Is there such a thing as line of sight HF propagation?

[MDK2]

As opposed to NVIS? I haven't had any reason to drive up north to Fort Collins or any of its surrounding communities, and thus be in line of sight of WWV. But if I lived in within, say, 20 or 30 miles of their transmitters, could I expect to hear them well at all times and frequencies because I'd be within line of sight? (I'm about 75 miles away as it is.) I tried googling around for an answer, but the closest I came was videos of people listening to stations while they were close enough to transmitter towers to be able to video record them on their cell phones.

I do plan to test this at some point, but I seldom have any reason to go up that way. Maybe I'll make a special trip of it just for that reason. But until then, I'm posing this question here.

[Stretchyman]

Do you mean GROUND WAVE as a method of propagation?

If so then YES for ANY freq under 2MHz or so....

I've just been messing aroung on MW (A.M. I think for you lot!) and can put out a decent groundwave with 5W and will cover a 50mile radius

Str

[Token]

Yes, all RF, regardless of frequency, is subject to line of sight propagation at all times, Sky Wave and Ground Wave propagation are frequency and condition dependent.

In application this means that anytime you have real line of sight to the source, and the source has high enough radiated power towards you to overcome the space loss between you and the source, you will receive the signal.

There are some situations where multipath can kill the signal (destructive addition of the energy from more than one path), however those should be the exception.

T!

[MDK2]

Do you mean GROUND WAVE as a method of propagation?

If so then YES for ANY freq under 2MHz or so....

I've just been messing aroung on MW (A.M. I think for you lot!) and can put out a decent groundwave with 5W and will cover a 50mile radius

Str

No, I mean specifically HF. I used WWV as an example for two reasons. One, they have frequencies all across HF, 2.5 MHz to 25 MHz. Only the 2.5 one is technically a MW frequency. Two, they're close to me but not close enough for line of sight. There are FM transmissions from that area that I can hear without external antennas, although they're very rough.

And I used the termed line of sight specifically, although I guess it's synonymous with ground wave. I know from videos I've seen that if you're right there by an HF transmitter, you can hear the signal well on your receiver. But I'm trying to find out how far you have to go away from them before normal HF propagation conditions take over. Where I am, I can generally only get WWV on 2.5 at night and 5 during the day, with faint hints of it on 10 and 15 when conditions are favorable (although WWVH is much more copyable on those frequencies). I've gotten only the faintest copy of 20 MHz on one occasion, and never 25. That's doubtless as much a reflection on the limitations of my equipment as it is my QTH, which places me well within the skip zone of the higher frequencies.

[MDK2]

Yes, all RF, regardless of frequency, is subject to line of sight propagation at all times, Sky Wave and Ground Wave propagation are frequency and condition dependent.

In application this means that anytime you have real line of sight to the source, and the source has high enough radiated power towards you to overcome the space loss between you and the source, you will receive the signal.

There are some situations where multipath can kill the signal (destructive addition of the energy from more than one path), however those should be the exception.

T!

Terrific. Thank you!

[Josh]

Typically, the lower the freq the farther the groundwave will go, all else being the same. Also the angle of radiation off the transmitter and receiver antennae come into play. And the height of the ionospheric layer the signal reradiates from comes into play at times. As you've seen, the freqs that are under the muf and above luf can be employed via nvis, wich is variable on a hourly to seasonal basis. As an example of stable hf line of sight work, two 10/11m beams pointed at each other can do 50 miles or more of very stable contact. A friend used to talk to his wife via 11m ssb from Council Bluffs Ia to Sioux City Ia (about 94 miles) via a 11m vertical atop his tower fed by his Ic706, her car had a Ic706 also and some mobile cb antenna, 100w out at each end.

To pull off the same thing at 2.5Mc in the daytime would take the typical  broadcasting tower at each end - and keep in mind that short yet efficient antennas at 2Mc and 27Mc are very different from each other as you can easily understand. Also one must consider vertical radiation is most desireable for MW work as the D layer absorbs vertical polarity least at MW/ambc freqs - explains why most every ambc antenna is a vertical that is wavelength calculated to cover the locality as well as possible.

As an example of MW daytime groundwave abilities, an ambc station in Omaha Ne has coverage out to Des Moines Ia to the east, Kansas City to the south, and into Sioux Falls SD to the north, from a vertical fed with 1kw.
http://radio-locator.com/info/KCRO-AM
http://radio-locator.com/cgi-bin/patg?id=KCRO-AM&h=D

It doesn't hurt that some of the most conductive soil in the US is under their tower.

[MDK2]

Interesting Josh, thank you.

[Token]

And I used the termed line of sight specifically, although I guess it's synonymous with ground wave.

No, line of sight and ground wave are not synonymous.  They are distinct modes of propagation.

Although a gross oversimplification ground wave can be seen as RF that creeps along the ground, and so can follow the curvature of the earth.  It goes out and follows the earths surface past the visual horizon.

Sky wave is, naturally, RF that departs the source antenna and reflects off a portion of the ionosphere or something high in the atmosphere before being received.  It goes up before it comes down someplace..

Line of sight means you have a direct path, essentially visual path (not limited to human visible wavelengths of light), from your antenna to the other antenna with no obstructions.  Trees, buildings, etc, don't count in this case as obstructions to the "visual" path until the frequency of operation is high enough that the RF cannot penetrate (see through) the "obstruction".

You can be in line of sight and also be in ground wave (you can also be in line of sight and in sky wave), but if you are in line of sight and no ground wave (or sky wave) is present you will still receive the signal, assuming the signal is leaving the source with enough ERP, in your specific direction (taking into account things like radiation pattern), to overcome free space loss and deliver a signal sufficiently above your MDS.

In actuality even with no sky wave and no ground wave you can (and often do) still get propagation beyond the visual horizon.  Scatter and refraction will bend a line of sight signal to some extent.  The Radio Horizon is most often something beyond the Visual Horizon, even at frequencies taken as truly line of sight limited.  I can't count the number of times I have seen an I or J band (J band is up to 20 GHz) radar track a target that was just behind a hill and not visible, or was well below the curvature of the earth for the geometry.

I'll leave the argument of "is aircraft scatter sky wave?  Or is it line of sight with scatter?" to someone else.  

T!

[MDK2]

Thank you. I didn't think that the terms truly meant the exact same thing (maybe analogous was the word I should have used), but I figured that there's a reason why I can get some of Fort Collins' radio signals but not the upper WWV frequencies.

[Pigmeat]

I used to vacation at Atlantic Beach, NC. In the town of Newport, about 20 miles away, surrounded by brackish swamp perfect for groundwave props was a religious SW broadcaster on 5920 kHz. running about 50 kW. Never heard a peep from it, even while driving by the town. When I was at home they would blast in the same time frame. Occasionally I could hear the VOA out of Greenville, NC about 60-70 miles to the N.N.W., same type of terrain, but they were running 500 kW. By the fluttery quality of the signal, I'm fairly certain when I heard Greenville down there, it was on the home leg of it's multi-hop path around the world. 

[ve6jy]

During severe solar storming though, all bets are off. A real life HF 80m example -  A friend lives 13 km from me - we both have real antennas and a low noise environment.  We are completely line of sight and can work with microwatts on VHF/UHF.  Yet conditions have been bad enough that on 3.7 mhz between us even several kw just barely gets a readable signal.  During normal conditions, a few watts would be plenty.  This has happened several times, to varying degrees and had I not experienced this, I'd be skeptical as to how we couldn't get a signal those few miles.

Don
VE6JY