[Elecraft] Resonant vs. Non-resonant antennas

[Al Lorona]

Here's a true statement: If an antenna is resonant, it radiates no better and no worse than a similar-sized antenna that isn't resonant.

First, let's define what 'resonance' is. Remember how impedance Z consists of a resistive part and a reactive part? We usually write it like this:

Z = 14 - j80

where Z = impedance
          resistance R = 14 ohms, and
          reactance X = 80 ohms (which in this case is capacitive because of the negative sign)

'Resonance' simply means that the reactive part is zero (0). The resistance can be any value, but if the reactance is zero, that's a resonant system. So this:

14 - j0

represents a resonant system.

It turns out that whether your antenna Z = 14 - j80, or if your antenna Z = 14 - j0, the resulting radiation is the same. I'm assuming that there's probably a network somewhere in the system to match each of these impedances to 50 ohms.

An antenna doesn't have to be resonant. But if it happens to be resonant, it won't radiate any better or worse than if it weren't. Resonant and non-resonant antennas can radiate equally well.

You might say, "Well, I choose *not* to use an antenna matching network, so I'd rather my antenna be resonant, because that'll result in the lowest SWR."

Yes, but in the case above where the antenna was resonant, the SWR is still 3.57 to 1, so you would probably want to use a matching network of some kind. That doesn't necessarily mean an antenna tuner... you could have a balun, a quarter-wave matching section, a trap, a coil, a capacitor or capacitors, a transformer, a particular length of transmission line, something like a delta feed or a gamma match, a stub, a resistor, or something else.

The thing that got me thinking about this was a thread where somebody was bothered that their AX1 antenna (or whatever it's called) wasn't resonant. Or maybe you avoid some other antenna because it's 'not resonant'. Just realize that whether the AX1 is resonant (reactive part equals zero) or whether it's not, it'll radiate the same.

Resonance is the magic that everyone is chasing, even though it doesn't buy you all that much. I'll always remember my dad saying, "I'll tell you how long to make your antenna: long enough to reach from where it begins to where it ends." He understood the relative unimportance of resonance.

Rhombics are non-resonant antennas, but boy do they radiate beautifully. Similarly, any other antenna that's not resonant can be made to behave beautifully, sometimes by using a matching network or antenna tuner or whatever.

"But I use one of those other methods you just mentioned, instead of an antenna tuner which has lots of loss." Careful, your matching device(s) could have just as much loss as plain old, properly sized capacitors and inductors.

"But I hate twiddling knobs." Okay, that's fine. Do what you have to do, but just don't claim that your 'resonant' antenna is any better than my 'non-resonant' antenna.

"But if the antenna's non-resonant, the SWR's probably gonna be higher, and that means more loss in the transmission line." So use open-wire line.

"But if I use open-wire line, I'm going to pick up more noise."  Not necessarily.

"But, but,..."


Al  W6LX/4

P. S. It's actually a false statement that resonance always results in the lowest loss with a given transmission line. We pretended it was true so we could make our point.