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Old 22-01-2009, 19:16   #1
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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.

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Old 22-01-2009, 20:07   #2
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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.

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Old 22-01-2009, 20:15   #3
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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.

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Old 22-01-2009, 20:54   #4
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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!
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Old 22-01-2009, 21:17   #5
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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.
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Old 22-01-2009, 21:27   #6
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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.
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Old 23-01-2009, 02:19   #7
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My VHF is Line-of-sight.

Its just needs an optometrist!

Notes on a Circumnavigation.

Somalia Pirates and our Convoy
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Old 23-01-2009, 06:09   #8
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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.

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