[opendtv] Re: Democrats Air Concerns About Analog Switchover

  • From: "Albert Manfredi" <bert22306@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Thu, 23 Nov 2006 13:46:00 -0500

Cliff wrote:

No, the stronger signals are coming from Baltimore even though I live15
miles closer to Philadelphia than to Baltimore. That's both stronger
digital and analog signals. If I point the antennas at Baltimore, there is
little improvement but I lose some of the Philadelphia stations

If your reception of Balt is anything similar to mine, and it probably is because we are at about the same distance from Balt (although on opposite sides), you would benefit from a whatever extra signal margin you can get. Because signal margin is what avoids the annoying freezups you mentioned. So it seems to me that you want to point your main antenna to the best signal source and go for all the margin you can get. For whatever reason, Baltimore is your best source of signal.

The 'antenna' on the UHF parabolic is a double bow tie that faces
the 7 ft. diameter reflector. I.E. the bow tie antenna IS pointing
at Baltimore.

Cliff, is that the way you would describe a satellite antenna pointing directly away from the satellite? The bow tie, a folded dipole, is supposed to be at the focus of the parabola. It's supposed to pick up all that UHF signal the parabola concentrates at its focus.

Think of the filament in an automobile headlight. In the headlight example, the filament is essentially a point light source, and the reflector turns that into a single beam, aimed back through the filament and out the front of the car. In this example you're doing the exact opposite. You want the antenna to concentrate what is essentially parallel rays coming from a distant transmitter into a single point of max signal density, and that's where the dipole should be.

If the dipole sensor is on the wrong side of the parabola, no matter how you say it, you are defeating the purpose of the reflector. It is not doing the concentrating of the EM rays from the intended source. The parabola is instead partially shielding the sensor from the source.

The specs for this antenna show that reception off the back side through
the reflector to the antenna is about 5 dB less than the front facing gain
which is 17 dB.

So the reflector isn't terribly effective, but it still gives you an extra 5 dB of margin. Truth is, I find that supposed 12 dB gain off the back amazing, for what is otherwise just a double bowtie. Not sure how they come up with that. At best, I'd expect around 3 dB of gain.

As for analog, I get very good pictures from most of the Baltimore U/V
stations, with extremely low noise at the bottom of the H. sync pulse
viewed on a waveform monitor. This is not the case for the Philadelphia
U/V analog stations. They are noisy and have occasional hits in them,
probably what most would call "Grade B" reception, or maybe worse.
This is the way it is year round, not just in winter. The reception has
been this way for 5 years, since I installed the antennas.

Well, then, looks like you meet the original definition of NTSC equivalency. In truth, I think ATSC can and will do a lot better than that, even at 3.3 b/s/Hz, and in my case it does. But of course, you have to take this digital cliff in consideration.


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