Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
VHF Nav/Com Signals  
User currently offlineDg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 2
Posted (13 years 7 months 1 week 5 days 1 hour ago) and read 2231 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 (13 years 7 months 1 week 5 days ago) and read 2182 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 (13 years 7 months 1 week 4 days 22 hours ago) and read 2129 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.
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 (13 years 7 months 1 week 4 days 20 hours ago) and read 2112 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 (13 years 7 months 1 week 3 days 3 hours ago) and read 2059 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.

Top Of Page
Forum Index

Reply To This Topic VHF Nav/Com Signals
No username? Sign up now!

Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)

Similar topics:More similar topics...
Most Common Nav/Com Headset? posted Mon Jan 3 2005 05:17:48 by KDTWFlyer
How Do NAV And COM Frequencies Work? posted Mon Mar 4 2002 19:20:10 by Bio15
HF Vs VHF Vs UHF R/T Frequencies Useage posted Fri Oct 20 2006 08:28:34 by BOACVC10
Marshalling Signals - Differences. posted Fri Aug 4 2006 09:25:59 by HAWK21M
Boeing Employees Registering For Airbusworld.com posted Wed Jun 7 2006 14:57:16 by Matell
VHF-C / Acars Incompatible? posted Fri Apr 7 2006 23:30:51 by IFEMaster
Why Pas Can't Have GPS On Com Airline posted Sun Nov 13 2005 02:54:48 by Airfoilsguy
How Do ACs Nav Today/flight Crew Test posted Sat Oct 22 2005 14:25:54 by D5DBY
The Future Of GPS. Will It Replace The Nav Radio? posted Tue Jan 11 2005 04:22:44 by Hmmmm...
NAV Autopilot posted Wed Dec 22 2004 12:47:14 by TBA04

Sponsor Message:
Printer friendly format