The VHF Omnidirectional Range (VOR) system is the backbone of instrument navigation in the United States. Ground stations broadcast on frequencies between 108.0 and 117.95 MHz, and airborne receivers decode those signals to tell the pilot whether the aircraft is on course, left of course, or right of course — and whether flying toward or away from the station. For an AMT working on the airframe side, understanding not just how the receiver works electronically but how the complete installation must be executed is critical to airworthiness. A poorly bonded antenna, a cable routed near an interference source, or a loose connector can render the system inaccurate or useless.
This article covers VOR receiver operating principles, the anatomy of the airborne system, and the specific installation requirements — antenna selection and placement, coaxial cabling, bonding, shielding, and post-installation checks — that keep the system performing within certified limits.
How VOR Works
Every VOR ground station simultaneously transmits two signals. The first is a reference phase signal, which is omnidirectional and has the same phase in all directions from the station. The second is a variable phase signal, which rotates at 30 revolutions per second and whose phase angle relative to the reference changes continuously as direction from the station changes. The airborne receiver compares the phase difference between the two signals. At magnetic north of the station, the two signals are in phase (0° difference). As you move clockwise around the station, the phase difference increases — at due east it is 90°, at south it is 180°, and so on. The receiver uses this phase comparison to determine the aircraft's magnetic bearing to or from the station, which is then displayed as a radial.
The cockpit instrument most associated with VOR is the Course Deviation Indicator (CDI), part of the VOR Indicator (also called the Omni Bearing Indicator, or OBI). The pilot manually rotates the Omni Bearing Selector (OBS) knob to set the desired course. When the needle is centered and the flag reads TO, the aircraft is on the selected course inbound; when the flag reads FROM and the needle is centered, the aircraft is on the selected course outbound. Full-scale CDI deflection typically represents 10° of course error (each dot represents approximately 2°). The TO/FROM flag is driven by a simple comparator circuit that determines whether the aircraft's radial is within 90° of the selected course (TO) or more than 90° away (FROM).
Airborne VOR System Components
The airborne VOR installation consists of several integrated elements: the VOR antenna, the coaxial feed line, the VOR/ILS receiver (often combined with other nav functions), the CDI or HSI display, and optional remote switching or audio filtering circuitry.
The VOR Antenna
VOR antennas must respond efficiently to horizontally polarized VHF signals, because VOR ground stations transmit with horizontal polarization. The most common design on light aircraft is the V-dipole (often called a "V-tail" or "rabbit-ears" antenna), which is tuned to the 108–118 MHz band. These antennas are typically mounted on top of the fuselage (dorsal) or on the vertical stabilizer. Top mounting is preferred because it provides a clear line-of-sight to the horizon in most directions with minimal airframe shadowing. Some aircraft use a blade-type or fin-mounted antenna, which can be more aerodynamically streamlined.
Antenna placement is governed by several constraints. The antenna must be located as far as practical from engines, propellers, and large metal structures that could cause reflections or blockage. A separation of at least one wavelength (approximately 2.7 meters at 110 MHz) from other antennas is a general rule of thumb to reduce mutual coupling. The antenna must also be mounted on a structurally sound surface capable of withstanding aerodynamic loads at the aircraft's Vne, with appropriate doublers or reinforcing plates as specified in the manufacturer's Structural Repair Manual or Supplemental Type Certificate (STC) data.
Coaxial Cable and Connectors
The antenna is connected to the receiver via 50-ohm coaxial cable (RG-58/U or RG-400 are common for VOR applications). The cable's outer shield must remain continuous and undamaged, because any break or kink in the shield will degrade signal quality and can introduce noise. The cable must be routed with smooth, gradual bends — the minimum bend radius specified by the cable manufacturer must be respected, typically six times the cable's outside diameter. Sharp bends can crack the dielectric and compromise the characteristic impedance, causing reflections and signal loss.
Connectors are typically BNC or TNC type on the antenna and BNC on the receiver input. Every connector must be properly crimped or soldered, then weatherproofed at the antenna end. A loose or corroded center pin in a coaxial connector is one of the most common causes of intermittent VOR reception. When routing cable inside the airframe, it should be secured with clamps or tie-wraps at intervals that prevent chafing against structure, and it must never be routed in the same bundle as high-current wires (battery cables, landing light circuits) that could introduce interference.
Bonding, Shielding, and Interference
Proper electrical bonding of the antenna mount to the airframe is essential. The antenna's ground plane must make low-impedance electrical contact with the airframe skin. Bonding resistance at the antenna mounting point should typically measure less than 1 milliohm (per AC 43.13-1B guidance on bonding), achieved by removing paint and anodizing under mounting hardware and using star washers or serrated bonding hardware. High bonding resistance creates a floating ground plane, which degrades antenna efficiency and can introduce static noise into the audio and nav signals.
The receiver chassis itself must also be bonded to the aircraft structure. In all-metal aircraft this is generally accomplished through the rack-and-panel mounting system, but in composite airframes, dedicated bonding straps or wires must be installed because the structure does not conduct electricity. Composite aircraft require particular attention: a conductive ground plane (a metallic mesh or foil embedded in or bonded to the composite) must be incorporated around the antenna mount and connected to the aircraft's grounding system.
Shielding of the coaxial cable's outer braid must terminate properly at both ends. Do not trim back the braid excessively at the connector; full 360-degree contact around the connector body is required. Partial shield termination (the so-called "pigtail" ground) introduces impedance discontinuities and degrades high-frequency shielding effectiveness — a significant issue in the VHF range.
Why Correct Installation Matters
Navigation errors caused by installation deficiencies are insidious because they may not be obvious to the pilot. A VOR receiver that consistently reads 5° off due to antenna shadowing or a corroded connector will give no flag warning — the TO/FROM indicator will still work, the CDI needle will still deflect, but the aircraft will fly an incorrect course. In IMC, this is a serious safety hazard. Furthermore, 14 CFR Part 91.171 requires VOR equipment used for IFR flight to be operationally checked within the preceding 30 days by one of several approved methods (VOT, certified airborne checkpoints, ground checkpoints, or dual VOR check). If the installation has introduced a systematic error, the aircraft may technically fail this check or, worse, pass the check by luck and then fail in the field.
From an airworthiness perspective, any modification to the navigation system installation — including antenna replacement, cable rerouting, or receiver substitution — must comply with the approved data: either the aircraft's Type Certificate Data Sheet (TCDS), a Supplemental Type Certificate (STC), or a Field Approval under 14 CFR Part 43. The work must be recorded in the aircraft maintenance records with a return-to-service entry signed by an appropriately certificated technician or repairman.
Key Numbers and Rules
- VOR frequency range: 108.0–117.95 MHz (VHF band, horizontally polarized)
- CDI full-scale deflection: typically ±10° from selected course (each dot ≈ 2°)
- IFR VOR check interval: 30 days (14 CFR §91.171)
- Maximum allowable VOR check error: ±4° for VOT or airborne checkpoint; ±6° for ground checkpoint; ±4° average for dual VOR check
- Antenna bonding resistance: generally less than 1 milliohm (reference AC 43.13-1B)
- Coaxial impedance: 50 ohms (RG-58/U or RG-400 typical)
- Minimum antenna-to-antenna separation: approximately one wavelength (~2.7 m at 110 MHz) as a practical guideline
- Composite airframes: require an installed conductive ground plane at the antenna mount
Common Test Traps
- Confusing polarization: VOR uses horizontal polarization, not vertical. Many other aviation communication antennas (VHF comm) also use vertical polarization, so don't mix them up when selecting or positioning antennas.
- Forgetting the ground plane in composites: Students often assume that antenna bonding is automatic in composite airframes the way it is in metal airframes. It is not — an explicit conductive ground plane must be engineered and installed.
- Using incorrect coaxial impedance: Some technicians substitute 75-ohm cable (common in consumer electronics) for 50-ohm aviation coaxial. The impedance mismatch causes signal reflections and degraded sensitivity.
- Pigtail shield termination: Connecting only a short wire from the coaxial braid to the connector body rather than maintaining 360-degree circumferential contact significantly degrades shielding at VHF frequencies.
- Skipping return-to-service documentation: Any maintenance or alteration to the nav installation — even replacing a coaxial connector — must be logged. Failure to document makes the aircraft unairworthy regardless of how well the work was performed.