Posts: 810
Joined: Tue Sep 07, 1999 10:21 am

VHF Nav/Com Signals

Sat Oct 26, 2002 3:04 am

Hey guys,

Why are VHF Nav/Com signals rarely disrupted by storms and other atmospheric phenomena, whereas LF/MF waves are subject to many disruptions (i.e. mountain effect, coastal effect, night effect etc..). I know much of difference is that LF/MF waves are reflected off the ionosphere back toward the receiver, and VHF signals are line-of-sight, but why?

Similarly, why are radar signals frequently attenuated but Nav/Com signals remain uninterrupted?
Posts: 120
Joined: Wed Feb 16, 2000 4:25 pm

RE: VHF Nav/Com Signals

Sat Oct 26, 2002 4:20 am

I don't know a lot on this subject, but as for radar it depends on the distance to the object that the RF is reflecting off of and density.
A large thunderstorm with large amounts of precip, may absorb and scatter the RF, so that there is signal degrading at the receiver, resulting in showing a shallow penetration of the storm on the radar screen, often by a black hole (no precip) or an attenuation bar behind the storm..

RE: VHF Nav/Com Signals

Sat Oct 26, 2002 5:40 am

Frequencies above approximately 30 mHz are line of sight, while frequencies between 3 and 30 mHz are subject to reflection by ionospheric layers (E and F layers), and below 3 mHz it is basically only ground waves for transmissions, although night effect is present.
VHF is above the frequencies that cause "static" (i.e. industrial 50/60 Hz)... also, the higher the frequency, the less power required for transmission.
(s) Skipper
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

RE: VHF Nav/Com Signals

Sat Oct 26, 2002 7:53 am

VHF signals are in fact somewhat affected by atmospheric disturbances, but as mentioned, not to the same degree as HF signals. The type of modulation is as much a factor as frequency.

VHF Coms and Navs utilize Amplitude Modulation (AM) so any disturbance that affects the amplitude of the signal will be heard. FM signals are much more immune to these affects. I've witnessed complete loss of VHF nav and com systems due to precipitation static made worse a maintenance error that caused some bonding straps to be missing.

Over the next several years, aircraft communications will be transformed into digital systems to allow for additional channels and resistance to atmospheric disturbances.

Weather Radar transmits at a frequency that is easily absorbed by water molecules. That's what makes it useful in detecting precipitation. Doppler radars further detect phase shifts caused by turbulence.

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Joined: Mon Nov 12, 2001 10:34 am

RE: VHF Nav/Com Signals

Mon Oct 28, 2002 12:46 am

And before anyone asks, amplitude modulation is used for VHF communtcations because, if two stations transmit at the same time, distortion is clearly generated, whereas if frequency modulation was used, only the higher powered station would be received.

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