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VHF Nav/Com Signals  
User currently offlineDg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 2
Posted (12 years 2 months 4 days 17 hours ago) and read 1585 times:

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?

4 replies: All unread, jump to last
 
User currently offlineFr8tdog From United States of America, joined Feb 2000, 120 posts, RR: 0
Reply 1, posted (12 years 2 months 4 days 16 hours ago) and read 1536 times:

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..


User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 2, posted (12 years 2 months 4 days 15 hours ago) and read 1483 times:

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.
xxx
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


User currently offlineAirplay From , joined Dec 1969, posts, RR:
Reply 3, posted (12 years 2 months 4 days 12 hours ago) and read 1466 times:

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.



User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 4, posted (12 years 2 months 2 days 20 hours ago) and read 1413 times:

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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