The 'critical frequency' is the highest frequency that gets reflected
when it is aimed straight up at the ionosphere. So it is a measure of
the electron density above a particular location. It is not dependent
on the path as it is a straight up and down value that quantifies the
state of the ionization above that location. This is a first step in
predicting MUF or MOF(the Maximum-Observed-Frequency which I prefer to
use).
The next important piece is the knowledge that as the angle decreases
from vertical the reflected frequency increases. So at 90 deg maybe the
critical frequency is 8mhz, but when a signal 'reflects' off the same
part of the ionosphere at a 45 degree angle the highest frequency that
comes back down may be 11mhz. a common approximation is 1/cos or the
secant of the angle times the critical frequency.... often times an
extra fudge factor that varies from 1 to about 1.2, but it gets in the
ballpark anyway.
MUF or MOF is a path dependent value. It depends not only on the state
of the ionosphere but also the path between the end points. To get from
critical frequency to MUF you need to calculate the angle of incidence
and apply that to the critical frequency as described above... repeat
that at each reflection point and pick the lowest of the reflected
frequencies and that is the MOF... the maximum frequency that may just
barely get reflected. Apply fudge factors again and get the lower
frequency called MUF that traffic handlers basically defined to be a
safe frequency to get good strong signals.
Since MUF or MOF is path dependent you can not make a map of it directly
unless you anchor one end of the path. This is what my MOF/LOF
prediction program does when making a world map.
http://www.k1ttt.net/software.html#mofwin or to use it on-line:
http://www.k1ttt.net/mofjava/mofjava_new.html
What you may see for a MUF map is what is often called M3000, or some
similar value which is the MUF calculated using a single 3000km hop with
that point as the center. So you would basically draw a 1500km radius
circle around the point and that would be the frequency that any
locations on opposite sides of the circle could use. Divide the path up
into 3000km hops and pick the lowest MUF of all the reflection points
and that would be the MUF for a longer path... it gets much messier than
that because the height of the ionosphere affects the angle and hop
length, the ionosphere can be tilted, and its not an exact science by
any means yet, so at best it gives you something to look for... beacons
and oblique sounders give more real time information.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: propagation-bounces@contesting.com [mailto:propagation-
> bounces@contesting.com] On Behalf Of Kuhns, Arthur G CA
> Sent: Wednesday, October 06, 2004 19:48
> To: 'NW7US, Tomas'; propagation
> Subject: RE: [Propagation] Critical vs. Maximum Usable Frequency map
>
> Hello group, First time posting.
> I am looking at these maps and reading the instructions and I am still
in
> the dark as to how to use them.
> I will tell you first what I believe is the way to use them and then
you
> can
> tell me if I am correct.
> I am looking at the map 'Near Real Time F2 Critical Freq Map. I see
these
> numbered contours. The numbers represent the max usable freq?
> Second the contour passing over Calif is numbered 08. So the max
usable
> freq is 8mhz?
> Now having that knowledge. How would I use such a map to determine
best
> propagation to say the middle east or Europe?
> Art KK6PJ
>
> Arthur Kuhns
> Calif. Army National Guard
> Repairs and Utilities Supply
> Camp Robert, CA. 93451
> Commercial Phone: 805-238-8880
> Email: arthur.kuhns@ca.ngb.army.mil
>
>
> -----Original Message-----
> From: NW7US, Tomas [mailto:nw7us@hfradio.org]
> Sent: Sunday, September 26, 2004 12:23 PM
> To: propagation
> Subject: [Propagation] Critical vs. Maximum Usable Frequency map
>
>
> On Sun, 26 Sep 2004 06:09:16 -0000, David Ackrill
> <dave.g0dja@tiscali.co.uk> wrote:
>
> > If you want the local F2 Critical frequency (the one where signals
go
> > almost straight up and down, try looking at this map.
> > http://www.spacew.com/www/fof2.html
>
> Excellent map for this.
>
> Also of interest are the "live" (using real measurements) HAP maps
from
> IPS:
>
> North America HAP maps:
>
http://www.ips.gov.au/Main.php?CatID=6&SecID=4&SecName=North%20America&S
ub
> Se
> cID=1&SubSecName=HAP%20Charts
>
> North America Ionospheric map:
>
http://www.ips.gov.au/Main.php?CatID=6&SecID=4&SecName=North%20America&S
ub
> Se
> cID=3&SubSecName=Ionospheric%20Map
>
> The best jump off point for IPS HF-related data is:
> http://www.ips.gov.au/Main.php?CatID=6
>
> These HAP maps are "Hourly Area Predictions" of communication
frequencies
> based on current observed conditions in the North America ionosphere.
> Because of the variability of the ionosphere this type of prediction
can
> only be made for the current hour.
>
> For paths that are more typical of "DX" (3000 km paths), there is also
> this map:
>
> http://www.spacew.com/www/realtime.html
>
> 73 de Tomas, NW7US (AARØJA/AAAØWA)
>
> : Propagation Editor for CQ, CQ VHF, and Popular Communications :
> : Quarterly Propagation Columnist for Monitoring Times Magazine :
> : Creator of live propagation center - http://prop.hfradio.org/ :
> : Member, US Army Miltary Affiliate Radio Service (MARS) AARØJA :
> : 122.93W 47.67N / Brinnon, Washington USA - CN87 - CW/SSB/DIGI :
> : Website, software, database design - http://newwebmakers.com/ :
> : Washington State Army MARS, State Army MARS Director - AAAØWA :
> : 1Øx56526, FISTS 7Ø55, FISTS NW 57, AR Lighthouse Society 144 :
> _______________________________________________
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