VAL Avionics NAV 2000 Installation Guide download pdf (Page 230)

SV-XPNDR-261/262 Installation, Configuration, and Testing

11-12 SkyView System Installation Guide - Revision R

Antenna Ground Plane

When a conventional aircraft monopole antenna is used it relies on a ground plane for correct

behaviour. For ideal performance the ground plane should be very large compared to the

wavelength of the transmission, which is 275 mm. In a metal skinned aircraft this is usually easy

to accomplish, but is more difficult in a composite or fabric skinned aircraft. In these cases a

metallic ground plane should be fabricated and fitted under the antenna.

As the ground plane is made smaller, the actual dimensions of the ground plane become more

critical, and small multiples of the wavelength should be avoided, as should circles. Rectangles

or squares are much less likely to create a critical dimension that resonates with the

transmissions. The smallest practical ground plane is a square around 120 mm per side; as the

size increases the performance may actually get worse, but will be better by the time the

ground plane is 700 mm on each side. Anything much larger than that size is unlikely to show

significant further improvement.

The thickness of the material used to construct the ground plane is not critical, providing it is

sufficiently conductive. A variety of proprietary mesh and grid solutions are available.

Heavyweight cooking foil meets the technical requirements, but obviously needs to be properly

supported.

Antenna Cable

The SV-XPNDR-262 is designed to meet Class 2 requirements with an allowance of 2 dB for loss

in the connectors and cable used to connect it to the antenna. The SV-XPNDR-261 is designed

to meet Class 1 requirements with the same 2 dB allowance. Excessive loss will degrade both

transmitter output power and receiver sensitivity.

Allowing 0.25dB loss for the connector at each end of the antenna cable assembly leaves an

allowance of 1.5dB maximum loss for the cable itself.

An acceptable cable:

 Has less than 1.5 dB loss for the run length needed

 Has a characteristic impedance of 50 Ohms

 Has double braid screens or has a foil and braid screen

Once the cable run length is known, a cable type with low enough loss per metre that meets

the above requirements can be chosen. Longer runs require lower loss cable. Consider moving

the SV-XPNDR-261/262 closer to the antenna to minimize the losses in the antenna cable –

subject to the limits identified above, the SV-XPNDR-261/262 can be at any distance from the

SkyView display(s) without affecting performance in any way.

Note: Low loss cable typically uses foamed or cellular dielectrics and foil screens. These

make such cables especially prone to damage from too-tight bends or from momentary

kinking during installation. Once kinked, these cables do not return to full performance when

straightened.

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VAL Avionics NAV 2000 Installation Guide Page 230