed by separate antennae. May have been a bit of overkill. Separate antennae, if electically isolated from one another, diplexed into a single input can produce similar results: reducing selective fading before it reaches the receiver. When I lived in Rockford, I rented a two bedroom home on a private estate west of town. The rental property included several acres on a hilltop, and access to the private lake on the estate. Of course, I went antenna crazy. And using multiple antennae into the BC-794, was able to mitigate a good deal of the selective fading throughout most of the HF spectra. Each antenna was connected to an RF preamp with a gain of 2-6db. The outputs of the preamps were combined through resistive pads (for isolation) into the RF input of the BC-794. The result was nothing short of amazing, with fading distortions dramatically reduced, and program listening, was quite pleasant. Even my wife was no longer critical of SW listening. Make no mistake, it wasn't FM quality. But it was fine wideband (when conditions permitted) AM quality. And though there WAS some latent fading remaining, it was, by far, less objectionable, and often barely noticeable than a single antenna on the same receiver. Diversity, at least in this case, is something to be implemented with affirmative action.  |
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