UHF mountain bounce

[bd348]

I was able to communicate with the family at camp over most of this road as I traveled north ( blue dot ).  At this point there is quite a ridge in the way.  As the miles grew to the north I was losing them, around four trail miles north of that intersection just south of the blue dot.  I think I was still talking to them at that lodge, for example, despite the ridge in the way.

Given the positions shown here, was the UHF signal perhaps going along the valley north-east from the blue dot, bouncing off a mountain, then going down the other valley south to the green dot?

It was pretty impressive no matter what.  If it matters: Midland MTX-275 15W mobile with a quarter wave on the Jeep hood, going to/from a Wouxun KG-S88G 5W handheld with a half wave in the trailer.  If I keep this up I may put a better antenna on the trailer.

[bd348]

This was Tray Mountain Road, near Helen, GA.  And the good stuff further north was closed...!

[axorlov]

Yes, my experience is the same. UHF goes surprisingly far in the mountains, with no line of sight. But it is lousy in the forest and rolling hills. Better, higher antenna on the trailer would help for sure.

[marcspaz]

Geek Alert:  Sorry...  LOL

 

So, your total distance is just about 100 yards past 1 statute mile and the ridge line is creating about 650-660 foot barrier.  That is not really an unusual experience and should be repeatable.  The explanation can get a little complicated, as to why.

 

Misconceptions about the Line of Sight coverage reflect the thoughts that UHF waves travel only in straight lines.  This is not true.  First and foremost, radio signals do follow the curve of the earth (to a degree, literally).  Second, RF photons diffract more strongly than optical photons because it has a longer wavelength than the optical photon. This is expected and predicted within classical electromagnetic theory and by quantum theory and why RF LoS is further that visual LoS.

 

If you have an antenna that has a pattern with takeoff angles between 5 degrees and 20 degrees, you are going to have great success with talking longer distance and talking around objects.  This is because a huge amount of radio signal is reflecting off of stuff in the atmosphere, as well as being refracted by the atmosphere.  However, the further you go, the wider the photons grouping gets and the more diluted the signal gets (fewer parts per million) and the harder it is for a receiver to do anything useful with it.  Using 15w as an example, the usable energy is going to be cut to 3.75w (1/4) within the first mile and 0.9w (1/16th) within the 3rd and 4th miles.

 

So, at a mile... its likely that it would work with that type of obstacle.  At 4 miles, ehh... not so much.

 

 

[Lscott]

8 hours ago, bd348 said:

Given the positions shown here, was the UHF signal perhaps going along the valley north-east from the blue dot, bouncing off a mountain, then going down the other valley south to the green dot?

That's possible. There are such things as passive repeaters, nothing more than a huge sheet of metal acting as a reflector. The mountains could be doing that.

What might be more likely is some diffraction effects. Light waves, and RF waves, will experience diffraction when passing a sharp edge. In your case it could be the ridge at the top of the mountain. Enough energy diffracted downward to make communication possible.

Warning only math geeks should try to read this.

  https://apps.dtic.mil/sti/pdfs/AD0117276.pdf

This isn't as bad and the illustrations make the explanations easier to understand.

  https://www.ihe.kit.edu/img/studium/Wave_Propagation.pdf

 

[WRUU653]

I am completely intrigued by this post. Here’s why, last Thursday the wife, dog and I took a walk around a nearby lake to see the water level (at this moment at 98%) and I was able to break the squelch on the local repeater using my KG-935+.  With the wife moving ahead and the dog dragging me along I didn’t have an opportunity to stick around the spot I was at to try and make a contact and it has baffled me ever since. There is no way I thought I could hit that repeater from my location.  I was back in the canyon when I did this. I tried again when we got back to the truck by the lake but no luck which didn’t surprise me. I’m looking forward to going back and trying this again. The only other thing I could think of was I hit a non listed repeater back there with the same tones and frequency but that seems almost as unlikely. 

[PartsMan]

More than a few times my Chinese GMRS junk has performed fine in situations the internet experts say it could not.

I have even out performed VHF ham radios a couple of times.

[WRFP399]

So I live in the mountains here in Alaska. I have found that GMRS signals (UHF) do very well at moving down the deep valleys flanked by tall/steep mountains. I assumed this is reflection/refraction. I can also confirm that GMRS (UHF) signals get chewed up by heavy forest.   Further to that point, there is a commercial UHF passive "repeater" on a ridge behind my home. It is just a large flat metal "billboard".

 

In my experience being in the mountains and away from urban areas "increases" the performance of the radio through better signal to noise ratios. Urban areas tend to drown out weaker signals but out in the back country were there is less RF noise that weak signal comes through strong.

 

[Lscott]

More info on passive repeaters.

Passive Repeater.pdf

[WRUU653]

4 hours ago, WRFP399 said:

Further to that point, there is a commercial UHF passive "repeater" on a ridge behind my home. It is just a large flat metal "billboard".

 

This blows my mind, a picture truly is worth a thousand words. Thanks for sharing.

@Lscott I can always count on extra reading material from you sir. Many thanks  

 

 

Edited March 22, 2023 by WRUU653
Word correction

[WRUU653]

Looking over the passive repeater pdf from @Lscott I had a moment of recollection, I have worked on radio, cell and television towers on many occasions as an electrician but on the power side of things and I suddenly remember seeing one of these passive repeater panels. I never knew it’s purpose. Very cool to finally know the story. As a side note I loved going to these locations, I would take photos from some ridge with a lake below or some such scenery text it to my brother with the caption “my office today”. 

[nokones]

Those reflectors are normally used for operational fixed microwave control paths.

[Lscott]

You also see small ones at the top of some communications towers. They are angled at about 45 degrees. The antenna is at the base of the tower pointing up at the reflector. Some people call these “periscope antennas”.