Dxers Unlimited
Dxers Unlimited's mid week edition for 14-15 October 2003

By Arnie Coro

Radio amateur CO2KK




Hi amigos, you are most welcome to the mid week edition of Dxers Unlimited reaching you via
short-wave and also the world wide web streaming audio from www.radiohc.cu. I am am Arnie Coro,
radio amateur CO2KK, your host here in Havana, and here is item one: Just as I told you during the
weekend edition, the high speed solar wind from a coronal hole has impacted the Earth's
magnetosphere , and during Tuesday morning here in Havana, the three hourly K index went up to 5 ,
while the effective sunspot number hit bottom low figures. But, as expected the disturbances
associated with such conditions were most noticeable at latitudes higher than 40 degrees North. Item
two: Radio is certainly a fascinating hobby that children as young as six or seven years old may
enjoy at the same time as their grandads... I still remember very well how my granpa Paco helped me
to finish winding the coil of one of my first crystal sets. Si amigos, radio is a nice hobby for
people of all ages, and that's why it has survived the impact of many new technologies, including
the now ever present computers. As a matter of fact, computers and radio get along quite well, as
many radio amateurs and short-wave listeners have discovered while exploring new communications
modes, like PSK31, MFSK16, Slow Scan Television, Fast scan low resolution television etc. And
talking about fast scan low resolution TV, there is a very easy to install software that allows both
receiving and transmitting the 30 lines standard low resolution fast scan TV signals, something that
some radio amateurs are just starting to do on the 10 meter band. Item three: The VBTTFD antenna
continues to capture the attention of scanner radios owners, as many of them realice that it is a
very low cost and practical antenna for monitoring . The 30 to 150 megaHertz version of the VBTTFD
is just three meters or a bit more than 10 feet in length, and it actually uses less horizontal
space, because the optimum way to install it is sloping at a 45 degree angle.... More about an
optimized version of the VBTTFD later in this mid week edition of Dxers Unlimited, the radio hobby
program that provides you with the most up to date HF plus low band VHF propagation forecasts... And
don't forget that ANY RADIO HOBBY RELATED questions that you may have will be answered both direct
to your e-mail or postal address and also on the air, so that other Dxers Unlimited's fans around
the world may also learn more about the hobby.

My sound engineer today is Jose Izquierdo, I am Arnie Coro in Havana, back with you in a few
seconds, after a short musical interval .

.......

This is Radio Havana Cuba , the name of the show is Dxers Unlimited, and let me tell you that I have
received no less than twenty e-mail messages asking about the low VHF band wide band terminated
dipole antenna, also known as the VBTTFD, that acronym standing for VHF, Broadband , Tilted
Terminated Dipole, now let me repeat it again, VHF, Broadband, Tilted Terminated Dipole.... At your
request amigos here is some additional information... The actual length of the antenna is easily
calculated by the formula 100 divided by the lowest frequency we want to receive in megaHertz... so,
if we want the antenna to pick up signals starting at 30 megaHertz then the length of the antenna is
100 divided by 30, or 3.3 meters. In actual practice, and following the advice of my good friend
Jose Angel, CO2JA, the latest version of the VBTTFD is slightly longer, so that it can also double
as an antenna for the 12 and 10 meters amateur bands... Jose recomended that the antenna be cut for
a lower frequency of 24 megaHertz, but I must say that I really rounded up the number to 25, so 100
divided by 25 , gave an antenna length of 4 meters.

The 4 meters long VBTTFD will cover the frequency range from about 23 megaHertz all the way up to
150 megaHertz, but the actual radiation pattern of the antenna will change noticeably according to
the frequency of operation and the way that the antenna is installed. The tilt angle is also
important, and tests showed that the antenna works best when the tilt angle is as near to 45 degrees
as possible. Some high school trigonometry will show you that the actual footprint of the VBTTFD
antenna is very small... And don't forget that with the 4 meters long version, your antenna systems
will cover the 12, 10, 6 and 2 meters amateur bands for both transmit and receive. The actual
frequency range covered by this antenna still amazes me and others that have tested it. As a matter
of fact, several professional radio engineers that have seen my two VBTTFD's in actual operation
haven't left my home without the complete , detailed, building instructions.

Regarding the very important for this antenna system "terminating resistor", values between 800 and
900 ohms are near the optimum, according to the most recent highly sophisticated antenna modelling
software simulation. The 800 value works nicely with a specially built 16 to 1 balanced to
unbalanced transformer,that matches standard 50 ohm coaxial cable very well, although I always
recommend using the VBTTFD with a homebrew PI network antenna tuner, that is very easy to build and
that provides a perfect one to one matching between the antenna and the radio. Some builders of the
VBTTFD have connected two commercial 4 to 1 baluns in order to achieve the 16 to 1 transformation,
and that approach seems to work too... Matching the lower cost TV type 75 ohm coaxial cable requires
a different impedance transformation ratio, but I must tell you all amigos, that at least two VBTTFD
's are working with the 16 to 1 balun feeding 75 ohm cable runs that are connected to a homebrew PI
network antenna tuner, actually two tuners, or matching networks as they should really be called...
One of them matches from 23 to 60 megaHertz and the other one from 60 to 150 megaHertz. With the 23
to 60 megaHertz antenna matching network installed, the 4 meters long VBTTFD is used for receiving
by just peaking the two variable capacitors for maximum received noise using the full coil in order
to match between 23 and about 40 megaHertz, then you will select half of the coil in order to tune
from 40 to 60 megaHertz.

This will give you coverage of a very interesting area of the VHF spectrum, where many propagation
phenomena take place, from sporadic E skip to tropospheric ducting, from Trans Equatorial
Propagation or TEP to standard F2 layer Dx at the peak of the solar cycle, without forgetting the
ever present meteor scatter propagation mode .

The easy to build matching network will allow you also to transmit on the 12,10, 6 and 2 meter bands
with very low standing wave ratio.

Using the shorter version of the VBTTFD, the one that is about 10 feet long, I was able to work
several Argentinean stations on 6 meters during the last spring equinoctial DX season. The VBTTFD
gave received signals of about the same intensity as a reference half wave dipole installed at the
same height above ground. On both 12 and 10 meters, with my QRP rig , running 4 Watts maximum power,
I have worked 4 continents using the longer version of the antenna, the one that is 4 meters in
length. So amigos, I am leaving for the upcoming weekend edition the detailed description of the
high VHF band VBTTFD that also provides coverage for the 70 centimeter UHF amateur band. If you have
any doubts about this low cost, easy to build with household wire and other easy to obtain
materials, antenna system for scanner radios and amateur bands transceivers, just send them to
arnie@rhc.cu, again , very easy to remember arnie@rhc.cu, or VIA AIR MAIL to Arnie Coro, Radio
Havana Cuba , Havana , Cuba ....

.......

Si amigos, yes my friends, you are listening to your favorite listener oriented radio hobby show,
the one and only that does include all the aspects of this wonderful way of using our spare time :
RADIO !

Why I don't bore you with long lists of possible station catches that may or may not be heard at
your particular location, well , I am sure that you realice that HF propagation is very much connect
to geographical locations , so those references given by some Dx programs should be taken up with
caution, as in many occasions you won't be able to pick up the signals of those stations at your
QTH. Dxers Unlimited is a technicall minded program , YES, because radio is a technical hobby... and
that's why I use up a lot of on the air time conveying to you technical information that will
certainly help you and other Dxers Unlimited's listeners enjoy the hobby a lot more. When I recomend
that you learn how to solder electronic components , circuit boards and connectors, I do it because
you can be sure that after you know how to solder,then you will be able to do minor repairs , fix a
desoldered connector, or even go ahead and build your own equipment, from a simple antenna tuner or
matching units that will require just a few soldered areas, to a complex transceiver kit that after
you assemble and test if will become the pride and joy of your ham radio station.

There are more than 73 ways of having a good time with you radios amigos !

Stay tuned and learn more about them , right here at your favorite radio hobby show !

......

You have questions, and Arnie tries to answer them is now tied at the number one

Preference of our listeners.... and here is today's question, that came in from Austria, listener
Otto from Vienna wants to know how to connect an end fed antenna for optimum reception, He says in
his letter that he can hang a nice antenna from his apartment building to another one about 30
meters away, but that the antenna must then be end fed , because it is impossible to install a
coaxial cable or a pararell open wire transmission line. Well amigo Otto, here is one rahter easy
way of doing it...Make the antenna length as near as a half wavelength at your favorite operating
band as possible, then use a pararell tuned circuit to provide the impedance transformation between
the very high value at the near end of the antenna wire and the coaxial cable. I have installed such
antennas many times, a 20 meter long section makes an almost perfect 40 meter band system that will
require just one 10 meters long counterpoise.

Si amigo Otto, end fed antennas are practical, but do remember that at the end of a half wave wire
the high voltage present can be very damaging if you don't take adequate precautions. End fed half
wave or multiple of a half wave antennas fed right at one end of the wire are about the simplest
skywires that you can use, and if properly installed they will perform quite nicely !

And now as always at the end of the program , here is Arnie Coro's Dxers Unlimited's HF propagation
update and forecast...Solar activity continues between very low and low, the effective sunspot
number is now hovering 40, and the particle count on the solar wind stream was rather high around 16
Hours UTC Tuesday. Expect some interesting AM medium wave and Tropical bands propagation during the
next 24 to 72 hours amigos, and don't forget that disturbed propagation doesn't exactly mean bad
propagation, as you may find quite a few stations that you are not able to hear during normal
conditions, present when a disturbance is in progress. Don't forget to send your signal reports, QSL
requests, comments about the show and radio hobby related questions to arnie@rhc.cu or VIA AIR MAIL
to Arnie Coro, Radio Havana Cuba, Havana, Cuba, see you all at the weekend edition of Dxers
Unlimited Saturday and Sunday UTC days amigos