On Sun, 23 Mar 2014 09:24:09 -0400, Jerry Stuckle
wrote:

On 3/22/2014 7:13 PM, Jeff Liebermann wrote:
My point about listening to aircraft is that there's little difficulty
hearing aircraft that are overhead, and plenty of difficulty hearing
aircraft near the horizon. Therefore, the antenna should have most of
its gain towards the horizon, and less gain above the horizon to near
overhead. At low frequencies, the discone does that. At the high end
of the range, it's quite the opposite.

There is? I remember back in the 70's a United Airlines pilot who would
regularly work 146.52. I had no trouble chatting with him from almost
300 miles away, even though he was only using a 1.5W HT. And out here
on the east coast, it's impossible to use an HT on any 2M repeater
frequency without bringing up multiple repeaters, even when using 1W
at about 3,000 feet or above. From 10,000 feet, forget it almost
anywhere in the country.

Yep. You can be heard for quite a distance from an airplane. The
problem is hearing anything as the chances of co-channel interference
is high.

I help maintain an ADS-B listening station (1090 MHz) with a good
view of the ocean. 200 Nm average range. I designed the antenna
specifically for the purpose and location:
http://802.11junk.com/jeffl/antennas/AMOS-5-1090MHz/index.html
The actual antenna system is somewhat more complex.

Also a VHF AIS receiver at a local hilltop. Great view from near the
coast from about 2,000 ft.
http://www.marinetraffic.com/ais/details/stations/112
(Note that it's NOT on Mt Umunhum. The receiver was moved to Bonny
Doon to eliminate weather xmitter interference). Average range is
about 200 Nm, and much more when ducting is available. If you're near
the coast, listen on 161.975 MHz and 162.025 MHz for the AIS traffic.

And BTW - commercial planes generally fly at around 7mi (35,000-37,000
ft.) high - not 5Mi (26,000 ft.) as you claimed.

True. The problem is that near the coast, most of the aircraft
traffic is on takeoff or approach and at much lower altitudes. Here's
a typical altitude profile for an KLAX to KSFO flight:
http://802.11junk.com/jeffl/crud/flight-profile.jpg
Data capture was from:
http://flightaware.com/live/flight/VRD935/history/20140323/1600Z/KLAX/KSFO/tracklog
which will change throughout the day. Notice that for a 500 Nm
flight, it doesn't stay above 30,000 ft for very long. By the time I
see the incoming data, it's usually between 10,000 to 15,000 ft. Of
course for cross country, they stay above 30,000 ft for much longer. I
just took a look at the raw data from the local ADS-B receiver. One
flight at 30,000 ft and everything else below about 15,000 ft over a
10 minute period.

Incidentally, until you mentioned radio range in an aircraft, I never
bothered to check how much TX power the aircraft was using for ADS-B
(1090 MHz). Looks like they deliver some serious power at altitude:
http://www.ads-b.com/PDF/UAT%20SARP.pdf
Table 12-1: Transmitter power levels
Transmitter Minimum power Maximum power Intended minimum
type at PMP at PMP air-to-air ranges
Aircraft (Low) 7 watts (+38.5 dBm) 18 watts (+42.5 dBm) 20 NM
Aircraft (Med) 16 watts (+42 dBm) 40 watts (+46 dBm) 40 NM
Aircraft (High) 100 watts (+50 dBm) 250 watts (+54 dBm) 120 NM
Note that the range is for air to air, not air to ground.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558