# What is the distance at which my VHF communication radio can reach ATC?

I just installed 2 new TKM MX11 radios and a new Garmin 340 audio panel. I also put in new coax cable and 2 new antennas in my 1964 Skylane. What kind of distance from ATC should I expect at 4 or 5 thousand feet?

• Why the downvotes on a first question? Ridiculous. May 31, 2017 at 7:33
• Hello Joel Segal, welcome to aviation.stackexchange.com! May 31, 2017 at 11:42
• No idea why the downvotes, I thought it was quite a reasonable question, regardless of being Joel's first or not. May 31, 2017 at 14:39
• Possible duplicate of How do you calculate the maximum distance to communicate with tower control? May 31, 2017 at 20:43
• @Simon I've no opinion on the quality of the question, so I didn't vote either way. But up or downvotes should be based on the quality of the question, not on whether or not the user is new. May 31, 2017 at 23:33

The TKM MX11 has an 8 watt transmitter which makes it a Class 4 transmitter as defined in RTCA DO-186B MOPS for Airborne Radios Communications Equipment. That specifies a maximum range of 100 miles.

In an area of relatively level terrain, you should see a range of about that 100 miles if you're at 5000 AGL or above. Below that, range will be reduced due to the curvature of the earth and multipath interference. At 2000 feet above level terrain, 40 to 50 mile range is typical based on an analysis I did about 20 years ago for a former employer.

One factor you can't control is the height of the ATC antenna. The higher it is, the greater the range. Even a 50 foot difference in the height can affect the range by several miles.

• Do you know if the 100 miles are specified as nautical or statute miles? Anecdotally, I have received radio transmissions near ground level from aircraft at ~87 NM (~100 statute miles) distance that were at 5000 ft.
– J W
May 31, 2017 at 20:45
• The wording in DO-186 is vague, so I looked up TSO C-169a which calls out the MOPS and defines the classes. It states: Transmitter used in a 25 kHz channel separation environment and intended to operate with a range of 100 nautical miles. Similarly, when flying at 5000 ft along the Gulf coast I've been able to communicate with UNICOM from 100 NM, but I've not been able to at 120 NM except where the ground station antenna was significantly higher than local ground level. May 31, 2017 at 23:56

The range of radio communication traffic depends on a number of factors.

VHF radio communication is approximately line-of-sight; the radio waves to not follow the curvature of the earth very well. This means in general that if the antenna of the other side is below the horizon, or behind a mountain you will have no reception.

A second aspect is the power of the radio transmission from transmitter to receiver. This depends on the power of the radio, losses in the cables, transmission antenna gain in the direction of transmission, attenuation in the transmission medium, free space path loss, receiver antenna gain in the direction of reception, losses in the cables and sensitivity of the receiver.

Also in the above, the noise levels need to be taken into account; AM transmissions are sensitive to radio noise.

The line of sight distance can be calculated as:

$\sqrt{2Rh_{ac}+h_{ac}^2} + \sqrt{2Rh_{atc}+h_{atc}^2}$

Where $R$ is the radius of the earth, $h$ the height of the antenna, subscript $ac$ the aircraft and subscript $atc$ the air traffic control antenna.

For a aircraft height of 3500 ft and an ATC antenna at 50 ft the line-of-sight distance is approximately 81.5 nautical miles.

With a rated output power of 8W for the MX11, an assumed cable loss of 2dB, estimated transmission antenna gain of 2 dBi (half dipole with some obstructions around it), receiver antenna gain of 2.14 dBi and a ICAO recommended receiver sensitivity of 20 microvolts per meter for ATC receivers, the transmission range is approximately 375 NM.

The transmission will be limited by the line-of-sight distance about 81 NM.