Every once in a while, I answer amateur radio questions on Quora, and sometimes I get requests to answer them. Well, a request popped up in my inbox this morning, and I just clicked on the link. I got a real chuckle from the two replies (see below)….Dan
Does a resonant antenna work better than a non-resonant antenna?
Reply #1 from “Retired EE with aerospace and biomedical experience & 8 patents.”
Yes a resonant antenna properly matched to the transmitter or reciever will work better a non-resonant antenna. Any antenna discussion MUST include the frequency or frequency band in order to be more detailed.Reply #2 from “Physicist, Electronic Engineer, Musician (1968-present)”
No. It may be easier to match a feedline to it, but once a current distribution is achieved in it, it radiates just as well as a non-resonant dipole.
So, antenna gurus, which one do you think is right?
UPDATE 6/14/20
A third fellow replied:
In order to avoid confusion, I offer a definition of a resonant antenna. (First of all, I am assuming you are not referring to an uncommon antenna type that has no resonance, no standing wave.) Irrespective of the feedline characteristics (assuming there is a feedline—could be fed directly by equipment), at its feed point, an antenna presents a certain impedance value as a function of frequency. The frequency for which this impedance is resistive (contains no reactive component) is the resonant frequency.
The objective when feeding an antenna is to transfer as much power as possible to it. As with any type of load, this is accomplished when the source and load impedance values are “matched”. Following from the previous paragraph, a resonant antenna needs to be fed with source having a resistive impedance equal to that presented by the antenna.
At all frequencies other than its resonant frequency the antenna presents a reactive impedance component (inductive or capacitive) that adds to the resistive component. If the source impedance is still resistive, it is no longer matched to that of the antenna, so the power delivered to it is reduced. In other words, any series reactance of the antenna adds to the resistive (called radiation resistance) component to reduce the amount of current flow; so the antenna does not work as well.
This does not mean, however, that a non-resonant antenna cannot work as well. If the source impedance is adjusted to add reactance that is equal and opposite to that presented by the non-resonant antenna, the power delivered to it will equal that of the antenna’s resonant frequency. A properly impedance “matched” non-resonant antenna works (radiates) just as well as a properly matched resonant antenna.
The equipment used to maintain this impedance “match” over a relatively wide frequency range is called an Antenna Tuner. Inasmuch as mounting and adjusting an antenna tuner attached/connected directly to the antenna feed point is normally frightfully awkward, it is normally at the radio equipment end of the feedline. In this configuration, the feedline and antenna are automatically tuned simultaneously. There are implications for the most desirable type of feedline to use, but that is another discussion.
(Note: As the frequency is changed, the resistive portion of antenna feed point impedance varies as well, but this change is relatively minor and is also compensated for by the antenna tuner.)
I like this answer.
Define “works better”.
More efficient?
More useful radiation pattern?
Less stressful on final amplifier circuits?
Something else?
An aerospace and biomedical engineer doesn’t really have any credentials for the topic.
As a EE, I agree with the EE. If you get the power to the antenna, it’s there. That’s why tuners help.
So the aerospace has the answer kinda right, but bypassed the point that the antenna doesn’t include the matching circuits (they aren’t the real antenna).
And when you get down to the details, what is a resonant antenna? Some may get close, but aren’t really perfectly resonant. An SWR of 1.5:1 isn’t resonant. But it may be close enough for amateurs.
So, in an answer of perfection, you really will never get there, so it doesn’t matter. In an answer to reality, matching circuits can steal some of your power, reducing the power to the antenna. That’s not great, but it’s better than the transmitter cutting power back because it can’t match. Or feedlines melting, or spikes occurring in the system.
Getting it close is quite often good ‘nuf!
Use the antenna with the pattern you want, then match it to the feedline you want.
The classic example is the 5/8 wave vertical antenna. A nice bit of low angle gain and not resonant. Use a high-quality coil to make it (electrically) a 3/4 wave antenna so it will match 50 Ω coax.
Ask any engineer at any AM broadcast station. Those antennas are all kinds of non-resonant lengths, usually for very good reasons. Sometimes because of tower height limits, but the antennas work just fine.
Credentials? BSEE with a concentration in signals and systems. My early work was in DSP before I moved to software and search engines. I think I got a C+ in the fields and waves class. I was pretty happy with that. Rice didn’t have grade inflation.
Hah! I think I got a B in Fields, although that probably was a little inflated. I envy the kids today. I think that with all the math software available now, I think it would be easier to envision fields now.
Really doesn’t matter as long as the feed line is matched to 52 ohms. There are certain antennas where the matching circuit can be very lossy so radiated power is much less but the pattern will remain nearly the same.
Since credentials seem to matter just a BSCS here.
… if we are talking about non resonant antennas, actually it’s the radio that needs to be matched to 50 ohms… not the feedline.
The feedline may be something else. Current assumptions are that a non resonant antenna be fed with a 9:1 toroidal balun matching unit or some balanced ladder-line ( the Zeppelin style feedline) showing 450 ohms or maybe 600. Currently the best option for an imprecise and changing impedance situation.
What is the actual ‘resistive’ impedance? Who the heck knows? It changes with every change in frequency so the true (resistive) impedance is just a guesstimation at best because of the many changing parameters including height above ground, ground conditions etc.
Probably the only guaranteed way to feed a non-resonant antenna with as little inefficiency as possible with a tuner is to bring the end right to the operating position and feed it with the tuner as as an end fed non resonant long wire. Grounding or counterpoise throws in another set of changing conditions.
The advantage of a ‘resonant’ antenna system is it’s a known quantity frequency or feed-point wise… be it 50 ohms for a well matched vertical, 72 ohms as is in the case of a simple dipole or 2400 ohms as in the case of the EFHW antenna.
I am going to go with “it depends.”
Ah! The old “gotta have a ‘resonate’ antenna or nothing” debate! One of the most popular articles I wrote for eHam–and a chapter in two of my books–is titled “Resonance Schmesonance.” (See: https://www.donkeith.com/n4kc/article.php?p=32 ) And I think the title tells you where I stand. I’m no guru by any means. I was in broadcast radio for a quarter century but not on the engineering side. But as Walter Underwood points out above, very few AM broadcast antennas are resonant. They usually have what we would call a “tuner” somewhere near the base of the tower/antenna to make the antenna SYSTEM a match for the output of the transmitter, therefore allowing efficient transfer of power from the transmitter to the transducer.
Just this week, I had a chat with a ham on two-meter FM who was trimming a quarter-wave mag mount to go on his vehicle. Cut, test, cut, test, cut, test. But he was frustrated because he could not get his SWR below 1.3-to-1 for the repeater input we were on. Of course, I told him to quit cutting, that 1.3 was just fine. Chasing resonance or 1-to-1 SWR is silly!
73,
Don N4KC
http://www.donkeith.com
http://www.n4kc.com
Great responses. Thanks, guys!
I agree with the above: resonant antennas are easier to match at the feedpoint. If properly matched, a non-resonant antenna will radiate just as well as a resonant antenna. However, the pattern of the non-resonant antenna might or might not be what you want.
Note that I said “match at the feedpoint”. If you don’t match at the antenna feedpoint and instead use a tuner located adjacent to your transmitter, your tuner can indeed make your transmitter see a 50-ohm purely resistive load. This is what a lot of hams do on HF. What’s happening on your feedline in this situation can get complicated. If a resonant antenna happens to be a perfect 50 ohms with zero reactance at its feedpoint and your feedline is a perfect 50 ohms, then you don’t need a tuner and your system *might* perform better than a non-resonant antenna whose reactance you are trying to cancel in your shack. But before the fans of resonant antennas proclaim superiority, they should remember that in reality no one’s “resonant” antenna is ever perfect. It’s just less non-resonant than someone else’s. The difference might or might not be significant.
Also, remember that wherever the match takes place — at the antenna feedpoint or in your shack next to the transmitter — tuners are not built with theoretically ideal capacitors and inductors. Real-world C’s and L’s have resistances themselves. Again, the consequence might or might not be significant.
I think you’re right on, Chuck, when you say that no “resonant” antenna is perfect, and that all antennas are just more or less non-resonant. The way that I’d say it is that some antennas are more resonant than others.
When answering the question, “Does a resonant antenna work better than a non-resonant antenna?” you also can’t discount the loss induced by the feedline. There’s more loss in a coaxial feedline than in a parallel feedline, such as ladder line or open-wire feedline.
And, if you’re using a “resonant” dipole or vertical, you’re going to have a mismatch somewhere. If you feed it with 50-ohm coax, the mismatch is going to be at the antenna feedpoint. If you feed a dipole with 72-ohm coax (and I’ve done this), then the mismatch is going to be at the transmitter.
So, as Bob, K0NR, opined, I think that the final answer its, “It depends.”
No one ventured to talk about antenna aperture. That is, the ratio of the antenna element to the wavelength you are trying to support. So, there is a limit to how non-resonant an antenna may be, and still be an effective radiator or receptor for that frequency.
Imagine using a 5 inch long antenna for 80 meters. You might be able to feed it, but even if you manage to get a lot of power into that tiny antenna, the mismatch with the surrounding medium for that frequency will kill your signal, especially on receive.
Think about the folks that are trying to work the 2200 M band with a reasonable antenna (not an 1100 M long dipole or 550 M vertical). Their effective radiated power is a tiny percentage of their transmitter power, even when the system is well-matched to the transmitter.
So, yeah, an antenna doesn’t need to be resonant to be effective, but there are limits to how far you can take that.
Yes indeed, “it depends”.
I agree with KB6NU. A real-world system inevitably has mismatch(es) somewhere. The number of parameters in a real-world system is so large that ultimately the amateur operator must experiment and see what works (or doesn’t). Expect the unexpected. Theory and modeling are important, but they will take you only so far. There’s a reason why commercial MF/HF installations are expensive; they leave less to chance.
N8SBE is correct, too. Another way of looking at the point he makes is “radiation resistance” of the antenna. Radiation resistance is a good thing. It arises from the photons you are launching from the antenna. The more of those, the better. An electrically short antenna will have a low radiation resistance compared to the ohmic resistance of the antenna and connector materials, dielectric losses, ground losses, etc. Consequently the efficiency of an electrically short antenna is low. Sure, you can make it appear to resonate by canceling reactance, but you’re adding losses in the process. This is why loop antennas for transmitting HF are usually built with copper pipe that has been silver-soldered. Every ohm counts.
You guys come close, but the bottom line is that a resonant antenna will indeed work better than a non-resonant antenna. One needs to look at the total system not just the antenna itself. A non-resonate antenna will require an antenna tuner and will have high coax losses. In QST Feb 2003 they reviewed antenna tuners with reported losses as high as 47%. With a resonant antenna a tuner is unnecessary and coax losses are minimal. So a resonant antenna will always perform better than a non-resonant antenna and its because of the associated losses and not because of the antenna, unless your loading a 40-meter dipole with an 80-meter signal.
I’m not so sure about that. I think that a doublet antenna, fed with a parallel-wire feedline can be as effective or even more effective than a resonant dipole.
An infinitely short center-fed horizontal dipole antenna will exhibit the same pattern and characteristics on any frequency, as will the same type and configuration of antenna that is resonant at the same frequency, provided all other conditions are identical and that both antennas are fed with lossless feedline. Of course, both an infinitely short dipole and a lossless feedline are useful fictions. Nonetheless, this thought experiment demonstrates that a resonant antenna is not per se superior to a non-resonant antenna.
You assume the tuner is separated from the antenna by a long feed line. A remote tuner breaks that assumption.
As someone who spend many years designing electrically short (i.e. non-resonant) antennas for commercial applications, the big problem with electrically short and non-resonant antennas is efficiency.
What I mean by efficiency is the amount of energy radiated from the antenna versus the amount of energy deposited into the antenna. You can get excellent efficiency from a resistor, but not very good radiation efficiency.
Below 1 GHz, it’s fairly common that an electrically short antenna will have an efficiency less than 10%. This is also based on antenna theory for electrically small antennas with a paper by Hanson being one of the classics in this area. I can provide a reference if anyone cares.
A resonant antenna will, in general, always perform much better than a non-resonant antenna due to efficiency., assuming the resonance is due to the electrical size of the antenna. There are all kinds of tricks you can do to increase the effective electrical length of an antenna, but many of these tricks do not improve efficiency and many (e.g. folding an antenna back on itself) can greatly degrade efficiency. In general, the best efficiency you can expect is about 70% for far-field coupling.
I am chairman of my local IEEE Microwave Theory and Techniques society, but beyond that I am merely an RF engineer
Y’all keep on about transmitting antennas, but I have a suspicion that resonance is more important in receiving antennas. Trying to pluck a nearly nonexistent signal out of the ether is difficult in the best of circumstances. Having an antenna that is immersed in the EM field _and_ resonant at (or near) the frequency of interest seems like a big plus.
I’m drawing analogies with stringed instruments, where a string tuned to, say G, vibrates in sympathy with acoustic vibrations at that frequency or, indeed, harmonically related frequencies, but is non responsive to other pitches.
Since the antenna is a transducer (string) immersed in the EM field (air), I would expect the coupling to be similar. I realize that there is some ambiguity between the “antenna” and the “antenna system” – but I consider all components which interact with the field to be part of the antenna.
I would appreciate other people’s thoughts and comments on receiving antenna resonance. 73s
Wow, I just found this, as I was looking for a definitive answer about antenna heights, in relation to the wavelength of the signal. But, I was also curious about using my tuner to transmit on 12 or 17 with my tri-band beam, and some of the responses talk at that mismatching. But I am still puzzled. When I use my tuner to match on 12 or 17, do I still point my beam in the classical direction, or is the pattern scrambled because it is not on the correct frequency ? I now understand I may not be at full power, because of the additional tuner circuits, but what is the impact on the radiation pattern ?
And back to my original question, how high does an HF antenna have to be ? It seems most say 1 wavelength is good, 1.5 is the maximum. Is that true ?
I’m not really an antenna guru, so I’ll leave a more detailed answer to someone who’s more knowledgeable than I am, but I would say that using your beam on a band that it’s not designed for will affect the radiation pattern, but I’d still point it in the same direction.