Dxers Unlimited weekend editions
by Prof. Arnaldo Coro Antich
radio amateur CO2KK
Radio Habana Cuba
Dxers Unlimited's weekend edition for 20-21 February 2010
By Arnie Coro
radio amateur CO2KK
Hi amigos radioaficionados around the world enjoying the best short wave
propagation conditions we have had since 2005...
Yes, you heard it right, according to many experts , the current revival of
solar activity has already made possible even 10 meters amateur band openings
via the F2 layer, something that can only happen when the number of free
electrons per cubic centimeter is pretty high .
This weekend the Sun has taken a short break,the number of sunspots has gone
down, but not to the zero level, so propagation conditions are going to be good
but not exceptional for the participants of the big ham CW radio contest that is
taking place during the weekend.
Item two: short wave listeners are also enjoying the higher frequency band
openings... a quick check during Friday local morning here in Havana, found the
16 meters international short wave broadcast band wide open to Europe and at the
same time to South America.
I also heard a few stations on the rarely used until now 21 megaHertz or 13
meters international broadcast band.
It is quite probable that many stations will make arrangements to start using
the 13 meters band for the really long paths starting soon.
At the same time that this enhancement in short wave propagation on the higher
frequencies is taking place, we are already observing signs of an increase in
ionospheric absorption on frequencies below 5 megaHertz, so the upcoming spring
equinox of the Northern Hemisphere, and autumn equinox of the Southern
Hemisphere may well be the last time that AM medium wave and Tropical Band Dxers
will enjoy the very low signal absorption of those frequencies that is
characteristic of the years when the solar cycle is at its minimum.
Item three: How much power is required to be able to enjoy a nice two way
amateur radio contact using CW Morse Code Radiotelegraphy ?
Well amigos, the answer to this question is frequency dependent, because the
higher you move up in frequency, the less power will be required to make a two
The reason for this very easy to realize effect , is that at the higher
frequencies there is less ionospheric absorption of radio waves.In plain
language that means that a 2 Watts QRP or low power amateur transmitter
operating on the 10 meters band will produce a much stronger signal at the
receiving end than when it is operated on the 80 meters band.
Now that the higher frequency ham bands, 15, 12 and 10 meters are making a much
expected come back, amateurs are going to have excellent opportunities of
completing the contacts required for many QRP awards by operating on the bands
from 21 to 29 megaHertz.
Si amigos, yes my friends, oui mes amis... the joy of QRP or low power operation
is second to none, your low power station can be heard half way around the world
with good signals when the higher frequency bands are open..
So gone are the days when we had to crank up the power output of our rigs on 80
and 40 meters to work DX.
Transmitters running between 2 and 20 Watts can now be heard very well ,
especially when connected to efficient antenna systems,like the easy to homebrew
HALF SQUARE phased verticals.
More about low cost antennas for amateur radio and short wave listening use in a
few seconds, when Dxers Unlimited's weekend edition continues after a short
break for station ID...
I am Arnie Coro, radio amateur CO2KK your host here in Havana !!!
This is Radio Havana Cuba, the name of the show is Dxers Unlimited, and here is
now more information about a very easy to homebrew, low cost, highly effective
The HALF SQUARE phased verticals that was derived from the BOBTAIL CURTAIN
Many years ago , during solar cycle 20, I installed a Bobtail Curtain array for
the 15 meters band... It was really very easy to homebrew, requiring just a
measuring tape, enough wire , an air spaced high voltage capacitor and a
The Bobtail curtain is an array of three quarter wave verticals that are
connected by means of a single wire transmission line.
The antenna is fed at the bottom of the center vertical , using a tuned circuit
that is resonated to the center of the band where you want to operate. My 15
meters band Bobtail Three Elements array made possible many nice DX contacts, as
well as giving me the opportunity to hold lengthy technical talks with some of
my friends at a time that the Internet didn't exist.
The Bobtail was hung between two steel pipe masts, and the bottom of the
vertical elements were seprated from my home's reinforced concrete roof by about
one meter or roughtly three feet...
Despite the low height above the roof, I kept receiving wonderful reports from
the stations located within the antenna's high gain pattern area. I soon noticed
that the Bobtail was a highly directional antenna, and that lead to moving one
of the masts in order to optimize coverage towards Europe with the “ main beam
My Bobtail was made using household number 12 PVC insulated wire, for both the
three vertical elements and the top phasing line... It was fed using 50 ohms
coaxial cable, but I remember once testing the antenna using a 75 ohms coaxial
cable feedline with exactly the same results, something that could be done with
the vacuum tubes RF power amplifier used by the rig at CO2KK .
The Bobtail went down when the solar cycle's activity was so low that the 15
meters band was behaving like 2 meters … and I forgot about that antenna until
quite recently , when several Cuban radio amateurs , enthusiastic about the
upward shift in solar activity taking place, asked me about the antennas that
could be built at low cost and with readily available materials.
Those converstations lead to a review of the antenna that is the close cousin of
the Bobtail , that is known as the HALF SQUARE....
More about the HALF SQUARE low cost , low take off angle, easy to install
antenna in a few seconds amigos...
You are listening to Radio Havana Cuba, the name of the show is Dxers Unlimited,
and yes, sure, we do QSL a hundred percent amigos... send your signal reports
and comments about our programs to inforhc at enet dot cu, and we will be
sending you our nice QSL card for your collection.
You can also send your request VIA AIR MAIL to Arnie Coro, Radio Havana Cuba,
Havana , Cuba... And now as stated earlier more information about the HALF
SQUARE short wave antenna system.
As I have just said earlier, the half square was derived from the more
sophisticated and also more directional BOBTAIL phased array.
The Half Square uses just two vertical antennas as opposed to the Bobtail that
The Half Square uses direct coaxial cable feed , and doesn't require any type of
matching device, in contrast to the Bobtail that needs a resonant tuned circuit
at the feedpoint .
It is true that the Half Square outperforms even expensive commercial antennas,
and this is due to the fact that when properly installed it provides a near
perfect match to the 50 ohms coaxial cable, and shows a very nice low take off
angle vertical radiation pattern, exactly what is needed to work DX.
As a matter of fact, the Half Square antenna can be installed using masts that
will barely place the bottom of the vertical elements just about one meter or
three feet from the ground, and the antenna will still be able to radiate a
signal that will run very close to the horizon, reaching the ionosphere at the
almost perfect incidence angle required for really long range propagation of the
I have made Half Square antennas for the 2, 6, 10 , 15 and 20 meters band,
following a set of very simple formulas to calculate the length of the vertical
elements and of the single wire transmission phasing line that connects the two
antennas so that they will generate the desired radiation pattern.
After many years of using the same formula, I came across another set of
formulas that claims that ir will produce a better horizontal radiation pattern,
and sure enough, the new formulas were right on target, enhancing the
performance of the antenna a bit more.
The length of the two vertical radiators that I use as a start up measurement is
0.28 of a wavelength, again zero dot twenty eight of a wavelength at the
operating frequency where the antenna is going to be used.
The phasing line length that I use is 0.47 of a wavelength, again slightly
different from the typical 0.25 of a wavelength for the vertical element and 0.5
or half of a wavelength for the phasing line.
You won't be dissapointed with the HALF SQUARE and one interesting finding I
made quite by accident was the the antenna will still work on the adjacent bands.
For example the 15 meters band Half Square will also operate on 17 and 12 meters
, if you use a standard antenna tuner to suppress the reactance present at the
The 15 meters band HALF SQUARE will also load up on 10 meters but it is next to
impossible to find out the horizontal radiation pattern that it will be
generating... anyway, the 10 meters band Half Square is so small and easy to
install that when the band finally starts to open up on an everyday basis and is
full of DX from one end to the other, I am sure that many of you will add one
more antenna mast and install the additional 10 meters band Half Square !!!
In an upcoming edition of Dxers Unlimited I will be describing the step by step
building instructions on how to build a 2 meters band Half Square antenna that
is ideal for portable work and also for participating in hidden transmitters fox
hunts because it has very sharp nulls !!!
And now amigos, as always at the end of the program when I am here in Havana and
can make the optical Sun observations and scan the bands to learn about the
current propagation conditions, here is Arnie Coro's Dxers Unlimited's HF bands
propagation update and forecast.
Solar activity is very low, with now two sunspots active regions in sight, the
Wolf number as determined by the Catania Sicily Solar Observatory expert Enrico
Catinoto , was 34 on the 18th of February at 8 thirty five in the morning local
time,both sunspot active regions are located at the Sun's southern hemisphere,
in contrast with the recent upsurge in solar activity that took place at the
Sun's northern hemisphere.
Solar flux is hovering around 85 units, and it may increase a little if another
active sunspot region shows up, or more activity is generated by the two already
existing ones. HF propagation conditions on the 20, 17, and 15 meters bands will
support daytime DX , while night time Dxing will require you to move down to 40
and 80 meters, don't forget to send your signal reports, comments about the
program and radio hobby related questions to inforhc at enet dot cu or VIA AIR
MAIL to Arnie Coro, Radio Havana Cuba, Havana, Cuba
Amigos if you read this script, please drop me an e-mail to inforhc at enet dot
cu. I had to post the script one day in advance because during the weekend I
will be helping local radio amateurs that will be working the Cw contest , so
won't be able to record the show early Saturday afternoon as I usually do . Your
comments will be most appreciated !!!
plicas of the circuits published in the very early
days Radio Amateurs Handbooks... in my case, the breadboard MOPA was
assembled as a teaching aid for the amateur radio training program , so
it is not a replica. That's why it uses a modern high tech solid state
power supply made from recycled silicon rectifiers that came from broken
compact fluorescent light bulbs, and large electrolytic capacitors
recycled from broken down computer power supplies...
For those of you familiar with power supply safety, that's why I
included the one to one isolation transformer. I can operate the solid
state power supply in two modes... as a voltage doubler, or as a voltage
tripler.... to provide plate and screen grid voltage to the two vacuum
tubes used by the breadboard transmitter...
The rig is crystal controlled, operates on the 40 meter band and is
capable of delivering up to 20 watts of nice and clean CW signals , that
so far have allowed me to work four continents despite the very poor
propagation conditions that have prevailed for the past three months,
and that now , fortunately are changing for the better....
Ah... before I forget, in the interest of safety, I did spent a lot of
time making a chicken mesh protective cover for the breadboard rig....
My ham radio license study group can see the rig's parts clearly , but
there is no danger of electrical shock... By the way , in the early days
of ham radio, people were not so aware of the dangers of electrical
shock, and there were quite a number of fatal accidents due to people
coming in contact with the high voltage power supplies...
If you ever plan on building a breadboard receiver or transmitter, be
sure to include into your project's list of materials, everything
required to build a protective cover to prevent accidental contact with
the rig's high voltage !!!
For those of you that already have obtained your amateur radio license,
let me add one additional warning, regarding the use of vintage replicas...
Present day radio frequency spectrum regulations require that additional
filtering be provided to those vintage replica rigs in case the
transmitters are going to be used on the air ... This will require
building of buying a radio frequency filter designed so that the rig
will then comply with the rules and regulations regarding the out of
band energy radiated .
In my case, the 20 Watts breadboard MOPA transmitter uses a PI network
for the plate load, and it is connected to a low pass filter that has a
very sharp cut-off above 20 megaHertz. The low pass filter was built so
that similar home brew rigs could be operated up to the 20 meters band
without worrying about non compliance with what the International
Telecommunications Union's regulations describe as “ non essential
radiations”, a fancy description for harmonics , out of band and
spurious emissions !!!
More about the 20 Watts breadboard MOPA, built mostly from recycled
components in an upcoming edition of Dxers Unlimited.... And if you want
to know more about this two old vacuum tubes plus several not so old
silicon rectifiers transmitter , just send me an e-mail to inforhc at
enet dot cu, again inforhc at enet dot cu, and I will send you a file
with a full description of this nice rig that may soon be duplicated by
another local Havana radio amateur operator.... who came to visit me
just to see the MOPA , after we made a two way contact on 40 meters and
he could not believe that is was a homebrew transmitter !!!
Seasons greetings amigos, yes, Seasons greetings and best wishes for a
nice 2010 to all Dxers Unlimited listeners around the world, and to
those of you that read the scripts published on my blog, let me say that
2010 will see the blog updated much more often, as I have now decided to
upload the scripts to the Blog as soon as I finish writing them.... the
blog is at http://dxersunlimited.blogspot.com
My two meters band high gain seventeen
Also , I am going to look back and see if the old scripts that were not
uploaded to the blog can be made available too amigos...
And now a short final item before the HF plus low band VHF propagation
Help preserve Planet Earth from the highly polluting electronic junk...
before throwing away to the garbage dump a piece of electronic
equipment...think about the possibility of recycling at least some parts
The wonderful Master Oscillator Power Amplifier 40 meters CW transmitter
described here today , was built using mostly , about ninety percent of
it, from recycled electronic components that came from compact
fluorescent light bulbs, a 286 vintage computer, a nineteen fifties TV
set that was not a collectors item and several other sources , including
a FAX machine and dot matrix printer... they all provided not only the
electronic components, but also much of the hardware required to build
the rig amigos !!!
Ready now to copy our twice weekly HF plus low band VHF propagation
update and forecast.
Solar activity still rated as LOW, despite the existence of five active
solar sunspot regions at the time I was writing this script, around
13:00 UTC Tuesday , that was 8 o'clock in the morning local time.
Solar flux will be soon reaching 85 units, and maybe moving up to 90 ...
The daytime maximum useable frequency curve is already showing a very
nice upward displacement, so be on the lookout for DX on frequencies
above 15 mega Hertz just after your local sunset. Don't forget to set
aside a little time to send me your comments about today's program ,
signal reports and any radio hobby related question you may want to ASK
ARNIE.... send mail to inforhc at enet dot cu, again inforhc at enet dot
cu or VIA AIR MAIL to Arnie Coro, Radio Havana Cuba, Havana, Cuba .