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Author Topic: Interesting phenomenom  (Read 18004 times)
N7DMA
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Posts: 17




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« on: April 30, 2013, 06:57:58 PM »

Hi, all:

I have a weekly CW practice chat with a friend who lives about 4 miles away. We used to use 2 meters, until his rig started going wonky, so we decided to give 40 meters a try. We were both QRP, at around 5 watts. I observed deep fading, going from 559 to about 229... slow, long and deep fades. On 2 meters, I'm used to flutter from passing planes and such, but I would have thought that on 40, we would be chatting via ground wave.

My antenna is at about 25 feet, hidden in my attic, and his at about 30 feet, in the clear. My first thought was some sort of multi-path distortions, but at such long, deep fades, I can't help but wonder what causes this. We both live within the city limits, but in spite of urban clutter, we have a pretty clear shot of each other. Kinda has me scratching my head, as we both observed this.

Maybe NVIS interference, or space aliens, or....? I discount traffic, as I think  the fades would have been more rapid. But the fades were so pronounced, I just have to wonder what else could cause this.

Any thoughts?

Thanks,

Karl
N7DMA
Tucson, AZ

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KL7CW
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Posts: 58




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« Reply #1 on: May 07, 2013, 01:22:08 AM »

Karl,
      Usually 5 watt signals at 4 miles on 40 meters (for mostly line of site paths) are reasonably strong (and not usually subject to fades) if the antennas are not of opposite polarization.  Occasionally if dipole antennas are exactly end to end a deep null of possibly 20 or more dB will occur.  If either of the above is the case, then the path might be more subject to fades.  NVIS propagation is a possibility, although this would be more likely on 80 or 160 meters. Another possibility is multipath reception with one signal being the direct signal, and the other being reflected from a large building or mountain perhaps many miles away.  If this is the case the fades could be explained by changes in atmospheric conditions (humidity, temperature, etc.).  The velocity of propagation of the reflected signal will change with time so it may arrive either in phase, or out of phase with the shorter path direct signal. 
     You could research propagation forecasts for the date and time of day and probably either eliminate NVIS or possibly retain that as a possible explanation.  If the fades occur often, or at various times of the day and night, you could probably eliminate NVIS propagation since the conventional wisdom seems to be that NVIS on 7 MHZ does not occur often, especially during our rather modest sunspot numbers during the current solar cycle.
      Perhaps other folks will post some more ideas. 
                Rick   KL7CW    Palmer, Alaska 
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K1CJS
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« Reply #2 on: May 07, 2013, 09:40:31 AM »

Could be simply that the antennas are pointing each in different directions, IE, his is E-W and yours is N_S.
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N7DMA
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Posts: 17




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« Reply #3 on: May 07, 2013, 09:02:48 PM »

Thanks for the replies.

Missing details are as follows:

Time was around 1930 mst. Original post frequency was 7.122 mHz. We repeated the experiment this past Monday, 5/6/13, same time. This time, we alternated between 7.120 and 10.120 mHz. Phenomenon was again observed on both frequencies, so I guess we can rule out NVIS.

Both antennas are oriented broadside to N-S, his being a dipole, in the clear at around 30 feet, and mine being a homebrew knock-off of the Ultimax 100 (24' radiator, fed with a 9:1 Un-Un), mounted in my attic at around 25'.

Local topography include the Santa Catalina mountains, max height around 9,000' asl are about 15 air miles north of me, add a few miles for Mike, south of my location. Tucson is in the valley, around 2900' asl, and we have the Rincon mountains about 10 miles east of me 9500' asl, the Santa Rita mountains, about 9500' asl about 40 miles south, and the Tucson Mountains, abt 2500' asl 15 miles west.

Same thing observed this week, but we bumped the power up to 30 watts after initial contact was established at 5W. Still noticed the fading, though not as pronounced at this power level (as to be expected).

Originally, I was leaning towards multi-path distortion due to NVIS, but I believe that can be discounted after additional experimentation. Now, I'm leaning more towards some sort of local phenomenon, not to discount atmospheric disturbances, due to changing humidity. It's been pretty dry here, but the humidity does increase after the sun sets. Then again, maybe our signals are bouncing off the mountains like a pinball, so other multi-path propagation isn't out of the question. A more probable solution.

Hmmmm... interesting. And more to ponder. This is the kind of thing that keeps me experimenting in radio (among other stuff!). I love a mystery! 38 years in the hobby, and still something new! The magic of radio!

Sorry to take up so much bandwidth with this, but it's fun to try to figure it out!

Any further thoughts are greatly appreciated!

73,
Karl
N7DMA
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GW3OQK
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Posts: 140




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« Reply #4 on: May 08, 2013, 03:23:13 AM »

Its interesting Karl. How about trying a separate vertical antenna for receiving and see if its the same.  Have you ever tried well after dark? And are any of your dipoles end-on to the other station?

When I was experiencing magnetosphere echoes on 80m I used http://www.spacew.com/www/fof2.html to see if it helped regarding critical frequency for NVIS.
73
Andrew
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AH6RR
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« Reply #5 on: May 09, 2013, 01:22:42 AM »

At that time of day or I should say night the 40M and 30M bands are getting longer so deep fades are normal. So what you are experiencing is some to very little NVIS if you were to try at say 1300-1500 hrs your time it would be all NVIS and very little QSB (fades). At 1900 there is very little D layer absorption so the signals are passing the D layer on their way to the F layer.
Here in Hawaii the 40M band is long around 1700 HST and local work can be challenging at best but working DX is no problem.
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NU1O
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« Reply #6 on: May 26, 2013, 01:04:01 PM »

At that time of day or I should say night the 40M and 30M bands are getting longer so deep fades are normal. So what you are experiencing is some to very little NVIS if you were to try at say 1300-1500 hrs your time it would be all NVIS and very little QSB (fades). At 1900 there is very little D layer absorption so the signals are passing the D layer on their way to the F layer.
Here in Hawaii the 40M band is long around 1700 HST and local work can be challenging at best but working DX is no problem.

I can see the band changing from short to long having an effect on skip propagation but why would the changing band effect a 4 mile ground wave signal?  Also, they observed the same thing on 30 meters.

N7DMA:  I think local phenomena is the most likely reason.  Either the mountains causing multi-path reception or perhaps the temperature changing as the sun goes down.  If possible try some other times.  Maybe early morning, midday, and well after sunset.


I spend a lot of time on 10 meters and try to work distant ground wave signals whenever the opportunity presents itself. I typically don't see fading on ground wave signals but when the signal traverses water there can be fading.  I recall working a station on Long Island and our signals had to travel across Long Island Sound and there was fading.  I have also observed this phenomenon at Cape Cod when receiving signals from across Cape Cod Bay.  Back before cable TV the fading could be observed in the VHF band while watching TV stations from Boston. I believe it usually occurred at night. I've never used an an HF rig from my Cape Cod QTH but I did bring a 2 meter HT on several trips and there was significant fading which I don't observe at my home QTH which is about 70 miles from Long Island Sound and about 90 miles from Boston.

If you collect more data please report your findings.

73,

Chris/NU1O
« Last Edit: May 26, 2013, 01:10:08 PM by NU1O » Logged
NO2A
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Posts: 769




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« Reply #7 on: June 03, 2013, 07:07:09 PM »

Hi Karl,what might be happening is both your signals are going straight up,and missing each other. Being just 4 miles away is really close for a 40m qso. If you could load your dipoles on 160 or 80m,I think this would work much better. I realize that may not be possible for you both,as physically short antennas can cause lots of rf problems at the shack. If you can`t use 2m,or 440mhz,maybe 6m would work.
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WX7G
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« Reply #8 on: June 05, 2013, 03:05:55 PM »

I think the fading is caused by NVIS and ground wave signals cancelling. A test of this is to repeat the test at 10 PM or later. If the fading is gone it was due to NVIS propagation.
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W7KB
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Posts: 57




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« Reply #9 on: July 13, 2013, 07:35:39 AM »

Have you two considered trying your cw practice on 6 meters rather than 40? ...Dennis W7KB.
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