Radio Havana Cuba
Dxers Unlimited
Dxers Unlimited's week end edition for 7 – 8 November 2010 Sunday and Monday UTC days
By Arnie Coro
radio amateur CO2KK

Hi amigos radioaficionados around the world and orbiting Planet Earth aboard the International Space Station... welcome to the weekend edition of Dxers Unlimited, that is now heard Sundays and Mondays UTC days, just after the top of the hour newscast... Here is now our first item of today's show …. Minimum parts count radios, both receivers and transmitters are a lot of fun to build, and capable of providing you with surprising performance …. A low parts count solid state regenerative receiver , using just three common low cost transistors was recently tested at my workshop... I ran a very comprehensive sensitivity test, and found out that the radio was capable of picking up very weak signals, as low as one microvolt, or one millionth of a volt at the antenna terminals !!! This low parts count receiver is not a bare bones radio... because it was designed to keep the number of electronic components at a low figure, but always preserving the performance of the set, something that has proven to be an excellent compromise. The receiver tunes across the AM broadcast band, and from 5 to 12 megaHertz by switching between two easily homebrew coils. The 5 to 12 megaHertz segment of the short wave spectrum was selected as a good compromise, that includes the 6, 7, 9 and 11 megaHertz international short wave broadcast bands , also known as the 49, 41 , 31 and 25 meter bands...
It also tune to the upper part of the 60 meters Tropical Band, and provides reception of the 40 meters amateur band. The low parts count receiver is powered by four rechargeable nickel metal hydride
batteries, with a nominal output at full charge of around 5.5 volts DC, that will last a very long time before needing a recharge... If you want to learn more about this simple, yet effective receiver that can be built by anyone that is capable of properly identifying electronic components, reading a circuit diagram and soldering parts to a printed circuit board, just send an e-mail to inforhc at enet dot cu... again inforhc at enet dot cu...
Now here is item two > More about the Super Islander single band amateur transceiver , designed so that it can be built and repaired by the radio amateur … not requiring expensive test instruments and the know how of an electronics guru to build it and keep it working... The Super Islander's receiver has evolved from the original CO7PR Islander transceiver design that made possible for a large number of Cuban radio amateurs to go on the air on the 40 meters band with a CW and Double Sideband Rig that is compatible with even today's most advanced technologies.
Nowadays the Super Islander receiver in its version 5.1 is an all solid state direct conversion receiver that has proven to be very sensitive, and also free of cross modulation problems caused by the super power international shortwave broadcast stations that operate above seven point two megaHertz, that is the 41 meters international shortwave broadcast band.
Using a well designed dual tuned bandpass input filter and attenuator, ahead of the low gain
radio frequency amplifier stage, the Super Islander receiver has been tested side by side to a very expensive professional transceiver , and it was demonstrated that its performance was really amazing to say the least. There are several versions of the Super Islander's receiver module... one uses a very simple NPN silicon transistor product detector, another version uses a Field Effect Transistor, an FET or FET as the product detector... yet another version tested here with also very good results uses a homebrew broadband double balanced product detector with four computer type switching diodes and two broadband trifilar wound transformers using the ferrite rings recycled from Compact Fluorescent Lighbulbs circuit boards... I also tested a two diodes product detector... After building the four prototype front ends... it was more than logical to ran laboratory type comparative tests regarding both sensitivity and inmunity to cross modulation. I found out that the version of the Super Islander with the four diodes double balanced product detector was the one providing the highest performance , although the other three options work quite well too... Nevertheless I now recommend the Super Islander Version 5.1.1 receiver circuit that uses a resistive signal attenuator between the antenna and the receiver input, followed by a bandpass dual tuned circuit filter that feeds the emitter of a grounded base NPN RF silicon transistor amplifier... The gain of the RF amplifier stage is controlled by means of a potentiometer, something that is required because the Super Islander does not have an Automatic Gain Control circuit... The signal from the RF amplifier is fed to a winding of the input broadband transformer of the DBM or dual balanced mixer...
Another port of the DBM is fed with a signal level of around plus seven d b M from the now classci three transistors VFO that operates from 3.5 to 3.6 megaHertz and has a frequency doubler
stage at the output.
The audio recovered at the output of the product detector is routed to a multi position switch, that is located at the front panel of the transceiver, used to select the routing of the audio directly to the audio amplifier module, or to a pair of audio filters... one peaking at 700 Hertz for CW reception and the other that has a low pass response below 2.5 kiloHertz.
The construction of the variable frequency oscillator is the most critical part of this project, and my advise is to obtain help from an experienced radio amateur that has already built one... The VFO is placed inside a steel box, so that it is totally shielded from any external electromagnetic fields !!!
More about the Super Islander transceiver project in an upcoming edition of Dxers Unlimited, and don't forget that I have already at hand partial schematics of the Super Islander modules that can be sent via e-mail... Send your request for the Super Islander schematics to inforhc at enet dot cu....
again inforhc at enet dot cu....

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Si amigos, yes my friends, this is the weekend edition of Dxers Unlimited coming to you from Havana...radio amateur emergency nets are a most valuable contribution to the protection of life and property when a natural disaster or a large scale man made accident happens. Amateur radio operators often provide the first on site coverage after something really bad happens.. And once again we saw this happening during the recent volcanic eruption and tsunami that caused loss of life and damages to Indonesia... Here in the Caribbean, where I am located, the radio amateur emergency nets of the Dominican Republic where carrying on training exercises and testing equipment Tuesday evening, getting ready for the approaching storm that is expected to produce large amounts of rainfall in Jamaica, Haiti , the Dominican Republic, the Turk and Caicos Islands....
Once again the 40 meters and the 2 meters bands are the first choice of experienced operators that have participated in previous emergency situations... The 80 meters band provides night time links at short distances, but not many stations are equipped with the required very long half wave dipole antennas needed to succesfully operate on the 80 meters band. During the peak moments of a hurricane related emergency, that usually come just before the storm makes landfall, and typically
about 6 to 12 hours after the core of the storm has left the area, the 2 meters band is used for direct contact between shelters, civil defense authorities, command posts and search and rescue team. The 40 meters band is used to handle emergency traffic regarding the need to evacuate persons from isolated areas that can not be reached via 2 meters... At night, 40 meters stops from providing the near vertical incidence skywave short distance contacts, and this is the moment when cooperation between radio amateur associations form different countries show up... by providing long distance relays to points located too close in the affected area for skywave 40 meters band night time communications … This long distance relays provided much needed communications just after the January 12th of 2010 Haiti earthquake, and also proved to be very useful during the extremely active 2008 tropical hurricane season, when Cuba was impacted by three powerful hurricanes, Gustav ,Ike and Paloma …
Si amigos, a 50 or 100 Watt class short wave ham transceiver equipped with a simple half wave inverted V dipole and powered from a 12 volts car battery makes a very effective emergency station that can help a lot to convey much needed aid to people requiring it, as well as assuring that vital weather data is received by the national weather services .