Cruisers Forum

Join CruisersForum Today

  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
Thread Tools Search this Thread Rate Thread Display Modes
Old 22-01-2009, 19:16   #1
Registered User

Join Date: Jan 2009
Posts: 101
VHF line-of-sight or not

I'm sure I can get alot of comments on this one. VHF IS lin-of-sight and so is HF if you only consider the ground wave of the transmitted signal. What makes HF useful for long range communications is that it is much more easily reflected or refracted from the different layers of the inosphere than VHF. HOWEVER, VHF has its enhanced propagation modes also. Infact, all frequencies have enhanced propagation modes up into the microwaves which includes RADAR. Admittedly, the enhanced modes of propagation occure much less often the higher you go in frequency.

Enhanced modes of propagation on VHF and above are called "Sporadic E", "Aurora", "Ducting" and "TE". You can also get strange reflections from heating of the surface of the water. Sporadic-E is an ionized cloud of the E-layer of the ionospere which can cause reflection of VHF and higher frequencies over several hundred miles much the same way that HF frequencies are propagated. Aurora is reflection of VHF signals off of the Aurora Borialis. These signals are very weak and may not be detectable on FM, but may be dectected on the upper HF frequencies. Often the reflected signal sounds like a whisper. Ducting can cause long distance communications (100's of miles) with little or no loss of signal strength. TE or transequatorial propagation occurs between stations located approximately the same distance from the equator (north and south) and usually occurs in the VHF range.

Using FM modulation on VHF would make these modes of propagation more difficult to detect, but they can happen. Ducting of a RADAR signal might give a flase reflection, but the odds of it happening over any length of time would be rare. However, it could happen.

For those using the Amateur Radio frequencies on VHF and above with SSB or CW instead of FM the oportunity for use of these modes of propagation become much more of a possibility.

Use of the numerous satelites available for Amateur Radio communications using SSB, CW, FM or digital communications becomes a real source for communications over large portions of the earth. Most of the frequencies available for use by satelites are in the VHF and above frequency range. This means that only a Technician license is required as long as you use the right equipment, which at times may only have to be a VHF/UHF (dual band) FM handheld unit. Satelites available for this type of communications are called LEO or Low Earth Orbit satelites and are only available for short periods of time. Other satelites give hours of available communications but require high gain antennas and the ability to track the satelite.

It is wise to remember that all frequencies have enhanced propagation modes, but these modes become less of a concern the higher in frequency you go. On HF, the enhanced modes (reflection from the ionosphere) are absolutly necessary for reliable long distance communications, but if those modes are not available, as changes occur in the ionosphere between day and night, certain HF frequencies are only good for Line-of-sight.

Hal is offline   Reply With Quote
Old 22-01-2009, 20:07   #2
Senior Cruiser
senormechanico's Avatar

Cruisers Forum Supporter

Join Date: Aug 2003
Boat: Dragonfly 1000 trimaran
Posts: 6,813
I regularly used skip with my HAM rig to do phone patches from the West coast of Mexico to a HAM friend in St. Louis. He made a local call to our daughter.

The best tropo ducting call I made was from near Manzanillo to La Paz B.C.S., a distance of about 470 miles. After talking on the Manana net 14.340 Mhz USB) a fellow HAM called and said the tropo was really rocking and asked me to call him on Channel 16 VHF. Using VHF high power (25W) it was perfectly readable. When we tried low power (1W) we could hear and identify each other's voices, but we couldn't quite understand the words.

Steve B.

senormechanico is offline   Reply With Quote
Old 22-01-2009, 20:15   #3
Senior Cruiser

Cruisers Forum Supporter

Join Date: Jul 2006
Location: Chesapeake Region and Maine
Boat: 42' Bob Perry sloop
Posts: 4,047
Images: 4

Unlike VHF, HF groundwave propagation isn't strictly line-of-sight. It's very much dependent on frequency.

For example, here are some typical distances for groundwaves at HF frequencies used by hams:

30 mHz 10 miles
21 mHz 20 miles
14 mHz 30 miles
7.0 mHz 45 miles
3.5 mHz 60 miles
2.0 mHz 80+ miles

You can see that as frequency decreases, groundwave coverage increases. That's why, for example, broadcast AM stations can regularly be heard over long distances, as they operate between 0.55 and 1.6 mHz.

btrayfors is offline   Reply With Quote
Old 22-01-2009, 20:54   #4
Registered User
Cowboy Sailer's Avatar

Join Date: Aug 2007
Location: on the boat. Gulf Coast
Boat: C&C 38'
Posts: 351
Images: 2
Great job! Good tutorial. I remember about 50 years ago that we started getting clear comunications over our radios (VHF) from a group of shrimpboats on our same frequency but about 800 miles away. My boss was invited to come down and go shrimping and he had a great time. Ever so often we would talk to our distant buddies from an oil rig to a shrimpboat like the were next door!
Jerry and Denver
Happy Old cruisers!
Cowboy Sailer is offline   Reply With Quote
Old 22-01-2009, 21:17   #5
Registered User

Join Date: Jan 2009
Posts: 101
OK Bill, you got me on that one. There is always an exception to the rule. The old 500 KHz emergency frequency had a ground wave distance of about 250 miles or more. The aircraft NDB stations can be heard over long distances also and the ELF frequencies used to communicate with submarines do so over the entire earth, but they use the ground wave to do so.
Hal is offline   Reply With Quote
Old 22-01-2009, 21:27   #6
Registered User

Join Date: Jan 2009
Posts: 101
Here is another one I better get to before Bill gets me again. There are certain experimental stations using DSP enhanced receiving techniques that are proving that ground wave at 500 KHz can be as great as 600 miles or more. Not your usual communications techniques, but it might be in the future when all of the data are in. However these distances are obtained at night when the ionized absorption layers of the ionospher have disapated. That is why you here so many more distant AM broadcast station at night.
Hal is offline   Reply With Quote
Old 23-01-2009, 02:19   #7
Long Range Cruiser
MarkJ's Avatar

Cruisers Forum Supporter

Join Date: Dec 2007
Location: Australian living on "Sea Life" currently in England.
Boat: Beneteau 393 "Sea Life"
Posts: 12,827
Images: 25
My VHF is Line-of-sight.

Its just needs an optometrist!

Notes on a Circumnavigation.

Somalia Pirates and our Convoy
MarkJ is offline   Reply With Quote
Old 23-01-2009, 06:09   #8
Senior Cruiser

Cruisers Forum Supporter

Join Date: Nov 2005
Location: Out there doin' it
Boat: 47' Olympic Adventure
Posts: 2,943
Originally Posted by Cowboy Sailer View Post
Ever so often we would talk to our distant buddies from an oil rig to a shrimpboat like the were next door!
Yeah, I've had that too - Pacific coast of Central America, was 5/5 talking to a ship over 600 miles away on Ch 16.

Lodesman is offline   Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
I Sold My Boat! Sight Unseen! clausont Off Topic Forum 15 30-07-2009 10:44
fuel line sight glass gettinthere Engines and Propulsion Systems 3 25-11-2008 10:16
Found this sight about many types of electric motors to power boats videorov Electrical: Batteries, Generators & Solar 1 18-03-2008 05:15
funny smart sight unbusted67 Liveaboard's Forum 0 22-02-2007 13:44
Question on sight reduction sneuman Seamanship & Boat Handling 2 14-10-2005 08:41

Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 08:45.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2019, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.