01.pdf


Aperçu du fichier PDF 01.pdf

Page 12345


Aperçu texte


CHAPTER 1

Propagation

1. INTRODUCTION
I will try to answer the following questions in this
chapter:
• When can I work DX on the low bands?
• Are conditions on 40, 80 and 160 meters the same or
similar?
• Are conditions very variable?
• Are conditions predictable?
• Is there any good propagation-prediction software?
• Are there any other tools to help me catch the difficult
ones?
• What are crooked paths, when do they happen and what
causes them?
• What directions should I aim my antennas?
In the last five years we have seen more publications than
ever before covering propagation, antenna modeling and digital
communications. It should come as no surprise that the availabil­
ity of powerful computers is a major reason for this evolution.
Speaking of antennas and digital communications, the more that
is being published on these subjects, the more we all benefit.
I cannot, however, say the same thing about publications
concerning propagation, especially related to the low bands.
The number of variables influencing low-band and especially
160-meter propagation appears to be so vast that scientists
have only discovered the proverbial tip of the iceberg. So far,
no one has come up with a forecasting system that works.
At best, we seem to be able to correlate some of the
known parameters with actual observations. But the full “why
and how”—the global picture—is still missing. But don’t let
this scare you off, the elements of mystery and discovery on
the low bands makes for half the excitement and fun there!
A number of publications (Ref 101, 103, 104, 105 and
167) cover the basic principles of radio propagation by iono­
spheric refraction, primarily on HF. Let me recommend in
particular Robert Brown’s (NM7M) excellent books: The
Little Pistol’s Guide to HF propagation and The Big Gun’s
Guide to Low-Band Propagation (Ref 167 and 179). These
books are must-reads for anyone who wants to have a more
than casual understanding of propagation.
The existence of books of this caliber makes it easy for
me, since I will not have to explain the basics of propagation.

CH1.PMD

1

This chapter on low-band propagation is thus not meant to be
a general-study book on propagation. I have written it for the
dedicated low-band DXer, who tries to understand the “how”
of propagation (not necessarily the “why”) to help him work
an elusive new country or maybe to generate a better contest
score.
This chapter is mainly based on observations, your
observations and mine. You will soon find that the basic rules
that govern propagation on the low bands are rather simple.
You need a path in darkness, one that exhibits (most often)
sunrise and sunset peaks. These are the simple but very
important ground rules.
Testimonies of literally hundreds of low band DXers are
used to identify what may seem like odd propagation phenom­
ena. I will mention possible or probable mechanisms that
govern these phenomena. These can be widely accepted, or in
some cases more speculative and yet-to-be-proven mecha­
nisms. What is important is that you be able to recognize
“odd” phenomena and circumstances and that you know how
to take advantage of them. That’s what this chapter is really all
about.
I will try to cover propagation on the three low bands
(40, 80 and 160 meters), explaining similarities, but also
pinpointing important differences. Understanding the basic
mechanics is very essential. If you realize that on 160 meters
the opening over a 18,000 km path will occur maybe one day
a week, and then only during a specific time of a “good”
year, and that the opening will last maybe 3 to 5 minutes, you
will realize how important it is to know when you have to try
to make that contact.
A spectacular example to illustrate this was my QSO on
160 meter with ZL7DK in early Mar 1998. 1998 was a good,
low sunspot year. I knew I had a 3 to 5 minute window both
in the morning as well as in the evening. After observing the
two windows for almost two weeks, one morning the weak
signals from Chatham Island finally came through and I was
able to work them. That morning “I had the skip.” Other
mornings their signals made it into England, France or Ger­
many, without any spillover in Belgium. Why? In addition to
just noting these facts, I will try to describe and explain a
mechanism that seems to fit these facts.
Over the years numerous HF propagation programs have
Propagation
1-1

2/17/2005, 2:39 PM