|
|
|
|
AE4YW On Dipoles, SWR and Resonance of Antennas My old
grandmother use to say, "Son, you can hear everything around here but good
ham a fryin". There is a lot of
wisdom in that statement. I
am going to apply that to the building and use of dipole
antennas.
Now I am a retired broadcast engineer. I worked to keep television and radio transmitters on the air. I worked on everything from "whistles" (low power transmitters less than 1,000 watts) to 1 megawatt ERP stations powered by twin Eimac Klystrodes (at $52,000 a copy, management expected us to be a bit careful with those). I have over 35 year's bench time. I said this only to let you know that I have seen a circuit or two in my time. I don't pretend to know everything, but I do know a few things. So lets start on
the subject of dipoles by getting on down to the nearest music
store. When you build a dipole, all you are really doing is making a tuning fork. No more, no less. You cut the radiators to vibrate at a specific frequency. In my youth, I would take my slide rule and pencil and do the actual math. (You youngsters who have never even heard of a slide rule look around you. The buildings, bridges, and machines were all built with one as there were few electronic calculators before the 1980s) These days with all my dead brain cells, all I know about math is that ought from ought is ought and tails from tadpoles is frogs. That being the case, I just use an online dipole calculator. I am not trying to detract from long division as "Miss Ruby" (and her ruler) was a great advocate of that discipline. It is just that a dipole calculator is easier for me to manage. If you want to divide the frequency into 468, just help yourself. (468 / f = total length in feet) Once you have your dipole built to the lengths given in the calculator, and have checked it several times with your ruler, you are done. You cannot trim it, or add to it. Only at this length is it at resonance. If you trim it or add to it you take it out of resonance and you ruin it. I full well realize it is the practice to prune them, but that is not a correct thing to do. It will only operate full out at the frequency of design by staying the length which it was cut. You see, the problem is not with the dipole at all. The problem is with the radio! The dipole is 75 ohms impedance. It always will be. The radio is trying to work a 75 ohm load with a 50 ohm output. There is no real fix for this. Pruning destroys the frequency integrity of the dipole. The only possible solution is to build a new Pi-L circuit in the radio, taking it to 75 ohms and feeding with RG6 or RG11. That and that alone would give you maximum ERP. I follow a different drummer. I take the lid off the radio and talk to the final. I simply say, "Mr. Final, you're just going to have to get use to it!" Then I key the mike and let the final hang on. I am not going to mess up my dipole just to satisfy a $20 SWR meter and a very stubborn 50 ohm radio. Radios are very bossy, and most hams eventually give in and buy an antenna tuner or prune and destroy their dipole. Both are "out of sight out of mind solutions". Not me, I am a "let em eat cake" man. After 30 years of running dipoles at 1.5 to 1 SWR with no tuner and no pruning, and not blowing a single final, I continue on. If you take a commercial grade field strength meter (like my old Hewlitt Packard) and take readings on your dipole at 50 and 100 yards, and then go back and prune it for 1.1 SWR and take another reading, you will find that your output just dropped 50 to 75%! A mighty hefty price to pay I might add, just to satisfy an SWR meter. Why? Because you're out of resonance!!!! You now have an air-cooled dummy load. Here is a "chiseled in stone fact", SWR has nothing to do with propagation, and it only has to do with reporting an impedance mismatch in the system. I have a 1,000 watt dummy load. If I hook up an SWR meter to it I get 1.1 to 1 SWR. Perfect. But it does not and never will transmit worth a tinkers "darn" (morally correct version). So much for believing SWR is useful in transmission guessing. What I am telling you is "if it's below 2, it will do". The old Generals never worried about SWR anyway. They weren't silly enough to use transistors. Anything under 5 to 1 worked with tubes. Tubes worried almost as much about SWR as a Hell's Angel does about bad breath. So after your talking all over the place with you new cut "tuning fork", even while putting the squeeze on your final's "laticimals", is there anything else? How do you make a dipole hum like a ten penny nail, hit at an angling blow, with a greasy ball ping hammer? Why you broad band it of course! Larger radiators! (Which reminds me of spring and the return of cleavage) There are many schemes and ways to do this. Wide ribbon cable, multi wires with separators, bazookas, and so forth have been tried, some to very good result. I am a simple man, with simple needs so I use an ultra light radiator 3 inches in diameter made by welding beer cans together into strings and mounting them on cheap PVC. If you would like to see such a contraption, just go to Ebay and type in "Cantenna" (*). (See editor note at bottom of article) With a very broad band antenna you can work a whole band at a time with flat SWR across the band (SWR does count above 2 to 1). No need for retuning ever few degrees of dial movement which comes in real handy while contesting or moving from CW to voice. This is also important for CBers who get the "CBjebies" and need a break. I love CBers. They break their radios reading "CB Tricks" and bring them to me to fix. What's not to like? Don’t be afraid to experiment! As Ross Perot said "You got to build something!" Perhaps after you master the dipole, there is a Struber Curtain, a Yagi, stacked log periodics, or maybe even a long wire rhombic in your future, who knows? For you SWR retentives, try a standing 2 to 1 rectangle full wave loop. It has an impedance of 51 ohms so your SWR won't even move, satisfying the stubbornness of radios. I will leave you with what my old "Elmer", Wiley Almond, left me with all those years ago. He tossed me a AA battery and said "See if you can master the power of the AA". When I asked what in the world he meant he said, "1 watt son, 1 watt. Anybody can do it with 2 KW, let me know how far you can manage to get with just 1 watt." While I still haven't conquered the AA, I can say I have 41 states and 4 continents with that 1 watt. I have a bit of time left, maybe I'll make it. You see, it's never about power; it's always about the antenna! The term
"Cantenna" now usually refers to a WiFi antenna made from a can.
In the article, it referes to multiple aluminum cans, like beer or soda cans, connected end to end to form a dipole resulting in larger diameter conductors which in turn results in much greater bandwidth. "Cantenna" also refers to the old Heathkit dummy load in a paint can.
  Hamuniverse.com uses Green Geeks Web Hosting! |
