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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