Radio Habana Cuba
Dxers Unlimited
Dxers Unlimited's mid week for 4-5 October 2005
By Arnie Coro
radio amateur CO2KK
Hi mis amigos radioaficionados around the world !!! This is the mid week
edition of your favorite radio hobby program, coming to you from Havana. I
am Arnie Coro,radio amateur CO2KK, and once again we see amateur radio
operators providing emergency communications during the course of a weather
related event. At the time you are listening to this program, Mexican ham
radio emergency nets are on the air, as tropical storm Stan is hitting
southwest Mexico. Stan has already caused more than thirty deaths in El
Salvador , due to the enormous rainfall area that is part of this weather
system. In Central America too, amateur radio operators are active to help
coordinate disaster relief activities. At a recent meeting of the
International Amateur Radio Union Region II Executive Committee, that was
held in Mexico City, and where the President of the Cuban Federation of
Radio Amateurs participated, the need to continue developing the emergency
preparedness plans for each member nation was emphasized. CO2RP, Pedro
Rodriguez, the President of FRC, as our national amateur radio association
is known, presented a report about how Cuba dealt with the recent hurricanes
and how amateur radio nets operating on HF and VHF frequencies helped to
deal with the communications needs, before, during and after the hurricanes
made landfall...
Item two: More about the NVIS antenna systems that I have developed here
precisely to optimize short range communications on the 80 and 40 meters
band. As we don't have here in Cuba a 60 meters amateur band yet, I could
not try the new antennas on the 5 megaHertz frequency range, but I am sure,
due to experimental simulation results using antenna modeling software that
the two new antennas should work very well on that band too. A detailed
description of Arnie Coro's SUPER CLOUD WARMER antennas later in this
edition of Dxers Unlimited...
Item three: Spotless or almost spotless Sun, optical sunspot count at ZERO,
but somehow the HF bands are still working !!! As always at the end of the
show our exclusive and not copyrighted HF plus low band VHF propagation
update and forecast ...
Stay right on this frequency or if you are connected to our streaming audio
available from 05 to 07 UTC from www.radiohc.cu, stay on line... Dxers
Unlimited will continue in just a few seconds...
........
Near Vertical Incidence Skywave, NVIS, is a HF propagation mode of great
interest to many users of the radio frequency spectrum... NVIS provides
immediate, easy to set up communications links at relatively short distances
without the need to use any type of repeater system whatsoever... A link
between two HF stations operating between 3 and 8 megaHertz, depending on
the time of the day and the solar activity, will provide a single channel
voice or data link second to none, and as a matter of fact, according to our
experience in Cuba during the past SIX... yes you heard it right, the past
SIX hurricanes that have made landfall here, is that the NVIS mode is very
reliable, and can be used even with relatively low power HF stations... My
own practical experience here as Emergency Coordinator for IARU Region II
Area C, is that the minimum power that must be fed to the special NVIS
antenna system is in the range of 25 Watts, although we have had the need to
use the NVIS links with 10 Watt stations, as that power output is what was
available at some locations... At the request of several dozens of Dxers
Unlimited listeners that wrote to arnie@rhc.cu, I am now going to describe
one of my NVIS antenna prototypes, that has proven to work very well and can
be built using standard antenna materials... Listen carefully...
Here is Arnie Coro's NVIS Cloud Warmer Special ...
My first prototype was built for the 40 meters band, and the required
operating bandwidth was between seven and seven decimal fifteen megaHertz,
that is from seven thousand kilohertz to seven thousand one hundred and
fifty kilohertz... The antenna was mounted using two rugged and well guyed
masts that are exactly five meters high...These masts are made of steel pipe
and the use a single set of four guy wires... The antenna itself is a
folded dipole, that is placed using two pulleys, one at each mast, so that
you can lower and raise the antenna at will, and you will later understand
why the pulleys are required.
I also use a third support mast at some installations, but this one is made
of heavy PVC plastic pipe and is located at the feedpoint, that is the
center of the dipole.
The folded dipole separation between the two wires that form it is 30
centimeters of one foot, and the wire used is copper multi stranded so
called seven dash eighteen or to make it easier to understand, this is a
wire made out of seven wires of number eighteen AWG size or 1.2 millimeters
diameter bare copper wire... I use a total of six fiberglass spreaders to
keep the folded dipole lower and upper wire at a uniform distance from each
other.
A second wire, that acts as a reflector element is placed 60 centimeters or
two feed above the ground, exactly below the folded dipole element... This
reflector element is cut to be 5 percent longer than the half wave dipole,
but in actual practice, when there is enough time, I test the effective, the
real resonance of the wire that is held by end insulators between the two
masts, to be sure that it is actually resonating to a frequency five percent
lower than seven point zero seventy five megaHertz, that is the center of
the operating band...
The half wave folded dipole is then placed between the two masts and a
balanced to unbalanced radio frequency transformer is installed between the
antenna and the coaxial feed line...
What you have built so far, is nothing more than a close spaced YAGI antenna
that for all practical purposes is radiating the center of its radiation
pattern straight up, at a 90 degrees angle...
But, beware that this antenna will not work properly if you don't adjust it.
.
The folded dipole is a balanced antenna element and it has roughly a 300
ohms impedance when in free space... This antenna element is now close to
the ground and to a reflector, so you won't see anything like 300 ohms
impedance at the center of the antenna when you connect an impedance bridge
or an antenna analyzer ...
Your typical impedance measurement will show any value between 80 and 30
ohms, according to all the tests I have made...
Suppose that you install the antenna , that is the two end masts, the
reflector element close to the ground , and the half wave folded dipole
element, your antenna analyzer shows an impedance of 50 ohms... Then I must
say that you are a very lucky person indeed !!! As nothing else has to be
done, but to the connect enough 50 ohm coaxial cable to reach your
transceiver !!! But, this seldom happens... so now be ready to listen more
carefully in order to learn how to rather easily adjust this antenna...
Standby for a few seconds... I'll be back in a moment...
.....
You are listening to Radio Havana Cuba, the name of the show is Dxers
Unlimited, and yes amigos, today I am providing Dxers Unlimited's fans that
are also amateur radio operators with detailed information about one of my
NVIS Cloud Warmer antenna prototypes, specially designed to give you that
extra powerful short range signal that is required during communications
emergencies that have to be handled on the HF bands, when VHF and UHF
repeaters have failed due to heavy winds, towers going down, or emergency
generators just running out of fuel !!!
Now lets suppose that your NVIS two element YAGI shows an impedance higher
than 50 ohms, for example , 70 ohms... Then here is what you do... Simply
lower the folded dipole element by using the pulleys, as it moves down,
closer to the reflector several things will happen at the same time, I am
sure that many antenna gurus will raise their eyebrows, but this simple
procedure works... as you lower the antenna closer to the reflector, what
you want to happen, happens ! And what you really want to happen is that the
antenna's impedance will go down and that it will be a better match to your
50 ohm coaxial cable feedline... Si, sure, the antenna's radiation pattern
will change , but just a little bit, nothing to fear , and of course that
the bandwidth of the Yagi will also change a bit too, but that is something
that you don't need to worry about. Your two element wire Yagi, pointing
straight up, will provide the very much needed during the emergency NEAR
VERTICAL INCIDENCE SKYWAVE or NVIS radiation pattern ... All the talk so far
applies to a 40 meter NVIS antenna system, so if you are going to use this
system on 60 meters or 80 meters, another set of antenna measurements and
antenna masts heights would be needed.
My own practical experience of perhaps more than 20 tropical storms and
hurricanes related emergencies during the past several years , have shown
that during the local daytime hours the 40 meter band provides the best NVIS
coverage, and during the evenings sometimes it simply vannishes, and you do
have to move down to the next lower frequency amateur band that is available
in your country, which in my case is 80 meters...
YES, SI, OUI, a complete detailed description on how to build your own NVIS
antenna systems, with several different options is now available as a humble
Dxers Unlimited's contribution to help amateur radio help others when
emergencies arrives.. Send for your Arnie Coro's Dxers Unlimited's NVIS INFO
PACKAGE... send mail to arnie@rhc.cu, again via e-mail only to arnie@rhc.cu.
.. in the near future if a suitable
sponsor is found, I may be printing a little booklet dealing with amateur's
radio role during communications emergencies, with special emphasis on
antennas for HF, VHF and UHF, as well as a lot of valuable information
gathered during the past four decades !!
And now amigos as always at the end of the program here is our exclusive and not copyrighted HF plus low band VHF propagation update and forecast... For the past three days ZERO SUNSPOT COUNT... absolutely BLANK SUN, but things may change during the next few days,as solar active region 808 rotates into view.. The HF maximum useable frequency curve is showing the typical slow rise and fast decay of periods of very low solar activity. Best bands for HF Dxing during the daytime are between 15 and 22 megaHertz, and evening Dxing is now rather limited to the 5 to 12 megaHertz range, but be aware of real good conditions on the AM broadcast and Tropical broadcast bands too... Also there are good chances for VHF transequatorial propagation to show up on the 10 and 6 meter bands too... And before going QRT, a reminder that a hurricane related emergency is in progress in Mexico, so amateur radio operators must keep the frequency range between 7050 and 7070 as clear as possible as the Mexicans are using 40 meters for their emergency nets. See you all at the weekend edition of Dxers Unlimited, and don't forget to send me your most appreciated opinions about this program to arnie@rhc.cu or VIA AIR MAIL, a postcard will be fine, to Arnie Coro, Radio Havana Cuba, Havana, Cuba