Critical Frequency and MUF
 

I thought I understood about critical frequencies and MUFs and such, but
an event this evening has me puzzled.

I was on a military net on a frequency somewhat above the 75-meter band.
According to the "Near Real Time F2-Layer Critical Frequency Map" at
http://solar.spacew.com/www/fof2.html, the critical frequency in this area
at that time was below 3 MHz, which would have put it WELL below our
frequency of operation. Yet, I was able to communicate with another
station about 70 miles away, with weak-but-clear (Q5 copy) both ways. And
I'm only running 5 watts (he was running 100 watts I believe).

70 miles seems a bit far for groundwave, and the critical frequency seemed
much too low for NVIS, so how was I able to communicate reliably with this
other station?

My antenna is a half wave dipole for the frequency in use, at elevation 17
feet above ground. My ground elevation is about 250 feet above sea level.

Would that big solar storm of the last day or so, the one that made all
the news about how it might knock out satellites and power grids and such,
have anything to do with it? I would have thought that if anything, that
would have hurt not helped.

Seems very strange...

Critical Frequency and MUF
 

"C. J. Clegg" wrote in message
...

I thought I understood about critical frequencies and MUFs and such, but
an event this evening has me puzzled.

I was on a military net on a frequency somewhat above the 75-meter band.
According to the "Near Real Time F2-Layer Critical Frequency Map" at
http://solar.spacew.com/www/fof2.html, the critical frequency in this area
at that time was below 3 MHz, which would have put it WELL below our
frequency of operation. Yet, I was able to communicate with another
station about 70 miles away, with weak-but-clear (Q5 copy) both ways. And
I'm only running 5 watts (he was running 100 watts I believe).

70 miles seems a bit far for groundwave, and the critical frequency seemed
much too low for NVIS, so how was I able to communicate reliably with this
other station?

My antenna is a half wave dipole for the frequency in use, at elevation 17
feet above ground. My ground elevation is about 250 feet above sea level.

Would that big solar storm of the last day or so, the one that made all
the news about how it might knock out satellites and power grids and such,
have anything to do with it? I would have thought that if anything, that
would have hurt not helped.

Seems very strange...


i would not call that far for ground wave if the band is reasonably quiet.
but also the very disturbed conditions could have greatly enhanced the muf
locally, there is a great auroral opening on 6m now and also some sporadic e
enhancements reported due to the storm, so it could have been some of that
helping things along.

Critical Frequency and MUF
 

On Thu, 14 Dec 2006 19:41:33 -0500, "C. J. Clegg"
wrote:


I thought I understood about critical frequencies and MUFs and such, but
an event this evening has me puzzled.

I was on a military net on a frequency somewhat above the 75-meter band.
According to the "Near Real Time F2-Layer Critical Frequency Map" at
http://solar.spacew.com/www/fof2.html, the critical frequency in this area
at that time was below 3 MHz, which would have put it WELL below our
frequency of operation. Yet, I was able to communicate with another
station about 70 miles away, with weak-but-clear (Q5 copy) both ways. And
I'm only running 5 watts (he was running 100 watts I believe).

70 miles seems a bit far for groundwave, and the critical frequency seemed
much too low for NVIS, so how was I able to communicate reliably with this
other station?


Do you know what f0F2 was at the path midpoint, or are you depending
on interpolation?

In this neck of the woods, there is abundant sporadic E, and of such
intensity that ionsondes are showing two, three and four
"reflections". How do they measure f0F2 in the presense of long
running high intensity Es that prevent rays passing through to the F2
layer?

Owen
--

Critical Frequency and MUF
 

On Fri, 15 Dec 2006 02:25:42 +0000, Owen Duffy wrote:

Do you know what f0F2 was at the path midpoint, or are you depending
on interpolation?

Well, the path midpoint was pretty much straight up, or maybe up to 20
degrees off of vertical. foF2 all along the path was just a bit under 3
MHz. Frequency of operation was well above 4 MHz.

In this neck of the woods, there is abundant sporadic E,

I don't really know much about sporadic E, have a vague idea what it is
and that's about all, but doesn't it assume that there is an E layer to
begin with? This was about 7:30 PM, sunset was around 4:45 PM, and the E
layer would have been pretty much gone along with the sun, wouldn't it?

Critical Frequency and MUF
 

On Fri, 15 Dec 2006 01:04:42 +0000, Dave wrote:

there is a great auroral opening on 6m now and also some sporadic e
enhancements reported due to the storm, so it could have been some of that
helping things along.

As I mentioned to Owen, I don't know a lot about sporadic E, but I assumed
that it required that there be an E layer to begin with, and wouldn't the
E layer be pretty much gone along with the sun? (this was 7:30 PM.)

Anyway, doesn't sporadic E mainly help higher frequency communications,
like 10 or 6 meters or above? This was somewhat above 4 MHz, above 75
meters, considerably below what I would have thought would have been
affected by sporadic E. But, like I said, I don't know very much about
sporadic E.

Critical Frequency and MUF
 

On Thu, 14 Dec 2006 23:52:22 -0500, "C. J. Clegg"
wrote:

On Fri, 15 Dec 2006 02:25:42 +0000, Owen Duffy wrote:

Do you know what f0F2 was at the path midpoint, or are you depending
on interpolation?


You missed my point, it isn't so much as the exact location of the
virtual midpoint, but is there an ionosound measureing that point, or
is the map constructed by interpolating measurements from just a small
number of ionsondes, and is the interpolation sound?

Well, the path midpoint was pretty much straight up, or maybe up to 20
degrees off of vertical. foF2 all along the path was just a bit under 3
MHz. Frequency of operation was well above 4 MHz.

In this neck of the woods, there is abundant sporadic E,

I should have added "at this time". But sporadic E is most likely
around the solstices, so you should be seeing a bit in the northern
hemisphere.


I don't really know much about sporadic E, have a vague idea what it is
and that's about all, but doesn't it assume that there is an E layer to
begin with? This was about 7:30 PM, sunset was around 4:45 PM, and the E
layer would have been pretty much gone along with the sun, wouldn't it?

Sporadic E does not depend on sunlight, it is one of the mysteries of
Es.

Owen
--

Critical Frequency and MUF
 

On Fri, 15 Dec 2006 08:09:45 +0000, Owen Duffy wrote:

You missed my point, it isn't so much as the exact location of the
virtual midpoint, but is there an ionosound measureing that point, or
is the map constructed by interpolating measurements from just a small
number of ionsondes, and is the interpolation sound?

Oh, OK, I get it.

I have no idea. I use the map at http://solar.spacew.com/www/fof2.html
and I don't know how they get the information they use to build that map.

Sporadic E does not depend on sunlight, it is one of the mysteries of
Es.

Doesn't sporadic E normally affect the higher frequencies, 15 meters and
above, rather than the lower frequencies around 75 meters?

Critical Frequency and MUF
 

On Fri, 15 Dec 2006 09:08:09 -0500, "C. J. Clegg"
wrote:

On Fri, 15 Dec 2006 08:09:45 +0000, Owen Duffy wrote:

You missed my point, it isn't so much as the exact location of the
virtual midpoint, but is there an ionosound measureing that point, or
is the map constructed by interpolating measurements from just a small
number of ionsondes, and is the interpolation sound?

Oh, OK, I get it.

I have no idea. I use the map at http://solar.spacew.com/www/fof2.html
and I don't know how they get the information they use to build that map.

Sporadic E does not depend on sunlight, it is one of the mysteries of
Es.

Doesn't sporadic E normally affect the higher frequencies, 15 meters and
above, rather than the lower frequencies around 75 meters?

Here is an ionogram from the station in Canberra this morning about
1.5 hours before sunrise.

http://www.vk1od.net/lost/ionogram.gif

Note the E layer at 100km height, very well established. It is so
intense, that there are multiple E - earth - E - earth "reflections".
You can see the F layer above it, and f0F2 looks between 2 and 3MHz.

(I believe that the existence of such a strong E layer before sunrise,
and the height in the 90km to 100km range characterise sporadic E.)

Sometimes the E layer reflection is so intense, that there isn't a
hint of the F layer. I don't know how they assess f0F2 under those
conditions, but they still produce the statistics.

I think you may be reading more accuracy into the maps than is safe.

As for "normally affecting higher frequencies", here you can see the
ionsonde discovering the Es layer at up to about 10MHz. When you take
into account oblique paths of a couple of thousand km or more, the Es
here with critical frequency up to about 10MHz should reflect 50MHz
ok. Now exploitation of that depends on such a patch of Es in the
right place, angled the right way, between two stations attempting
communication. There have been some big openings on 6m here in the
last week or so, not huge, just the seasonal openings typical around
the solstices.

Owen
--

Critical Frequency and MUF
 

Owen is correct- and anyone operateing 10& 6
meters will tell you the same thing! Have
been on 6 for around 45 years now, an can attest
that "E" openings can occur ANYTIME, day, or nite!
And, the "E" layer is ionized , to a degree, 24
hours per day (There are more than a few running
"Scatter", with power levels of 100-1KW makeing
contacts, when the band(s) are literally closed ).
Es is the "bread and butter" of the VHF operator,
rangeing from 35-400 miles for super intense
openings, out to 1200, or so miles, when the band
is barely open. Figure stations (on scatter), from
Montana, New Mexico, Colorado, and Wyoming being
copied with NO major opening, into Reno, Nevada!
And, keep in mind, you also get Double hop, 2
adjacent (or more) layers , giveing you cross
country capabilities. One further thing: IF
you shea short E skip on a band (say 10 meters),
odds are great that 6 is open; Same for 6 meter
short skip, often means that 2 meters, or at least
F.M.Broadcasts (88-108 MHz) will be open!. F2
is associated with sunspots-Es seems to get worse
as sunspot activity increases!Might you find this
interesting? Jim NN7K

Owen Duffy wrote:

I don't really know much about sporadic E, have a vague idea what it is
and that's about all, but doesn't it assume that there is an E layer to
begin with? This was about 7:30 PM, sunset was around 4:45 PM, and the E
layer would have been pretty much gone along with the sun, wouldn't it?

Sporadic E does not depend on sunlight, it is one of the mysteries of
Es.

Owen
--

Critical Frequency and MUF
 

Jim - NN7K wrote:
Couple errors (I ain't a professional typeist)!


rangeing from 35-400 miles for super intense

Make that 350-400 miles!

One further thing: IF
you shea short E skip on a band (say 10 meters),
odds are great that 6 is open; Make that


"HEAR", not "shea"! Jim