At 10/1/2010 03:18 PM, Matt Jenkins wrote:
Thanks for the explanation. I think I understand it. I have a couple
more quick questions.
What is the difference between co-channel and adjacent channel?
Co-channel means the same frequency, so if you're on channel 31,
you're protecting a channel 31. The adjacent channel rules assume a
certain amount of receiver selectivity.
Does that mean if I am more than 68 km from a station I can operate
a fixed TVWS Base station at up to 600 meters HAAT?
No. This was what IEEE 802 proposed. The FCC's Order referenced it,
and then simply said that the maximum ground HAAT was 75 meters, full
stop. Such is the difference between engineers playing with formulas
and lawyers in a hurry to draft something that they barely understand
while making political compromises.
- Matt
On 10/01/2010 11:56 AM, Fred Goldstein wrote:
At 10/1/2010 02:27 PM, Matt Jenkins wrote:
What are the headings for your chart? I don't understand it....
Eudora had trouble with cut-and-paste of the original document.
The first column is height above average terrain, from x to y
meters (10 but less than 30, from 30 but less than 50...). The
second is the proposed distance outside of the protected contour of
a co-channel station. THe second (the small distance) is the
proposed distance outside of the protected contour of an
adjacent-channel station.
So IEEE 802's proposal (in a 2009 Petition) was to allow antennas
above 600 meters HAAT only if more than 68 kilometers outside of
the protected contour of a co-channel station, or 426 meters
outside of the contour of an adjacent-channel station.
Not that those calculations were perfect; sometimes being precise
isn't the same as being accurate. TV broadcast interference is
usually measured at a fixed height, I think 10 meters above
ground. If the antenna is 500m above average terrain, it is
probably more than 30 meters above ground. It might even be on a
rather tall tower. In that case, the signal level near the ground
will not be the same as the signal level in a straight line. So
there is probably no likelihood of adjacent-channel interference.
I remember an FM station (WMSC) that came on the air around 1970, 2
channels away from two another ones (WKCR, WFUV) whose protected
contours it was within. You had to protect second and third
adjacent channels, which normally meant 4-channel spacing, because
receivers near to the antenna would be clobbered (>20dB
stronger). In this case the new station was about halfway up an
existing 1000-foot TV mast. So its signal strength at the height
that counted was so low that it did not violate the interference
rules for second and third adjacent channels. It is currently
licensed for 1W ERP at 205m HAAT. (But one of the
second-adjacent-channel licensees has still given them grief at the FCC.)
On 09/30/2010 08:13 PM, Fred Goldstein wrote:
At 9/30/2010 10:37 PM, Jack Unger wrote:
Fred,
I'm sorry to seem dense but I don't understand your explanation
below. I'd appreciate it if you would re-explain. The FCC said:
"transmit antenna used with fixed devices may not be more than
30 meters above the ground. In addition, fixed devices may not
be located at sites where the height above average terrain
(HAAT) at ground level is more than 76 meters".
I'm trying to reconcile that with your statements. Could you
please re-explain more clearly or by using better actual numbers
(both HAAT at ground level and antenna height above ground)?
Thanks in advance,
jack
Sure. In the Order itself, the FCC explained the origin of the
76 meter HAAT limit. They explained that they didn't want any
antennas more than 106 meters AAT. That's the maximum antenna
HAAT I referred to. Since antennas are allowed to be 30 meters
above ground, they subtracted 30 from 106 and got 75. See
paragraph 66 of the Order:
"We find that limiting the fixed device antenna HAAT to 106
meters (350 feet), as calculated by the TV bands database,
provides an appropriate balance of these concerns. We will
therefore restrict fixed TV bands devices from operating at
locations where the HAAT of the ground is greater than 76 meters;
this will allow use of an antenna at a height of up to 30 meters
above ground level to provide an antenna HAAT of 106 meters.
Accordingly, we are specifying that a fixed TV bands device
antenna may not be located at a site where the ground HAAT is
greater than 75 meters (246 feet)."
The Order cited an IEEE 802 Petitition
http://fjallfoss.fcc.gov/ecfs/document/view?id=6520201311 which
called for HAAT to be a factor. But they didn't call for a ban
on operation above 75 meters; rather, they wanted co-channel
separation to increase with height:
less than 3 meters |
6 km 0.1 km
3 Less than 10 meters* 6.9 km 0.256 km
10 Less than 30 meters 10.8 km 0.285 km
30 Less than 50 meters 13.6 km 0.309 km
50 Less than 75 meters 16.1 km 0.330 km
75 Less than 150 meters 22.6 km 0.372 km
150 Less than 300 meters 32 km 0.405 km
300 Less than 600 meters 45.7 km 0.419 km
600 meters or higher 68 km 0.426 km
That's rational. On the other hand I'd prefer allowing fixed
devices at any ground elevation, to allow everyone to subscribe,
so I'd suggest instead that they maximum ERP be decreased in
order to limit interference to the same level. So maybe 6 dB
from 76 to 150 meters and 10 dB to 300 meters, though that's a
guess; I haven't run the calculations. And I'd allow directional
antennas, professionally installed, to have ERP measured in the
direction of the protected contour, with no reduction in ERP if
it's clear to the distance the above chart.
I'm thinking about a petition to that effect. I have real
subscriber sites in mind.
--
Fred Goldstein k1io fgoldstein "at" ionary.com
ionary Consulting http://www.ionary.com/
+1 617 795 2701
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