RADIO HABANA CUBA
DXERS UNLIMITED
MID-WEEK EDITION
TUESDAY, January 15, 2002
By Arnie Coro CO2KKSend your comments, questions and ideas to: arnie@radiohc.org
Hi amigos, radioaficionados! Welcome to the mid-week edition of the radio hobby program you are always waiting for... YES this is Dxers Unlimited on the air, and on the world wide web at our Dxers Unlimited website.
I am Arnie Coro, your host here in Havana and here is Item One: Solar cycle 23 continues to show a lot of activity, solar flux again well past the 200 units mark, so we are enjoying very nice DX conditions on the higher frequency bands... More about HF propagation as always at the end of the show...
Item Two: Another compact antenna, its made from 300 ohm TV type twinlead, or if you prefer, the DCTL can be made from a homebrew two wire transmission line... The amazing facts about this antenna are its very small size and rather nice performance.. it is also a low cost skywire that can be installed using a single mast and some nylon or dacron rope... More about the DCTL antenna later in today's edition of Dxers Unlimited.
Item Three: A quick demonstration of how the new digital communications modes work was arranged Monday afternoon at my QTH for two radio amateurs that had never seen them working... I used the no physical connections approach to leave them amazed of how computers and radios can be teamed to provide excellent keyboard to keyboard radio communications...
AND, as always at the end of the show, our exclusive and not copyrighted HF plus 6 meters propagation update and forecast... Stay tuned for more radio hobby related information coming to you from Havana, Margarita Delgado is at the mixer board and doing double duty as producer of the show... Irma Veitia is in charge of the correspondence, I am Arnie Coro back with you in a few seconds
Here is Item Two in detail: The DCTL antenna, one that I must admit, the first time I saw it, I thought it was yet another inefficient compact antenna... BUT, I WAS WRONG, and after testing one of them on the very popular here in Cuba 40 meter band, known as "LA BANDA," or in English "THE BAND" among Cuban radio amateurs, I had no choice but to change my first opinion about this sort of DELTA LOOP made of 300 ohm TV twinline twisted and connected in a very special way not to one, but actually connected to two different length stubs also made from 300 ohm twinlead.
The complete diagrams and specs for the DCTL are available right here on the Dxers Unlimited website.
The DCTL is not the type of antenna that can be described properly using words, you have to see it in order to realize how the length of 300 ohm TV twinlead that forms the DELTA LOOP is interconnected to the two stubs and the transmission line that goes to your transmitter or transceiver. YES, this is a COMPACT antenna by all standards, without the complications that other compact antennas have regarding the use of special high voltage variable capacitors, as is the case of the otherwhise excellent magnetic loops. The DCTL is much easier to homebrew, and can be installed using just one main support pole, with the lower wire of the DELTA LOOP simply tied to stakes driven into the ground.
BUT, what does the DCTL acronym stands for? DCTL means, Distributed Capacitance Twisted Loop, and precisely the distributed capacitance all along the loop, plus that available via one of the two stubs is what makes possible a RESONANT antenna of such a small size... OK, this is a single band antenna, it has to be carefully built and then trimmed to operate on your favorite amateur band, so we are not talking about a wideband skywire that can be used from 3 to 30 megaHertz! BUT, for many amateur radio operators that live where space for installing antennas is very difficult to find, the DCTL Distributed Capacitance Twisted Loop is a practical solution, and YES, it can be installed indoors due to its small size, especially on the bands from 30 meters UP!
NOW a warning: If you install a DCTL indoors, then the power applied to the antenna should be limited, in order to reduce the exposure to the radio frequency energy due to the proximity of the radiating element. Keeping the power at less than 25 watts will still bring lots of contacts when band conditions are good, and RF currents will not be harmful.
THE BIG PROBLEM with the DCTL is learning how to do the rather complicated interconnection between the four lengths of 300 ohm TV type twinlead involved in the construction of the antenna. They are the antenna itself, the two stubs and the 300 ohm transmission line that connects to your station.
The DCTL, the Distributed Capacitance Twisted Loop, is an inexpensive, effective, resonant alternative that can fit in a 8' tall by 12' or so wide space (like a wall, or balcony), even on 80 meters. Here are the details. The DCTL was developed by Jim McLelland, WA6QBU, and was published in CQ and 73 Magazines in mid and late 1994. The construction material couldn't be simpler - 300 ohm twinlead. (Dimensions here are for the 80m version; tables and formulas follow.)
Using a 28' length of twinlead, the opposite wires are connected to a shorted 4' 6" stub of 300 ohm twinlead. The remaining two ends of the 28' twinlead connects to an open capacitive stub, 30" in length, also made of 300 ohm twinlead.
Now that gives you an overall picture of the antenna. Lz has a shorted end, while Lc is left open on the non-connected end. The critical part is in the connections. Each "side" of twinlead is in a different color, and the connections are shown below. The Ll (large loop) connection, properly made, puts a "twist" in the twinlead of 1/2 turn. These diagrams assume that NO twisting is made in the twinlead, and what the resulting connections look like. Use an ohmmeter to ensure you are NOT connecting the same wires! Use heat shrink or other GOOD insulating material -- the voltages and heat can get very very high at the junction points. Good soldered connections are also critical.
Why, if the antenna is resonant, is a tuner required? Well, it's not. But your usable bandwidth (2:1 swr points) is pretty narrow, at 1/100th of the design frequency. So, adding a tuner stretches it out a bit, in a fairly reasonable fashion. If your tuner requires a 50 ohm coaxial input (like mine does) you can use a 6:1 balun in line. Trimming the open capacitive stub 6" will raise the resonance point approximately 100 kHz for 80/75 meters.
(Note: Capacitive stub becomes pretty useless above 30m, in fact the whole antenna gets pretty darn small!)
Remember, adding the capacitive stub lowers the resonant frequency! Here are some pre-calculated dimensions. Remember that the total loop length can be divided by three (in a delta configuration) or four (in a square configuration) to give you an idea of space required.
Here are the formulas:
The total length (Ll and Lz, or Lt) = 130 / F(MHz)
The shorted stub (Lz) = 27 / [(2xF(MHz))-2]
Loop length (Ll) = Lt-Lz
Capacitive tuning stub (Lc) = 24 x (1 / [F(MHz)/2]**2)
Band Freq. Lt Lz Ll Lc (to lower resonant freq. by 100 kHz) 75 meter 4.0 32'6" 4'6" 28'0" 6" 40 7.3 17'10" 2'2" 15'8" 1 3/8" 30 10.15 12'10" 1'6" 11'4" 1" 20 14.44 9'0" 1'0" 8' 1/2"
No, I don't forget that the world is not totally wired, so for those of you that have no access to the world wide web, I have already printed several copies of the files that you can request by sending e-mail to arnie@radiohc.org or a postcard to Arnie Coro, Radio Havana Cuba, Havana, Cuba, requesting the DCTL ANTENNA info package... Remember that this is a single band antenna, that does need an antenna tuner, but it is so compact and efficient that for many persons it is a viable option that makes the difference between operating an amateur radio station and not been able to be on the air!
The DCTL ANTENNA, Distributed Capacitance Twisted Loop, a practical solution for radio amateur operators that have too little space for installing antennas!
This is Dxers Unlimited's mid-week edition... and here is Item Three: Monday late afternoon I was at home listening to the 15 meter band. It was wide open and in the frequency range between 21070 and 21100 I could pick up several digital modes stations... Then the doorbell rang, two of the local amateurs wanted to see if I had in my junkbox several capacitors they needed for a homebrew receiver in the works... We went to my shack and they heard the sounds from the 15 meter band digital stations, so the conversation turned away from the capacitors to the digital modes...
Just for fun, I put the computer's microphone right next to the radio's loudspeaker, and clicked on the MIX W program ICON on the desktop of the machine... Then, another click at the MODE selection box, and yet another one at the PSK31 line... It took but a few seconds to see several PSK31 signals on the waterfall display of the MIX W digital modes software, then to the amazement of my visitors, I clicked on top of one the slowly moving lines and CLEAR, PERFECT TEXT, started to appear on the computer's screen... We monitored a QSO of an amateur station from Tenerife in the Canary Islands, near the coast of Africa with a station in the United States of America, then, another click and we saw a local ham CO2AV Argelio calling CQ on PSK 31, then we saw a different type of signal on the waterfall display, and my friends asked what it was...
"It's a PACTOR station," I told them, "so we need to change the MIX W mode..." In a matter of seconds, the PACTOR station was providing perfect copy too... "Arnie," they asked me, "how is this possible, the microphone is picking up our conversation, the noise, King Coro barking...?" For those of you not familiar with him, King Coro is my Bitchon Habanero dog and he was barking loud to his dog friend across the street who goes by the name of Junior...
Well, the fact was that the MIX W digital communications mode software was providing perfect copy of those signals even without a direct cable connection from the radio to the computer... I then spent some time explaining to Reynaldo and Juan Carlos that the decoding of the digital modes was made possible by the use of computer synthetized razor sharp audio filters, something that can be done by software....
We also tried decoding CW Morse Code Signals, one of the other modes available in the MIX W program menu, and sure... we picked up several CW QSO's in progress running at different speeds...
BUT, what really left my friends Reynaldo and Juan Carlos really surprised was when after tuning in a PSK31 digital mode signal that was calling CQ, I placed my amateur radio transceiver's microphone right next to one of the computer's loudspeakers and pressing the PTT with one hand, started to answer the CQ by typing the reply with the other hand on the keyboard...
YES... we received a reply and a 579 report from a station in Canada... only to show that today's digital modes software is able to produce this kind of amazing results.
NOW, imagine, the performance of the digital modes achieved when a properly built and installed interface between the computer and the transceiver is in place!. MIX W and other similar digital modes software can be downloaded from several sites, some of them like MIX W are shareware, while others are offered as freeware by the authors. Surfing the WEB's search engines with the keywords "digital amateur radio" "PSK 31" and "MFSK16" should bring back many URL's of sites that provide information about the digital modes and downloading of programs designed to interface computers with amateur radio equipment.
DIGITAL COMMUNICATIONS MODES, one of the more than sixty four ways you and I can have a good time playing with our wonderful toys ... RADIOS!
And now amigos, as always, at the end of the show, here is Arnie Coro's Dxers Unlimited HF plus 6 meters propagation update and forecast... Solar activity was moderate, and may go into high at anytime now... Solar flux is now again well above the 200 units mark, telling us that the ionosphere is receiving a very powerful stream of X rays and ultra violet light energy, keeping it very well ionized.
Such high levels of solar flux keep the 10, 12 and 15 meters amateur bands wide open many hours per day... and will still make possible 6 meter band openings, although they are not going now to be as frequent as during December... The effective, smoothed, sunspot number on January 15 was 185, and the A index was 9, the number of sunspots on Monday was very near 190 and my optical observations show that this may increase on Wednesday... The winter sporadic E season is about to come to an end now.
See you at the weekend edition of the Dxers Unlimited, amigos, and in the meantime, if you already have your own amateur radio station on the air, monitor 10 meters for DX, and maybe you will find me calling CQ DX on 28.5 megaHertz, my favorite spot on 10... see you there amigos for a nice chat!
Arnie Coro CO2KK
Havana, Cuba
January 15, 2002Postal address:
Arnie Coro, "Dxers Unlimited"
Radio Havana Cuba, Havana, CUBA
e-mail: arnie@radiohc.org
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