[opendtv] Re: VHF vs UHF coverage
- From: "Dale Kelly" <dalekelly@xxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 18 May 2009 14:59:31 -0700
Bert,
IMO, yours is a very interesting and useful analysis.
However, my concern in this post was an assigned UHF DTV channels ability to
replicate the stations analog coverage. It is necessary, in this instance,
to also compare UHF verses low band VHF propagation. Low band VHF's
generally more robust signal and it's ability to provide some amount of over
the horizon coverage, can't be replicated by UHF signals.
Therefore, UHF Translators / repeaters are needed to extend the new UHF DTV
coverage area to service the LB VHF stations viewers.
Also, thanks for the FCC document re: Replacement DTV translator stations. I
assume that it deals with this issue and now that I'm home again I'll read
it.
Dale
-----Original Message-----
From: opendtv-bounce@xxxxxxxxxxxxx
[mailto:opendtv-bounce@xxxxxxxxxxxxx]On Behalf Of Manfredi, Albert E
Sent: Wednesday, May 06, 2009 10:14 AM
To: opendtv@xxxxxxxxxxxxx
Subject: [opendtv] VHF vs UHF coverage
Dale Kelly wrote:
> Are you aware that the FCC originally calculated that many
> digital UHF stations, replacing many VHFs, would require up
> to 5MW ERP to replicate current coverage?
>
> After we convinced them of the physical and economic
> impractibility of constructing and operating such a high
> powered facility, they reduced the max UHF DTV power to the
> current 1MW level.
To guesstimate what my reception expectations of the VHF stations should
be, when they move back to high VHF on 12 June, I used the Hata model as
described here:
http://w3.antd.nist.gov/wctg/manet/calcmodels_r1.pdf
in the first three pages, corrected for long distances as described in
page 3, and I'm using the suburban parameters.
The max ERP for UHF digital transmissions is 1 MW. It seems to me that
the expected coverage of such a station would be similar to that of a 5
MW analog UHF, the digital being 7 dB less power than the analog.
So then my question was, what power levels would I need from a digital
high VHF station, to match the reach of that 1 MW UHF station? For my
specific setup? The 1 MW digital UHF is a known quantity for me, given
that a few stations in Baltimore are at or close to that level.
Using 80 Km (50 mi) range, 300 meter transmit antenna height, 1 meter
receive antenna height, 195 MHz for high VHF, and 615 MHz for UHF, path
attenuation for the UHF signal is 159 dB for VHF and 173 dB for UHF. The
UHF sees 14 dB greater path loss than the VHF.
So in this configuration, the VHF station would need to transmit with
ERP of 39.8 KW to match the coverage of the UHF 1 MW signal. That is
higher than what any VHF station in this area is planning to use. Not by
much, in two of four cases, but it's still higher.
So, I conclude that digital VHF coverage *could* be similar to that of 5
MW analog UHF stations. At least, with low receive antenna height. But
not much better.
Then I tried the same computation with a 10 meter receive antenna
height, and increased the range to 100 Km (60 mi).
Now the path attenuation for VHF is 148 dB, and 156 for VHF, i.e. only 8
dB less attenuation for VHF. So to match the UHF 1 MW signal in terms of
reception robustness, the VHF would need to be 158 KW ERP! Much higher
than anyone is planning around these parts.
So again, it seems to me that with the limited power being planned for
digital VHF stations, we're not going to see any increase in coverage
area compared with UHF at the planned ERP levels for many UHF digital
stations. Perhaps some terrain oddities would give VHF a bigger
advantage than what this Hata model predicts.
Bert
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