VHF Com aerials and composite aircraft

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Russell Phillips
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VHF Com aerials and composite aircraft

Unread post by Russell Phillips » Fri Jan 04, 2019 10:40 pm

The Whisper motor glider and Whisper X manuals suggest building a dipole aerial into the fin. This is visually and aerodynamically a great aerial ...BUT it does not perform well when an EFIS is installed in the aircraft. If the aircraft has no EFIS fitted then all is well BUT fit an EFIS and all hell breaks loose :evil:

The reason has been hard to pin point but with much help from Rainier (CEO of MGL) the cause has finally been explained. All EFIS screens emit electromagnetic energy (lots of it). If you want an EFIS then there is no way around this.
The simple dipole aerial uses the lower pole of the aerial as it's "ground plane". This normally works ok and even gives good figures on an SWR meter, BUT the coaxial cable running back to your radio inevitably goes somewhere within a metre or two of your EFIS. The EFIS interference climbs into the coax shield here because it is acting as part of the dipole's makeshift "ground plane".

The solution......an ugly external aerial with a proper multi poled ground plane like the one in the picture.
Use this link to calculate the various lengths:
www.m0ukd.com

Fine tune the aerial length using an SWR meter if you can get your hands on one.
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