On Sat, 22 Mar 2014 08:47:56 -0700, Jon Danniken
wrote:

I don't have any professional experience with aircraft antennas.
However, I have done some ADS-B 1090 MHz designs and tests. The basic
idea is that the antenna should have the most gain at the horizon and
somewhat above the horizon to get the most range. Commercial jets fly
at about 5 miles altitude maximum, so gain in the upwards direction is
less critical. That's quite opposite of what the discone and
biconical will do.

I thought the discone/biconicals were more of a horizon-looking antenna,
at least from what I have read on them?

After I ran the NEC2 models, that seems true for the low end of the
frequency range. They are suppose to look something like a broadband
version of a vertical dipole. However, as the frequency goes up,
additional lobes appear until at the top of the frequency range, most
of the RF is going straight up. A Biconical is somewhat better than a
discone at retaining a sane looking pattern and reasonable gain, but
not much better.

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.

Maybe I'll look at a switcher of some sort eventually, and plan on just
manually doing it for now, thanks.

Ummm... climbing the tower to rotate a manual switch doesn't sound
like a good idea.



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

Jeff Liebermann wrote:
Jon Danniken wrote:


I thought the discone/biconicals were more of a horizon-looking antenna,
at least from what I have read on them?

After I ran the NEC2 models, that seems true for the low end of the
frequency range. They are suppose to look something like a broadband
version of a vertical dipole. However, as the frequency goes up,
additional lobes appear until at the top of the frequency range, most
of the RF is going straight up. A Biconical is somewhat better than a
discone at retaining a sane looking pattern and reasonable gain, but
not much better.

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.

Aha, okay I got you now, thanks for that. I still need to figure out
how to read the "lobe pattern" charts.

Maybe I'll look at a switcher of some sort eventually, and plan on just
manually doing it for now, thanks.

Ummm... climbing the tower to rotate a manual switch doesn't sound
like a good idea.

Hehe, indeed it doesn't. I was thinking more along the lines of a relay
box, if such a thing is possible, or maybe there is actually a gadget
that does something similar.

Along those lines, if connecting antennae of differing frequencies
together is not something that works, how does an antenna with multiple
different elements, like something like a scantenna
(http://i.imgur.com/D3Aeb58.jpg) get away with it?

Jon

On 03/21/2014 07:54 AM, Jon Danniken wrote:
Ralph Mowery wrote:

Ok on the vertical dipole. For this antenna you need to run the feedline
horizontal from it for a couple of feet and then down.

The impedance of this antenna should be around 70 ohms and if I were you , I
would use some 70 ohm rg-6 coax back to the receiver.

The 300:75 converter is actually a balun that has a 4 to 1 ratio. It
normally does 2 things, changes a 300 ohm to 75 ohm inpedance such as many
TV antennaas were set for 300 ohms so the twin lead could be used. As
things changed over the years, the newer TV sets had a 70 ohm input for the
coax cable. The 300:70 could be used either way, 300 ohm antenna to coax or
coax to the old 300 ohm input of the TV.

Removing it from the vertical dipole (70 ohm inpedance) and using coax to
the receiver will probably help.


Thanks Ralph, I'll try that this weekend and see what the results are (I
have scads of RG6).

Well, I got up on the roof and reconfigured the antenna today, and got a
nice improvement from my setup. Unfortunately I wasn't very scientific
about figuring out what made the big difference, but I did remove the
4:1 balun and ran the coax out horizonally for ~1/4 wavelength. I also
soldered the old crimp connection at the base of the elements; they were
reading about six ohms from the twinlead to the tips before, now the
resistance is low enough to not be measured by my DMM.

After the work today, noise is down ~10dB, and I can make out a lot more
transmissions, with much higher clarity, than I could before. I still
occasionally get a transmission with a strong signal that sounds
garbled, but everything else is coming in very nicely.

Thanks for the suggestions, they paid off.

Jon

On 3/22/2014 7:13 PM, Jeff Liebermann wrote:
On Sat, 22 Mar 2014 08:47:56 -0700, Jon Danniken
wrote:

I don't have any professional experience with aircraft antennas.
However, I have done some ADS-B 1090 MHz designs and tests. The basic
idea is that the antenna should have the most gain at the horizon and
somewhat above the horizon to get the most range. Commercial jets fly
at about 5 miles altitude maximum, so gain in the upwards direction is
less critical. That's quite opposite of what the discone and
biconical will do.

I thought the discone/biconicals were more of a horizon-looking antenna,
at least from what I have read on them?

After I ran the NEC2 models, that seems true for the low end of the
frequency range. They are suppose to look something like a broadband
version of a vertical dipole. However, as the frequency goes up,
additional lobes appear until at the top of the frequency range, most
of the RF is going straight up. A Biconical is somewhat better than a
discone at retaining a sane looking pattern and reasonable gain, but
not much better.

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.

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

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

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