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By Don Keith, N4KC
Copyright 2018 by Don Keith
I’m not trying to get a war started. Or troll for comments. All I’m really trying to do is help others in our hobby—not just newcomers—understand one of the basic concepts of radio frequency transmission: antenna system resonance. Understand it and use it to their benefit so they may better enjoy our wonderful hobby.
There, I said it. RESONANCE. Notice, though, that I used the phrase antenna system resonance. I know of no one using only an “antenna” to send electromagnetic waves whirling off into the sky. They likely have much more between the output stage of their final amplifier (not just a “kicker” but the amplifier inside your radio) and the ionosphere. Much more than just what we typically refer to as an “antenna.” And all that stuff in the chain affects that fleeting Holy Grail we call “resonance.” (By the way, anyone who responds by talking about a “resonate antenna” will be promptly flailed with a length of old left-over RG-59 coax! “Resonate” is a verb. “Resonant” is an adjective.)
Let me start the melee with a strong statement: achieving a perfectly resonant antenna system is virtually impossible. By “antenna,” I mean a transducer—a thingy that changes one form of energy into another, and for our purposes, we’ll call it an antenna (but I really mean an “antenna system”) that changes AC current into electromagnetic waves. The system at resonance forms a more-or-less tuned circuit, which throws in equal doses of inductive reactance and capacitive reactance. At resonance, they cancel each other out, leaving only resistance. And I’m talking two kinds of resistance: loss and radiation. For the purposes of this little conversation starter, let’s ignore loss—which is not usually a big factor at HF frequencies—and concentrate on the good stuff…radiation resistance, something we all want to conjure up as much of as we can.
Now, ponder for a moment all the possible variations of capacitive and inductive reactance that might be swirling around from the rear end of your radio, the jumper to the above-mentioned “kicker,” all the guts inside the amp, the jumper to the antenna matching device (I’m philosophically opposed to the term “antenna tuner” because you are NOT tuning the antenna with that box at all!), the innards of the “tuner,” the feedline, the hank of wire or aluminum that we call an “antenna,” all the various connectors and insulators and maybe even a balun, your kid’s swing set, the neighbor’s metal-roofed garage, and even the sunspot-influenced stuff God put up in the sky above you. Gosh, even the dirt in your yard enters into the equation. And your neighbor’s yard. And all the way beyond the horizon. All part of your antenna “system!”
My, my, my. Hard to imagine it would ever be possible to bring those two forces of reactance to a point where they perfectly cancel each other out and allow every microwatt of power from your little transmitter to shoot off your antenna, launched in the direction of that highly-prized DXpedition. But it can happen. Not very easily. But, technically, it can happen.
That’s one of the things you bought that matching device for in the first place. You can make it appear to your radio that you have that perfect resonance out there, stretched between two shade trees or clamped to a mast on top of the tower. With those controls on the front of the matchbox, you can adjust its internal components, thus presenting to your transmitter the exact value of impedance—the two reactances canceling each other out—that the rig wants to see. (I know. The matching device can go out there closer to the feedpoint, too, and would work better in many situations, but that’s beyond the scope of this conversation starter.)
But just for grins, let’s say Jupiter aligns with Mars and you happen to cut the antenna perfectly, have precisely the correct length of feedline, the ground and earthworms in it present the right amount of conductivity, and Junior’s swing set is exactly the correct distance from the antenna feedpoint. Capacitive and inductive reactance balance beautifully, wiping each other out, leaving only the good stuff: delightful radiation resistance.
Whooppee! You have achieved a resonant antenna system! The much-worshipped standing wave ratio is 1:1 (Our inane fixation on SWR is fodder for another article. In fact, I wrote one. You can find it HERE or at my web site: www.n4kc.com). You are emitting about as much of your precious RF into space as you possibly could manage, based on the immutable laws of physics.
Then you go and do something dumb, like QSY up or down the band a couple of hundred kilocycles, chasing a DX station or to talk with a buddy. Aw, heck! The aerial is no longer resonant. Either Mr. Capacitive or Mr. Inductive have the upper hand. The needle on the SWR meter sways disturbingly upward. Impedance dips toward zero ohms or zooms toward the sky, abandoning “50 ohms” completely. You begin worrying about your considerable investment in your nice radio.
Aw, in truth it’s probably no big deal on 40 meters or lower wavelengths. A dipole cut for the middle of the band on 40 or above will probably still work okay from one end to the other. But it won’t be resonant. No, it won’t.
Heaven forbid you try to use that dipole on some other band that is not an odd harmonic of the one for which you cut it. Resonant? Not by a long shot. And maybe so far off that your radio spits and sparks.
Oh, you might be able to adjust that matchbox so the radio is fine with everything. But signals suddenly seem weaker. People ignore you when you call them. You scream and squawk but the DX stations no longer seem to hear you. The radio thinks there is a resonant aerial out there in the backyard, but you and the RF gods soon know different.
“But wait, OM,” you say. “You told us resonance did not matter. Were you trying to get a fight started after all?”
Well, I did say that, and maybe I should have clarified it a bit. Within reason, resonance is not necessary to communicate with relative effectiveness. The men who went to the moon used decidedly non-resonant antennas in their radio communications. Few AM broadcast stations have truly resonant antennas. They use capacitors and coils—sound like that “antenna tuner” on your desk?—to get a match to their towers/antennas. Only thing is, once they start transmitting on their assigned frequency and have everything set, they don’t have to change anything. As opposed to you, you QSYing, band-hopping fool, you. That is, they don’t have to adjust anything until something else changes, like the ground system starts to deteriorate.
So why do we work so hard to make resonant antenna systems if it doesn’t matter?
It actually does, in some cases. Even if you can get your matchbox to present a lovely 50-ohm load into a ten-penny nail, it will not be nearly as efficient a radiator as a dipole cut to frequency. You probably want to achieve something a bit closer to resonance than that!
If you use a feedline that has higher loss when presented with standing waves, then you would want your antenna to be a lot closer to resonance. Again, that is true, even if your lovely antenna matching device seems to have all that mess worked out. That’s coax I’m talking about, folks. Handy and pretty as the stuff is, it will not work nearly as well if presented with a load that is way off from 50 ohms of impedance.
There are alternatives. Open-air-dielectric feedline is the best. Or window line. It’s cheap, virtually ignores standing waves, and allows you to use one non-resonant-in-most-places antenna across a broad part of the radio spectrum. It has its quirks, too, I’ll grant you. But learn more and you will see what I mean. The same above-mentioned article talks about this stuff and why it can be an important part of your antenna farm.
So here is what I am saying:
1. It is very difficult to achieve perfect resonance in an antenna system, but it can be done.
2. But is it really worth it?
3. If you do devise a resonant antenna system, once you venture up or down the band a ways, or you jump to another band, you are moving farther and farther from Shangri La. Results may deteriorate rapidly.
4. An antenna matching device can allow you to use a very, very non-resonant antenna system, but the result may or may not be a good one.
5. With the proper circuitry (antenna matching components) and a feedline that is not a stickler for resonance, you can still use a very, very non-resonant antenna with very, very good results.
6. Learn more about SWR and resonance so you don’t become a slave to them, but so you can manage them in such a way that you get the results you want.
Look, maybe you are one of those guys who camps on one frequency on one band all day every day. Fine. Get that dang antenna as close to resonance as you can. Do not invest in a matching device. Feed it with coax. And knock yourself out. Brag that you don’t believe in “antenna tuners.” “I only use resonant antennas!” you expound. Good for you, Chief. Enjoy that narrow little sliver of spectrum while the rest of us flit about across a broad swath of the shortwaves, chewing rags and nailing DX.
See, we want to get a taste of every cycle of spectrum we have available to us. Few of us are able to put up a couple of dozen “resonant” antennas to do so. (To be fair, with a decent multi-band antenna like a hexbeam or fan dipole and enough dipoles to cover 160 through 30, I figure you could get by with six or seven antennas without being too far off resonance. A trap vertical could probably cover 160 through 10, but it ain’t gonna be resonant in many places on some of those longer wavelengths! Some guys even sell multi-band, no-radials-required verticals, but good luck filling your logbook using one of those.)
Fact is, many of us prefer not spending all our operating time worrying about “resonance.” Or fidgeting about standing waves. Wringing our hands as we glare at the SWR meter.
We’d much rather learn how to manage these things, take advantage of the science, and apply it so we can have a perfectly wonderful experience every time we throw the “ON” switch.
Besides, learning and experimenting with this stuff is where a lot of the fun happens anyway.
Don Keith N4KC has been a ham radio operator for more than fifty years. After a long career in broadcasting and advertising, he now writes full time and has published more than thirty books, fiction and non-fiction, on a wide range of subjects including amateur radio. See www.donkeith.com or www.n4kc.com for more info.