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Author Topic: Discriminating Between Ground Wave vs. Skip  (Read 368 times)

KG5YFO

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Discriminating Between Ground Wave vs. Skip
« on: March 10, 2023, 07:37:45 PM »

First forum post so my apologies if I use incorrect terminology.

I would like to know if there is any possible way for a receiving station to discriminate between a ground wave signal vs. skip. I am a software developer who works with RTL_SDR libraries. With software I can look at any element of a received signal more effectively than most spectrum analyzers. But I need a seasoned Elmer to tell me what signal elements could help me reach my goal of knowing if a signal has been skipped off the ionosphere or not. Any info or suggested resources would be helpful.
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K6AER

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Re: Discriminating Between Ground Wave vs. Skip
« Reply #1 on: March 10, 2023, 08:35:38 PM »

Generally speaking, a ground wave signal has very little multipath distortion. With ionosphere skip, multi path distortion can be scene very easily with a spectrum waterfall as light spots ascending or descending in frequency bandwidth vs amplitude as a result of multipath distortion and phase cancelation.

HP-88941A Vector Signal Analyzer was my work horse during the 4G development to study the effects of signal distortion in the field for the cellular industry.

Contrary to belief, terra bits of software has been written for spectrum analysis of this signal condition.

Start reading the ARRL Antenna Handbook.

HP has tons of engineering papers on the use of their equipment to study this signal condition.  Contact your local sales rep and he can guide you to the appropriate tutorials.

73 Mike K6AER
« Last Edit: March 10, 2023, 08:39:10 PM by K6AER »
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KG5YFO

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Re: Discriminating Between Ground Wave vs. Skip
« Reply #2 on: March 10, 2023, 08:54:02 PM »

Nice! Much better than “no, you cannot tell the difference”, which is what I was expecting. Note that the HP scope & spectrum analyzer division split off to become Agilent, then after 15 years their marketing geniuses decided to rename their well recognized brand to “Keysight”.

I will research their site and/or try to make friends with the Systems Engineer in my area.

I have the ARRL Antenna Handbook so will look there as well.

Thanks!
« Last Edit: March 10, 2023, 08:57:23 PM by KG5YFO »
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W1VT

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Re: Discriminating Between Ground Wave vs. Skip
« Reply #3 on: March 11, 2023, 03:23:16 AM »

The most obvious change is a polarity conversion.  Ionospheric skip results in the creation of circularly polarized waves from incident linear waves.
Set up two detectors, one for LHCP and the other for RHCP, to easily see this. 

Ground wave will change LHCP to RHCP if it bounces off a wall.  But vertical remains vertical and horizontal remains horizontal.

https://www.lz1aq.signacor.com/docs/hpsra/horizontally-polarized-small-active-receiving-antennas7.htm
1. The benefits to use  both VP and HP antennas

Receiving and transmitting with circular would be more popular if you didn't need to switch polarity between transmit and receive.

Zak W1VT
« Last Edit: March 11, 2023, 03:37:39 AM by W1VT »
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K9AO

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Re: Discriminating Between Ground Wave vs. Skip
« Reply #4 on: March 11, 2023, 07:14:13 AM »

If you use Linrad with a dual-channel SDR with cross-polarized antennas then you can actually see the polarization in the phasing window display. It'll show you as it uses an adaptive algorithm what the best signal strength is and the scope-type display will let you see if that optimized setting is straight vertical or horizontal, RHCP of LHCP or RH elliptical or LH elliptical polarization.

Recent changes to Linrad have made the dual-channel reception (and recording and playback) work with more SDR devices. SDRplay devices were one of these that now work well but the fixes I think also applied to a pair of RTL devices. I haven't tested this on RTL hardware but I have tested extensively using the SDRplay RSPduo.

You can also go to fixed phasing mode and set any of these in small increments too manually to see the effect.

I think you would find the above very interesting and helpful. IF you leave the adaptive phasing algorithm active you can watch the best polarization change around as the ionosphere changes. Daylight on mediumwave with ground wave you'll see it remains steady.

There are also many other demodulation mdoes that are exceptionally interesting and helpful, coherent mode 2 is one of them.
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WB6BYU

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Re: Discriminating Between Ground Wave vs. Skip
« Reply #5 on: March 11, 2023, 01:01:38 PM »

Quote from: KG5YFO

... knowing if a signal has been skipped off the ionosphere or not...



Let's start with a bit of terminology (which isn't always consistent
among various references).  That will help to determine just
what conditions you are trying to detect.

I would divide radio wave propagation into 3 modes:  the surface
wave
, which travels along the air/ground interface;  the
direct wave, which travels directly from transmitter to receiver,
and the sky wave, which bounces off the ionosphere (or passes
right through it instead).  Yes, there are others, like moonbounce,
tropo, or meteor scatter, but let's start with the easiest ones.

The surface wave must be vertically polarized.  That's what AM BC
stations rely on for local coverage, as the coverage is very consistent
over time.  However, the surface wave attenuates faster with distance
than the other modes, so it has a more limited range, depending on
frequency and transmitter power.  When ionospheric conditions permit
(usually at night), the reflected sky wave signal can be received
simultaneously, which causes fading (due to the relative phase
of the signals) which limits the useful range for "broadcast quality"
reception.

The direct wave polarization is determined by the transmitter antenna,
and whatever reflections or refractions happen along the way.  Often
it also provides a good stable signal, but is prone to multipath
interference (fading) due to moving objects like aircraft, or in some
cases trucks driving through the signal path.

The sky wave can be reflected back to Earth via the E or F layer
of the ionosphere.  It undergoes some polarization modification
along the way, so the polarization of the received wave is basically
unpredictable, even knowing the transmitter antenna.  We
often experience that as "fading", as the polarization changes from
that of our receiving antenna to being cross polarized and back
again.  This might take up to an hour over stable paths, or just a
couple minutes when conditions are changing quickly along the
path.

Note that the sky wave can also be reflected from the troposphere
(which we often refer to as tropo scatter).  That's not the same as
the ionosphere, which is one reason that I wanted to clarify your
exact requirements.


So if the received signal is vertically polarized, it may be surface
wave, but otherwise it isn't.  If it is pretty stable over time, then it may
be surface wave or direct wave, or sky wave under stable conditions
(depending how long of a sample you want to take to determine the
propagation mode).

If the signal strength does exhibit fading or other phase interference,
it maybe a combination of surface wave and sky wave, or it may be
direct wave with airplane interference, or sky wave over a less stable
path.

You really can't tell too much from a single input to an SDR: 
polarization typically would need to compare two inputs from antennas
with different polarization.  And a signal from a moving vehicle may
realy confuse the software, as path loss and reflections will vary fairly
quickly over time.


So it's important to determine exact what conditions you want
your algorithm to handle, and which you choose to ignore.

It certainly isn't a trivial task!

KA4WJA

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Re: Discriminating Between Ground Wave vs. Skip
« Reply #6 on: March 13, 2023, 12:19:20 PM »

Charles,
1)  A few others here have already given you the scientific approaches that you can use to differentiate between Surface Wave (also sometimes called "ground wave") and Skywave (what you referred to as "skip")

{to be clear, I'm not a software guy....nor do I play with SDR's (although my knowledge of them goes back decades, when a good friend was doing the independent 3rd part proof-of-performance testing of the B2 Bombers SDR comms systems)....so, I'm deferring to the others (and to yourself) on what the software, etc. can do....my comments are in the area of
 RF and radiowave propagation...}

And, I recommend that you reread what Dale, WB6BYU, wrote....as it is a very good explanation of what is going on with various signal propagation!
 
First forum post so my apologies if I use incorrect terminology.

I would like to know if there is any possible way for a receiving station to discriminate between a ground wave signal vs. skip. I am a software developer who works with RTL_SDR libraries. With software I can look at any element of a received signal more effectively than most spectrum analyzers. But I need a seasoned Elmer to tell me what signal elements could help me reach my goal of knowing if a signal has been skipped off the ionosphere or not. Any info or suggested resources would be helpful.

2)  Having said the above....there may be much simpler ways for you to make the determination...
Depending on the freqs being monitored / examined...
And, depending on the antennas you're using...
You may be able to quickly / easily determine with almost certainty which signals are surface wave, versus skywave, without any "software" nor "analysis" at all.

I apologize up front, if you already are aware of this....but, the wording of your question suggests otherwise...so..

So, as examples...

a)  Surface wave is almost entirely vertically polarized (probably should just write "entirely", but some will start a calculation showing some infinitesimal chance otherwise)....and on the range is VERY limited (even with excellent ground conductivity in your area of Texas, surfacewave / groundwave on HF bands is limited to < 40 - 50 miles on 80m, and < 20 - 25 miles on 40m....and maybe a dozen miles at best on 20m, and on any higher freq it is usually much less, but the directwave (line-of-sight) is used then...on 160m surfacewave range can be 50 - 100 miles with good ground conductivity (and over sea water, I've seen 200+ miles surfacewave on 160m)

b)  So, any signals from ranges farther than these are skywave (although you do get some troposcatter on 10m, more often than some think....remember it was Marconi himself that discovered troposcatter in ~ 1930....using 30mhz....but for this discussion, just ignore troposcatter)

c)  Also, any signals that are other than vertically polarized, are also generally skywave.

d)  As well as lower HF signals (such as on 80m and 40m) that are strong, and are close but > line-of-sight distance (such as that "no-man's land" range of 20 - 30 miles on 80m), are almost always skywave...
{fyi, this actually presents an example within an example...as a distance of ~ 20 - 25 miles, where the average 80m antennas are beyond line-of-sight, you can still have a surfacewave signal but it usually MUCH weaker than the skywave signal, and therefore not usually noticed....(if you have stations slightly closer, you can also have a directwave signal as well)...and, when the ionosphere's critical freq approaches the 80m band, you can see fading that has the surface wave signal in direct competition with the now weaker skywave signal, and you'll have very noticeable echos and warbles, etc...and as the crit freq drops below the 80m band the skywave signal will become very weak and you're left with just the surfacewave signal....}
 

3)  Charles, the point I'm driving at here is....since we don't know what signals / what bands you're examining, we don't know exactly what the best answer for you is.

But, if you're analyzing signals from, say 20m and from > a few hundred miles away, there is no need to use software... hi hi....just some further knowledge of propagation will go a long ways to determine these are skywave... :)

And, if you're looking at 40m or 80m, and some signals are from 30 - 50 miles away, and some are 100's or 1000's of miles away, here as well there's no need for software...as these are skywave...

As you can see on our HF bands, there is actually very little "surface wave" comms going on.  :)   (yes, on 160m there is some....and some VERY local comms, such as within 10 - 20 miles on 40m and 80m....but just about everything those ranges is skywave)



Okay, no more rambling from me today...
I hope this helps you out.

73,
John,  KA4WJA

P.S.  If I screwed up here, and not making sense...sorry, wicked stressed today.  :(
« Last Edit: March 13, 2023, 12:23:56 PM by KA4WJA »
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W1VT

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Re: Discriminating Between Ground Wave vs. Skip
« Reply #7 on: March 15, 2023, 08:00:17 AM »

The January 2022 QST  has a article describing ionosounders.  They display two channels.  The O mode or ordinary mode is RHCP. 
The X mode or extraordinary mode is LHCP.  Ionospheric skip shows an obvious dispersion between the O and X modes, while E skip does not.
« Last Edit: March 15, 2023, 08:02:22 AM by W1VT »
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