Jim Brown wrote:
> Can anyone point me to engineering references that discuss the
> relative susceptibility to RFI of DTV as compared to analog TV? That
> is, is DTV more or less susceptible to interference, or about the
> same? Yes, I would certainly expect "the cliff effect." I'm
> specifically interested in RFI from hams.
>
Relating what I've seen over the past few years from one location. I've
included all of the set up as each installation/location is going to be
different.
I have a Samsung 40", 1080i capable LCD set that is about 2 1/2 years
old. I run the legal limit on 160-10 and about 800 SSB and CW on six
meters. The TV antennas are located between 90 (Pointed S) and 95'
(Pointed NW) on the tower (Both with preamps mounted at the antennas) I
also have a DISH Network dish at 20' on the tower (to see over my
neighbors willow tree which STILL seems to be growing. There are
sloping dipoles for 75 and 40 anchored to the top of the tower with poly
rope putting the ends of the antennas just below the TV antennas. There
is also a half sloper on 160 with the feed pint within about 3-4' of the
TV antennas. The tribander is located at 100, a 7L C3i 6-meter yagi at
115, and a 14' cross boom at 130 holding a stacked pair of vertically
polarized 12's on 144 and 11's on 440. I can run up to a KW on 144, but
typically run between 50 and 160 watts all modes, including digital.
There are also side mounted Diamond duo-band antennas with the tops
about10' below the TV antennas. There is also a HyGain AV640 on a 40'
tower at the W end of my shop with crossed dipoles (TV) about 6' below
the antenna. On the N side of the shop is another Diamond dual band
vertical at 20' to the base and that one is about 20-30' NE of the
AV640. I run a KW SSB and CW on this system as well along with 800 PEP
on 6. Yes, I know that is well beyond the AV640's ratings for 6, but
they tell me it should handle it on SSB.
The TV stations are pretty much evenly distributed between low and high
VHF and UHF. They are all moving around but the distribution will stay
pretty much the same as it has over the last 5 years. IIRC (too lazy to
look it up again) but there will be 40 some channel two stations left
across the nation. So in general things are going to stay pretty much
the same with the distribution, but with many changes in any locality.
IOW the chart looks like they are playing musical channels. There are
no channel 2 stations in the area, but one will be deep fringe which the
big antennas at 90' will pick up. I've not had a chance to try that one
for interference susceptibility on this system nor have I actually
pointed the one antenna at it and probably won't. As to the crossed
dipoles under the AV640 I've never tried channel 2, 3 or 4 on that one
as there are none in range. The lowest reachable is channel 5 and there
I have absolutely no interference, nor have I seen any between 144 and
channel 12. On analog I did have some interference between the stacked
12's and the TV antenna pointed S when the stacked 12's were also
aligned with the TV antenna. So far, there's been nothing with Digital.
I also need to clarify the so called cliff effect. It really isn't a
cliff, but rather a very steep hill, or slope. It's a myth that you get
or don't get a station with nothing in between. Compared to analog
which slowly gets weaker and weaker with a poorer and poorer image,
digital is picture perfect up to a point. Then it may start by
pixilating and/or freeze intermittently, they sound may start breaking
up, or going out of sync with the image, but you definitely still can
watch until it gets really bad. It's just between starting to notice a
bit of pixelation and going out is a very narrow range so I can see why
some call it a cliff. There is also a lot of processing going on in the
TV set which may make up for a lost pixel here and there.
With all those qualifying statements and all things remaining equal, I
have seen DTV as less susceptible to noticeable interference than
analog. BUT this is basically one location with two transmitters and
two TV antenna installations. I've been watching Off the Air (OTA) HD
TV as well as standard definition (SD) digital before the local stations
went to HD. I am reliably watching HD stations out to 100 miles, or I
should say my wife does while I operate without problems. BUT with the
satellite system (I did have two big C-band dishes set up), I rarely
find anything of interest on the "networks" and watch the affiliates for
the local news.
Actually I get most of my news and weather off the internet and have
subscription RADAR along with aviation weather.
But again, I emphasize this is just one location and one family's
experience from one location.
73
Roger (K8RI)
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