Great perils have this beauty,
that they bring to light the fraternity of strangers.
—Victor Hugo (1802-1885)
Emergency HF Communications
By Jack L. Stone, Publisher
INTRODUCTION
ecent
discussions about NVIS antennas on the antenna discussion list opened two paths outside the usual technical
matter that participants usually focus on. One is the history of NVIS antennas and the use
of that mode during WWII on both sides of the battlefront. The second pathway started in
contributions concerning emergency communications. What connects both pathways is the
relationship between antennas and the uses to which we put them. Following is a portion of
an exchange on the topic of NVIS on the discussion list this past month kicked off by list
member Arnie Coro from Cuba:
From: Arnie Coro, CO2KK Dear amigos: During the hurricane season, our emergency nets operate on 40 meters during the local daytime hours using the NVIS mode. We use, as expected, half wave dipole antennas close to the ground, and although sophisticated antenna modelling software doesn't show a significant advantage, we do add a reflector wire below the dipole, usually about one meter (3 feet) above the ground. The separation between the "driven element" and the "reflector" is set at about 0.1 wavelength, something that sends the impedance at the center of the half wave dipole way down, a "problem" that I solved by making the driven element a folded dipole... The matching of the antenna to the 50 ohm coax , without an antenna tuner, is achieved by moving the dipole up and down the two short masts...We usually get around 1.2, 1.4 SWR without much trouble and operate the antennas straight into the transceivers without an antenna matching unit. One interesting fact about these emergency communications antennas is that they pick up much less noise than a standard half wave dipole placed between two masts at 10 to 12 meters above the ground, or used as an inverted V dipole with a single mast a around the same heights of 10 to 12 meters. These antennas are NO GOOD for DX at all !!! But at distances between 0 and 500 kilometers deliver a tremendous signal that is also characterized by little fading,as compared with a standard horizontal half wave dipole or an inverted V. 73 and DX Arnie Coro, CO2KK |
-73- |
From: Arnie Coro, CO2KK Maestro Cebik is, as always, absolutely right !!! And the real world, as always, much richer than the "modelled" world !!! Our hurricane season antennas work very well on the 7 MHz amateur band as soon as a certain level of ionization of the E layer is reached... then later the F1 makes its contribution to that mode of propagation too. As the E layer ionization is related to the JA angle (the angle of the Sun above the horizon) by a cosine 1/3 function if my memory doesn't fail... Our stations start to communicate using NVIS on 40 meters about an hour or sometimes two hours after sunrise.... this depending on the solar activity at that particular moment. I am advocating at the IARU International Amateur Radio Union Region II Emergency Coordination for some kind of lobbying with the telecommunications administrations so that all the nations in our continent will allow amateur radio use of a band on the 5 megaHertz region of the spectrum, precisely because it, in my opinion, is the best area for NVIS work all around... NVIS on 80 meters works well too, but ionospheric absorption of the 3.75 MHz signals is certainly much higher than on 5.0 and 7.1 MHz !!! The standard CO2KK Emergency antenna uses a folded dipole that makes a very good match when the skywire is close to the ground enough to produce the best possible "tropical band antenna effect" (in reference to the use of NVIS by broadcast stations in thinly populated areas of the Tropics where operation on the .54 to 1.6 MHz AM Broadcast Band was not efficient at all...) In countries like India, Brazil Indonesia, and also in Cuba, use of the 120, 90 and 60 meter Tropical Bands using special NVIS antennas provided skywave coverage of large areas with a single transmitter running 5, 10, 20 , 50 or even 100 kW. Our own station here Radio Rebelde, recently upgraded its NVIS coverage of the Cuban archipelago on 5025 kiloHertz (60 meters band) using a new 50 kiloWatt transmitter and a NVIS antenna system that according to my monitoring all along the main island of Cuba and the Isle of Youth provides an excellent coverage , "filling in" areas of the mountain regions of the archipelago where the AM and FM broadcast signals won't reach... (areas we call "zonas de silencio ") or silence zones, as people living there had very poor reception of the national broadcast networks... 73 and DX Arnie Coro, CO2KK |
The use of NVIS in Cuba to cover the island nation
with emergency broadcasts and communications is an interesting application of NVIS to
cover a region having a limited area, but filled with terrain that does not permit
line-of-sight signals that we in the US normally use. Although born and much used in
similar battlefield terrains, NVIS techniques are proving very useful to emergency work,
especially in storms of the Katrina caliber. It is not possible to say whether such a
system might have provided aid not only to the agencies working to provide life-saving
assistance after the fact, but also early warning and advice on evacuation of the New
Orleans region.
In Katrina, as in so many other storms, the cell and repeater systems failed for want of
power. Since each tower needs its own power, only a few need to fail and the entire system
is broken. Amateurs came to the rescue in many situations using emergency power supplies,
HF equipment, and simple antennas. Both shorter NVIS skip and longer normal skip played
key roles in rendering the communications assistance required. Operator persistence,
durability, and discipline made the ad hoc system work as well as it did until the
restoration of normal channels.
HOW ABOUT THIS?
Now just suppose that we had a well-designed set of emergency NVIS stations set up in
critical areas, that is, regions where we have traditionally experienced emergency
conditions that threaten human life or create significant suffering for large numbers of
people. Somewhere in the spectrum between 3 and 8 MHz, we might dedicate channels for
various types of communication—broadcast for citizens with pre-tuned fixed-frequency
radios, separate channels for various agencies, special channels for coordination among
agencies, etc. Modern techniques of message compression and frequency hopping could allow
centralized message handling within an affected region—as well as periodic broadcasts
with the critical information needed by every citizen. Moreover, the use of NVIS antennas
would limit the area covered so that outside the affected region, normal communications
might continue (or, as in many emergency situations, by temporarily suspended).
One of the beauties of such a system is that all of the components are close to the
ground. A broken wire or lead does not require a trip up a hundred foot tower in the
middle of a hurricane. Temporary repairs are not only more easily made; they are likely to
be more effective at the lower frequencies used. The right selection of antennas would not
require much acreage, as such things go, and the entire system might fit within a single
building.
In addition, one might set up emergency communications trailers for use in areas of
special need, that is, tactical headquarters. We might custom design each trailer for the
likely hazards through which it would pass on its way to the field location. An easily
erected NVIS antenna (and others) would put the central station in reliable contact with
the field station, even if out of the line of site.
Nothing in these notes, of course, prevents us from equipping either the central or field
stations with all of the usual VHF, satellite, and cellular equipment that agencies have
come to rely upon—often with a high frustration component. A NVIS system added to the
usual array of communications potentials might alleviate the frustration in some cases. If
such a system managed to save one person during an emergency when all other channels were
unavailable, it would be worth the effort.
These somewhat random thoughts on improving our emergency communications capabilities have
a point that is relevant to antenneX, since I doubt that the present thinking in US
federal agencies is flexible enough to lead to real progress in the near future on using
NVIS effectively as an adjunct to other communications methods. Many of our writers and
experimenters work in the HF region. The ease of obtaining reliable equipment and the
macro-aspect of antenna components make this an ideal part of the spectrum for the
experimenter with limit electronic and shop equipment. Many an engineer has turned to the
UHF-and-up portion of the spectrum and has seen within it the future of RF communications.
However, even if the details of these thoughts are well off target, the general theme
remains: we have not yet reached the limit of the uses to which we may put the HF region
or the technologies—both simple and complex—that may enhance those uses.
SPECTRUM CHANGES
Best spectrum use changes with time. Some applications that once used lower frequencies
have gravitated upward. The move is not new. Perhaps it began with RCA’s maneuver to
place FM broadcasting in the VHF region to prevent it from competing with investments in
MF AM broadcasting. Although modern broadcast radio has largely trivialized the FM band,
the move opened the way to thinking about the best part of the spectrum to place
television broadcasting when it finally emerged. Whether we like it or not, the AM
broadcast band has become talk-radio. Uses of the spectrum change over the decades, but
the spectrum itself opens new opportunities as we eventually learn more about it and how
we may more effectively use what we learn.
The history of antenna experimentation and development has been driven by the uses to
which we put the spectrum for which we design the antennas. The lower HF region has shown
us some new opportunities. We perhaps are still far from designing the ultimate antennas
to make the best use of these opportunities. What do you think?
THE PURPOSE OF THIS JOURNAL
In my column for September 2006, “The antenneX Mission,” I wrote about
a subject close to my heart — the purpose of this journal. It had been quite
some time since I had reminded our readers of our original purpose when we set
out some 20 years ago.
More than anything else, this journal has always been a “labor of love” for me.
It is the reason I founded the antenneX magazine almost 20 years ago, knowing
that running a magazine is a tough business and especially so since I planned to
do it without the traditional revenue support from advertisers or outside
investors. I wanted this magazine to always be able to write about any issue
without concerns of losing the support from any of those type revenues. I am
pleased to say, we have never steered from that course one iota. And, as a
consequence, we have covered some very controversial developments throughout the
past two decades. In the process, we have provided valuable factual information
to more than 200 countries, even to the most remote regions of the globe. Hence,
our readers are much more informed about the truth about all of the “new”
devices as their stories unfolded in the antenna and antenna-related field. For
more details, you are encouraged to read my column from September 2006.
Additionally, we were pleased to receive and publish in the October 2006 issue, a very special editorial reaction to my column written by one of our long-term readers and contributors, Dr. Jef Verborgt. Those two articles are located by the following links below:
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AN INVITATION TO CONTRIBUTORS
antenneX
thrives on the contributions of antenna experimenters, ranging from the informal
home shop construction project to the theoretical investigation of basic
antenna, feedline, and propagation phenomena. Over the years, we have published
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At the same time, antenneX has experienced continuous growth in its
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even better. How? By submitting an article every now and then based on your
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Among the engineering and researching readers, there are
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IN THIS ISSUE
We again include many fine articles by our great writing team from around the globe. Now, allow me to introduce this month's line-up of content:
OUR MONTHLY COLUMNS (plus this one you are reading by yours truly):
FEATURE ARTICLES IN THE LIBRARY:
Some Further Notes on the Gamma Match |
| In "Some Preliminary Notes on the Gamma Match," I began (but certainly did not conclude) a comparison between two methods of calculating gamma-match dimensions (specifically, the length of the gamma rod and the required series capacitance) with additional comparisons to a set of MININEC models. Once I translated the two calculation methods (the Healey-Wheeler and the Tolles-Nelson-Leeson systems) into handy spreadsheet formats, they stood ready to deliver any amount of required data on a moment's notice. However, modeling is far less handy in this regard, since each new case requires a new model. |
A Re-Examination of the Gamma Match Part I |
| There are very few antenna designs that produce a perfect match to any given feedline, coaxial or otherwise. Those that do are designed so as to compromise some other desirable antenna characteristic. Many of the common commercial designs may use any of the following matching systems e.g. The Beta and its close relative the Hairpin, Gamma or quarter wave sections to achieve the desired match. A matching system is a necessity to minimize losses along the feedline to the output transducer –the antenna. My aim is to review for the readers the ‘Gamma Match’ so as to understand how it operates and hopefully improve its performance. |
80–10
Meter Antenna System Impedance Matching: Part 3 |
| Previous
articles concerning impedance matching provided some insight to me but further testing and
measurement show other interesting experiences. In particular, this article addresses my
antenna system in areas that appear to be quite different than all the others reviewed. From exploring web sites it is apparent, as previously thought, that there are countless others who are still exploring the area of impedance matching and include equation sets and their approach to the area that date back to 2001. Indeed, impedance matching can be found as far back as 1940 (and probably further in history). Clearly, it is a very interesting and important area for all communication spectrum assignments, especially regarding field equipment and systems. |
Scattering of a Plane Wave by a Small Conducting Sphere |
| In this
paper, we shall discuss the scattering of a plane electromagnetic wave of angular
frequency that is incident on a perfectly conducting sphere of a radius when the
wavelength obeys. Then, we shall calculate the electromagnetic fields and Poynting vector
everywhere and interpret these fields in both the near and far zones. We review this well-known problem with an emphasis on details of the fields close to the sphere following the usual, briefer analysis that applies to the far zone. A possible interest in a study of the near zone in this problem is its relation to the near-zone behavior of broadcast antennas. Although the latter are not typically considered as examples of scattering phenomena, in a sense they are as we shall see. |
160-Meter PA Output Network |
| Whether
your PA is a hive of transistors or a single tube, it needs an output network to couple
its power to the load. Your PA works best when the output network is adjusted properly.
Otherwise you may end up operating illegally or your PA may burn up. What follows in this
article is not a rigorous, but a casual treatment of PA output network design. Designing an RF Power Amplifier output network requires correct impedance transformation, adequate bandwidth, minimal interaction between loading and tuning controls, and good harmonic suppression. The 160-meter band extends from 1800 to 2000 kHz. Relative to 1900 kHz, this is a +/- 100 kHz width of more than +/- five percent. This is a fairly wide band to cover. Then, Consider a simple output network consisting of only four components. |
What is a Slim Jim? |
| The "Slim Jim" antenna has grown so popular among new radio amateurs who need a good antenna to work 2-meter repeaters that is has acquired its own name. Indeed, many newer amateurs think of the slim jim as a unique antenna, and some have claimed extraordinary performance capabilities for it--all without a good sense of what it takes to make such comparisons, of course. The seeming uniqueness of the antenna has even engendered some fairly poor explanations of how the antenna works. Therefore, it might be serviceable to spend a little time looking at the slim jim and seeing what may be its correct electrical origins. |
Well, there you have it, folks—thanks for listening and remember, the reading lamp is always on for you in the reading rooms. If I can be of further help, I'm just a Stone's Throw! away.-30-
Best reGARDS, Jack L. Stone, Publisher
jack@antennex.com
October 2006 antenneX Online Issue #114
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December 31, 2010
