Ever since I wrote the article "Propagation of Long Radio Waves" originally
published in Amateur Radio magazine and now published by the Long Wave Club
of America I have intended publishing a similar article on Medium Wave
Propagation mainly applying to the 160 metre amateur band namely 1.8 to 2.0
Mhz and particularly applying to long distance propagation. Propagation in
this frequency range is nothing like as clear cut as with low frequency or
high frequency propagation.
In response to an e-mail from Walter Schulz VQ9TD/K3OQF/T5 I have set out
as much as I know on the subject with some speculation thrown in.
Propagation diagrams in this article have been added since the original
correspondence. For clarity the diagrams are exaggerated in height by a
factor of four.
Subject: Propagation
Date: Fri, 14 Nov 2003 16:23:23 -0500
Dear Mr. Adcock,
I have a copy off your paper describing Long Wave Radio Wave Propagation.
Have your written anything else on propagation subject pertaining to VLF to
3 Mhz range. I am interested in the propagation on 160 meters. As you know
the American Broadcast Band (MW) to 3 Mhz. is where a transition in
propagation occurs. I am looking for more information about propagation in
this region of the spectrum. Thank you for your valuable time. I hope to
hear from you soon.
Regards, Walter Schulz VQ9TD/K3OQF/T5
Thanks for your letter Walter and thanks for your interest, I am preparing
a rather lengthy reply. I think this will help you to put your thoughts in
order on these subjects. I hope you also read my supplement to the article.
73 John
Subject: Reply
Date: Mon, 17 Nov 2003 23:37:47 -0500
Hello John,
Thank you for the reply and the very interesting information. I will look
forward to any other information you may provide.
During the 1963/64 was stationed on Guam, M.I. at the Naval Communications
Station NPN and used to talk with Australia a lot on 20 meter USB at the
club station KG6AAY. Fond memories for sure. My next contact with your
country men were in Mogadishu, Somalia. HMAS Tobruk was there the same time
my tanker American Osprey was there. My ship was apart of preposition
squadron stationed at VQ9 land.
Back to the reason for writing you. Here is the back ground, I was
thinking that many of my friends at experimented with Beverage Antennas.
They notice the F/B ratio's were lacking at night. Measuring them during
the day on the MW 500 to 1700 Kcs. the F/B was pretty good.
Many radio amateurs over here use Beverage antennas for 160 meter
receiving. Many of them seem to think propagation conditions are the same
on 1.8 Mcs. as 200 Kcs. Nobody addresses the difference in propagation
characteristics. I simply do not believe what the current literature
describes about Beverage antennas work on 160 Meters, I do not think the
reception patterns are correct nor any of the F/B, gain figures, etc are
correct due to propagation. As you know at this frequency 1.8 Mcs skywave
becomes dominant. Now you have the background.
Hope this information gives you some idea what I am about in my tasks at
the moment.
73s Walter
The Main Reply
The short answer is no to your first question but to answer the question
in any detail I would need to write another paper or perhaps a book! I ask
many questions about things that don't completely add up and I often don't
get answers. There is a lot written on HF propagation basically covering 5
MHz to 30 MHz but when you go below that the standard explanations are
incomplete. Prediction charts in some amateur magazines often show the
frequency range on a linear scale going to zero. This is quite clearly
ridiculous but probably the formulae they use only covers HF phenomena and
therefore the charts are correct for that range only.
It was always obvious that the regular explanations given popular texts did
not cover frequencies below 3 MHz or did not properly explain what happens
below 3 MHz. Almost anything has been investigated somewhere but the
difficulty is to find it and make it applicable to our (amateur radio)
purposes. I will say a little bit about LF first.
"VLF Radio Engineering" by Watt Mainly covers true VLF, that is 9 to 30 kHz
but it does give some useful information to determine what happens up to
200 kHz if you put two and two together. The explanations given in watt
were the only explanations I have ever seen that made sense. They fully
explain why long distance propagation improves below 200 kHz and in
particular why long distance communications is possible on daylight and
night paths between 9 and 50 kHz. In fact I have never seen these
explanations any where else although I have no doubt there are similar
explanation given in many learned papers published within institutions. For
a signal passing through an ionized medium the loss in the medium increases
with decreasing frequency. The way in which LF signals are reflected by the
under side of the ionosphere is unique to LF and fairly well explained in
the article.
Even in texts like Watt it is necessary to read between the lines to make
sense of everything. For example if you look at the graphs in appendix E
(properties of the ionosphere) you observe that they all come from
different souses. For example there is a very good graph showing
conductivity (active loss in mhos per metre) for heights from 40 to 90 km
for both day and night while another graphs gives electron density to 1000
km but there is nothing to relate the last mentioned graph to the others.
Presumably the conductivity (real loss the reciprocal of which is
resistivity) rises to a peak at a certain height in the interface layer
between the ionosphere and the unionized atmosphere and falls off above
about 100 km while the susceptivity (unreal or reactive conductivity, the
reciprocal of which is reactivity) increases considerably with height up to
300 or 400 km. Active and reactive ionization in the ionosphere is referred
to in the text. There is no graph showing the active and reactive
components of the ionosphere with height (perhaps no one has ever made such
a graph). There must be great opportunities here for research students to
tie all these things together.
The lumped constant equivalent for reactivity is reactance and reactivity
is related to effective dielectric constant and this in turn the
refractive index of the medium.
I will give a bit of a run down on how the ionosphere appears to me at MF
with some reading between the lines.
The E layer is approximately 100 km above the earth and because reflection
takes place over a distance due to reducing refractive index with height
the virtual height is about 120 km. If there are E's present the E's
slightly above the E layer but because the E's layer is very thin and
intense (like a plane surface) its actual and virtual height is the same
and is also about 120 km. At night the E layer is not a layer but it is
actually the bottom of the ionosphere below the F layer. At night the E
layer has a critical frequency of about 360 kHz and a maximum usable
frequency (at low angle) of about 2.2 MHz. This would influence propagation
on 160 metres, in that, it would eliminate the very lowest path angle via
the F layer. The lowest angle F layer path would skip over the very low
angle E layer path and be slightly shorter than if there was no E layer.
For an hour or so after sunset while the sun is still shining on the E
region it could considerably affect distant propagation. Above the E layer
the ionization rises continuously with height to a peak at 300 to 400 km as
referred to above (the F layer). Below the E layer at night where the
ionosphere and the atmosphere meet there is a thin night time loss layer of
significant conductivity at MF and hence loss at MF at about 90 km. (This
night time loss layer is much less significant above about 5 MHz). See
diagram Fig. 2.
In the day time the E layer is a definite layer with a peak at about 100 km
and a critical frequency at between 3 to 3.5 MHz and a maximum usable
frequency very much higher. In the daytime below the E layer there is
another ionized layer, the D layer, that is mainly lossy. It has a height
of about 80 km and it has no critical frequency because its high loss
renders it non refractive. The effective height of the low frequency
reflection layer is on the underside of the D layer where it interfaces
with the atmosphere and this is about 70 to 75 km above the ground. See
diagram Fig. 1.
The low loss (reactive) part of the ionosphere, basically above 100 km, has
an effective refractive index less than one and even less than zero (unreal
or involving the square root of -1). How can this be that the waves can
travel faster than light. Actually the refractive index is less than 1
because the phase velocity increases inversely with frequency while the
packet or group velocity is slower than the speed if light. This is a
characteristic of a medium where the propagation velocity decreases with
decreasing frequency. Getting your mind around this is not easy but you can
only study the texts on the subject and take my word for it, Radio
Engineers Handbook by Terman is very good on this subject. It is in the
reactive component or low refractive index part of the ionosphere where
high and medium frequency waves are turned around on their journey into
space and returned to earth . See diagram Fig. 3.
Loss in the lossy part of the ionosphere is inversely proportional to the
square of frequency. Because of effects of the earth's magnetic field the
loss is higher than expected at the "gyro" frequency and lower than
expected higher and lower than the gyro frequency. In Melbourne the gyro
frequency just happens to be about 1800 kHz.
The loss layer at the bottom of the ionosphere causes most of the loss to
radio waves. Medium frequency waves below 2 MHz are almost completely
absorbed in this region during the day, even on short skip. At night the
loss layer has little effect above 5 MHz but over long to medium distances
it has an increasing effect below this frequency. At low frequency the loss
layer behaves like a lossy metallic mirror and reflects radio waves by the
speculum method. Speculum is Latin for mirror and in Roman days mirrors
were made of polished metal and not very good reflectors.
At LF, at least below 200 kHz and at a small fraction of a wavelength above
the ground, there is virtually no high angle signal which also means there
is no horizontal polarization transverse to the direction of the signal.
This is due to the nature of the ionosphere as a reflecting medium and
equally to the fact that the ground is a very good reflector at LF and
that, for the horizontally polarized component of the reflected wave is
phase inverted relative to the direct wave and therefore cancels. This is
well described in my article and other texts on the subject.
Now what about medium frequency between the BC band and 2.0 Mhz at night,
forget about day time for now. I divide this discussion into points.
1. Amateurs use both vertical and horizontal polarization on the band.
Radiation of horizontal polarization is poor from a small antenna but
can be as good from a large antenna as it is on HF. Signals passing
through the Ionosphere become circularly polarized (poly polarized)
at the receiving end and therefore contain both polarization
components.
2. Signals passing through the loss layer of the ionosphere lose energy
cumulatively for each skip through the ionosphere i.e., if you have 5
dB loss for each pass through the loss layer, that is 10 dB per hop
you will have 40 dB for 4 hops. (Actually this is just an example as I
don't know the value of loss at MF in the loss layer at night. I also
note that different prediction soft wares give different values for
night path loss at MF, does any one know?) Loss is greatest at the
lowest angle of signal path but also greater the more the hops. It is
doubtful that multi hop signals between the ground and the ionosphere
are possible on MF.
3. In addition, the presence of the E layer at night probably blankets
very low angle signals because it reflects signals to a shorter skip
at a low angle.
4. Chordal propagation is now well known and has been written about in a
number of papers however its exact effect and predictability in not so
well known. It would appear that it has different effects on different
bands on HF and MF. When the underside of the ionosphere is not
completely concentric with the ground and a wave path hits the
ionosphere at a point of a slope it can be reflected on a path where
it will travel above the E layer and be reflected cordially by the F
layer until it hits a suitable slope in the ionosphere and be
projected back to the ground. This will avoid cumulative losses when
the signal passes several times through the loss layer and losses from
several ground reflections. Slopes in the ionosphere at sunrise and
sunset are well known. See diagram Fig. 4.
5. I am now going to be more speculative but the following is based on
various reports I have heard from time to time. At MF the signal does
not penetrate as deeply into the ionosphere as at HF. The wave path
will be reflected from an ionization contour in the ionosphere between
the E and F layer at a point where the ionization is strong enough to
produce this reflection. Between here and say a distance away of 5000
km there could be several variations in ionization intensity. These
variations are like clouds floating between the more stratified main
part of the F layer and the E layer. This type of propagation is more
likely to occur when the signal starts or ends at a sunrise or sunset
point but can occur at other points in the darkness path as well.
In-between these points reflection of the signal will be sporadic and
usually weak because of spreading of the paths. The signal could also
be reflected back to ground at a fairly high angle. So what we finish
up with on long MF paths at the receiving end is a sporadic "patch
work" of signal angles, paths and polarizations. It should be
recognised that these frequencies are not used commercially for long
distance communications for very good reasons. This is not to detract
from amateur efforts to use the frequencies for that purpose.
6. Now about your beverage antennas. I presume the antennas are long and
fairly close to the ground. At LF at ground level signals are only
vertically polarized for both the ground wave and the ionosphere
waves. Also the ionospheric waves have a very low angle so that you
cannot differentiate between the two without very sophisticated gear.
As the signal wave travels forward the lower end of the wave front
drags backwards and becomes almost horizontal as it enters the ground.
This is an almost perfect situation for a beverage antenna. The wave
traveling along the ground will induce a traveling wave in the antenna
as it travels along the wire toward the receiver.
7. At HF as you know there is such a thing as a single long wire beam.
This is like one leg of a Vee beam. At MF the ground is not as
effective as at LF and with signals arriving at different angles the
beverage very easily pick up a mixture of polarizations at different
angles to the ground. It is therefor not surprising that the Beverage
may not work as well.
8. At my QTH I do not have much room in my suburban back yard, I use a
horizontal dipole parallel with the ground. Fed with the feeders
connected in parallel it makes quite a good antenna as a top loaded
vertical for transmission especially when used with a counterpoise.
When fed as a dipole I get good rejection of vertical polarization on
reception. This setup was the subject of an Article I wrote in
"Amateur Radio" magazine May to August 1971, "Home Station Antennas
on 160 Metres". I have recently made this article into an HTML file
and it available from my web page. I have found here is that ground
wave signals are always better received on the vertical. Most noises,
local and atmospheric are worse on the vertical than the horizontal.
Signals at say 1000 km are best received vertically polarised in the
evening before the E layer fades. At other times almost all
ionospheric paths are better on the dipole, even long DX. Horizontally
polarized signals close to the ground are received at a higher angle
than vertically polarized signals and this is consistent with my
postulation above that a lot of MF DX signals are not received at a
very low angle.
9. There is some very interesting software, which tries to predict signal
paths even at MF. It takes into account all known characteristics of
the ionosphere. I do not have this software but it has an instruction
manual with a lot of interesting information in it. The system is
called Skycom Pro by Prolab Pro and is obtainable at
http://www.spacew.com/www/proplab.html.
John Adcock VK3ACA
Hello John,
Thanks for your Email containing 9. items in it. I really very much
appreciate your comments and thoughts. They confirm my thinking regarding
to the Beverage antenna.
Yours points 6. and 7. are the conclusions that I have come to on this
side of the world. Up here in the states there are some who are more
interested in making money even when they know it doesn't work that way or
getting their name publish in an amateur journal.
I came to the conclusion that the Beverage is an E field antenna system.
However, on medium wave and especially 160 meters and above reception is
employing sky waves. Yes, you are exactly right its a single wire traveling
wave antenna that is terminated. W7EL Roy Lewellen confirms his software
only calculates the sky wave acting on a Beverage on 160 m. And that you
must use perfect ground for the calculation, that leaves me to believe the
program is somewhat inaccurate for modeling a Beverage on this frequency.
Thanks for the tips on Prolab, I have down loaded the manual. And I am
reading it with great interest.
I am presently writing up the paper "Some New Things I Have Learned About
Beverage Antennas". Upon finishing this will forward to you. Would like to
know your thoughts on it.
Second, you are right there is really little written on ELF, VLF, LW, MW
bands. I have a number books on the subject but there is really not much in
them.
Thank you again for your help, both of the papers are excellent and you
have made my day. Best Regards,
73 Walter
Supplementary notes to above:-
Hello again Walter
Your original request was for information on propagation in the MF range,
mainly 200kHz to 3MHz. In my reply I had forgotten about an important
article in "CQ" magazine which was very relevant to the subject. The
article is "The 160 Meter Band An Enigma Shrouded in Mystery" parts 1 & 2
by Cary Oler and Dr Theodore Cohen N4XX, CQ March and April 1998.
Most of the ideas in the above article are similar to those in the notes I
sent you including take off and reception angle and the use Beverage
antennas. A comparison between the above article and my notes is as
follows.
1. Attenuation of signal in the loss layer. Attenuation to long distance
signals at night are usually sufficient to make multi hop propagation
impossible. The article and my notes are similar on this point. This night
path loss is confirmed by the path loss shown by most prediction programs.
2. Reference to the "gyro frequency" is similar in my notes and the
article.
3. The negative effect of "Auroral Zone loss" is dealt with in some detail
in the article and is given some importance. I did not refer to this as I
have no experience with it.
4. According to the article long distance propagation takes place mainly as
a result of ducting between the E and F layers. This is backed up by using
the "Prolab software" which I referred to.
In my notes I attribute long distance to "Chordal Propagation". Coral
propagation is, in effect, a ducting except that it does not require an
increase in ionization in the E region on the way down. I would express
some doubt on there being a peak at the E layer and then a trough above the
E layer at night because all the plots of Ionization versus height I have
ever seen do not show this. However the effect is the same. In the article
the 160 metre signals having high launching angles and high reception
angles are explained as well as signals changing their direction or
deviating from the great circle. Patchy reception is referred to where a
signal from one station is strongly received in one area and not heard in
another.
Chordal propagation is an interesting concept and affects different bands
differently. It can occur anywhere the reflecting layer is sloping upwards
relative to the ground. I first heard of it when a German young man, Hans
Albrecht came to Melbourne just after the war (WW2). He was a strong
academic and returned to Europe to study. He published a paper on the
subject and sent an article to "Amateur Radio" at the same time. A number
of other articles have been published on the subject in overseas journals.
Coral seems to be known in European circles but does not seem to be as
well known in the Americas.
It is fairly obvious that propagation paths in the lower, less ionized
part of the F layer would have very unstable paths and would be likely to
come back to earth in an unpredictable or sporadic manner.
I hope you find this of interest. Please let me know when you finish your
article on the beverage.
73 John
References to articles on or related to Chordal Propagation.
Investigations on Great-Circle Propagation between Eastern Australia and
Western Europe. - Hans Albrecht. - Publication "Geofis. p. eappl." - 1957
Pages 169 to 180.
The Roll of Ionospheric - Layer Tilts in Long Range High - Frequency Radio
Propagation. - Sydney Stein. "Journal of Geophysical Research." - March
1958 - Pages 217 to 241.
Propagation Studies on 3.5 and 7 Mc. - Hans Albrecht. - "Amateur Radio
(Australia)." - April 1958 - Pages 2 and 3.
Low Angle Radiation. - L A Moxon "Wireless World" April 1970. - Pages
155 to 158.
The Influence of Chordal Paths on Signals to the Near Antipode of an HF
Radio Transmitter. - Gary E J Bold - "IEEE Transactions on Antennas and
Propagation." Volume AP-20, No 6, November 1972 - Pages 741 to 746.
The 160 Metre Band. An Enigma Shrouded in Mystery. Parts I and II. - Cary
Older and Dr. Theodore J Cohen, N4XX. "CQ" - March and April 1998. Pages
9 to 14 and 11 to 16 respectively.