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Author Topic: Resonant vs. nonresonant antenna's  (Read 13804 times)
PA3GMP
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Posts: 75




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« on: November 25, 2006, 10:53:49 AM »

DR OM's,

What are your thoughts about resonant vs. nonresonant antenna's? I know hams who swear that unless an antenna has precisely the right length (either physically or trapped) to be resonant, it's useless. I also know hams who claim that any dipole will do, the longer the better (within reason) as long as it's fed via a symmetrical feed line and connected to a proper antenna tuner.

I've used both with good results, and I really haven't found one to be consistenly better than the other. In fact I tend to put up an antenna the length of which is determined by the distance between whatever end points are convenient, I feed it via a ladder line and tune it using my trusty old MFJ portable tuner.

Without wanting to start a holy war on this subject... what are your points of view? Is resonant better than nonresonant, or is long-and-nonresonant better than short-and-resonant?

73 Frank PA3GMP/ZS6TMV
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AA4PB
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Posts: 12784




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« Reply #1 on: November 25, 2006, 02:30:37 PM »

There is absolutly no difference between the performance of a resonant and a non-resonant radiating element provided it is fed properly. Those who have bad experiences with non-resonant antennas probably tried to feed it with coax cable and a tuner at the far end. The mismatch on the coax causes a lot of loss which limited the performance. Feed it with low loss cable like open wire or put the tuner directly at the feed point and a non-resonant antenna will work find.

Actually, the tuner will tune any antenna "system" (radiating element plus feed line) to resonance just like inserting a loading coil into the element will tune it to resonance.
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W5CPT
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Posts: 557




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« Reply #2 on: November 27, 2006, 06:23:19 AM »

You can make contacts on either type of antenna, HOWEVER..  A non-resonant antenna means you can not tune to a frequency and start talking or sending CW.  You must tune the antenna system which takes time.  You can tune it on the air after trying to find a clear frequency close to the one on which you want to operate or use some external means such as a Palomar Tuner-Tuner or switch in a MFJ-259 (that is what I do). There are some automatic systems such as the Icom AH-4 or the SGC line that tunes the system for you, but remember it still takes some time. If you have any aspirations to do any contesting, a resonant antenna system is a must as you will be hard pressed to find a clear freq on which to tune.

Above all remember the first rule of Ham Radio which is "RF gotta go somewhere!"


Clint - W5CPT
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N3OX
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Posts: 8854


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« Reply #3 on: November 27, 2006, 07:14:26 AM »

"If you have any aspirations to do any contesting, a resonant antenna system is a must as you will be hard pressed to find a clear freq on which to tune. "

What about switched matching networks?  

http://www.n3ox.net/projects/lowbandvert

Not resonant on any band I use it on, but works quite well, and I don't have to do anything other than twist one knob, just like an antenna switch.

73,
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WW5AA
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Posts: 2088




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« Reply #4 on: November 27, 2006, 07:21:10 AM »

The best way is to put an antenna analyzer in line with a switch, the radio/amp to antenna tuner. You can tune up very fast and if you have a graphing analyzer, you can see how far you can move off and still have a good match. No QRM involved!

73, de Lindy
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PA3GMP
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Posts: 75




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« Reply #5 on: November 27, 2006, 11:37:49 AM »

> A non-resonant antenna means you can not tune to a
> frequency and start talking or sending CW. You must
> tune the antenna system which takes time.
That is a very good point, and especially important in a  contest or similar situation. However I'm not really a contester, not to mention pretty easy-going. :-) And even then there are several ways around that. For example I've got an old ARRL handbook with a nice design for a tuner with presets that are selected by the band select data from the rig.

Practical advantages of resonant antenna's aside, I'm mainly interested in any significant changes in performance between resonant and non-resonant antenna's. And the general consensus seems to be that these are negligible. Correct?

73 Frank PA3GMP/ZS6TMV
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WW5AA
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Posts: 2088




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« Reply #6 on: November 27, 2006, 12:39:16 PM »

Frank,

The practical point is that for TX a non-resonant (within reason) antenna is just as efficient as one that is resonant. RX is a totally different ball game. Even at resonants good TX antennas make poor RX antennas, mainly an element of signal to noise ratio. When discussing the merits of an antenna, all too often this issue fades into the background. A lot of hams would be happier if they realized this in the quest for that perfect antenna.

73, de Lindy

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NR9R
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Posts: 151




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« Reply #7 on: November 27, 2006, 06:12:13 PM »

Frank,

I think that your experience with non-resonant antennas joins the consensus.  A properly fed doublet or random wire antenna will perform very well and offer gain advantages at frequencies above the resonant length as compared to a resonant dipole. As mentioned above, the point of disagreement regarding the performance of such antennas lies in the matching method.

Typical antenna tuners on the market place a 4:1 balun on the output of the tuner where it interfaces with the balanced feedline.  Since such a balun can become lossy when it encounters high voltages, some argue that it is better to use a 1:1 current choke at the input of the tuner to prevent such losses in the balun.  This will come at the loss of impedance matching range however.  Either method is definitely a compromise to using a true balanced line tuner--that is pretty well agreed upon.

One disadvantage that occurs when feeding a doublet is that at frequencies corresponding to 1.25*(resonant frequency) the pattern is no longer broadside but begins to show a figure-eight pattern with lobes at about 45 degrees from broadside.  Although these lobes can show significant amounts of gain, it may not be in the direction you want.  

To sum up my opinion, I think the properly fed non-resonant doublet antenna wins over a trapped, folded or fan dipole.  The frequencies where losses become an issue (assuming the antenna is used above its resonant length) are also the frequencies which exhibit a large amount of gain, so you will come out ahead.  Even when considering the reduced matching losses of the resonant multiband antennas--they will only perform as a ½ wave dipole and not be able reach the same net gain.  

Hope this is useful,

Anthony
AA9OC          
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PA3GMP
Member

Posts: 75




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« Reply #8 on: November 28, 2006, 11:26:11 AM »

> The practical point is that for TX a non-resonant
> (within reason) antenna is just as efficient as one
> that is resonant. RX is a totally different ball game.

Lindy, do you mean that for RX there _is_ a significant difference in performance between resonant and non-resonant antenna's? Or is your point merely that a good TX antenna does not necessarily give good RX results as well? I agree with the latter in general, but how does it relate (if at all) to resonant vs. non-resonant antenna's?

73 Frank PA3GMP/ZS6TMV
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WW5AA
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Posts: 2088




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« Reply #9 on: November 28, 2006, 02:35:23 PM »

Frank,

Impedance matching in the case of a receiver antenna does not have quite the same meaning as in the transmitting case. Pickup Efficiency: Although the transmitting and receiving properties of an antenna are in general, reciprocal, there is another fundamental difference between the two cases that is of very great practical importance. In the transmitting case all the power supplied to the antenna is radiated (assuming negligible ohmic resistance) regardless of the physical size of the antenna system. For example, a 300 MHz half wave radiator, which is only about 50 cm (19 inches) long, radiates every bit as efficiently as a 3.5 MHz half-wave antenna, which is about 41 meters (134 feet) long. But in receiving, the 300 MHz antenna does not abstract anything like the amount of energy from passing waves that the 3.5 MHz antenna does. This is because the section of wave front from which the antenna can draw energy extends only about a quarter wave length from the conductor. At 3.5 MHz this represents an area roughly 1/2 wavelength or 41 meters in diameter, but at 300 MHz the diameter of the area is only « meter. Since the energy is evenly distributed throughout the wave front regardless of the wavelength, the effective area that the receiving antenna can utilize varies with the SQUARE of the wavelength. A 3.5 MHz half-wave antenna therefore picks up something like 7000 times as much energy as 300 MHz half-wave antenna, the field strength being the same in both cases. Thus, as my Elmer once said, the bigger the better. Although the longer antenna requires tuning for resonants at the higher frequencies (loss), the increased capture area increases gain many times over losses. ON4UN's book on low band DXing explains this much better than I can.

73, de Lindy
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NR9R
Member

Posts: 151




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« Reply #10 on: November 28, 2006, 03:23:03 PM »

Lindy raises an interesting aspect of antenna theory.  I would like to add to this:

As long as you are matching an antenna above its resonant frequency, the receive properties will be reciprocal, so I think in your case Frank, this is probably a non-issue.

Because many other factors such as directionality and polarization also contribute to an antennas signal-to-noise ratio, sometimes bigger is not always better.  160m and 80m enthusiasts often use small vertically polarized antenna arrays for receive.  As Lindy mentioned, the signal strength on such an antenna will be very small but since the signal-to-noise ratio is much improved, adding a preamplifier will overcome this.  These type of receiving antennas are also outlined in ON4UN’s book—a good book to have!

Anyway, this is probably a bit off of the original topic but I thought a little more info wouldn’t hurt.

73
Anthony
AA9OC
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AA4PB
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Posts: 12784




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« Reply #11 on: November 28, 2006, 06:50:09 PM »

The efficiency is related to radiation resistance (directly related to the physical size of the antenna) vs. the loss. If you "assume" the loss is zero then even a 1-inch long antenna on 80M will be 100% efficient and will radiate all of the transmitter power.

The problem is that real antenna systems don't have zero loss. You generally have some sort of feed line and some sort of matching system in order to provide a 50 ohm load to the transmitter. All of these things have loss.
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N4KC
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« Reply #12 on: December 01, 2006, 10:21:38 AM »

I'm really enjoying the comments on this thread!  Frank, thanks for asking a question many of us have wrestled with, and thanks to those who have answered rationally and clearly.  As a 45-year ham, I've only recently become interested in antenna theory after a period of inactivity.  A few things I've picked up that have helped my understanding of a very complicated area:

-- we need to talk about antenna "systems" instead of "antennas."  Everything from the output/input of our transmitters/receivers is part of what the oldtimers called the "transducer."  

-- There are an almost infinite number of factors that can apply to how well that system performs.  That includes feedline, tuner, antenna, height, soil, nearby obstacles, radiation angle, time of day, sunspots, and the state of various levels of the atmosphere.

--SWR is merely another factor.  1:1 SWR does not necessarily mean you have an efficient antenna system or that you are radiating as much RF into space as you might be able to.

--A truly resonant antenna system is only truly resonant on one frequency or certain multiple wavelengths of that frequency.  Move a little bit either way and it is no longer resonant.

--There's nothing wrong with working toward perfection but the truth is, it's darn nigh impossible to achieve.  We all just need to do the best we can, whether we are ever totally satisfied or not.  Can't believe even the most avid contester would not take 5 seconds or less to allow an auto-tuner to work when switching bands to a frequency range saved in the tuner's memory!

--Half the fun is in the tinkering and learning.  Keep some kind of standard of comparison--maybe a dipole or vertical--and see what works best.  If it's better, keep it.  If not, go back.

Maybe I'm stating the obvious, but it really is fun to put up some wire, give it "fire," and see it work better than anything I've ever tried before...to have someone compare my signal to another pretty good antenna and accuse me of turning on some power on the new hunk of wire.  Just wish I had more time/space/money/wire to play with!

73,

Don N4KC
www.donkeith.com

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PA3GMP
Member

Posts: 75




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« Reply #13 on: December 03, 2006, 07:13:43 AM »

Hi, Lindy,

> For example, a 300 MHz half wave radiator, which is
> only about 50 cm (19 inches) long, radiates every bit
> as efficiently as a 3.5 MHz half-wave antenna, which is
> about 41 meters (134 feet) long. But in receiving, the
> 300 MHz antenna does not abstract anything like the
> amount of energy from passing waves that the 3.5 MHz
> antenna does.

I'm still trying to get my head around this. :-)

When you compare the above 300MHz antenna to a 3.5MHz one, are you comparing them when used at the same wavelength (i.e. use a 300MHz antenna to radiate 3.5MHz) or do you use them both at the frequencies they are intended for?

If the former, how can a 300MHz antenna (which is much shorter than the wavelength) radiate as efficiently at 3.5MHz as the larger one?

If the latter, is the amount of energy picked up from the wave front during reception solely dependent upon physical antenna dimensions, or is the ratio between physical antenna length and the operating wavelength the determining factor?

As you can see I'm struggling with this. I can understand that any antenna, even a door knob, will eventually radiate the HF energy fed into it just like a 160m wire will, but there is obviously a difference in efficiency between the two. :-) Also I can understand how a large antenna picks up more energy from the wave front than a short one, but if that's the only factor why don't we use antenna's ten or more times the wavelength on microwave bands?

I'd appreciate further enlightenment. And thank you (and everyone else concerned) for raising a very interesting point.

73 de Frank PA3GMP/ZS6TMV
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PA3GMP
Member

Posts: 75




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« Reply #14 on: December 03, 2006, 07:22:44 AM »

Don N4KC opined with considerable skill:

> we need to talk about antenna "systems" instead of "antennas." Everything from the output/input of our
> transmitters/receivers is part of what the oldtimers called the "transducer."

Which may seem a trivial point, but it is not. Don is quite right. I once witnessed a discussion (which eventually erupted in... well, let's be kind and call it something less than a discussion) about whether or not propagation is reciprocal. While some hams only considered the propagational path (i.e. from when the wavefront left the transmitting antenna until it was picked up by the receiving antenna) others included the antenna itself in the equation, while yet others considered the entire antenna system including feed lines, baluns, tuners and even finals and pre-amps. Needless to see no consensus was reached. :-)

I also agree with Don that it's been an interesting and enjoyable discussion, and I am grateful to all who contributed!!

73 de Frank PA3GMP/ZS6TMV
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