Archived from groups: alt.video.ptv.tivo (
More info?)
In article <bob-92D13F.19572810052005@news.verizon.net>,
Robert Peirce <bob@peirce-family.com.invalid> wrote:
>Okay. I tested 11.1, First the good news. It is working tonight. It
>is possible there was a transmitter problem yesterday. At any rate,
>here are my readings:
>
>Degrees Signal [0-100]
>0 82
>7.5 87
>15 89
>22.5 90 The tower is supposed to be at 28. My rotor may be off.
>30 88
>37.5 81
That looks like a fairly symmetrical pattern, centered somewhere
around 18. Wouldn't surprise me if your rotor/aim is off by about ten
degrees.
Hmmm... a thought... when you're figuring directions, are you
remembering to compensate for the difference between true north and
magnetic (or compass) north? Here on the left coast, a compass needle
points around 15 degrees east of true north. The correction is
certainly different where you live. If you're figuring directions to
the transmitter on the basis of true bearings (from a map) but are
setting your rotor position based on a compass bearing, and you aren't
correcting for the difference between magnetic and true north, then
that's the likeliest cause of an error!
>45 71
>52.5 68
>60 & + Fluctuating from 40 down to 4 at 75 degrees
The falloff once you get past 37.5, and the sharp degradation beyond
60, suggests to me that you've got a pretty deep null at an angle of
about 50 degrees either side of your center.
Without knowing what the calibration numbers mean in decibels, it's
impossible to figure out the formal "half-power beam width", but it
looks to me as if you start losing signal pretty badly once you've
turned more than 30 degrees away from the transmitter.
That's not terribly good news, in your situation. If you've got a
good-signal lobe that's about 60 degrees wide, and you're trying to
capture signals from transmitters spread over a 90-degree arc, you're
going to end up with some stations which aren't in the effective part
of the main lobe.
If channel 11.1 is giving you the best signal when the antenna's
pointed at around 20, and if you're trying to receive it properly with
the antenna pointed at 52, then it's probably just about to fall off
of the side of the lobe, and is probably well below its optimal
strength.
>This looks pretty good. Last night 2.1 and 4.1 were reading around 71
>at 52 degrees. 68 is almost as good, but it falls off real quick beyond
>that point.
>
>Now the next problem is how can you count on these readings being there
>when you are trying to record?? In other words, if I set my antenna at
>52, which seems pretty good, but the power is down at the point when I
>am trying to record, I am not going to get the signal!
Yup. That does seem to have been a problem here in the SF Bay area.
The local stations' ATSC signals are coming out of different
transmitters and different antennas, much of the time, and are often
not operating at ideal power levels. There have also been some
reliability problems. I've heard people report days when most of the
digital locals were unavailable, when (e.g.) there was work going on
at the Sutro Towers site in San Francisco.
Not much one can do about transmitter-side problems, I'm afraid. One
hopes that things will get better as time progresses.
Now, as to your reception problems. I suspect that at least part of
it is that your antenna does seem to have a narrower horizontal
pattern (main lobe / beam) than is ideal for your situation, since you
need to receive signals from a fairly wide horizontal arc. The
solution is to have less horizontal directivity. This will cost you
some amount of signal strength in the forward direction (i.e. stations
at which you point the antenna directly will seem to have weaker
signals) but will increase the effective strength of the stations
which are off to the side.
You may be able to compensate for this by having more *vertical*
directivity. This would be done by using an antenna which has more
elements stacked one above another.
UHF-band SD/HD antennas are often of the "bow-tie and reflector"
design... one, two, or four bow-tie-shaped wire dipoles, mounted a few
inches in front of a wire-grid reflector screen.
Placing the elements side by side, horizontally, increases the
horizontal gain and narrows the beam in the horizontal direction.
Placing them above one another has no effect on the horizontal
pattern, but compresses the pattern vertically.
So, if you're interested in picking up the UHF-band channels, a good
antenna for you to use would probably be one which has several bow-tie
elements mounted one above another, in a single vertical line. This
antenna would have a broad, but shallow main lobe (i.e. not very
directional in the horizontal direction, but quite directional towards
the horizon).
For VHF-band, you'll probably just need a standard log-periodic
antenna, as these don't have a lot of gain on either axis.
Another, more expensive option would be to have two separate antennas,
covering two different portions of the horizon, and then combine their
signals somehow. Tricky and not all that easy to design and engineer,
except for some special cases such as "one channel on antenna A, all
others on antenna B."
--
Dave Platt <dplatt@radagast.org> AE6EO
Hosting the Jade Warrior home page:
http
/www.radagast.org/jade-warrior
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