Dxers Unlimited weekend editions
by Prof. Arnaldo Coro Antich
radio amateur CO2KK
Radio Habana Cuba
Radio Havana Cuba
22-23 June 2010
A solitary , single sunspot active region is traveling across the Sun while optical observers keep track of it. As a Cuban astronomer said a few days ago, it is nice to see at least one active sunspot region, rather than watch a totally blank solar disc.
The optical observations are matching pretty well with the microwave radiation measurements, as an all quiet solar alert continues to be in progress.
Item two: Front end selectivity is one of the missing features of the typical low cost short wave receivers... Designers under pressure of salespersons go into cost cutting measures when a new short wave radio is being developed, and among the first things they do to cut costs, is the removal of the input filters between the antenna and the first mixer.
The lack of front end selectivity can be rather easily solved by building and installing a bandpass input filter. It will take several hours of construction time, but it won't cost much...
Results can be really amazing when a well built input tuned filter is properly set up. For example, a low cost portable short wave radio, that uses a single conversion design, will show a dramatic improvement in the quality of reception if you add two tuned circuits that are lightly coupled between the two by means of a small capacitor.
I recently wound two coils using plastic pill bottles as coil forms, tuned them with a pair of variable capacitors removed from junked transistor radios, and installed it between my full size 40 meters band half wave dipole and the Grundig ETON FR 200 portable receiver.
As I was expecting the reception improved a lot, with a dramatic decrease of image frequencies. Connecting the FR200 single conversion low cost portable directly to the 40 meters band half wave dipole is out of the question, because the receiver will overload badly. But, when connected to the same antenna by means of the dual tuned bandpass input filter, it is like if you are listening to a totally different receiver.
The dual tuned bandpass input filter is a passive device, because it doesn't have any active components. In actual practice it introduces a signal loss, but the added extra front end selectivity more than compensates that slight loss of signal.
The tuning range of the variable filter will depend on the inductances of the coils and the maximum capacity of the variable capacitors... So there is a lot of room for experimenting, because you can wind as many coils as you wish... after all the coil forms are of very low cost … and if you can't find empty plastic pill containers, then you can cut small lengths of white PVC pipe and use them as coil forms.
The coupling between the bandpass tuneable input filter and the antenna is done by means of a three turns link wound at the bottom end of the first coil, and the coupling between the filter and the radio is also done in the same way...
In my latest version of this extremely useful accesory, I am using two one thousand ohms carbon potentiometers, in order to be able to attenuate those very strong signals that do need attenuation. One potentiometer is located at the antenna input, and the second one is located between the second tuned circuit and the receiver. Of course that we are talking about an experimental setup, so the second potentiometer is not really needed for regular use. You will only need the first one, the one located at the antenna input, that works as a very effective input radio frequency attenuator.
By the way this is a very nice weekend project, that will always work, as there are no chances to misplace components , so you simply can't go wrong... Anyway, if you want to see the actual circuit diagram and a photo of the prototype, just drop me an e-mail to inforhc at enet dot cu, and I will send you a small computer file that will help you visualize both the circuit diagram and the actual device.
Stay tune for more radio hobby related information, coming to you from Havana... I am Arnie Coro, back with you in a few seconds amigos
Si amigos, yes my friends, oui mes amis, the radio hobby revives itself very often... I keep a close watch about the introduction of new technologies that use radio waves as a support, and all I can say is that it is certainly amazing how the hobby continues to develop. Take for example the radio control hobby vehicles of all types … from the classic model airplanes to racing cars, sailboats, and even robots !!! Ham radio operators are at an advantage compared to the rest of the radio control hobby enthusiasts, because licensed amateur radio operators can use certain segments of the bands assigned to them to provide the control signals in an environment that is much more protected from harmful interference... I do have several anecdotes of radio controlled model airplanes starting to show a strange behavior and even flying away out of range of the ground transmitter sending the control signals, when an unexpected source of radio frequency interference disrupted the link between the model plane and the ground controller...
By the way, the use of radio control systems for operating model planes, boats, racing cars and robots is one of the 86 ways that you and I can ejoy this wonderful hobby , your hobby , my hobby, RADIO !!!
Recent sporadic E openings happening over North America have amazed newcomers to the VHF ham bands... especially those who have just started to explore the weak signals segment of the 2 meters band.
One interesting fact about the recent phenomenal 2 meters band openings was that it could be detected by picking up very distant FM repeaters, some of them located even fifteen hundred miles away. Here in the Caribbean we enjoyed nice 6 meters band sporadic E openings, but the lack of a proper 2 meters horizontally polarized antenna at my ham radio station was a big stumbling block when I tried to monitor around 144.2 to 144.4 megaHertz for CW and single sideband DX signals...
And now amigos as always at the end of the show, here is Arnie Coro's Dxers Unlimited's HF plus low band VHF propagation update and forecast. A single sunspot active region in sight, solar flux hovering around 70 units, and the daytime maximum useable frequency curve climbing at a very slow pace in the morning due to the very low solar activity that is expected to continue at least for the next two days... Expect more sporadic E openings , some of them that may be really exceptional as regards to length of the opening and its duration. Don't forget to send your signal reports and comments about the program, as well as radio hobby related questions, to : inforhc at enet dot cu, again, inforhc at enet dot cu