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Ο λόγος στασίμων 1:1 ενός κεραιοσυστήματος, δεν συνεπάγεται τον 1:1 λόγο μεταφοράς ισχύος, προς την κεραία!

Thursday, 19 March 2015

Antenna Lessons from the Old Timers!

Antenna Lessons from the Old Timers

Don Keith (N4KC) on May 24, 2007
View comments about this article! 


Lessons From the Old Timers
By Don Keith N4KC (www.donkeith.com)
Down here in Alabama, we know that three things inevitably lead to fistfights:

  • Where your loyalty lies when it comes to Alabama or Auburn football

  • Which rib joint has the best barbecue

  • And which route offers the quickest trip to the beach
On the ham bands and in Internet forums, there are at least three equally incendiary topics:

  • Contesters “crowding the bands”

  • The FCC dropping the Morse code requirement for new licensees

  • And whether an antenna cut to resonance radiates better than one that is not
Don't think these are hot topics? Then you have not been listening or reading! I'll leave the first two alone for now so I can foolishly—and at risk of “flaming,” personal attacks, and questions about my heritage—take on the third topic. I do such a silly thing primarily for three reasons:

  • We are enjoying an influx of newly licensed and newly privileged HF operators who might be able to benefit from a rational discussion on the subject.

  • We now have TEN amateur radio HF bands, and extended Advanced/Extra SSB privileges that challenge the bandwidth of most antenna system installations, especially on 80/75.

  • And as a student of history, I maintain that we can learn valuable lessons from those who came before us. The old timers who pioneered radio were correct on lots of things…including getting the most from limited antenna systems.
Here's the contention of many otherwise knowledgeable hams: you are always better off using an antenna that is cut to resonance for a particular operating frequency. That claim, many say, should be engraved in stone. It's one of the great truisms.
“Resonance.” It sounds like such a nice word. “We're in resonance on that issue, my friend.” “That topic resonates with the people!” Dictionary.com defines the word thusly: “To reinforce oscillations because the natural frequency of the device is the same as the frequency of the source.” Who could possibly argue with such a wonderful purpose? Don't all we hams want to reinforce oscillations? I know I do!
To cut to the chase, when talking about antenna systems, we call them “resonant” when the capacitive reactance present in the system is equal to the inductive reactance, and the two cancel each other out, leaving the impedance at the load point at its design value—typically 50 ohms. In that magic alignment by the gods of RF, the antenna is able to radiate into space most of the radio frequency energy that is sent to it from the transmitter via a feed line. Everything seems predestined to work well together. A dipole antenna hanging high between two trees will—on its design frequency and if properly constructed—present something close to 50 ohms impedance. Our typical coaxial feed line has a characteristic impedance of 50 ohms. The output circuit of your YaeKenEleIcomTech radio craves a 50-ohm load.
There we are! Resonance! Maximum transfer of energy occurs! Music swells, flags unfurl, the sun breaks through the clouds, and all is right with the world! We have achieved resonance!
But then, relatively new ham radio operator, you simply cannot leave well enough alone. You go and do something dumb, like touch the tuning knob and change frequency up or down the band in search of new people with whom to chat, or you go off down the band looking to chase some rare morsel of DX. Or, in a truly desperate move to find someone to talk with or to seek better propagation, you flip the switch to change to another amateur band entirely. Suddenly, your fancy, new transceiver is faced with an impedance value that is considerably removed from the Holy Grail of 50 ohms. The value may climb into the hundreds or even thousands of ohms, or drop to almost nothing, becoming disgustingly capacitive or inductively reactive. Suddenly, standing waves are introduced into the system as the RF energy encounters the ugliness of non-resonance. Energy is rudely deflected back down the coaxial feed line, all the way to the output of the transmitter from whence it came only a fraction of a second before, but it does not like the 50 ohms it finds there either. Zoom! Off it bounces once more, back up the cable, waving at its friends who are on their way back down already. But there are fewer and fewer of the reflected radio frequency waves now because some of them are being burned up—zapped energy—due to the loss in the coax.
Thankfully, if the carnage is too much—the standing waves too large a portion of the originally emitted energy—the transmitter does the only humane thing it can do. It shuts down. Then it refuses to operate at that wavelength ever again. You, dear operator, have no other choice. You return to the vicinity of the spot on the dial for which you originally designed that antenna, ignoring the limitless other frequencies and bands where others seem to play at will with no concern for the impedance encountered by their shiny YaeKenEleIcomTech radios.
But how do they do it? Gosh, there are ten amateur HF bands, and some of them are remarkably broad. Your transmitter only seems to like certain spots on those bands—those that are odd multiples of the design frequency of your nice dipole antenna, but those are few and far between and are mostly dead all the time. Finally, you ask another ham when he wanders down to where you are stuck, in the middle of the band. He is on the air, operating all over the spectrum, even though his signal is not really all that strong when compared to some of the others. Still, he seems able to move and transmit even when he is farther away than a few kilohertz in either direction, so you swallow your pride and ask him how he does it. Somehow, you manage to pull his answer out of the static and noise.
An antenna tuner! Well, of course! All you need is that wonderful device that allows you to show your expensive radio a nice 50-ohm match and all is right with the world. You can dash and flip all over the RF spectrum, working everything you hear. You had no idea the answer was so simple! Soon the box arrives from the manufacturer and you hook the “tuner” up between the rig and the coax feed line. You follow the directions and soon, after some spitting and sparking somewhere inside the radio as you learn to adjust the capacitor and inductor inside the shiny, new box, it shows you a wonderful thing on its sexy front-panel meter—a near one-to-one SWR! The rig's happy again. You go off to the hinterlands of each band, trying the thing out. It still balks in some places, but for the most part, it seems to load fine.
It should. The manufacturer's catalog said it would match almost any load. You've heard guys talk about loading to a bedspring, a hank of wire tossed out the window, a screen door. And it cost two weeks' salary. It has to be good!
Soon you are able to transmit on frequencies previously unavailable to you, using your high-hung, well-designed dipole all over creation. Sometimes you actually get a response to people you call, though they often lose you before the QSO is completed. You even work DX, though the reports are typically bad, and you never seem to be able to get through in the pileups for the really rare ones. The mic bites you when it touches your lip while you are talking. The XYL complains about the answering machine starting up by itself when you are on “that #%&*@ radio!” The neighbor lady stares at you angrily when you meet at the mailbox.
Hey, the sunspots are really bashful nowadays. The ionosphere sleeps most of the time. You only have a hundred watts. The bands are rife with static this time of the year. All ham stations have some RFI and the fact that your mic stings your lip confirms for you that the rig is making RF somewhere inside its box. You'll do a better station ground someday, even though you thought you had a pretty good one already. At least you are on the air, exercising those new privileges, having a blast in the world's greatest hobby.
But there is still that nagging suspicion that other hams are hearing and getting out better than you are. It can't be, though. You have a one-to-one SWR. The meter says so. That's the best you can do. The rig is happy. You work DX sometimes. You get through on the local roundtable most of the time. And all with that one dipole, the only antenna you will be able to put up for a while.
Then, one day you have a nice conversation on a band far removed from your antenna's design frequency, talking with a distant station who has a really big signal. You assume he is running power but when you ask what kind of amp he has, and that you've been considering getting one so you, too, can get out better, he tells you something that is hard to believe. Even though he has an amplifier, he doesn't even have the filaments turned on at the moment. He rarely uses the thing. Doesn't need to. You grin. The guy's clearly lying. He's what you call “arm chair copy,” one of the loudest signals on the band.
You ask about the antenna. He tells you it is a dipole, no higher or longer than yours. How about the tuner, then? Same make and model as yours. Lucky guy! He obviously lives in an RF hotspot, over great soil, maybe surrounded by saltwater. Nope. City lot. Rocky clay soil. Nearest saltwater is 500 miles away.
Then he casually mentions his feed line. It's something he calls “ladder line.” 600-ohm ladder line, homebrew, using bits of plastic coat hanger cut to 6-inch lengths to keep parallel runs of 14-guage wire an equal distance apart as it runs from the antenna feed point to the house. It runs right into the shack, through a feed-through in a windowpane, directly to the balanced output of his “antenna matching device.” For some reason, he makes a point of not calling the box an “antenna tuner.”
0x01 graphic
But what difference does this “ladder line” stuff make? You have some really nice RG-8X that the dealer said was perfectly fine for HF. And it is so easy to work with. “Ladder line” sounds ugly and not a little bit dangerous. And without a layer of copper shield to protect its insides, doesn't he have to be really careful where he runs the stuff to keep from frying neighborhood kids and small furry critters?
Then your new friend says he wants to tell you a few things about the old days so you will understand his preference for that old, outdated method of feeding RF to an antenna. You roll your eyes, check the station clock, and almost make up an excuse so you can tell him you have to QRT. But it's still a few minutes until net time so you humor the guy and listen to what he is anxious to tell you.
“Back in the early days of radio, hams had to find the easiest and most efficient ways of doing things,” he says. “Often they had to make whatever they needed. There was no coax back then. It had not been invented. They came up with air-dielectric feed line and found it worked very, very well. Nice, low loss. Cheap. Easy to make themselves. So the standard in those days was 600-ohm ladder line, two parallel runs of wire separated by some kind of non-conductive material every foot or so.”
600 ohms? Your ears perk up. You're still learning about all this impedance stuff, but you know 600 ohms is a heck of a long way from the 50 ohm match your rig wants. The 50 ohms your pretty run of coax presents. And a far cry from the impedance typically encountered at the feed point of a simple dipole antenna. You ask him the “SWR” question. Surely it was a problem, even way back then, when dinosaurs roamed the earth.
“Back then, the output circuits of the tube-type rigs they used had a great deal of matching range built in,” he explains. “Most of the inductors and capacitors we now find in our outboard antenna matching devices were a part of the transmitters way back in the early days. But even so, those guys not only didn't know much about standing waves, they didn't really worry much about it. The loss in that ladder line was so low, even if there was a mismatch at the antenna feed point, and even if there were standing waves on the line, the RF was eventually mostly all radiated. It didn't get lost in the feed line, like it can in coax.”
For some reason, you feel compelled to defend the honor of good, old coax. If ladder line was so good, why did everyone go to coax in the first place, once somebody built that better mousetrap?
“It is easy to use and work with, not a problem to run into a shack next to all kinds of other cables and metal, and the stuff works well in many instances,” your new friend acknowledges. “For unbalanced antennas or VHF and UHF, it's preferable by far. Remember, though, that back in the day, there were no 60-, 30-, 17- or 12-meter bands yet. Most ops used a relatively narrow range of frequencies, and the typical antenna farm usually consisted of a dipole for 80/75, a dipole or vertical for 40, and a tri-bander for 20, 15 and 10. Nowadays, with so many potential operating frequencies, few of us can manage antennas that are specifically cut to work on each of those bands. Thanks to the old-timers, we knew there was a way. And the way was the open-air-dielectric ladder line or mostly-open-air window line.”
Net time has come and gone but you still don't get it, so you ask him to clarify his position. “Wait,” you say, ignoring the tingle when the microphone brushes your lip. “You are saying SWR doesn't matter? That can't be!”
“Well, sure it matters, if it's high enough. But by simply using a much lower loss feed line, you make it much less a factor. Look in the ARRL `Antenna Book' at the comparative loss between different types of coaxial cable and ladder or window line. There is loss in any real-world conductor, but it is so much less in the old-fashioned stuff that it makes those trips up and down the feed line for reflected power much easier, and most of it gets radiated eventually by your antenna, not burned up in your coaxial cable.”
But what about that 600-to-50-ohm mismatch back at the rig? 12 to 1? Serious stuff! Your radio ain't gonna like that one bit!
“Best thing for a balanced, ladder-line-fed antenna system is what is called a balanced antenna matching unit, which is, of course, designed for matching a balanced antenna system. It does a nice, effective job of matching the 50-ohm output of your rig to whatever impedance you encounter. And believe me, that impedance will vary all over the place when you try to use one big dipole on all ten bands. But it is not really a worry. The low-loss line takes care of most load mismatches you'll see. Those standing waves eventually go dancing off into space to hopefully be reflected back to earth somewhere near that big dx-pedition everybody's calling.”
You check to make sure but you have a balanced output on your tuner…er…antenna matching device. Can't you just use that to match the antenna?
“Sure,” he says. “That's what I'm doing now, though I'm going to build myself a balanced tuner when I get the time and find the parts I need. It's an easy project, even for a beginner. You and I have a 4-to-1 current-type balun…a balanced-to-unbalanced transformer…in the ATU…antenna tuning unit, if you want to call it that…and the circuit inside the device will present a nominal 50-ohm load to your transmitter. Our balun is heavy enough to work fine at the power levels we use, but I have another much heftier unit I use when I throw on the afterburner. Some ATUs use voltage-type baluns or they simply are not built tough enough to handle the kinds of mismatches you may encounter on a very wide range of frequencies you will be able to operate on. Those don't give very good results and could even fail. By the way, I don't call them `antenna tuners' for a reason. You are not `tuning' the `antenna.' The typical way most folks `tune' an antenna is make it longer or shorter. What you are really doing with that box in the shack is matching your transmitter to the antenna system. It is an antenna system!”
How can you tell if the internal balun isn't “tough enough?” you ask.
“Smoke and flames,” he says and laughs. “Just make sure the thing is rated for much higher power than what you intend to run. By the way, there are other ways to do this thing, you know. Want to hear about them?”
You ignore the XYL screaming about your “Donald Duck voice” messing up “American Idol” on TV and tell him to go ahead. There are some logistical problems with open wire line, he admits. It needs to be kept at least a few inches away from other metal, cables, and the ground. He tells you about how some hams run the ladder line to a balun outside the shack and use as short a run of high-quality, low loss coax as they need to get inside the house and to the tuner. They usually have a one-to-one balun for this purpose since it's typically best to pass whatever impedance you encounter at the feed point to the tuner. If your system sees a very low impedance, you don't want to step it down any more. Matching devices do better when they are attempting to match higher impedances rather than lower ones.
Some fellows put a balun right there at the feed point, then run coax to the shack. That's not necessarily a good idea since there will still be standing waves as you move around the bands and they will still be dissipated as heat in the cable.
There are some who put a remote matching device at the feed point, tuning for 50 ohms, and then running coax. That works pretty well, but you still have to have an ATU that can stand weather, be light enough that it doesn't drag down your aerial, and has current running to it to remotely so you can change the capacitive and inductive parameters from your operating position in order to find the best match.
Other hams tape two runs of good, low-loss 50-ohm coax together and solder each of the two sides of the ladder line to the center conductors of the coax cables. Then the grounds are tied together on the matching device end and hooked to the station ground. Finally, the two center conductors are attached to the balanced output of the matching device. The twin-run of coax should be kept as short as possible, of course, but the 100-ohm impedance presented is little or no problem.
You think for a moment. You have saved your best question for last. You ignore the buzzing sound from your nearby stereo speakers as you speak into the microphone.
“But regardless of the feed line, using an antenna on frequencies where it is nowhere near resonant is not as good as having an antenna cut to resonance for that frequency, right? This is just a compromise and we pay a heavy price for trying to use just one antenna from 1.8 to 30 mHz.”
So you've said it. A cut-to-resonant-length antenna is always better. There is only a slight pause on the other end of the circuit.
“You've been listening to some of the guys on 75 meters, right? Or reading those forums on the Internet. First thing, don't think of it as an `antenna.' Think of it as an `antenna system.' There are lots of things that make up your antenna system—the output circuit of your transmitter, the cable to the ATU, the ATU, the feed line to the antenna, the antenna itself, the earth beneath it, the trees in your yard, the chain link fence at the back of the lot, a mountain a mile away, the atmosphere above you. Obviously you don't have much control over some of that. But you can bring a good portion of it into a state that is what we call `resonance.' You have two goals in the process.
“First, because of the way most of our solid-state radios are designed to work these days, you must present a load at the output of the rig that is relatively close to 50 ohms. Some radios are more forgiving than others, but a serious mismatch will either damage the rig or cause it to cut back power or shut off completely. Most amplifiers—and especially the new solid state ones—are just as picky about the load they prefer. If the mismatch is so great that the rig won't work, it's darn hard to make contacts!
“Your second goal in life is to cause the antenna…the wire-in-the-sky part of the system…to radiate as much of the power that you send it as it can. Yes, one way to do that is to trim the antenna so that it is non-reactive at a particular frequency—the one you use all the time—and close to 50 ohms. Then you can feed it with coax. Even then, you may want a balun at the feed point to try to keep common mode currents off the shield of the coax and stray RF out of the shack and house. That RF energy does you no good there. It just makes XYLs and neighbors really irritable and gives you a painful tingle sometimes. But remember, the farther you venture from the design frequency of the antenna, the greater the mismatch, and eventually, with coax, the standing waves will be high enough to cause loss of precious power. Yes, the antenna will work okay at odd multiples of the lowest design frequency, but how many of those actually fall within an amateur band? And yes, you may be able to dial in the right combination of inductive and capacitive reactance to please your radio, but you won't be throwing much of your original power in the direction of the DX station's antenna system.”
He offers to email you a diagram of the antenna system he built when he first began to experiment with the old hams' way of doing things. He promises it is cheap and simple, and that the dipole part of the antenna system is decidedly non-resonant—by itself—at an almost infinite number of frequencies. It is not even the best setup possible, he notes, but it is far better than what you are currently using. It is cut to be a total of a half-wavelength long (each leg is a quarter-wavelength and should be exactly the same length so it will be truly balanced) for the lowest frequency that is anticipated to be used…or that will fit in a yard. He maintained that it would work fine on most bands from there through ten meters when used with ladder line or window line, 300 to 600 ohm impedance, and a good quality one-to-one balun.
When the diagram arrives in your in-box, it looks like this:
0x01 graphic
After saying your 73, you sit back and think about what the nice Elmer has said. It does make sense. So much so that you invest in an antenna book and do some research on the web. Though you still see some of the “resonant antenna rules” posts and hear on the air lots of people preaching the gospel of the resonant antenna, you also see lots of information that backs up what the fellow said.
You Google W2DU and read excerpts from his book on the subject. You visit W4RNL's site and find a wealth of information. You purchase a good-quality one-to-one current-type balun, a spool of inexpensive window line, and a good, strain-relief center insulator designed for the open-air-dielectric feed line. Then, when you get the antenna built and up in the air, you marvel at what you have been missing all this time.
And you vow that from now on, you will begin listening to what the old folks say. Sometimes, while they were dodging dinosaurs and discovering fire, they actually figured out how to make simple antennas work much better!
http://www.eham.net/articles/16690


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