Communication, navigation and surveillance

Antennas and Coax

A Cessna 172 comes in with a squawk: Comm 1 is weak transmitting and receiving, and it got worse after a wet month. In the shop the radio puts its full power into a test load, and its audio checks good. The loss is somewhere between the radio and the air around its antenna.

Most weak-radio squawks end at the antenna base, the connector or the coax, and a technician who can inspect, install and bond an antenna fixes them without replacing a radio.

A Cessna 172 seen from the left front, with its antennas: two whips and a GPS antenna on top, a whip on the tailcone, the V-shaped VOR antenna on the fin, and three antennas on the belly.
The Cessna 172 in this lesson. Comm 1 is the whip on top of the cabin.
Full power
Into a test load

The radio puts out its rated power.

Weak both ways
On the airplane

The pilot hears the tower faintly, and the tower hears the pilot faintly.

Antenna

A conductor that sends and receives radio waves. When the radio transmits, the antenna turns its alternating current into a radio wave. When it receives, a passing radio wave makes a small current in the antenna.

Coaxial cable (coax)

The cable that carries the signal between the radio and its antenna: a center conductor, an insulator around it, a braided shield and an outer jacket.

Test load

A resistor that takes all of the radio's power without sending it out as a radio wave, so it tests the radio alone. Shops also call it a dummy load.

Learning Objectives

When you finish this lesson you can do the four things below. The check at the end asks about each of them.

  1. 1IdentifyMatch each antenna on a light airplane to its system, and explain why its length and polarization must suit that system.
  2. 2SelectChoose an antenna location and its coax from the installation data: spacing, a clear view, a ground plane, cable type and length.
  3. 3InstallInstall and bond an antenna: a doubler, a flat mount, a bond of 0.003 ohm or less, and a seal.
  4. 4InspectInspect an antenna system for water, corrosion and damage, and explain what a high VSWR means.

Key Facts

Quarter wave

A comm whip is about a quarter of a wavelength, about 2 feet. The metal skin supplies the rest.

Half a wavelength

Com 2 on top sits at least half a wavelength from Com 1, about 50 inches.

0.003 ohm

The most resistance allowed between an antenna base and a metal skin.

High VSWR

Power is coming back from the antenna system instead of going out.

Coax Impedance

In Routing, Coax, and Databus Wiring you learned how a coax cable is built. This lesson starts from there.

A coaxial cable cut back in steps: the protective plastic covering, the braided shielding that is the outer conductor, the white dielectric insulator, and the central conductor.
Source: FAA-H-8083-31B, Figure 11-94. Coaxial cable between an antenna and its transmitter or receiver.
Recall

What sets a coax cable's characteristic impedance?

Characteristic impedance

The opposition a coax cable shows to the radio signal, in ohms, set by the size and spacing of its conductors. Aircraft antenna coax is usually about 50 ohms, to match the antenna.

Wavelength and Antenna Length

An antenna sends and receives best when its length suits the radio wave. The length that matters is the wavelength.

Frequency

The number of cycles of the radio wave each second, in hertz. One megahertz (MHz) is one million cycles each second.

Wavelength

The distance a radio wave travels during one cycle. The higher the frequency, the shorter the wavelength.

Resonant

An antenna is resonant when its length suits the frequency, so the strongest signal goes out and comes in. A half-wave antenna is resonant.

Wavelength in feet = 984 ÷ frequency in MHz
127 MHz
Wavelength7.7 ft
Half wave46 in
Quarter wave23 in
02468101214
Drawn to scale on a 14-foot ruler: one wavelength of the wave (blue), the quarter of it a whip needs (orange), and that whip beside a 6-foot person.

This is the real table from AC 43.13-2B. Select a row to tune the drawing to it.

AC 43.13-2B Table 3-1, wavelength in feet equals 984 divided by frequency in megahertz: 75 MHz, 13.1 feet, quarter wavelength 40 inches; 125 MHz, 7.9 feet, 24 inches; 1000 MHz, 1 foot, 3 inches.

Source: AC 43.13-2B, Table 3-1.

Quarter-Wave Antennas and the Ground Plane

Most aircraft VHF comm antennas are a quarter of a wavelength long. The metal skin under the antenna supplies the missing quarter.

A quarter-wave antenna gets the efficiency of a half-wave antenna by using the conductive skin it is mounted on as the second quarter. That conductive surface is the ground plane. On a metal airplane the skin is the ground plane. On a fabric, wood or composite airplane the installer must build one under the skin, from an aluminum plate, wire mesh, heavy foil or wires spread out from the antenna base.

Two quarter-wave Marconi antennas. Left: the antenna, one quarter wavelength long, on a metal aircraft skin that is its ground plane. Right: the antenna on a non-metallic skin, with the ground plane under the skin.

Source: FAA-H-8083-31B, Figure 11-92.

Select a number on the figure

Each number names one part of the figure.

3412
  1. The whip: one quarter of a wavelength.
  2. The skin under the base.
  3. The ground plane under a composite skin: foil or wire mesh, bonded to the airframe.
  4. The mirror image: with a ground plane, the antenna acts as if this second quarter were there.

Most antennas need a ground plane of about 24 by 24 inches. Larger is better, and it must be symmetrical. Carbon fiber is conductive, but it has not been found adequate as a ground plane.

Marconi antenna

The handbook's name for a quarter-wave antenna that uses the skin as its ground plane. Guglielmo Marconi found in 1895 that a vertical wire, with the other side of his transmitter connected to the earth, sent his signals much farther. On an airplane the skin plays the part of the earth.

Polarization

Polarization is the direction of the electric field a radio wave carries, and it is set by how the antenna is mounted. A receiving antenna gets the strongest signal when it is mounted the same way as the transmitting antenna.

Vertically polarized

Mounted up and down, like a comm whip. VHF comm antennas are vertically polarized.

Horizontally polarized

Mounted side to side, like the V-shaped VOR antenna on the fin. VOR and glideslope stations send horizontally polarized signals.

0 degrees
Signal received

A whip antenna raked back at an angle from its base.
Source: FAA-H-8083-31B, Figure 11-90. Many antennas are canted for better reception.

Many vertical and horizontal antennas on aircraft are mounted at a slight angle, called canted. A canted antenna still gets a weak signal, instead of none, when the polarization does not match.

An antenna must have the same polarization as the ground station it works with.

Reflected Power and VSWR

When the antenna system does not accept all of the radio's power, the rest comes back down the coax. VSWR is the number that tells you how much is coming back.

Forward power

The power going from the radio up the coax toward the antenna.

Reflected power

The power coming back down the coax because the antenna system did not accept it.

In-line wattmeter

A meter in the coax at the radio that reads forward and reflected power.

VSWR

Voltage standing wave ratio. The forward and reflected waves add and cancel along the coax and make a standing wave. VSWR compares its highest voltage to its lowest. 1.0 is a perfect match. The higher the number, the more power is coming back.

Forward10.0 W
Reflected0.0 W
VSWR1.0
Limit2.0
  • Forward power
  • Reflected power
  • Standing wave on the coax

A high VSWR means power is reflected instead of radiated. Check the antenna, its base, the connector and the coax before you blame the radio.

The VSWR limit for an installation comes from its installation or maintenance data. The lab later in this lesson uses 2.0.

Antennas on a Light Airplane

Each system has its own antenna, sized for its own band and mounted where it can reach its station. Turn the airplane and select each antenna.

Drag to turn the airplane, pinch or scroll to zoom. Select a number, or the antenna itself, to read about it.

Select an antenna

Each number is one antenna. Its panel shows the real antenna from AC 43.13-2B.

Top or Belly

Where an antenna goes depends on where its signal comes from. Sort each antenna to the side of the airplane it belongs on.

Top

It needs the sky or the stations above and around the airplane.

Tap the bucket where the card belongs. On a computer you can also drag the card.

Belly

It needs the ground or the stations below the airplane.

Which bucket does this card belong in?

Photographs: AC 43.13-2B, Figures 3-9, 3-10, 3-12, 3-14, 3-18 and 3-19.

Antenna Location in AC 43.13-2B

An advisory circular (AC) is an FAA document that describes acceptable methods. AC 43.13-2B covers alterations, and its Chapter 3 covers antenna installation. This is its real page on location. Select a numbered paragraph to read what it means in the shop.

AC 43.13-2BChapter 3, page 25
AC 43.13-2B page 25, two columns: paragraphs 304c and 304d, 305 Antenna Selection, 306 Antenna Location a through h with the GPS note, and 307 Antenna Bonding a.

Paragraph

Select a numbered paragraph

The page is the FAA's own text. Each number sits on one paragraph about where an antenna may go.

Source: AC 43.13-2B, Acceptable Methods, Techniques, and Practices, Aircraft Alterations, Chapter 3, page 25.

Placing a Second Comm Antenna

The owner of the Cessna wants a second comm radio. Its antenna, Com 2, needs a location. Try each spot on the airplane and read the result.

Select a lettered spot to put Com 2 there. Drag to turn the airplane, pinch or scroll to zoom.

Select a spot

Each letter is a place someone has suggested for Com 2.

The airplane's own approved data and the radio's installation manual decide the final spot. If a hole falls over a stringer, go back to that data. Do not move the antenna on your own.

Coax Runs

The coax carries the signal between the radio and the antenna. Its type and length come from the installation manual, and the way it is routed decides how much signal arrives.

12345
  1. The radio in its rack.
  2. Clamps about every 2 feet, on the coax alone, not on another bundle.
  3. A bulkhead connector where the run passes a frame.
  4. A sweeping bend, no tighter than the manual allows.
  5. The antenna on the top skin.

Type

Use the cable type the installation manual names. Aircraft antenna coax is about 50 ohms.

Length

Keep runs as short as practical. Transponders, DME and other pulse systems need the precise length the manual gives, and the two cables from a VOR/LOC/GS coupler must be the same length.

Support

Secure the cable about every 2 feet along its whole length. Do not route it with or tie it to other wire bundles.

Bends and handling

Bend it no tighter than the installation manual allows; the handbook gives at least 10 times the cable diameter as a general practice. Never kink coax, step on it or drop things on it.

Acceptable

The run meets the installation rules.

Tap the bucket where the card belongs. On a computer you can also drag the card.

Not acceptable

The run must be corrected.

Which bucket does this card belong in?

You learned how to terminate and repair coax in Routing, Coax, and Databus Wiring. The same rules apply here.

Sources: FAA-H-8083-31B, Chapter 11, Transmission Lines. AC 43.13-2B, par. 306h. AC 43.13-1B, par. 11-120 and 11-121.

Installing an Antenna

Always follow the antenna manufacturer's instructions. The steps below are the FAA's typical installation, with the real words of AC 43.13-2B for each one.

WarningInstalling an antenna means drilling the skin. Check what is behind it before the drill turns: wire bundles, control cables, fuel lines and structure.

Checking the Bond

The skin is part of the antenna, so the bond between the base and the skin is part of the antenna too. AC 43.13-2B allows no more than 0.003 ohm between the base plate and the skin.

3 mΩ
0.003 ohm, the limit

A milliohm (mΩ) is one thousandth of an ohm, so 0.003 ohm is 3 milliohms.

You used the milliohm meter in Terminations and Bonding. It drives a known current through the joint on one pair of leads and reads the voltage across it on a second pair, so the resistance of its own leads does not count.

----mΩ123
  1. The milliohm meter. Its display reads in milliohms.
  2. The antenna base plate and the skin under it, where the bond is.
  3. Four leads: the red pair carries the test current through the joint, and the white pair reads the voltage across it.

Choose an installation above.

If an antenna is not properly bonded, its pattern may be distorted and gaps in its coverage may appear.

Source: AC 43.13-2B, par. 307a, 307b and 309b(7). The three readings are example readings.

Water and Corrosion at the Base

An antenna base sits in the airstream and gets rain, washing and deicing fluid. Water that gets under the base or into the connector corrodes the bond and the connector, and the radio turns weak both ways or works until it rains.

Select a number on the drawing

The antenna has been lifted off the skin. Each number is one thing to look for.

Found 0 of 6

CautionUse only the sealant and corrosion treatment the maintenance manual specifies. The wrong compound traps moisture or attacks the finish.

Sources: AC 43.13-2B, par. 307e and 312.

Antenna Length and Spacing

Two formulas give you the numbers on this lesson's antennas. Both use the frequency in megahertz.

984 ÷ MHz
Wavelength in feet

AC 43.13-2B, Table 3-1. Use it for distances, such as spacing.

468 ÷ MHz
Half-wave antenna in feet

FAA-H-8083-31B. Use it for the length of an antenna.

The antenna formula comes out about 5 percent shorter than half the wavelength, because the air at the ends of a real antenna makes it act a little longer than it is.

Worked example 1

VOR antenna arm at 113 MHz

  1. 1468 ÷ 113 = 4.14 feet for the whole half-wave antenna.
  2. 24.14 × 12 = 49.7 inches.
  3. 3Each arm of the V is a quarter wave: 49.7 ÷ 2 = about 25 inches.

Worked example 2

Com 2 spacing at 118 MHz

  1. 1Use the lowest comm frequency, 118 MHz: it has the longest wave.
  2. 2984 ÷ 118 = 8.34 feet for one wavelength.
  3. 3Half of that is 4.17 feet. 4.17 × 12 = 50 inches.

Your Turn

Your turn 1

Find the cut length of a quarter-wave comm whip at 127 MHz, in inches.

inches

Your turn 2

Find a quarter wavelength at the transponder's reply frequency, 1090 MHz, in inches.

inches

The Weak Radio

Two airplanes have radios that are weak both ways. Prove each radio into a test load, measure the antenna system, inspect, repair and measure again.

Predict

N590JM's Comm 1 is weak both ways and worse after rain. Before you measure anything, where do you expect the loss?

Steps done: 0 of 7. Your results record in the log beside the instrument. Open in a new tab

The lab is optional practice. Continue whenever you are ready.

Fixing Comm 1

Back to the Cessna with the weak Comm 1. This is the job, in order.

  1. Prove the radio. Put the in-line wattmeter and a test load on the radio and key it. It reads full forward power with nothing reflected, so the radio is good.
  2. Measure the antenna system. Move the wattmeter to the antenna coax and key again. VSWR reads 3.2, over the 2.0 limit in the installation data.
  3. Remove the antenna and inspect. You find white powder under the base, pitting in the skin and a green, discolored center pin.
  4. Clean and treat the skin and the base per the maintenance manual, and put a conversion coat on the bare aluminum.
  5. Install a new connector on the coax and new mounting hardware.
  6. Mount the antenna flat, tighten the screws evenly to the torque limit, and measure the bond: 0.003 ohm or less.
  7. Seal the edges with the sealant the maintenance manual specifies.
  8. Measure again. VSWR reads 1.3, the radio is normal both ways, and no radio was replaced. Record the work in the maintenance records.

Check

Five questions, three answers each. Each answer tells you why it is right or wrong.

4 of 5
Needed to pass
  1. 1IdentifyWhy a comm whip is a quarter wave long.
  2. 2SelectHow far apart two comm antennas on top must be.
  3. 3InstallThe bond limit between the base and the skin.
  4. 4InspectWhat a high VSWR tells you.
  5. 5InstallWhat a composite airplane needs that a metal one already has.
Question 1 of 5

0 of 5
Results Results
From this lesson
  1. 01A comm whip is a quarter wave; the metal skin is its ground plane and supplies the other quarter.
  2. 02Com 2 on top sits at least half a wavelength from Com 1; the installation manual has the final word.
  3. 03Install on a doubler, mount flat, bond to 0.003 ohm or less, and seal the edges.
  4. 04A high VSWR means power is reflected; prove the radio into a test load, then inspect the base, connector and coax.
Select any row to re-read its feedback.

Field Card

Each antenna is sized for its band: a wavelength in feet is 984 divided by the frequency in megahertz, and a comm whip is a quarter wave because the metal skin, its ground plane, supplies the rest. You choose a location and coax from the installation data: Com 1 and the GPS on top, Com 2 on top only with half a wavelength of spacing, the cable type and length from the manual. You install an antenna on a doubler, mount it flat, bond it to 0.003 ohm or less and seal it. You inspect the base and connector for water and corrosion, and a high VSWR tells you power is coming back instead of going out.

  1. 1You can now match an antenna to its system and work its length from its frequency.
  2. 2You can now choose a location and coax from the installation data: spacing, a clear view, cable type and length.
  3. 3You can now install an antenna on a doubler, mount it flat, bond it to 0.003 ohm or less and seal it.
  4. 4You can now inspect a base and connector for water and corrosion, and explain what a high VSWR means.

Length

Wavelength984 ÷ MHz
Antenna cut468 ÷ MHz
Comm whipabout 22 in
Polarizationmatch the station

Location

Com 1 and GPStop
Com 2 on tophalf a wavelength
Rule of thumb36 in apart
Final wordinstallation manual

Install

Doublerinside the skin
Mountflat, no overtorque
Bond0.003 ohm or less
Edgescompatible RTV

Inspect

Basewhite powder, pitting
Connectorgreen pin, braid
High VSWRpower coming back
Prove firsttest load
Rule

Prove the radio into a test load before you replace it.

Sources: Federal Aviation Administration (FAA) Aviation Maintenance Technician Handbook, Airframe (FAA-H-8083-31B), Chapter 11. Advisory Circular (AC) 43.13-2B, Chapter 3. AC 43.13-1B, Chapter 11. The 3D airplane is the AERO flight simulator's Cessna 172.

Read every page and pass the check to finish.