Dxers Unlimited Dxers Unlimited weekend special edition for June 16-17 2007 By Arnie Coro radio amateur CO2KK
Hi amigos radioaficionados around the world ! This is the weekend edition of your favorite listener oriented radio hobby program. A special edition of the program devoted to TEN of the most frequently asked questions about compact high Q magnetic loop antennas...I am Arnie Coro, radio amateur CO2KK, and as always it is my pleasure to share with you some time on the air and on the web . Today I'll not be able to provide our exclusive and not copyrighted HF plus low band VHF propagation update and forecast because I will be out of town when you are listening to this program Now here is our technical topics section totally devoted today to MAGNETIC LOOP ANTENNAS, for a very good reason... when properly built, and I want to underscore those two words, properly built, well, when magnetic loops are constructed the way they should, there is no doubt that they are a great and sometimes the only solution for those that can not install any other antenna types due to space restrictions... Some radio amateurs living in apartment buildings have magnetic loops that can be nicely disguised as artwork or even elevated in front gardens disguised as bird feeders !!! Please remember that today I'll not be providing our HF plus low band VHF propagation update and forecast,because of my travel schedule. Stay tuned for more radio hobby related information coming to you from Havana. Our e-mail address is [EMAIL PROTECTED], and VIA AIR MAIL you may request our QSL card by sending a postcard with your signal report and comments about the program to Arnie Coro, Radio Havana Cuba, Havana , Cuba. ..... This is a special edition of Radio Havana Cuba's radio hobby program.. the name of the show is Dxers Unlimited and YES, we do attempt to cover all the more than 80 ways you and I can enjoy this wonderful hobby, from sending fast scan low resolution TV on the 10 meters amateur band to picking up more FM stations for your log via meteor scatter propagation... from talking to another radio amateur across town with your 2 meter band FM handie talkie, to picking up signals from deep space probes!!! ANTENNAS... ANTENNAS... yes they are one of the favorite topics of Dxers Unlimited's listeners, according to the correspondence that I receive here.. Today our technical topics section,as I said earlier, will be devoted to the magnetic loops, and some frequently asked questions about them... Let's start with a definition !!! 1. What is a Magnetic Loop? A Magnetic loop also known as an STL or small transmitting loop is nothing else than a large coil with a capacitor added to bring the system to resonance. It is unlike a large resonant loop (a more common antenna type) in that the loop size has to be smaller than 1/4 wavelength and does not resonate on its own, it does need an external capacitor in order to make the inductor capacitor circuit to resonate on a specific frequency.Again, see the difference, the loop must be less than 1/4 of a wavelength, and it needs the ""help"" of a capacitor to resonate to the operating frequency. The loop is brought to resonance by attaching a capacitor to the two open ends of the loop. The loop is tuned by changing the value of the capacitor. An increase in capacitance will bring the resonance of the loop lower in frequency, a decrease in capacitance will make the loop resonate higher. Now here is question number two 2. How well does it a magnetic loop perform compared to other antennas? A Magnetic Loop's performance seems to be dependent on the care and quality of its construction, placement above ground, and other factors. A generalization of performance would place it a bit below that of a dipole, and in some instances exceeding the performance of a dipole. It usually will far surpass the performance of most mobile vertical antennas, and other compromise configurations Question number 3 frequently asked about magnetic loops 3. Why some people build magnetic loops as squares instead of circles or octagons? Aren't circles more efficient? Yes, circles are more efficient, by about 10% as compared to a square loop! But they are harder to make when your working with one or two inch copper. I would rather just build the loop a little bigger to gain back the loss. For instance, according to loop modeling software, it only takes a extra 50 centimeters extra length or for that matter making the loop circumference of an extra 12 millimeters more diameter of pipe to gain back the lost efficiency. Also, squares are easier to make and have less solder joints than octagons. Now the fourth question about magnetic loops here at this special edition of Dxers Unlimited.. 4. Is copper better than aluminum for making magnetic loops? Absolutely and for the most part. It has over a 20% difference in efficiency according to some modeling software. More efficiency equals more performance. It's also a lot easier to solder, and can be found at your local hardware store. However, aluminum is far lighter. In a few seconds we will continue with this special edition of Dxers Unlimited devoted to magnetic loop antennas... You can send your radio hobby related questions to [EMAIL PROTECTED], or VIA AIR MAIL to Arnie Coro, Radio Havana Cuba, Havana , Cuba ...... NOW question number 5 about magnetic loops 5. How important is loop circumference? How important is the size of the conductor diameter? Loop circumference relates directly to efficiency and performance The larger the circumference of the loop, the better your loop will work -- however, if you exceed 1/4 wavelength, things seem to get mysterious. It's a balance, try to make your magnetic loops as close to about 85-90% of the wavelength in circumference, and that seems to be the magic number. . NOW a very important question follows, number 6 most frequently asked question about magnetic loops. 6. Do you really need to solder or weld all the joints on a loop? According to almost all the literature I have seen on loops, all connections must be soldered or welded. Please read on because I do not believe it's that simple. It appears that mechanical connections create resistance in the conductor which cause major losses in efficiency. It is interesting to note that all the loop articles I have seen and read do not discuss mechanically built loops, only welded versions. I myself have built both mechanical and soldered loops and have not found major variations in performance if the mechanical connections are well made. . I have also built loops mechanically, tested them for some time, and then soldered them together to see if there would be a measurable change. As of yet, as long as the mechanical connections are well made, there seems to be no measurable difference, especially if you keep the magnetic loop antenna indoors,without exposing it to the weather, and let me add that many radio amateurs have to use their magnetic loop antennas inside their homes due to the infamous CCR restrictions, especially at new urban developments. 7. Can one use regular air variables from an old transmatch for tuning? Most loop literature would say that the wiper connections on commonly found air variables are too lossy. Also voltage capacity is an issue. Any capacitor used should be able to withstand high voltages in the 3-4KV range if you are running 50-100 watts. If you are using welded stator or butterfly capacitors with a high enough voltage rating there should be no problem or significant losses. However, high voltage stator or butterfly capacitors with a large capacitive range usually cost about the same as a nice used Jennings Vacuum Variable. I personally use the vacuum variables. A cheap alternative to expensive high voltage variables is the use of coax for monoband utility. Take a good piece of coax, separate the braid from the conductor for a couple of inches and attach each side to each open end of the loop. Use varying lengths of coax to create different capacitance values for tuning. Trim the coax till you reach your desired resonance. This works very well if you don't mind being stuck on one band. You can also change the coaxial stubs when you want to change frequency pretty easily. I have heard opinions put forth that braid in coax is too lossy for good loop performance for capacitors, but I have not found this to be the case at all. 8. How well does a loop made of coax work? When using coax, either the ground braid or the center conductor can be used. Unless you are using one inch hardline the diameter of the braid is simply too small for high efficiency. Have I tried making a magnetic loop out of the outer braid of RG-8U and other types of coax? Yes. Does it work well? In my opinion the answer is no. One inch or even half inch copper pipe will work much, much better. I would not recommend this strategy at all since every experiment of this type yielded poorly performing loops. Also, some opinions believe that the braid in coax is too lossy because it is not one piece of metal but strands of copper with minute gaps between the pieces introducing excessive losses. You can certainly build a magnetic loop using half inch coaxial cable for receive only applications, and such antenna works quite well according to my experience, but don't try to work DX with it connecting your ham radio transceiver to such a loop ! 9. How do you compare copper or aluminum strap to pipe? Modeling software claims a square strap is equal to one half the size of full diameter pipe. 10. Does a loop have a gain pattern? What does it look like? Does it matter? A vertically positioned magnetic loop has a figure eight gain pattern along the plane of the loop itself. In other words, think of two doughnut shaped patterns radiating from the the vertical portion of each vertical side. This pattern creates a null where the two donuts meet in the center of the loop. So a loop with its vertical sides placed north and south, will radiate best north and south. The same is true if you rotate the loop east and west. Loops can be placed on rotors and the loop can be turned for best signal strength or it can be used to null out offensive interference. Magnetic Loops also radiate RF at all angles, so they are well suited for both high and low angle reception and transmission. This is advantageous for both high angle NVIS work, and DX. Loops that are mounted horizontally radiate in an omnidirectional pattern and do not share the benefit of a vertical polarization. I would recommend vertical placement.NOW ... a very common question ... And now an extra question, that was added after the initial script was written, in order to answer an e-mail message that came in just as I was going to wrap up the program... So this is question number eleven about magnetic loops 11. Why are commercial loops so expensive? Magnetic Loops require high voltage capacitors for more than QRP power level operation. These capacitors tend to be very expensive. A brand new Jennings vacuum variable can cost at least five hundred dollars (they are a lot cheaper used). High voltage stator and butterfly capacitors also cost a lot of money. So the final product including the capacitor can be quite expensive. Also, commercial manufacturers desire to build the best possible loop, so they employ all welded construction on loop joints. This also adds to the cost. And now amigos, let me tell you once again that this show had to be digitally recorded one day in advance of the day that it is going on the air, because I had to travel to the provinces to attend a scientific event. Hope to have you listening to the next edition of Dxers Unlimited, and don't forget to send me your signal reports and comments as well as any radio hobby related questions to [EMAIL PROTECTED], again [EMAIL PROTECTED], and VIA AIR MAIL send a postcard or letter to Arnie Coro, Radio Havana Cuba, Havana , Cuba... ---[Start Commercial]--------------------- Preorder your WRTH 2007: http://www.hard-core-dx.com/redirect2.php?id=wrth2007 ---[End Commercial]----------------------- ________________________________________ Hard-Core-DX mailing list Hard-Core-DX@hard-core-dx.com http://montreal.kotalampi.com/mailman/listinfo/hard-core-dx http://www.hard-core-dx.com/ _______________________________________________ THE INFORMATION IN THIS ARTICLE IS FREE. It may be copied, distributed and/or modified under the conditions set down in the Design Science License published by Michael Stutz at http://www.gnu.org/licenses/dsl.html
