by Pat Dyer
5315 Silvertip Drive
San Antonio TX 78228
As the number of WTFDA members interested in the 30 to 50 MHz band has grown tremendously in
recent months, and as this frequency range is the only one in the club's sphere of interest
that encounters any great amount of F2 DX, some explanation of that layer and its behavior is
felt in order.
As far as VHF is concerned the most striking trait of F2 skip is that it is a daytime phen-
menon (though sometimes it may peak in late afternoon or early evening with a magnetic storm
onset). The difference in day and night ionograms show this best in the figure below.
At night the F1 and F2 layers effectively merge. Unlike the other lower layers, it does not
almost completely disappear. This is due to the comparatively low particle density at F2
height (on the order of one ten-billionth to one-trillionth that of sea level) with the
fewer free electron collisions slowing recombination.
The principal ionic constituents of the F region are O+ and N+ (singly ionized atomic oxygen
and nitrogen). The molecular ions (e.g., O2+ in the E region) have been further acted on by
solar radiation, with the 910-980 Ångstrom (Lyman continuum) and 350-200 Ångstrom ranges the
most responsible. Temperatures of 1400-1500° K are found, but this isn't much in terms of
calories with so few particles involved.
Unlike Es, the F2 layer shows a wide variety of heights (250-400 km) that depend on time of
day, season, solar level, and geomagnetic latitude. Thus, angles of incidence (and M factors
from the Secant Law) are not as simply determined. In addition, signal reflection (more pro-
perly "refraction") takes place over depths of tens of kilometers, compared to the 1 km or so
depth found in Es.
All else being equal, DX signals via F-layer propagation will normally be stronger than Es
ones, since the neutral particle density is much lower (i.e., fewer electron collisions and
thus less absorption). This is easily attested to by those who have heard a 35-MHz pager by
each mode (e.g., from here KMD 342, Fresno CA).
Solar activity control of the F2 layer over a sunspot cycle is such that the typical maximum
electron densities may vary from a low of some 500,000 per cc to a value 2 or 3 times that
(or, as is the case with the very large Cycle 19, a factor of 6 or 7 times it). These higher
densities occur at progressively higher altitudes. The limit for F2 MUF's (not counting TE
or transequatorial scatter, to be covered later) have ranged up in the 70 MHz region. The
Japan-Okinawa region is well noted for such high levels due to a proper combination of geo-
magnetic and geographic location.
Geomagnetic control of F2 is very pronounced, especially near the geomagnetic equator where
at some distance on each side "bulges" build up amounting to tilts; see the figure below.
This situation produces unexpectedly high MUF's (due to the very low angle of incidence) and
the F2F2 mode (double-hop without any intermediate ground reflection and absorption losses).
This path may be somewhat critical as either too much or too little ionization at either end
will cause an overshoot or undershoot of the other bulge.
Seasonal variations of VHF F2 are very pronounced. In the mid-northern hemisphere the season
(in high solar level years) may run from August well through May when MUF's of over 30 MHz
are possible. In leaner years, this is shortened to September through April. The U.S. F2
season (i.e., over 30 MHz possible on east-west U.S. paths) is somewhat shorter, by about a
month on each end. In the period overlapping with the Es season(s), it is very likely that
linkage with F2 openings will extend them to many unexpected areas (see February 1973 VUD).
Geomagnetic storm conditions affect the F2 layer in two main ways: (1) If storm onset occurs
in the afternoon period, MUF's often will be driven to values many percent above normal for
the time of day, year, etc. (2) The "normal" MUF's the following day will likely be lowered
F2 IN DEPTH (continued) May 1977
by up to 50% or more (this may take several days to recover). Each storm does have its own
details. Often in the recovery stage the north-south MUF's will build again to unexpectedly
high levels for a time. The disturbed geomagnetic field acts in such a way as to lessen the
electron densities in some places while compressing the existing Ne's (thus MUF's) in others.
(Geomagnetic storms, are the result of the interaction of streams of ion particles from the
active solar regions with the earth's magnetic field.) In low solar active years, these rare
storms may be the only chance for any VHF F2 DX openings (e.g., late October and around the
Thanksgiving holiday season in 1973) as the MUF's are pushed up. This has been the trend for
the past few years.
National Bureau of Standards radio station WWV (on 2.5, 5, 10, and 15 MHz) provides geophysi-
cal alerts, given by voice at 18 minutes after the hour (revised daily at 0400 GMT or when-
ever sudden condition changes warrant a modification). These give the solar-terrestrial
conditions for the previous day (i.e., solar flux, magnetic A-Indez, solar activity such as
major flares, and magnetic storms, if any) and a forecast of the expected solar activity for
the coming day along with magnetic field expectations (storms, etc.) prior to July 1971 this
was given once an hour in a rather cumbersome code (see Bob Cooper's article in Mar '68 QST).
Though these forecasts are primarily aimed at the h.f. users (e.g., shortwave broadcasts, any
point-to-point telephone circuits, etc.), the VHFer can take notice of unsettled or disturbed
condition announcements and act accordingly (look for F2 or aurora, depending where you are).
However, many a good unexpected F2 opening in 30-50 MHz has been the result of a WWV-classed
"minor" storm. For additional information on the WWV services, write to, National Bureau of
Standards, Time and Frequency Division, Boulder CO 80302.
The WWV signals themselves can be of use to VHF DXers, though much has been lost in that res-
pect with the silencing of its 20 and 25 MHz outlets in February. For instance, the second
harmonic of the 25 MHz transmitter must have radiated several milliwatts as it was often
heard here on Es at 50 MHz (providing a good beacon and calibration spot). Depending on
your distance from Fort Collins (the antenna site), good estimates of the Es and F2 MUF's
could be made by noting the 15, 20, and 25 MHz outlets. By theory at least, from here,
WWV-25 on F2 implied a transcontinental MUF of 50 MHz. At night, after all F2 has gone,
the remaining 15 MHz signal provides an excellent Es beacon for that area.
Closely related to F2 is transequatorial scatter (TE). This was discovered by radio amateurs
in the late 1940's (Cycle 18) and was intensively studied as part of the IGY (Cycle 19).
It was found that TE is associated with the ionogram phenomenon known as "spread F," wherein
the echo is spread out in duration to several times the length of the original pulse. This
implies some sort of scattering by irregularities in the F region. These irregularities form
at or after sundown near the geomagnetic equator. On oblique paths, the most pronounced ef-
fect on the signal is the rapid flutter (10 Hz or more, but not as bad as auroral flutter).
It has been found that TE may well affect frequencies as high as 1.5 times the daytime F2
peak MUF in the region. During sunspot peaks TE may last at 50 MHz for a considerable time
past local midnight.
The best paths are those that cross the geomagnetic equator at almost right angles (e.g.,
Central America to South America, Japan to Australia, etc.). The beet time of year is near
the equinoxes (March and September). With the March-April period there is often enough Es
around to permit paths to link up from the southern U.S. to an already ongoing TE event in
However, North America is likely one of the poorest places for TE, as the geomagnetic equator
dips as far south over South America (well under the geographic equator) compared to other lo-
cales (geomagnetic equator north of geographic from about Hawaii to North Africa).
Unlike normal F2 mode, TE may not exhibit obvious effects on the lower frequencies; for
example, 50 MHz "open" when 35 MHz apparently is not. Thus there is not a simple, reliable
way to see a TE "MUF" (although Bob Cooper found that the flutter fading in the Virgin Isles
was likely due to a very rapid swing in the MUF of the path).
It has been suggested that one of the reasons for a lack of low band VHF TV channels in sou-
thern South America was because of the mutual CCI that would occur with the Venezuelan,, etc.
with the almost nightly TE there many months of the year.
It is hoped that this article has been (and will be) of use to those DXers only experienced
in the Es and trop propagation modes. As always, direct comments are welcomed.
This was a composite accumulation of several "features" that I had run in
my monthly VUD VHF Utility DX Columns.
Page last modified November 3, 1999