So LOS for VHF/UHF isn't all that important? - Page 1
Forum section: electronics / rf-microwave

author: W9GFO
author_rank: Regular Contributor
timestamp: March 31, 2018, 11:52:11 pm
content: Everything I can find to read about VHF signals tells me that an obstruction such as a 120ft hill, will block the signals. I am trying to understand why it is that I can hit a repeater 12 miles away using my little HT on VHF (and UHF), even though there is a hill (more like a ridge line) 1/4 mile away that is 120 ft higher than the LOS path. What phenomenon is permitting me to make contact? I would like to understand it better but Google is not leading me where I need to go.

author: rhb
author_rank: Super Contributor
timestamp: April 01, 2018, 12:22:35 am
content: Refraction caused by the topography and variations in moisture level in the atmosphere are the main contributors. LOS is the high frequency approximation. Reality is more subtle. There have been recent professional papers on seeing around a corner by means of edge diffractions.Sorry, but I can't offer any details. I'm accustomed to it in elastic wave propagation in the 5-200 Hz range. So I didn't pursue reading the paper.

author: ikrase
author_rank: Regular Contributor
timestamp: April 01, 2018, 03:03:12 am
content: Like many things, this is incredibly relative.146 MHz VHF will reliably penetrate obstacles that stop 5.8 GHz signals cold.Two immediate possibilities would be reflections (if there is something around that can reflect) and the Fresnel zone (it's not a *line* of sight, it's more like a football shape of sight.)

author: VK5RC
author_rank: Unknown
timestamp: April 01, 2018, 03:09:07 am
content: Sometimes 'knife edge' diffraction over a ridge.

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2018, 03:27:21 am
content: Do some VHF and UHF transmitter hunting and you will experience what is going on. With a good beam antenna and an airport you can learn what makes standard aircraft non-stealthy.Refraction around edges and reflection from surfaces are enough to get beyond line of sight but multipath and fading become a significant issue. And just because you bring up the repeater, it does not mean that your signal is intelligible.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 01, 2018, 10:59:44 am
content: Quote from: W9GFO on March 31, 2018, 11:52:11 pmTraditionally, it was thought that VHF/UHF were limited to "line of sight", but that was really "an educated guess" back in the early1930s.VHF does have a "ground wave" which will follow smooth variations in ground height to some extent, although it is nowhere as pronounced as at low frequencies."Knife edge" diffraction is a real thing at these frequencies so sharp hill tops will are more likely than not to exhibit this effect.By the way, it is a very real effect at light frequencies, too, which is where it was originally identified.That is why it is a good idea to extend your coverage of sunscreen slightly inside your collar, short sleeves, etc.Tropospheric propagation at VHF/UHF, is another thing that happens, giving ranges out to the thousands of km when "ducting" is present.If ducting is contributing to your ability to access the repeater (though I doubt it,with the sort distance), be prepared to lose it when your weather changes.

author: W9GFO
author_rank: Regular Contributor
timestamp: April 01, 2018, 11:08:46 pm
content: Here is an article that I came across on this subject;http://arundale.com/docs/ais/PathlossCalculationsforAmateurs.pdfAnd this websitehttp://www.ve2dbe.comthat calculates coverage based upon your location, antenna height, power etc...When I enter my data into this calculation the 120 ft hill between myself and the repeater apparently has no significant effect on the signal.

author: denverpilot
author_rank: Regular Contributor
timestamp: April 02, 2018, 07:27:11 am
content: Also we have HT users in mountain repeater country accessing them with HTs a lot further out than 12 miles.Because it really is LOS.And we drive the receivers way down toward the noise floor with pre-amplification carefully balanced with filtering.But. Frankly. They usually sound like crap and it gets old. Even a decent quarter wave rubber duck makes a huge difference in their signal quality by bringing things up just a couple of dB at the receiver site.Nothing worse than having to tell someone new and excited about the hobby that their $30 HT from China just sounds like crap. Audio chain wise (on many of them) and RF signal strength wise... not to mention the ones that sneak out of the factory with their deviation set all wrong.And mobile... ugh. Reflections, picket fencing in and out of the squelch threshold, in and out of terrain holes. Just sounds like crap.We try, but when the mountain range the repeaters sit on is 25 miles away from the geographic center of town, there’s just not a lot we can do without multiple receiver sites. And that’s wonderful but a whole different engineering game and price tag on the repeater side of things.100 miles out, hill topping, with a real 50W radio and a properly installed high gain antenna is really common too. It’s gotten harder over the last couple of decades with the general site noise and city RF noise though.One site is nearly 5000’ over average terrain and people are amazed when we tell them that after filtering and combiner losses on a shared DUAL 8-bay VHF dipole array, one TX, one RX, stacked vertically (yes, it’s over twenty feet tall at VHF on a tower leg) with a pattern favoring the direction toward the city, the repeaters average about 7W to the antenna, and can be heard 100 miles out. Everywhere.By far my favorite site. But not cheap either. 11,440’ MSL and town is roughly 5,300-6,000 MSL. View is great from up there, too. Working up there all day can make people hypoxic and dehydration is a big deal.Nothing beats altitude if you can get it.