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Author Topic: 2m SSB Propagation over terrain?  (Read 7505 times)
KD8PGB
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« on: February 17, 2016, 02:05:07 PM »


 So I am working with a buddy who is 50 miles away, and he is about 3 miles behind a 600ft hill,  and I am about 15 miles from a 400ft hill..those are the two main objects in our paths.

For sake of argument if we were to put up a 4-7 element Yagi at each end at about 60ft AGL (still just beneath the tree cover for me), would we have fairly reliable communication over 2m SSB? I have read that 2m SSB proposes well when both stations have horizontal polarity antennas, but I never hear mention of terrain and or obstacles.

Any thoughts on the reliability of this connection? I don't want to burn a fair amount of cash on antennas and such just to find out the terrain is blocking us.

Dan W5DMH (formerly KD8PGB, need to update that here)

 
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WB2WIK
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« Reply #1 on: February 18, 2016, 09:45:44 AM »

Need the lat/lon coordinates for both sites, very specifically, to do a terrain profile and see what results.

Also, re the "4 to 7 element yagi," there's a lot of differences there.  A close spaced 4-L and a wide spaced 7-L can have as much as 4-5 dB gain difference; add that up on both ends, and it can easily make or break a communications path.  Ditto for transmission line loss.  Ditto for transmitter power level used.  Big difference between 100W output and 25W output, especially when you double it to improve the path by 12 dB. Wink

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KD8PGB
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« Reply #2 on: February 18, 2016, 02:50:45 PM »



I already ran a profile...thats how I know the hill heights and locations.

I am well aware that line loss, output power and transmit power are factors.

I had hoped I would get some responses from 2m SSB guys that are routinely communicating over terrain and could interject their experiences about reliability over terrain with horizontally polarized antennas and similar terrain.


Dan W5DMH

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N9DG
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« Reply #3 on: February 18, 2016, 03:55:03 PM »

I would think that if there is 25W or more on both ends that a 50 mile path with modest yagi's on each end should be quite reliable. The elevation above the horizon of 600 foot hill 3 miles away isn't that much. And the 15 miles from a 400 foot hill even less so. I'd be more worried about close to the antennas on each end obstacles like numerous trees buildings etc. Being below the tree cover is potentially more of a problem depending how thick the tree cover is. One or two trees fairly close doesn't hurt all that much on 2M, a thick forest canopy certainly can.
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W8JX
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« Reply #4 on: February 18, 2016, 06:50:15 PM »

I think if you both get a decent 2m co linear like a Hustler G7 up as high as possible that 2m FM will work as about as well as SSB here. 2m tends to creep a bit over/past edges or hills a lot moreso than 440 which is pretty much line of sight. The trick here is to try to get up at least 40 feet or more (higher is better) and it can help when trying to pick up a little "ducting" more regularly to extend range a bit. If this was 75 miles over same terrain it would be a much bigger challenge. BTW a G7 up at forty feet or more is better a small yagi at 30 feet or less here. 
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You can embrace new computing technology and change with it or cling to past and fall further behind everyday....
KD8PGB
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« Reply #5 on: February 19, 2016, 01:28:17 AM »


 Thanks guys, that is encouraging information,  I think we'll give it a shot and see what we get.


Dan W5DMH
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KD8PGB
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« Reply #6 on: March 22, 2016, 12:12:49 PM »

 So we have done a little testing with Yagi's  I have a 4 element M2 and my buddy has a 7 element M2. Mine is at 25ft and his is at 10ft ....dramatic improvement over verticals! We have been testing a 22 mile path with hills in the way and a lot of QRM.  Just over a 24 hour period it has been very reliable and substantially stronger signals than what we had with verticals.

We will test with this for the next few weeks until my tower goes up at my other location (50 mile path), then I will put up probably a 12 element on a rotor at 60-70ft, now that will be interesting testing

I almost forgot to mention, over 2m SSB with the Yagi antennas it would seem that power is not much of a factor....we found at 6pm yesterday the propagation from him to me was better than me to him but that was to be expected...what surprised me was that he dropped his power all the way down to 5 watts and was still coming in strong. I on the other hand increased my power from 50watts all the way up to 200 watts and the increase in db was tiny, only 2-3 db SNR  improvement.... however at 5am this morning, I was being received at 15db SNR which was a vast improvement, but I believe that was due to reduced noise/qrm...not improved signal itself...maybe some more experienced hams can chime in....I also noticed through the day today, the SNR has been declining all day...every couple hours its a bit worse...

I would not think that atmospheric conditions would be impacting us much when our path is so short at 22 miles?

 
 Grin
 
« Last Edit: March 22, 2016, 12:32:05 PM by KD8PGB » Logged
G8HQP
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« Reply #7 on: March 22, 2016, 01:30:47 PM »

Unless you have a 'line of sight' path (and at VHF that means no obstructions near the path - in the Fresnel zone) you will find that the atmosphere has an effect, because to some extent you will be relying on refraction due to temperature/humidity vertical profile.
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KD8PGB
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« Reply #8 on: March 23, 2016, 03:58:20 AM »

Unless you have a 'line of sight' path (and at VHF that means no obstructions near the path - in the Fresnel zone) you will find that the atmosphere has an effect, because to some extent you will be relying on refraction due to temperature/humidity vertical profile.

I had originally thought our path was to close...at 22 miles...but after your post I read up on 2m propagation and found this tropospheric ducting is quite common for just out of LOS (and further) very interesting reading and explains our initial test results..
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KT4NR
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« Reply #9 on: March 23, 2016, 07:17:29 AM »

As this unfolds this might make for a great QST article.
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K6JH
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« Reply #10 on: April 14, 2016, 03:30:04 PM »

Here in Los Angeles I used to have a 2M vertical at the top of a 50 ft tower. 10 watts from an FT-227R. I was chatting with a mobile friend FM simplex about 60 miles away, with some small mountains in between. I lost him a bit as he got very close to the base of the mountain in-between, but for the most part it was solid copy all the way home.

We even got a break from a guy down in San Diego, perhaps 120 miles away. Although he probably was running beams.

Perhaps this was aided by a temperature inversion, but the point is don't underestimate what can be done. Try it!
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73
Jim K6JH
ONAIR
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« Reply #11 on: April 14, 2016, 09:24:51 PM »

Here in Los Angeles I used to have a 2M vertical at the top of a 50 ft tower. 10 watts from an FT-227R. I was chatting with a mobile friend FM simplex about 60 miles away, with some small mountains in between. I lost him a bit as he got very close to the base of the mountain in-between, but for the most part it was solid copy all the way home.

We even got a break from a guy down in San Diego, perhaps 120 miles away. Although he probably was running beams.

Perhaps this was aided by a temperature inversion, but the point is don't underestimate what can be done. Try it!
  That's what I like about SoCal!  Lots of areas of high elevation, where you can just go portable and put up a small 2m/440 beam.
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K6JH
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« Reply #12 on: April 15, 2016, 02:34:24 PM »

Quote from:  link=topic=107763.msg919530#msg919530 date=1460694291
That's what I like about SoCal!  Lots of areas of high elevation, where you can just go portable and put up a small 2m/440 beam.

Yeah, I used to do that in my younger days. Just hop in the truck and drive up to the mountaintop.

Unfortunately the Forest Service is getting more persnickety about locking the gates to the choice places. Including demolishing the old radar pedestal that was up on Mount Gleason that I bolted antennas to during VHF contests back in my 20's.
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73
Jim K6JH
ONAIR
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« Reply #13 on: April 16, 2016, 01:23:52 AM »

Quote from:  link=topic=107763.msg919530#msg919530 date=1460694291
That's what I like about SoCal!  Lots of areas of high elevation, where you can just go portable and put up a small 2m/440 beam.

Yeah, I used to do that in my younger days. Just hop in the truck and drive up to the mountaintop.

Unfortunately the Forest Service is getting more persnickety about locking the gates to the choice places. Including demolishing the old radar pedestal that was up on Mount Gleason that I bolted antennas to during VHF contests back in my 20's.
    Darn!  Are they closing up other places as well?
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KA4WJA
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« Reply #14 on: April 20, 2016, 09:55:18 AM »

Dan,
I'm glad you took the time to experiment and that you've found out how well VHF works (other than 2m FM!)....
The more you use it, the higher you put your antennas (not forgetting to consider coax loss), etc. the more you will enjoy it!
But...


But, I think you may be confusing propagation here....this is not tropo ducting...(nor even very likely knife-edge refraction over your hills), but rather like most vhf/uhf comms beyond line-of-sight, is tropo-scatter...and even then you'rr not that far beyond LOS...
{of course, you can also have experienced some local tropo enhancement (commonly just called "tropo"), but this is definitely not ducting!}

Unless you have a 'line of sight' path (and at VHF that means no obstructions near the path - in the Fresnel zone) you will find that the atmosphere has an effect, because to some extent you will be relying on refraction due to temperature/humidity vertical profile.

I had originally thought our path was to close...at 22 miles...but after your post I read up on 2m propagation and found this tropospheric ducting is quite common for just out of LOS (and further) very interesting reading and explains our initial test results..

Perhaps you might like some details?
Here is what I wrote explaining 156mhz Marine VHF-FM propagation to non-hams / non-tech sailors:
Quote
....For land-based, terrestrial VHF communications, I've gotten used to the VERY simple and easy-to-calculate formula here, which can be easily done in your head in seconds....but this gives results in statute miles....

 VHF Radio line-of-sight distance (in statute miles) = √2 x Af (where Af is the height, in feet, of Antenna)
 Line-of-sight communications range between two VHF radio stations (in statute miles) = √ (2x Af1) + √ (2x Af2)....

Quote
2) Beyond "line-of-sight"?? How is that possible??

Is this something exotic?? Actually, NO....No, it is NOT exotic at all, to the contrary it is normal and common....and is actually VERY reliable and VERY predictable (out to the distance where path loss exceeds the available transmit powers and antenna gains).....And, this is called "tropo-scatter"
"Tropo-scatter" might sound exotic and rare, but it is anything but!!It is simply how almost all VHF and UHF signals (radio, TV, data, etc.) travel beyond line-of-sight..."Tropo-scatter" is simply the spreading of the radiowaves in the troposphere, caused by the natural irregularities in our atmosphere...


 The troposphere extends from approx. 1000' - 3000' above sea level up to approx 35,000' above sea level....with most of our "scattering" occurring between 10,000' and 30,000' (although a "tropo-duct" is usually very narrow and defined, "tropo-scatter" uses most of the troposphere, especially when using wide antenna beamwidths, as we do, on our boats)...
 The atmosphere above the troposphere is fairly stable, has little humidity, and has a fairly even temperature cline, hence little irregularities to "scatter" VHF signals...


 In addition to radio and TV broadcasting, VHF and UHF "tropo-scatter" has been in regular use by both military and commercial users for 60 some years now....and while some of the legacy commercial tropo-scatter communications links have been replaced by newer satellite data links, there are still many in operation.....not to mention the 10's of 1000's of ham radio operators that use "tropo-scatter" daily (some unknowingly, of course)...and if you've read about "over-the-horizon-radar" (whether military surveillance, or for weather), well this is "tropo-scatter-radar" (primarily UHF, but some VHF as well)....
 FYI, the first reported use of VHF "tropo-scatter" was way back in 1928, by Guglielmo Marconi, using a 30mhz system, for a link between Sardinia and the Italian mainland!!! (but, the elegance of this mode of propagation wasn't examined much until the early/mid 1930's...)


Let me be perfectly clear here:


VHF (and UHF) "Tropo-scatter" is a 100% / always-there, communications mode/path, and is NOT any special mode, does NOT rely on any special conditions to exist at all....It is simply the way that VHF, UHF, and SHF signals propagate beyond the direct-wave (beyond line-of-sight)....


 For these reasons, most laypersons will erroneously call this "ground-wave" (which it is NOT at all like, as VHF and UHF signals have no real "ground-wave" at all!!), but "tropo-scatter" is simply the way VHF, UHF, and SHF signals propagate beyond line-of-sight...and since the phrase "tropo" is in there, some will think that this is relying on some atmospheric enhancement and therefore could not be thought of as reliable, but to the contrary, it does not rely on any special atmospheric conditions, and hence is 100% reliable (up to the distance where the received signal no longer is useful, when the transmit powers and antenna gains, fail to over-come the "tropo-scatter path loss")

 Wide-bandwidth modes such as FM Voice are not the usual means of utilizing "tropo-scatter", due to their inherent noise-limited receivers and need to attain threshold in order to have any real useful signal-noise performance....but, if significant transmit power and high-gain antennas are employed, wide-bandwidth "tropo-scatter" can accomplish many wonderful things (such as provide multiple voice circuits and multiple high-speed data links to/from North Sea oil rigs and the mainland, etc.)...

 With our limited transmit power (25 watts), and low-gain (dipole) antennas, our wide-bandwidth Marine VHF, is rather limited in its "tropo-scatter" performance...
 (typically about 50nm at best...of course slightly longer ranges are possible with antennas mounted up high, such as tall towers of USCG stations or NOAA weather broadcasts, etc.)


According to some old references, VHF "tropo-scatter" path loss at the 55nm (100km) range is approx. 168db....which would not allow most Marine VHF communications, +44dbm - 168db = -124dbm, below the typical -117dbm for 12db SINAD....which means no communications...
 But, over sea water, there are times when this path is usable, although it is NOT reliable, which is why the 30nm - 35nm range is typically thought of as the "maximum" normal / reliable range for most pleasure boat-to-pleasure boat Marine VHF-FM comms...




3) "Tropo" / "Tropo-enhancement" (also sometimes referred to as "non-ducting-tropo")
This is the most common means of "enhanced" VHF (and UHF/SHF) communications....and is noticed by many mariners along coastal areas, especially in summertimes....

 The technical reason for these fairly common, springtime and summertime coastal "tropo-enhancements" are due to daily temperature inversions in the troposphere.....where instead of the air temp descending as altitude increases, there is a slight rise in temperature at a higher altitude in the troposphere...
"Tropo-enhancement" (especially along the edge of hi-pressure areas, as well in coastal areas, or along island chains / archipelagos) typically covers rather broad areas, and can make signals from a few hundred miles away very strong....although there is typically also some fading, and while these signals are fairly strong, they do fall off in strength at typical inverse-square rates, meaning unlike in a "tropo-duct" these "tropo-enhanced" signals from 400 - 500 miles will be weaker than those 200-250 miles distant...
 These are common in areas of high-pressure, especially in humid air, along the edge of a high-pressure area, etc.

 Alhough, "tropo-enhancement" can effect signals from great distances and allow for VHF
 communications farther than a few hundred miles, it is rare that these conditions extend more than 500 - 750nm, and when it does, it is using the highest part of the troposphere...and, with our 25 watt FM radios, it is unlikely that most will experience "tropo-enhancement" farther than that....
 And, for us using Marine VHF, typical "tropo-enhancement" will show good signals from 100nm out to 300-400nm.....and rare instances out to 500nm - 750nm....





4) "Tropo-Ducting":
If you are communicating on VHF Marine radio farther than 500nm - 750nm, then you're likely using a "tropo-duct", but again except for some specific locales (such as Portugal / Morocco to Canaries, S. FL to Long Island, California to Hawaii, etc.) a "tropo-duct" is rare, and is also usually geographically fairly narrow....meaning that the distance covered might be quite long (1000 - 2000 miles), but the "duct" is typically fairly narrow (< 100 miles wide, and many times <50 miles wide...), and almost always over water, in areas of stable hi-pressure, but unlike "tropo-enhancement", a "tropo-duct" always uses the very lowest part of the troposphere (typically only 1000' - 3000' above sea level)....just like its name implies, it acts like a "duct", long and narrow, and close to the ground...


 Typical "tropo-ducting" provides very strong distant signals, and many times these distant signals are as strong (or sometimes even stronger) than those in your local area...and a sure sign that you are working stations in a "tropo-duct" is that stations along the duct are all about the same signal strength, rather than the more distant stations being weaker....if you are working a station > 500nm away, and they are as strong (or stronger) than other stations along the same path that are only 100nm away, then you are working in a duct....
 BUT...
 But, as I write above, except for some specific geographical areas, "tropo-ducting" is rare!!!
 So, most "beyond tropo-scatter" range VHF communications is via "tropo-enhancement", not tropo-ducting"...



While there is a lot more to all of this, I hope this helps you out.

73,
John,  KA4WJA

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