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Author Topic: Short skip on 7 mHz?  (Read 567 times)
KB1RDL
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« on: June 08, 2009, 08:30:13 PM »

I noticed some unusual propagation on 40 meters tonight, very short skip.

Seems that the skip zone was less than 100 miles.  My QTH is in Hartford CT and tonight around 2000-2100 local time I could work RI (3905 Century Club) and hear MA (didn't get a chance to work them, but they were coming in 59+60 dB for a while) all within 60-70 miles.  Then some time after 2100 EST they were all gone and it was "back to usual".  This was my first time observing this phenomenon.

Normally I don't hear stations closer than 300 miles on 40 meters.  My antenna is an EFHWA (wire) which I wouldn't normally describe as NVIS.  I've used it to work Europe.

Does Es happen on 7 mhz?  Just curious.
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N4JTE
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« Reply #1 on: June 08, 2009, 08:45:12 PM »

First of all welcome to 40 meters, every night is an adventure, prop is differant every night. but what the hell is a EFHWA antenna??
Bob
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N6NKN
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« Reply #2 on: June 08, 2009, 08:49:28 PM »

It has happened several times here in the west coast in the past few weeks. Stations in our 40 meter round table group that are usually in the noise were S9+. Lasts for a few minutes to about an hour. It's a real treat when it happens.

Rick N6NKN
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G1YHE
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« Reply #3 on: June 08, 2009, 10:11:59 PM »

EFHWA = End Fed Half Wave Antenna, perhaps.
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KB1RDL
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« Reply #4 on: June 09, 2009, 03:28:49 AM »

EFHWA - End Fed Half Wave Antenna.

http://www.aa5tb.com/efha.html
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WA3SKN
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« Reply #5 on: June 09, 2009, 04:28:19 AM »

As you transition locally between day and nightime conditions in the early evening, strange and wonderous things happen on all the bands... especially 40 meters!
Quit trying to determine if it E-skip or F2, and operate!  Take advantage of the situation!
Predictions are predictions, actual operations may vary.
73s.

-Mike.
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WB4TJH
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« Reply #6 on: June 09, 2009, 04:43:05 AM »

Short skip on 40 meters is quite common this time of year in the late afternoon. I live in Sarasota, Florida, and lately have had 40 meter roundtables with hams in St. Petersburg, 50 miles north, Tampa, next to St. Pete, Ft. Myers, 80 miles south, and several other cities, all from 40 to 120 miles or so from me. Most of the signals have been very strong. The conditions have lasted maybe an hour, and then the signals fade away like smoke in the wind.
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KB1LKR
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« Reply #7 on: June 09, 2009, 07:22:02 AM »

"but what the hell is a EFHWA antenna??"

End Fed Half WAve [wire] antenna?

How high is the antenna over the ground and is it horizontal, sloped, or other? It may have some high angle lobes if it's not real high (say << 20m/65ft) which combined w/ odd propagation conditions, may be getting you the short hops.
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N3OX
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« Reply #8 on: June 09, 2009, 07:51:27 AM »

"Does Es happen on 7 mhz? Just curious. "

Sure.  

Last night around sunset and a bit later here 6m was open with very loud signals to VE9/VE1/VY2 ... so there certainly was some serious Es happening.  The cloud, if it was a single one, appeared to drift southeast and eventually the Maritimes disappeared and Midwest at slightly longer range came in a bit later.

If you got on the air when a strong, area of E layer ionization was right above your station, I think it could provide some very short skip on 7MHz, and all in all the timing was right.  

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB2WIK
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« Reply #9 on: June 09, 2009, 11:07:32 AM »

I've never heard of Es at 7 MHz.

I've never even heard of it at 14 MHz.

Does anyone know more about this?

All the experimental data, stuff Arecibo did, etc, was in the VHF spectrum between 30 and 300 MHz, I can't find anything documented for lower frequencies...

Would be interesting to know more.
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N3OX
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« Reply #10 on: June 09, 2009, 11:36:19 AM »

"I've never heard of Es at 7 MHz.

I've never even heard of it at 14 MHz. "

Yeah, but I think it works over all HF.

Here's an ionogram of what happens when (I think pretty strong) Es is going on:

http://www.qsl.net/wa5iyx/ionogram/82852330.gif

Compared to the latest one (unfortunately several days ago, maybe last night will come out later) out of Massachusetts:

http://digisonde.haystack.edu/latestFrames.htm
 
In the Es one, I think the strong echo at 100km and near multiples is from the E layer and re-reflections (I guess off the earth and back ? )

If so, it seems like E layer echoes during sporadic E happen all the way down to the lowest frequencies they measure.

If you're not sitting right under the cloud, though, you might not notice anything odd.  I would have had 59+20dB sigs out of W0's when the cloud was off that way anyway.  It's just that burst of vertical incidence skip that might be noticed as the cloud goes right overhead.

I don't think Es gets any less localized on the lower HF bands than it is on the high ones, so I suppose you got a little focused enhancement in an otherwise wide open band via F2 if you were on last night.

Not quite as dramatic as 6m being full of 59+20dB VY2's and VE1's
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K8GU
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« Reply #11 on: June 11, 2009, 05:45:22 AM »

My money is on Es, too, if it's a major departure from the day-to-day behavior you've observed recently.  A lot of the short skip on 7 MHz and up in the afternoon/evening during summer in low sunspot years is Es.

A few words about frequency:  

In the "bulk" sense, we get ionospheric reflection whenever the refractive index goes to zero.  To normal approximation, this gives the familiar "below the critical frequency" for vertically-propagating waves or MUF for obliquely-propagating waves.  There is a lower cut-off to this behavior, as well, that is dictated by the collision and gyro frequencies---beyond the scope of what I'm talking about here.  This is part of what makes 160 (and to a lesser extent, 80) meters interesting.  Look here for more info:

http://en.wikipedia.org/wiki/Appleton-Hartree_equation

The E-region echoes in the ionogram that Dan posted actually "shadow" the F-region echoes up to 6 MHz or so, making them the dominant propagation mode for those frequencies with reflection points over the ionosonde.  Ionosondes have pretty crude spatial resolution (think about the difficulty in producing a narrow-beamwidth, broadband HF antenna for a reasonable cost).  So, this ionogram is probably a valid assessment of what's happening within a few hundred km of Millstone Hill.

The large ionospheric radars at Millstone Hill, Arecibo, Shigaraki, Kototabang, and Jicamarca (I only mention the low- and mid-latitude stations because those are the ones I'm familiar with), were designed to perform "incoherent scatter" measurements of electron and ion densities and temperatures.  VHF is sufficiently above the plasma frequency, affords high gain with reasonably-sized antennas (when you have 18,432 elements, this matters) and (at the time of construction) an optimum balance between high-power transmitting components and low-noise receiving components.

It turns out that many ionospheric irregularities, and certain forms of Es are such an example, exhibit irregularities on smaller scales.  That is, an Es "cloud" is not just a sheet of ions floating in the sky.  It is a turbulent medium full of small scale waves, which create strong radar (Bragg) backscatter at VHF, as well.

Anyhow, this is a far departure from the original topic, but since Steve asked, I had to comment.
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KB1RDL
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« Reply #12 on: June 12, 2009, 08:56:33 AM »

The antenna is a sloper, about 45 ft off the ground at its highest end and about 20 at the lowest.

But normally I do not hear or experience short skip like that.  The other station was coming in 59 as well.
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N5LRZ
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« Reply #13 on: June 12, 2009, 10:16:56 AM »

Buy or borrow an ARRL Antenna Book.  Any fairly new issue will do.  They can be gotten for a very reasonable price at Hamfests.

In it you will find a nice explanation of propogation and how the D, E, and F layers of the atmosphere as they become ionized change propagation distances.
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