Jim,
I do not have any specific "reference" documents to which I can point you,
but you may be interested in two anecdotal experiments I have performed at
my QTH.
1. The ice dispenser in my refrigerator seems to contain a brush-type
motor. If the TV in the adjacent family room is tuned to ANY of the local
VHF analog stations (channels 2, 7, 9, or 12) and the ice dispenser is
operated, the picture always suffers from "tearing" and fast-moving noise
bars across the screen. Obviously, I have learned to dispense ice during
commercials only. I recently ran the ice dispenser while watching the
digital version of these same stations (via my Channelmaster converter
box, with S-video output) and had no interference at all. Now, it must be
said that all of the digital transmissions were on UHF, so I'll have to
repeat this experiment after the changeover, as Ch. 7, 9, and 12 will all
return to their original channels as digital. Only Ch. 2 is staying up at
UHF, as Ch. 51.
2. A better test is that which I ran on my new UHF installation. I had
always sworn that I would never put a TV receive antenna on my big VHF/UHF
tower, as I did not want to have TVI issues when xmitting on the higher
bands. Well, I have been forced to eat my words because I very much
wanted to continue watching two stations in the Quad Cities area (of IL
and IA), ahich is about 100 miles to my SE. Those stations are presently
on analog channels 6 and 24, with 24 being WQPT, a PBS outlet that is
independant from the PBS service we have in Iowa and which has a lot of
programming I enjoy. The contour lines of digital WQPT fall nearly 40
miles short of my location (info provided to me by none less than the
station manager!), and which I confirmed with the FCC plots. Aside from
waiting for tropo duct openings, my best bet seemed to be to increase
antenna height. The tower that I use for over-air TV is a 40 footer, with
10 feet of mast atop it and a big Winegard LP VHF/UHF antenna and rotator
hauling in the signals. That remains in use for most of my TV sigs, but I
now have a very large Winegard UHF-only antenna mounted at the 75 foot
level of my big tower. I also have one of their pre-amps mounted about
half-way down the tower. This has solved my viewing of "weak" signal WQPT
very nicely. I have better than 90% reliable copy of the WQPT digital
signal with this system.
Naturally, I had to test it for robustness. The specs at Winegard's web
site for their pre-amp (19 dB average gain across full Ch. 14 to 69 UHF
band with 2.6 dB NF) says it can withstand more than 90,000 microvolts of
signal. It is in a plastic weather-proof housing, and that is why I put
it half-way down the tower - get it out of the direct field of my antennas
that are clustered around the top of the tower. I ran my tests by
transmitting on 6m, 2m, 70cm, and even 223 MHz. The power level on the
first 3 bands was 100 to 150 watts, and I tried both SSB and FM. NO
PROBLEMS at various in-band frequencies that I use. The only case of
interference to a digi-TV signal came when I transmitted on 223.500 with
30 watts: I blanked out the screen on some channels and caused pixelated
lock-up on other channels. A few were not affected at all. The test
setup was an Insignia converter box (the Zenith box as sold by Best Buy)
connected to the inside end of the coax coming from the Winegard pre-amp.
The output of the converter was on Ch. 3 to a Toshiba TV.
I do not have detailed test data at this time. I am simply recalling the
"gross" results of the test. In short summary, I can say that under some
circumstances, there is the chance for continued interference between ham
operations and DTV. I need to carefully note which digi-channels got
creamed so that I can better figure out the probable cause. The good
news, of course, is that moderate power on 6 and 2 meters seems to have no
effect. As we know, the digi-channels will be changing next month, so the
ones getting killed now may go away.
I should end by saying that 223.500 MHz, FM mode, is a popular frequency
in the Cedar Rapids area, with more than a dozen stations active. We use
it like an intercomm. Most rigs are lower powered than mine, the typical
being an ancient Midland 13-509 running 10 crystal-controlled watts. My
rig is an ADI 237, synthesized at 30 watts.
I fully agree that there is nothing like well-documented, refereed papers
in offical journals to provide information of high confidence. I suspect
that there must be some such papers out there right now, and more will
surely follow. However, I think if enough hams run well-documented
experiments, some usefulness will come from such information. Of course,
details about antenna types, antenna separation and polarity, transmit
power, frequencies used, and all that are needed to make any such
experiments viable. My account is short on details in many ways, but I
hope it povides some usable feedback to you or someone on the list.
73, Dale
WA9ENA
"Jim Brown" <jim@audiosystemsgroup.com>
Sent by: rfi-bounces@contesting.com
01/22/2009 01:32 AM
To
"RFI List" <rfi@contesting.com>
cc
Subject
[RFI] RFI Susceptibility of DTV?
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.
73,
Jim Brown K9YC
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