[opendtv] Re: Transition report
- From: "James Albro" <jalbro@xxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Sun, 14 Jun 2009 19:14:01 -0400
Bert -
I don't think your specific case provides very much insight regarding the
effectiveness of ATSC broadcasting on VHF-HI channels.
As I understand it, you have two antennas, an Antennas Direct DB4 and a
Radio Shack double bowtie over screen. Antennas Direct claims the DB4 has
almost 14 dB gain for UHF channels. They make no claim as to gain at high
VHF. Channel Master, which makes both 2-bay and 4-bay bowtie-over-screen
antennas, says they are UHF only and lists the VHF-HI gain as "N/A". For
viewers who need to receive both UHF and high VHF channels they offer a
Digital Advantage antenna, essentially a UHF corner antenna with gain that
has a VHF dipole grafted on the back. The high VHF gain is only 2 dB at
best and it's non-directional (as you would expect). The addition of the
VHF-HI dipole increases the antenna width to 47 inches, compared to their
UHF bowtie-over-screen antennas that are only 25 inches wide.
So what you have are antennas with elements that are too short for the high
VHF band, that probably have negative gain at VHF-HI frequencies; which
therefore don't work well (or at all) for receiving high VHF stations. Why
does the failure of these antennas to receive channels 7 and 9 for you
indicate that the stations' power levels are too low? I'm actually sort of
surprised that you could receive WUSA at all with your setup.
Jim Albro
----- Original Message -----
From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Sunday, June 14, 2009 6:09 PM
Subject: [opendtv] Re: Transition report
> Just as I have been fretting. The draconian power cut, when moving to
> digital VHF, was simply too great.
I have two updates, one of them from across the pond.
Given that my one problematic channel, ABC7, which moved back to VHF,
was pixellating continuously, the problem seemed to be a slight deficit
in signal margin. The one really nice thing about using indoor antennas,
if that's even an option, is that you can mess with them easily.
So, in the downstairs system which uses the AntennasDirect antenna farm,
I figured that the reflectors incorporated with the quad dipole stacks
were simply too close, and too small, to be of much help for VHF. Simple
test: just prop up two cookie sheets, side by side, some distance behind
the antenna, to act as a VHF reflector. And now ABC7 comes in. Almost
perfectly solid. (But of course, without spousal approval.)
I didn't try upstairs, but I expect the same result. The Radio Shack
double bowtie does not have a VHF reflector. (In truth, even its UHF
reflector is too close to be valid, I think.)
Still, the simple fact is that when ABC7 was UHF, they had built in
enough signal margin to make it quite robust EVEN for indoor recpetion.
When they went down to VHF, they expected some sort of propagation
miracle from the lower frequency, and the miracle didn't materialize.
Surpise. So they are forcing people to install a more complicated
antenna system than they could get by with when the signal was UHF.
That's plain silly.
As timing would have it, Rome (the real one) is going through the same
thing now. But not all of stations at the same time. Anyway, it turns
out that there too, they expect miracles. Whereas the analog stations
were duplicated on top of two hills, one to the South and one to the
North, the digital multilexes are split, half from one hill, half from
the other.
The main RAI channels are unfortunately only available from the hill way
to the South, or so I gather from their web site. So I figured no way
would my mom and sister be able to use their existing antennas, in their
respective apartments. And in fact, they couldn't. My mom was already
was using an outdoor antenna, it was pointing to the close-by hill. My
sister used rabbit ears.
So what did they do? They called an "antennista." In my mom's apartment,
he reaimed the antenna, presumably to the hill to the South. And in my
sister's, he repaired the outdoor antenna that had already been
installed, but was not being used.
So that's the way these things are really done, as opposed to the hype
about paper clips. The same problems cropped up here and there.
Bert
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