DG_pilot
Topic Author
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?
 
fr8tdog
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..
 
Guest

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.
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
 
airplay
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.

 
411A
Posts: 1788
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.

Who is online

Users browsing this forum: CanadaFair, Google [Bot], kabq737, ordpark, RushmoreAir, ytz and 1 guest

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos