RADIO HABANA CUBA
DXERS UNLIMITED
WEEKEND EDITION
SATURDAY, MAY 19, 2001
By Arnie Coro CO2KKSend your comments, questions and ideas to: arnie@radiohc.org
Hi amigos! Welcome to the weekend edition of Dxers Unlimited... I am Arnie Coro, your friend here in Havana, and now here is item one: disappointing solar rotation... all indications are that solar activity during this particular Carrington rotation has just peaked at a much lower level than expected, and that has taken its toll as daytime Maximum Useable Frequencies barely pass the 25 megaHertz mark...
Item two: What's a TRF radio, Arnie? A question sent in by a listener that tells me he is picking up Dxers Unlimited twice weekly on 9820 kHz during the just after 0330 UTC re-run of the show... Amigo, I'll answer your question right here on this same frequency in a few seconds...
My sound engineer and producer is Margarita Delgado, I am Arnie Coro... stay tuned.
This is the weekend edition of Dxers Unlimited, the radio hobby show that covers all aspects of this wonderful pastime, from monitoring natural radio signals on the extra low frequencies, to picking up the Spaceship-to-Earth communications links, from enjoying amateur radio on a shoestring budget to making friends half way around the world operating PSK31 teletype with very low power... YES, there are more than 50 ways of enjoying this wonderful hobby amigos!!! And here is one of them, learning about radio and electronics, so that you can homebrew some nice radio equipment...
So, here is the question again: Arnie, what's a TRF receiver?
Well my friend, a TRF receiver is a radio that amplifies the radio waves straight through, without any frequency changes.... as simple as that -- the Tuned Radio Frequency receiver simply boosts the signals picked up by the antenna and delivers those signals to a detector... The TRF, or Tuned Radio Frequency receiver, was one of the very first circuits used during the early years of radio, but today you won't find many TRF radios except perhaps in some very specific short range applications like garage door openers and electronic lock systems for cars...
Believe it or not, the old TRF circuit is still in use, and it does work quite well under specific operating conditions. But you won't find a TRF receiver for sale by a major manufacturer of short wave radios, and there are many reasons why the TRF circuit is not used anymore for certain applications where selectivity, the ability to separate between signals, must be good. Radio experimenters still homebrew TRF receivers, and let me tell you that they can deliver excellent audio quality, but their main drawback is lack of selectivity. In the old days, TRF or tuned radio frequency receivers used several vacuum tube stages of amplification, each with its own tuned circuit, and those who used those radios had to peak each individual stage by carefully moving the variable capacitors back and forth until they obtained the maximum signal output...
You can homebrew a TRF radio receiver using either vacuum tubes, bipolar transistors of field effect transistors, and they will all work, especially if built for the AM broadcast band... My favorite TRF receiver uses two pentode vacuum tubes as RF amplifiers, a diode detector and a triode and a pentode forming a high gain audio amplifier... And let me tell you that if delivers an outstanding quality when tuned to the local AM medium wave broadcast band stations. My good friend Jose Antonio Fuentes, the Chief Engineer at Radio Progreso National Network, always asks me how his flagships 50 kilowatt AM stations are sounding on my TRF receiver, and I always tell him that 880, 890, 900 and 640 kiloHertz sound just beautiful on the TRF receiver....
A more modern version using two Field Effect Transistors, a diode and an integrated circuit audio amplifier works very well, too, but it lacks the romance of the glowing in the dark vacuum tubes... this newer version is definetely not a nostalgia radio!!!!
Now our ever-popular antenna section today will describe once again, at the request of many Dxers Unlimited's listeners, one of the easiest to build wire gain antennas... One that provides excellent results despite its low cost and the fact that it requires just one tall mast, tree or building for its installation... This antenna is known as the asymmetric dipole; it is certainly a single band antenna by design, but with some extra care, you can make it work nicely on two bands. One of the most popular combinations for the asymmetric dipole is the one that is designed to operate on the 40 and 15 meter bands.
The asymetric dipole, as it name clearly says, has one of its legs or arms longer than the other... In the case of the KK-4015, my own two-band antenna design, it has two wire elements on the short side that are one quarter wavelength on 40 meters and one quarter wavelength on 15 meters. These two wires are connected together and they are installed so that they are at the highest elevation above ground... Then the other arm or leg of the antenna is formed by a single wire that is three quarters wavelength long on the 40 meter band... YES, this is a rather long antenna, and requires some real estate for installing it, but it is not installed horizontally; instead you install the antenna sloping down at a 45 degree angle in the direction you want to work the DX.... The KK4015 antenna is fed via a coaxial balun choke coil with 50 ohm coaxial cable, and you must use pulleys so that the antenna's length can be trimmed for optimum operation....
You can make similar dual band asymmetric dipoles for 10 and 15 meters, and they will not require so much space, but the beauty of the KK4015 is that it is an antenna that works well both during the daylight hours when 15 is open, and at night when the 40 meters or 7 megaHertz band is delivering the DX... Of course during the peak months of the solar cycle 15 meters will remain open even well after midnight, but this is something that happens only during a short period. My KK4015 antenna is beaming to Europe, and it has a rather broad horizontal pattern, so it covers from Eastern North America all the way to North Africa with good results... My dual band assymmetric sloping antenna is a low cost solution for radio amateurs who can not afford to install a Yagi or Quad antenna...
If you want to know more about the KK4015 and similar dual band low cost wire antennas, just drop me an e-mail and I'll send you the Asymmetric Dipoles INFO PACKAGE... If you don't have e-mail, then drop me a post card; remember to include your return address, and specify that you want the Assymmetric Dipoles INFO PACKAGE. Send your e-mail request to arnie@radiohc.org and our postal address could not be easier, just send a postcard with your signal report, comments about the show and the request for the Assymmetric Dipoles INFO PACKAGE to Arnie Coro, Radio Havana Cuba, Havana, Cuba.
Homebrewing the simpler radios is a lot of fun, and if you are able to work DX with them, then you enter into the FUN squared category... I'll tell you more about YET ANOTHER simple transceiver a little later in today's edition of Dxers Unlimited. Stay tuned right on this same Radio Havana Cuba frequency... I'll be back in just a few seconds...
You are listening to the weekend edition of Dxers Unlimited and here is our next topic for today in detail: Simple radios, the ones that you can assemble using standard, common electronic components, are a lot of fun... The latest single band ham transceiver I am testing now uses a minimum parts count approach, yet delivers a hefty 5 watts to the antenna, and the receiver is fairly sensitive... NO, NO, NO... it won't compare with a sophisticated oriental black box costing at least one kilo dollar or very near that figure... BUT, this one you can make yourself, repair yourself, and make as many as you wish for the newcomers to the hobby who are studying for the ham radio license test...
The whole idea behind this radio is what electronic designers sometime jokingly refer as the KISS approach... KISS K, I, S, S, standing for "keep it simple, stupid," and let me tell you that it works.
The transceiver is built forgetting about miniaturization... no attempt is made to make it smaller than needed for easy assembly... Let me describe it.... it uses a very easy-to-make antenna input attenuator, a double tuned bandpass input filter and a four diode broadband mixer which works as a product detector, then the audio recovered by the detector is amplified by a low noise transistor and an integrated circuit audio amplifier...The signal for local oscillator injection comes from a simple two-transistor VFO, or variable frequency oscillator.... As you see parts count is kept to a minimum in this direct conversion receiver that works very well on the 160, 80, 40 and 30 meter bands if proper care is taken while building the most critical part of it, the VFO....
The transmitter uses the same VFO and three more transistors to make for a good gain distribution per stage, the stages are broadbanded, and require no tuning at all.... The whole transceiver is packaged in a small box and the controls are very simple, just a receiver audio volume control, and the VFO tuning control. As you may realize, this transceiver has the flexibility of VFO operation, so you can always try to find a clear spot on the band to operate. Six transistors and an integrated audio amplifier is all it takes to be able to enjoy CW Morse Code operation on any of the four bands mentioned, 160, 80, 40 or 30 meters. Do remember that this is a single band transceiver.
This is an excellent project for a radio club... the KK-ONE transceiver, as I have named it, will also receive Single Side Band signals, so it can be used for cross mode contacts, that is, you can have a two-way QSO with a station that is using CW as you are, or with a station that is on SSB and can copy your CW signal... More on the KK-ONE transceiver in an upcoming edition of Dxers Unlimited. If you want to learn more about this simple no-frills, low-cost, easy-to-build and adjust 5 watt single band transceiver, just drop me an e-mail to arnie@radiohc.org or send an AIR MAIL postcard to Arnie Coro, Radio Havana Cuba, Havana, Cuba.
OK, back to the questions and answers section. The question today, sent by a listener in Iowa, USA:
Arnie, you talked recently about the TRF or tuned radio frequency receivers, when asked by a listener what TRF stood for... Now my question is, what happens if I add regeneration to the RF stage or stages of a TRF receiver?
GOOD QUESTION, AMIGO!! If you add regeneration to the RF amplifier stage or stages of a TRF receiver, two things will happen: The receiver's gain will increase significantly, and the bandwidth will be narrower, up to the point that the stage begins to actually oscillate and the receiver will not work any longer... Just at the edge of oscillation, the receiver will show its maximum sensitivity and minimum bandwidth. BUT DO NOT CONFUSE a TRF receiver with regenerative RF stages with a REGENERATIVE RECEIVER -- they are two rather different circuits, as in the TRF a separate detector which is not regenerative at all is used... It can be an infinite impedance detector (one I like very much by the way) or a standard diode detector.
In the regenerative receiver circuit, the detector stage is capable of coming very near to the point of oscillation, and even past that point -- that's why a single stage regenerative radio can work so well. due to the tremendous gain that can be achieved with that circuit... I hope this answers your question amigo from Iowa, and do keep experimenting with both TRF and Regenerative radios, they are a lot of fun.
AND if any of you needs to learn more about regenerative radio receivers, just send me an e-mail to arnie@radiohc.org and I will send you as an attachment Arnie Coro's Regenerative Receivers INFO PACKAGE, with the circuit diagrams of vacuum tube, bipolar transistors and Field Effect Transistors radios for you to experiment! If you don't have e-mail, just drop me a post card via Air Mail to Arnie Coro, Radio Havana Cuba, Havana, Cuba...
And just before going QRT here, as always at the end of the show, here is Arnie Coro's exclusive and not copyrighted, in the public domain, at the service of all radio hobbyists worldwide, our HF propagation update and forecast: Solar activity is low and is expected to oscillate between low and moderate during the next two to three days... Actual solar flux Saturday was very near 140 units, and was moving down, as scientists now think that the peak activity for this solar rotation has just passed. Sunspot number is 109, down from Friday's 137 and Thursday's 125. The actual effective sunspot number, the figure you want to use for calculation propagation with any software package, is very near 90, and let me tell you that normal summer time propagation is setting in now, so we will see lower daytime maximum useable frequencies due to the expanding ionosphere.
For 6 meter band ham radio operators, and TV and FM band Dxers, sporadic E clouds will continue to provide very interesting DX starting just after sunrise and sometimes coming out of nowhere in the middle of the night! Short wave listeners should avoid the lower frequencies during local evening hours, as they are becoming noisier due to the summer thunderstorms.
See you at the mid-week edition of Dxers Unlimited, and don't forget to send your e-mail messages directly to me at arnie@radiohc.org.
Arnie Coro CO2KK
Havana, Cuba
May 19, 2001Postal address:
Arnie Coro, "Dxers Unlimited"
Radio Havana Cuba, Havana, CUBA
e-mail: arnie@radiohc.org
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