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I have a few questions on the types of antenna used for tower-aircraft comms.

What type of antennas are used today for tower-aircraft comms? Are they directional or omni-directional?

What's the range of data rate (in kbps) that these antennas can provide?

Also, is there any safety issues related to power transmission between tower and aircraft? What's the power limit for this transmission? I know that the tower controls the plane until the cruising altitude and after that it hands it over to enroute. Before enroute takes control, how does the tower ensure that the transmit tower is not too high while ensuring it can still reach the plane?

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    $\begingroup$ The tower does not control until cruising altitude. Shortly after takeoff they are handed off to either a TRACON or an en route controller. TRACON can be located at the airport, but usually is not. $\endgroup$
    – TomMcW
    May 3, 2018 at 5:58
  • $\begingroup$ Going the other direction, radio in the aircraft have decent output power. For example, the Avidyne IF540 GPS Moving Map/Nav/Com is spec'ed for: VHF COM Power Output 10 Watts nominal (12V power source) and Optional 16 Watt (for 28vdc installations only) $\endgroup$
    – CrossRoads
    May 3, 2018 at 14:11

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They are omni-directional: Anyone, in any direction, can hear the broadcast.

They are analog (non-digital), AM signals, so there are no bits, and thinking in terms of kbps or data-rate doesn't make sense.

These have similar transmission power to many AM radio stations. Do you have safety concerns about local AM radio stations that broadcast baseball games?

Finally, the tower does not "control" the airplane. The pilot on-board is the one in control. The tower may simply give traffic directions, such as what heading to take, or what altitude is appropriate.

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    $\begingroup$ "These have similar transmission power to many AM radio stations." Are you sure? I don't know about US, but broadcast radio stations tend to have quite high power outputs. A more apt comparison would probably be a cell phone, at least in terms of power output... $\endgroup$
    – user
    May 3, 2018 at 11:28
  • $\begingroup$ "Although most car phones have a transmitter power of 3 watts, a handheld cell phone operates on about 0.75 to 1 watt of power." ... "Many cell phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts)." Power of a Cell Phone Transmitter - The Physics Factbook hypertextbook.com/facts/2006/EbruBek.shtml So personnal handheld phones would seem to be pretty lower power. The cell towers are likely higher power. $\endgroup$
    – CrossRoads
    May 7, 2018 at 12:00
  • $\begingroup$ Cell Tower Power Output Although the FCC permits an effective radiated power (ERP) of up to 500 watts per channel (depending on the tower height), the majority of cellular or PCS cell sites in urban and suburban areas operate at an ERP of 100 watts per channel or less. Human Exposure to Radio Frequency Fields: Guidelines for Cellular Antenna Sites fcc.gov/consumers/guides/… $\endgroup$
    – CrossRoads
    May 7, 2018 at 12:03
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What type of antennas are used today for tower-aircraft comms? Are they directional or omni-directional?

The antennas are omni-directional however on the aircraft the mounting is important to maintain line of site and the FAA covers it in this AC. This is in reference to <12,500LB aircraft but the logic applies to all planes.

d. VHF Com 1 should be mounted on the top of the aircraft since this will provide the best unobstructed location. VHF Com 2 can also be mounted on the top, provided there is at least 1/2 wavelength (of the antenna operating frequency) distance available between antennas.

e. If Com 2 is mounted under the aircraft, a bent whip may be required to provide ground clearance. Bent whips may not provide the best performance because of proximity to the aircraft skin. Signal reflection and obstruction is more of a problem with such locations. Extension of the landing gear or flaps may also impact belly mounted antenna performance.

f. Antennas need to “see” with a direct lineof-site to the source. Antenna patterns can be disrupted by landing gear or vertical stabilizers, for example. When mounting antennas, try to locate them in areas where line-of-sight view is not obstructed.

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It would seem the ATC outputs pretty low power!

This is from a General Dynamics Air Traffic Control radio data sheet

RF Output Power:

Low Power Transmitter 2-12 Watts with co-site filter, 2-20 Watts with out co-site filter High Power Transmitter 12-35 Watts with co-site filter, 12-50 without filter

https://gdmissionsystems.com/-/media/General-Dynamics/C4ISR-Technologies/Radios/PDF/radios-cm300-350-v2-vhf-uhf-air-traffic-control-transmitter-datasheet.ashx?la=en&hash=357448B534D17E0AD21A91485D60C3C110972654

I don't know if this would be a radio for an airport control tower, which talks to in a range of about 10-15 miles, or an Approach radio which talks to you over a wider range, like 30 miles or more.

The datasheet does say "these rack mounted transmitter and receiver systems are specifically designed to meet the dynamic mission requirements of air traffic control centers, commercial airports," so perhaps both?

When I flew from just west of Boston south to Nantucket, a distance of 98 nautical miles as the crow flies, I started with Boston Approach for flight following, got switched to a 2nd Boston Approach, got switched to Providence Approach (I flew a little more southerly to go over Martha's Vineyard and less over the ocean), then a 3rd Boston Approach, and finally Nantucket Tower when about 8-10 miles from the airport. Same going back.

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  • $\begingroup$ The increase in pure path loss going from 10 miles to 30 miles is something like 7-8 dB for a line-of-sight transmission. While significant in absolute terms, that's not a huge difference in practice. $\endgroup$
    – user
    May 7, 2018 at 14:11
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    $\begingroup$ For VHF/UHF airband, 10-15W is enough to reach pretty much any aircraft above the horizon; if they're below the horizon, they'd be talking to a different station--or using HF, for oceanic flights. You can't directly compare to terrestrial radio power levels, which will need far, far more power for the same distances. $\endgroup$
    – StephenS
    Dec 31, 2018 at 0:41

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