Topband: skew path

[Bill Tippett]

W1FV wrote:
>From my experience in New England, the 80 and 40 meter "skew" path openings
around local sunrise do appear to be strongly correlated with the sunspot 
cycle.

        I agree...especially noticeable on 80 meters.

W1FV wrote:
>Last year, for example, I would estimate I worked 150 to 200 JA's on 80 
meters, and the vast majority of these were on the southwest skew path 
(the rest were short path at sunrise and a few were via the southeast long 
path at sunset). 

W8JI wrote:
>JA's on many occasions arrive here from southwest paths early in 
the openings, and while they generally have more QSB than on the 
direct path they certainly do not have flutter. It is common to see 
an opening via both paths, with different fading rates.

        Not surprising to me since the true bearing of JA from the East
Coast is around 330 degrees  It skirts the northern auroral zone and is
probably skewed south as a result.  You always have to look at paths as
function of geography relative to auroral zones from each QTH.  From
Colorado, JA is 315 degrees and from California it is about 300 degrees.  
That in combination with the fact that the path is farther from the 
northern auroral oval for western stations accounts for the direct path 
being the predominant mode from there in my opinion.

W8JI wrote:
>I've watched the aurora levels and A and K indices, and have a 
difficult time finding any correlation between solar activity and 
signals...let alone propagation in general on 160.

        I see very high correlation with the Kp index for geomagnetic
skewing and little correlation for the SSW/SSE long path.  Geomagnetic
skew is almost a given when the signal path skirts the edge of the
auroral zone.  From Colorado, Magnetic North is 13 degrees which means
the direct path to Europe (~40 degrees) had to go right through the 
edge of the auroral oval.  I found that my 70 degree Beverage was my
primary Beverage for working Europe most of the time.  Only when the
geomagnetic field was Kp 2 or less was the 40 degree Beverage best.
I was very surprised to see geomagnetic skew to Europe when I moved to
NC...but after looking at a DX-Aid plot, I saw that Kp of 5 results in 
the auroral oval dropping to about 50 degrees.  After seeing that, I 
was much less surprised by the geomagnetic skewing to EU from here.

W8JI wrote:
>A word of caution. It takes a fairly large array and a good signal to 
measure skews of 45 degrees or less with any reliability, unless 
you are using pattern nulls. With 350 feet of broadside antenna 
spacing, I can resolve about 20-30 degrees of shift without resorting 
to interferometer techniques. I really don't believe a single Beverage 
would be accurate at less than 45 degrees of skew when you 
consider the wide response angles. We have to be very careful, 
especially when signals are weak. We might be nulling the noise 
more than peaking the signal when we change directions!!!

        I agree for 160.  Unfortunately 160 long path signals are so
weak that they are usually audible on only one Beverage so it is difficult
to make any comparisons.  However, on 80 and 40 meters, long path signals 
are much stronger and stations using rotary antennas have confirmed the
directions.  I believe the 160 mode is identical to what is regularly seen 
on 80 and 40 but much more rare because signals are so much weaker due to 
higher absorption on 160.

        Here are some very simple rules for working the sunrise long path 
mode on 40-160:

1.  Look at your great circle map and identify the slice from ~30 degrees
to ~330 degrees.  Target stations with direct bearings within this slice 
should be workable on the sunrise long path.  I believe this to be true
independent of where you are located in the northern hemisphere...NA, EU 
or JA.  The OPTIMUM date will be at the time of year when 20-30 minutes 
after their sunset coincides with 20-30 minutes before your sunrise...but 
other dates are possible at reduced signal levels.  

2.  The predominant direction will be ~210 degrees independent of the 
target bearing...make sure you have a good RX antenna in this direction.
This direction has been verified by literally tens of thousands of QSO's
over a 40 year period on 40/80 meters by stations using rotary antennas.

3.  Long path propagation IS influenced by:
        
        a. stage of the sunspot cycle (high flux is definitely better)
        b. season (roughly Fall Equinox through Spring Equinox).
        c. time of day (before sunrise...how much before is a function of
the specific band...generally the 80/160 peak is about 20-30 minutes 
prior to local sunrise)

4.  Long path propagation IS NOT strongly influenced by the state of the
geomagnetic field.

5.  Be patient and persistent on 160...but be prepared.  I've made only 4
sunrise long path QSO's in 17 years of operating on 160 but each one of
those was a new country/zone.  80 meter LP is much easier with many more
QSO's in over 50 countries.  40 meter LP is extremely common...during
contests it is probably easier for West Coast stations to work Eastern EU
and Asia (excluding Far East) via LP than via short path before EU sunrise...
at least that was true for me from Colorado.  

6.  The sunset long path follows similar rules but in reverse...i.e. the
peak is 20-30 minutes after sunset and the predominant direction is 150 
degrees.  I've only made 2 of these QSO's on 160 from NC and zero from 
Colorado....again it is much more common on 80 and 40.  I recall XZ0A 
making quite a few 80M QSO's with NA on this path but only 2 on 160M.

        We can debate ad infinitum the path the signals actually follow 
around the world...it is an academic discussion that really doesn't
matter to me.  I frankly doubt anyone can conclusively answer that question
until they do time-synchronized soundings to determine signal delays and 
approximate the distance traversed.  What really matters to me is that I 
know when to listen for these signals and in which direction to have a 
Beverage pointed for those very rare cases when they occur on Topband.

                                                73,  Bill  W4ZV

P.S.  Compare these characteristics which I feel distinguish geomagnetic 
skewing from what I called long path above.  They are quite different.

1.  Target areas for geomagnetic skewing are generally in directions which
skirt the auroral zones.  From NA, JA will be skewed south, VK/ZL will be
skewed north, EU will be skewed south and stations in the south Atlantic
or south Indian Ocean will be skewed north (like VK0IR was).

2.  The amount of skew is a function of geomagnetic disturbance level as
indicated by Kp (forget Ap since it is yesterday's average of Kp and not 
a realtime measure).  For Kp of 0-2 little skew will be noted, for Kp 3-5
40-50 degrees is not unusual and for Kp 6-8 it can be as much as 90 degrees
in the directions indicated above.

3.  Geomagnetic skew can occur at any time of day, any season and any part
of the solar cycle.  I believe this is because it is related to geomagnetic
disturbance levels and not daylight/darkness relationships governed by time 
of day or season.

4.  Geomagnetic skew is generally an impediment to working new areas of the
world.  You will work the same areas you normally would via direct path
but it may be more difficult.  This is in contrast to long path which allows 
you to work areas in the northern hemisphere that would otherwise be very 
difficult if not impossible to work because of their location on the other
side of the northern auroral zone.  By the time I left Colorado, my 80M 
DXCC totals were well over 300.  In looking over my list of long path 
countries worked (total ~50), I estimate that ~25 would not have been 
possible except via the long path mode (primarily Middle East, Central Asia 
and SE Asia) because direct path bearings required traversing the northern
auroral oval which was extremely difficult from Colorado on 80M.  On 160,
I would not have 3W5, 4S7, S21, or XZ0 if not for the long path mode.