Line Of sight Question ?

[BOBRR]

Hello,

Got to thinking about this a bit, and realize I don't have a good understanding of LOS.

The Tower freq. at a nearby airport (Hanscom) is 118.5 MHz.

This is am.
Correct ?

Being am, can one consider that the propagation is Line-Of-Sight ?

If it was, e.g., fm, would it still be mainly LOS ?

How should one think about LOS propagation for am and fm ?
Thanks,
B.

 

[doctordialtone]

This is am.
Correct ?

Correct!

How should one think about LOS propagation for am and fm ?

Consider listening to AM Radio late at night. It depends.

"Several different types of propagation are used in practical radio transmission systems. Line-of-sight propagationmeans radio waves which travel in a straight line from the transmitting antenna to the receiving antenna. Line of sight transmission is used to medium range radio transmission such as cell phones, cordless phones, walkie-talkies, wireless networks, FM radio and television broadcasting and radar, and satellite communication, such as satellite television. Line-of-sight transmission on the surface of the Earth is limited to the distance to the visual horizon, which depends on the height of transmitting and receiving antennas. It is the only propagation method possible at microwave frequencies and above. At microwave frequencies, moisture in the atmosphere (rain fade) can degrade transmission.

"At lower frequencies in the MF, LF, and VLF bands, due to diffraction radio waves can bend over obstacles like hills, and travel beyond the horizon as surface waves which follow the contour of the Earth. These are called ground waves.AM broadcasting stations use ground waves to cover their listening areas. As the frequency gets lower, the attenuation with distance decreases, so very low frequency (VLF) and extremely low frequency (ELF) ground waves can be used to communicate worldwide. VLF and ELF waves can penetrate significant distances through water and earth, and these frequencies are used for mine communication and military communication with submerged submarines.

"At medium wave and shortwave frequencies (MF and HF bands) radio waves can refract from a layer of charged particles (ions) high in the atmosphere, called the ionosphere. This means that radio waves transmitted at an angle into the sky can be reflected back to Earth beyond the horizon, at great distances, even transcontinental distances. This is called skywave propagation. It is used by amateur radio operators to talk to other countries, and shortwave broadcasting stations that broadcast internationally. Skywave communication is variable, dependent on conditions in the upper atmosphere; it is most reliable at night and in the winter. Due to its unreliability, since the advent ofcommunication satellites in the 1960s, many long range communication needs that previously used skywaves now use satellites.

From Radio propagation - Wikipedia

AM vs FM - Difference and Comparison | Diffen

 

[TampaTyron]

LOS is generally based on frequency, antenna type, power out, and antenna height....... Not as much modulation type (yes, I know it does have a small impact on receiver performance). TT

 

[prcguy]

One way to look at VHF/UHF/microwave line of sight is the transmitter is a light bulb and your surroundings are a combination of light reflecting surfaces and light absorbing surfaces. Some are like mirrors to light and others might as well be painted flat black.

One one hand, line of sight should mean if you can't see the light bulb you don't have line of sight communications but in reality you can see reflections off many things like buildings, bridges, water tanks, etc, that will fill in around corners and refract off mountain tops and scatter down the other side.

As mentioned in another post, its the frequency range that determines line of sight and not the modulation mode like AM, FM, SSB, etc. They would all have the same propagation at the same frequency but some modes have an advantage when signals are weak.

 

[BOBRR]

Hi,

Much thanks to all,

Just the info i was looking for.

appreciate the excellent explanations,
Bob

 

[vagrant]

Line Of Sight Calculator

As previously noted, one must simultaneously consider:
- The wavelength (frequency) they are going to monitor.
- The output power of the transmitting station
- The quality, or lack thereof on the receiving station (Radio, Coax, Antenna)

 

[prcguy]

There are separate calculators for free space loss and radio horizon that take into account frequency, antenna gain or loss, feedline loss, etc. However these calculators are only accurate in "free space". One thing the free space and line of sight calculators do not consider is ground bounce, multipath and signal cancellation in the circuit and this cannot be accurately calculated.

You can have clear line of sight to a distant station with binoculars or a telescope, plenty of signal overhead according to the line of sight calculators and still have a weak signal. This is because there can be a parallel but different length path between antennas that can give up to 6dB more gain in the circuit if signals arrive in phase or up to about 20dB loss when signals arrive 180 degrees out of phase. Somebody at NASA screwed this up on a Mars rover and it only got a fraction of the distance from the main landing craft that they expected because the engineer used "free space" line of sight calculations and didn't add headroom in transmitter power or antenna gain to make up for potential ground bounce interference and signal cancellation.

Years ago I had to do a field study for a large (the largest) direct to home satellite company in the US where in some areas another company was allowed to have a terrestrial system on the same frequency which would potentially interfere with the satellite reception. My job was to measure the precise signal levels from the terrestrial transmitters at specific locations. After consulting with one of my mentors, a senior staff scientist for Raytheon and after going through a bunch of napkins at a restaurant from doing calculations on them, it was determined to get the last word in potential interference levels, my receive probe antenna would have to move up and down at least 15ft to identify and measure most peaks and nulls from ground bounce, and this was at 12GHz.

Sure enough during field testing, I could watch the signal go up and down 10dB and more and multiple times over 15ft as I moved my receive antenna up and down at each location. Had we simply calculated the potential interference levels they could have been off by at least 10dB. At lower frequencies for aircraft it would be a much wider span of heights between peaks and nulls signal gain or loss and the only way to really know what your path loss is to measure it.

The setup in the picture is good for measuring precise field strengths around 12GHz with the ability to move up and down to find peaks and nulls from ground bounce. A similar setup for aircraft band or other VHF/UHF testing might have to move up and down 50ft or more to get accurate readings and factor out ground bounce interference.

Line Of Sight Calculator

As previously noted, one must simultaneously consider:
- The wavelength (frequency) they are going to monitor.
- The output power of the transmitting station
- The quality, or lack thereof on the receiving station (Radio, Coax, Antenna)

 

[vagrant]

True, it can all go to hell in a handbasket, but as a general rule of thumb for the frequency the Op noted it seemed good enough. I do not often mention phasing unless they're using a handheld radio on one end or both. Still, your point is quite valid as I have plenty of phasing issues with select local friends on 2m with our verticals. It's an issue on both ends.

LOS experiments can be fun. Several decades ago and friend was at 10,200' and I was at 8800' ASL. We were separated by almost 1.5 miles. We did some testing with timing on hearing gunfire, car horn blowing, whistles and mirror signaling. (We avoided the three interval) Fortunately, we had some handheld CB radios at the time to coordinate. When I later climbed to the higher elevation another day, I enjoyed a conversation using the handheld CB to a guy about 90 miles away. I would have a blast now with my amateur license. I have the gear, so I probably should. It would probably weigh about the same as the full sized VHF camcorder and batteries I hauled up there along with a couple of beers and the CB. It would be a good prelim to see if I can hustle up Mt. Fuji in the next couple of years.