Is resonance really where you get best antenna performance?

[The DB]

I know, there are a lot of people that will quickly say yes and move on. I was once one of them.

I note that it is interesting that no one that said tuning for "resonance" is always the best has been able to actually show this. They believe it to the core and can often give hypothetical examples, but no one has ever actually demonstrate this, and they refuse to question this belief. Here I am doing just that.

I have, for a while, been saying that resonance is not the determiner of an antenna's peak gain, although I have been somewhat careful with my wording because I am aware that when I say this, I am going against the grain, and because of that, even here on wwdx, people will sometimes trash you or discount what you are saying or just call you an idiot.

The thing is, I don't tune antennas like most others. While I do use an antenna analyzer (I currently use a NanoVNA) I tune the antenna based on field strength, not where one variable or another is whatever. This has led me to a different view of what is going on that most people never actually see. I have been doing this for quite some time, and have been posting about this for several years on this very forum, as well as others. There are maybe two others at most that I have ever seen even refer to field strength.

Now I'm not saying other methods are wrong. You want to tune for X=0, the you have my blessing. The same goes for SWR. I'm just saying that what you will see from doing these are different than what I see when using field strength, that is all.

To start I am going to post a graph. This graph is a center fed dipole mounted 30 feet off the ground. It is made out of copper, an the frequency used is 27.2 MHz. The graph shows the gains from approximately two feet shorter than resonance to approximately two feet longer than resonance for a change of four feet from one side of the graph to the other. It is important to note that the resonant point is not in the direct center of this graph but offset a bit, as is SWR.

[photo=large]6692[/photo]

Here we see gain go from 7 dBi at the antennas shortest length to 7.2 dBi at the antennas longest length. There is no peak at X=0, instead the curve continues like nothing has happened.

[photo=large]6687[/photo]

The numbers on the left are for gain, the numbers on the right are for X. Here we clearly see where X crosses X=0 line and we see what the gain is at the point.

I also did the same with SWR for those that want to see it.

[photo=large]6688[/photo]

I will point out that the low SWR point and where X=0 are about an inch and a half apart from each other. They do not line up on this antenna. Also, it is hard to see on this graph, but the SWR low point is 1.55, which makes sense for a center fed dipole at this height.

So far, this is what you would expect to see for SWR and reactance if you plugged the antenna analyzer directly into the antenna. However, most real world installations have something else in play, namely coax. This is fine for what I have shown, but to show accurate data on what happens to said gain, I have to include the feed line. Unfortunately, to do this we need to do some additional math... I used Open Office Calc (an Excel clone) to do this math.

[photo=large]6686[/photo]

Yep, it was a pain in the... And that isn't even all of it.

To start with I am going to take the first graph and re-scale it so I can better show what happens after we add coax to the mix. The only difference is the scale on the left side of the graph.

[photo=large]6693[/photo]

Now lets add some coax. The antenna is 30 feet up, so lets add 35 feet of LMR-400 to run straight down to the radio below.

[photo=large]6689[/photo]

We see how the coax effects gain. Its hard to see here, but peak gain does not line up with either SWR or X=0, and is in fact on a longer part of the antenna then either of those. Not much longer, but longer none the less.

We also see that the very small amount of losses in this coax (0.222 dB into a perfect match) has more of an effect on peak gain and the overall gain curve than very large changes in X (which scales from almost -150 to over 100).

Now lets add in a longer section of coax, in this case 100 feet of LMR-400. I'll leave the 35 foot length in as a comparison.

[photo=large]6690[/photo]

Here we see more of the same. In this case, 100 feet of LMR-400 (which has 0.635 dB of loss into a perfect match), the peak gain point is actually on the other side of the X=0 point and is approaching the low SWR point. Making the coax longer will simply push the peak gain length closer and closer to the low SWR point.

For comparisons sake, 18 feet of good quality RG-58 has 0.358 dB of loss. This is the best you can expect to see with most mobile antenna (read magnet) mounts that you buy with rare exception.

For base antenna comparisons, 100 feet of good quality RG-213 will have 1.049 dB of loss. This amount of loss is right at about the upper limits that I try and use. Anything beyond that and I really try to convince the person I am working with to get a better feed line.

As someone who uses field strength as the primary method of tuning antennas, I have seen this very thing happen time and time again. The coax you choose affects field strength in general, as well as peak gain, more than anything else out there, including SWR and X. The changes to performance seen even over what is a very wide range of X values pales in comparison to even a very small change in coax losses.

Now to put one last lens of perspective on all of this. On all of the comparison charts I listed above, you won't actually notice the difference in real world performance no matter if you choose X=0, the low SWR point, or the peak field strength point for tuning an antenna. The difference between all three of these points in all of the charts above is so insignificant that you aren't even approaching anything you will ever notice. In fact, you would have to increase the differences in the worst case scenario by many orders of magnitude to ever get close to seeing such a difference. This is why I have said on multiple occasions to tune to whichever you see fit, and don't worry to much about it. Performance wise you won't benefit from changing how you tune your antenna. That being said, there is the benefit of you potentially learning something new, and while that may not necessarily help your antennas performance, you may become better at working with them over all...


The DB

 

[Handy Andy]

Good to see this...

I'd like to comment but I'll await to see if other wish to chime in.

You did good in the explanation - but this is difficult to "power point" but I can easily see what you're getting at.

Though hard to "visually demonstrate" it is not hard to visualize as modeling in ones' head from whom has asked "Is resonance the ultimate - or is there something more" - this tells me there is something more.

• that 72 ohms versus the 50~52Ω representations -
• one is more true to form in free air, while another, as a means to match antenna puts the operator in a position of losing a working value of radiation as a resistance and pattern development (mobiles unable to suspend a feed point) and are forced to use mono-pole Marconi-style with the counterpoise developing the opposing image via vehicle chassis or radial designs ground images for home - as an example
• makes me ask then; about the validity of 50Ω - is it only a working value meant to simply reduce problems of SWR matching because of the type of connections - coax and output - used?

 

[sp5it]

You can skip coax from measurement by using 180° coax or use measurement device can treat coax as transparent.
Mike

 

[TheBlaster]

I will try and follow this but it is a little complex so far.

Myself, I tune for the lowest price.

I build a well known design single element antenna, a bit longer than needed, that will be within 10pct of getting a very low SWR and I look at the internal SWR meter on my radio and trim it down with my ground radials - if required - installed, till the meter almost not moving in the middle of band where I want to operate. I get the SWR as low as I possibly can.

Then I am heard all around the world on a single wire element and 100W - so low SWR on a known design definitely works very well.

Now to try and understand your post.... so your 7dB +/- 0.2dB or so first graph.. Are these 7dB readings coming directly from a field strength meter scaled in dB you are placing near the antenna in reality ? Does that field strength vary along the length of the antenna or is it the same from say the centre of the dipole, to 5 feet along or 10 feet along or at the end - equidistant from the radiating element ? i.e. is the field strength changing with distance away from current or voltage centres ? I thought field strength is measured in... W/m2 or V/m

Maybe your dipole is just starting to slowly reach the optimal of a 5/8 wave antenna (albeit horizontal) and why you gain goes up as it gets longer but further from resonance.

Maybe angles of the gain need to also be considered although measuring those is going to be tricky and that won't be possible in the near field anyway as many antennas far field peak angle of radiation is based on summation of the reflected ground wave as well.

Quite interesting gets the brain ticking over if nothing else.

Also to add as one example a 5/8 wave is not a resonant antenna so I presume you are not tuning to resonance with a non resonant antenna and just merely tuning for best impedance match at your feed point to ensure maximum power transfer. The latter seems to be the case for a non resonant antenna so you can forget resonance in that case... and just go with lowest SWR as being a good way to tune.

Maybe the - closer to a 5/8 wavelength gain - counters the coax losses you have.. and explains why despite shifting from resonance of the dipole, the gain from optimized length makes field strength increase even though SWR is increasing. Maybe.. I don't know just throwing some thoughts that pop into my head around. It seems there are interdependent factors, antenna gain vs resonance vs swr (and on the coax too) competing for the specification of maximum field strength.

I will counter my own possible thoughts here , just for additional confusion..a horizontal 5/8 wave dipole may never reach a situation of more gain than a 1/2 wave horizontal antenna or also compared to the small "technical gain" you get vertical 5/8th wave over a 1/2 wave or 1/4 wave vertical... the reason (reflection in far field) of the lowered angled focus may not be realized. Maybe you should model a 5/8 wave horizontal dipole vs 1/2 wave horizontal dipole and see what is presented.

I am sure I have just written mainly nonsense with something that might inspire further input from someone who knows what they are taking about.

 

[Justme]

. If you have a dipole and its longer than a half wave you can get lobes which can give you a certain advantage and gain in certain directions. This usually only happens when it is lengthened by multiples of a wave length/

Something that happens with my FD-4 from Fritzel the OCF antenna.
1/2 wave on 80 1 wave on 40 etc and 4 wave lengths on 10 and on 6 meters 13 wavelengths long where it works just fine.
Is it bad to have more nulls? nope depends on where the lobes are directed to in my case very nice to all continents.
Is ist a bad antenna? nope worked all continents in hone with it on 100 watts.
Are there better antenna's ? yes but i don't have room for a large tower and beams.
Adding my verticals i gt all over the globe with them not more you can wish for 100 watts and yes i can make 1000 watts here, but prefer just running 100 watts.

Worked today bunch USA/Canadian stations, Thailand, China Australia, Philipines, EU, Middle east, south America in a few hours trying from 10 to 20 meters.
All from the Netherlands.
Is the Imax 2 K a bad antenna? not ideal for just 11 meter though it does fine there.
It works good from 10 meter to 18 MHz. on 15 it is a 1/2 wave and on both i worked New Zealand as far i can get, half way around the world, 100 watts phone.
Without conditions 1500 watts and a large beam won't help either.

 

[groundwire]

Tune for resonance, then match feedpoint resistance to transmission line. Thats how it is

 

[338_MtRushmore]

Thanks for sharing your thoughts, that is interesting.

I'm not certain any of our measurements are fully understood by anyone. I've heard very few plausible arguments for tuning x=0 with a feedline. Most of these arguments are from very intelligent people who use unrelated theories in attempt to prove this one.

I really enjoy Walter Maxwells swr dip theory because I know there is some truth there, I just can't argue against it.

Field strength readings are just too much trouble for the possible gain. It just isn't worth it to me.

 

[AudioShockwav]

Sticky.

73
Jeff

 

[freecell]

antenna gain is not determined by calibrated field strength measurements, only directivity. antenna gain can only be determined if both antenna directivity and radiation efficiency are known values. stay away from anyone talking about the gain of an antenna and not referencing both of these parameters or variables.

antenna gain = maximum directivity* X radiation efficiency
* = at any elevation, radiation or take-off angle.

if max dir and rad eff are unknown there's no "antenna gain" to talk about and any quoted "antenna gain" figures are disingenuous, that is to say dishonest at best.

 

[AudioShockwav]

Now to put one last lens of perspective on all of this.
On all of the comparison charts I listed above, you won't actually notice the difference in real world performance no matter if you choose X=0, the low SWR point, or the peak field strength point for tuning an antenna.

The difference between all three of these points in all of the charts above is so insignificant that you aren't even approaching anything you will ever notice. In fact, you would have to increase the differences in the worst case scenario by many orders of magnitude to ever get close to seeing such a difference. This is why I have said on multiple occasions to tune to whichever you see fit, and don't worry to much about it. Performance wise you won't benefit from changing how you tune your antenna. That being said, there is the benefit of you potentially learning something new, and while that may not necessarily help your antennas performance, you may become better at working with them over all...

I like threads that make you think.
Thanks DB.

73
Jeff

 

[The DB]

I am sure I have just written mainly nonsense with something that might inspire further input from someone who knows what they are taking about.

Actually, I think you did bring up something that I should have covered better in my first post...

Now to try and understand your post.... so your 7dB +/- 0.2dB or so first graph.. Are these 7dB readings coming directly from a field strength meter scaled in dB you are placing near the antenna in reality ? Does that field strength vary along the length of the antenna or is it the same from say the centre of the dipole, to 5 feet along or 10 feet along or at the end - equidistant from the radiating element ? i.e. is the field strength changing with distance away from current or voltage centres ? I thought field strength is measured in... W/m2 or V/m

While I use field strength when tuning antennas, in this case I used an antenna model to get a gain in dBi as my starting point. I thought I mentioned this but as I go back and reread what I posted, I clearly didn't. I can actually get a dBm measurement using a spectrum analyzer, or a dB measurement with the output signal as its comparison using the second port on my NanoVNA (commonly called S21). In any case, as the output is in some form of dB measurement, adding a feed line losses will be applied the same way and have the same effect no matter how the number was measured/calculated, as long as it is in decibels.

You can skip coax from measurement by using 180° coax or use measurement device can treat coax as transparent.

True. That being said, this technique does have its limits, and the availability of antenna analyzers that do a much better job of this built in is growing. Even still, I don't get what this has to do with the point I was trying to make...

Tune for resonance, then match feedpoint resistance to transmission line. Thats how it is

Hey look, a true believer...

Like I said above, tune your antenna how you want, its not like it will make a large enough difference that you will actually notice it...

Thanks for sharing your thoughts, that is interesting.

Thank you.

m not certain any of our measurements are fully understood by anyone. I've heard very few plausible arguments for tuning x=0 with a feedline. Most of these arguments are from very intelligent people who use unrelated theories in attempt to prove this one.

This is part of why I started the thread. In my opinion, the situation is even worse than you posted here.

I really enjoy Walter Maxwells swr dip theory because I know there is some truth there, I just can't argue against it.

I'm not at a level to be able to argue for or against everything he said myself. I know there are some people that think they are, but I have yet to see anyone actually convince me of it yet. M. Walter Maxwell is still the cornerstone of my knowledge set. I have tried to test some of what he said using my own methods and techniques, with varying degrees of success. I have yet to demonstrate anything he has stated is wrong. What I have posted in the first post above he has actually written about, and while not intended to agree/disagree with him, this clearly agrees with his writings, and if I may be so bold, explains some of what he wrote in a different (and hopefully more understandable) way.

Field strength readings are just too much trouble for the possible gain. It just isn't worth it to me.

Speaking from experience, I don't blame ya.

Sticky.

Wow, thanks, didn't expect this.

antenna gain is not determined by calibrated field strength measurements, only directivity. antenna gain can only be determined if both antenna directivity and radiation efficiency are known values. stay away from anyone talking about the gain of an antenna and not referencing both of these parameters or variables.

Ever hear of an anionic chamber? Guess what one of the things they can do with such a device... Thats right, measure direct antenna gain, and they can do it without knowing the antenna's efficiency and directivity. To say this can't be done is just plane wrong. Also, you can get the antenna's gain by using Maxwell's Equations. This is how all modeling software calculates gain, and while the software I prefer to use can also calculate the antenna's efficiency and directivity, these are actually reverse calculated from already knowing the antennas gain, among other things.

Also, the gain I posted above came from antenna modeling, it wasn't directly measured and calculated directly from field strength. Further, as I mentioned above, using either a spectrum analyzer or my NanoVNA, I can directly measure some form of field strength in decibels, and while the gain figures will be different, they will function exactly the same and be affected by the same amount as the gain figures I used from the modeling software. Decibels are designed to function like that, and if they don't, they are useless not only for my purpose above, but also for any purposes that involves antennas.

I like threads that make you think.
Thanks DB.

Glad to help.


The DB

 

[Greg T]

This is an extremely interesting read. I started back in 1964 talking on an old RCA Mark VII and a 5/8 ground plane. Actually, it was my brother's setup, but he let me use it. As I watched him set it up and tune the antenna, he would try to explain as much as he could to an 11 year old. The one thing that stood out to me as he explained was that " you tune for lowest SWR to protect the transmitter, BUT, the lowest SWR does not necessarily mean the best performance for the antenna. I never did understand that until many years later.

 

[Handy Andy]

Yes - this is a thinking thread...

And I'm just going to leave this here - because what I find interesting...I'll leave some thoughts about...

There are all Center Fed 1/2 W/L or 2 1/4 W/L sections fed in the center
So if I'm allowed...
(...Thank you...)​

The above diagrams are only representational - but it is interesting to note that as we change the Angle of one of the radiators - only one for simplicity's sake...

Feed point impedance changes...we all seem to agree on this...

The "true" Marconi is closer to 36 ohm Feed point impedance while Vertical Dipole is 72~75 Ohms...

The paradox is the middle picture - having the idealized feed point impedance to match that which most transmitters made today (in the CB Real) use - which is 50 ohm.

Can't always get that on a vehicle...nor at home - seen a lot more with 90 degree or if droopy you're already at 1/2 Wave long on one vertical radial.

The Marconi gets you to use "earth" as half the image, but you also "image" into your neighbors stuff.

So to make the rather unwieldy vertical dipole or the lowly Marconi to work - most stick with 1/4 Wave Lengths for the typical CB antenna. But then you also have the matching problem of 1/2 wave End-Fed - its' not 50 ohms nor even close - requires some form of transformation to occur to offset the feed point impedance - again depending on the efficiency of this mechanism...

But that also puts you in a position of not having the most idealized feed point impedance in which to hook your antenna to the coax and also maintain a low SWR (or does it need to be low?)

Even though 72 or 36 gives you about 1:1.4~1.7 SWR (How you look at it) you're not that far off from the 50 ohm - but you have to suffer a trade off of the Radiation resistance - Field strength - and Field Pattern.

So again the "Make It Resonate or Gosh Darn the torpedoes" crux keeps coming up.

Again, I'll just leave this here...

 

[freecell]

"Ever hear of an anionic chamber? Guess what one of the things they can do with such a device... Thats right, measure direct antenna gain, and they can do it without knowing the antenna's efficiency and directivity. To say this can't be done is just plane wrong. Also, you can get the antenna's gain by using Maxwell's Equations. This is how all modeling software calculates gain, and while the software I prefer to use can also calculate the antenna's efficiency and directivity, these are actually reverse calculated from already knowing the antennas gain, among other things."

"Also, the gain I posted above came from antenna modeling, it wasn't directly measured and calculated directly from field strength. Further, as I mentioned above, using either a spectrum analyzer or my NanoVNA, I can directly measure some form of field strength in decibels, and while the gain figures will be different, they will function exactly the same and be affected by the same amount as the gain figures I used from the modeling software. Decibels are designed to function like that, and if they don't, they are useless not only for my purpose above, but also for any purposes that involves antennas."

first of all when analyzing antenna systems there is no such thing as an "anionic" chamber.

an "anechoic" chamber is what you're groping for. all that an anechoic chamber provides is an enclosed space or room designed to completely absorb reflections of either sound or electromagnetic waves. (an-echoic meaning "non-reflective, non-echoing, echo-free") secondly, this chamber does not have the ability to determine which part of total antenna resistance is from radiation resistance and which part of total antenna resistance is from loss resistance which disposes with the idea that it is able to determine antenna radiation efficiency.

i don't remember any inputs in any version of nec for antenna radiation resistance or antenna loss resistance. (if i'm mistaken here then please feel free to correct me) in a simple full size self resonant 1/4 wave antenna the formula for determining antenna radiation efficiency is:

radiation efficiency = radiation resistance / radiation resistance + loss resistance. this is one of two variables required for an accurate determination of the "gain" of an antenna.

the three images in andy's post are all representative of self-resonant antennas, meaning that they are capable of self-resonance solely based upon their physical dimensions with regard to wavelength and operating frequency, a property of all end fed 1/4 wavelength radiators including dipoles and inverted v antennas. this also applies to any radiator of any odd multiple 1/4 wavelengths.

andy also posted:

"But then you also have the matching problem of 1/2 wave End-Fed - its' not 50 ohms nor even close - requires some form of transformation to occur to offset the feed point impedance - again depending on the efficiency of this mechanism..."

the most effective and efficient method for feeding the base (high impedance) of an end fed half wave vertical is from the opposite end (high impedance) of an end fed quarter wave vertical.

https://www.worldwidedx.com/threads...ernative-view-point.31799/page-44#post-758740

just so there is no misunderstanding, an end fed 1/4 radiator operated against one or more 1/4 elements is by definition center fed, ground mounted or otherwise.

 

[The DB]

first of all when analyzing antenna systems there is no such thing as an "anionic" chamber.

Yes, I meant anechoic chamber. It was after a long day of overtime and I was tired. I am doing the same thing again so if I make such a mistake again I will correct it when I am more awake.

an "anechoic" chamber is what you're groping for. all that an anechoic chamber provides is an enclosed space or room designed to completely absorb reflections of either sound or electromagnetic waves. (an-echoic meaning "non-reflective, non-echoing, echo-free")

Yes, that is how they are designed.

secondly, this chamber does not have the ability to determine which part of total antenna resistance is from radiation resistance and which part of total antenna resistance is from loss resistance which disposes with the idea that it is able to determine antenna radiation efficiency.

Never said it did, nor does it need to.

radiation efficiency = radiation resistance / radiation resistance + loss resistance. this is one of two variables required for an accurate determination of the "gain" of an antenna.

You seem to think that this is the one and only possible way to get an antenna's gain. I'm sorry but you are mistaken. I mentioned two different methods in my previous post and all I get is a few lines on one that clearly demonstrates that you don't understand how anechoic chambers are actually used. I visited one as part of a ham group touring a commercial antenna testing facility. They had an antenna array they were testing and they showed us how their system takes the many calibrated field strength readings from inside the anechoic chamber and shows not only the radiation pattern, but the antenna's gain as well. It was an FM broadcast phased array that used phasing to steer the pattern away from an undesired direction. You can spout your beliefs that it doesn't work this way all you like, but I was there, I've seen it happen, and I've listened to a rather lengthy explanation on how it works. There was no mention of "radiation resistance" or "loss resistance", and I wouldn't expect there to be. If these have any relation to how anechoic chambers actually function, its something that if used is calculated from the results, not measured within.

Speaking of "radiation resistance" and "loss resistance", aside from the two of us, I don't think I've seen anyone use both of them together in the same post. Its largely ignored in favor of only looking at reactance/resonance. I would say, based on both antenna modeling and experience, that the R component, and more specifically relationship between radiation and loss resistance has more of an effect on an end fed 1/4 wavelength antenna than getting it right at X=0. I can go on and on about my experimenting with, and thoughts on this topic, but I won't do that here.

I know your original (pre-edit) post talked about antenna modeling and efficiency, and while I don't remember the exact wording, I think I remember the gist of it. Here I will post a screen shot and give an explanation of how 4nec2 reports gain, among a few other bonuses.

[photo=large]6694[/photo]

This is a photo of the model used in the graph above with the antenna in its longest state, not quite two feet longer than the resonant point. For what I am showing, this doesn't really matter as we are looking at how the software shows certain data and where it gets it from.

There are two different lines that has to do with efficiency. The first is simply called "Efficiency". This info is given directly in the nec2 output file, and only includes the losses from within the antenna. The fact that this antenna is made out of copper wire, for example, is where the efficiency stat is calculated from in this case. I don't think there is anything else in this model affecting this. It also factors in the losses with matching networks and such. Anything from within the antenna itself, from before the radiation actually happens is shown here. This is the best analog the modeling software shows to your explanation of "radiation efficiency".

The second line in question is "Radiat-eff" or literally "radiation efficiency". This is calculated differently. It takes all of the directions that the software calculates gain for, then averages them. This gives a number less than one (at least it should be) and is in decibels. From here they use a formula to determine the percentage lost. This is listed in the nec2 output file as two numbers, one the output, and the other tells you weather this was in free space or over an earth, where an additional calculation is needed (simple division so nothing major). This method includes all forms of loss, including losses from factors outside of the antenna itself, such as ground reflection losses.

Its strange that most modeling programs I've seen don't include either of these, yet its not difficult to get this information when looking directly at a nec2 output file.

One additional line that may be of interest to you is the line called RDF. It stands for "Receiving Directivity Factor", and is calculated with the exact same formula that I think I've seen you post for calculating directivity in the past.

While we are on the topic of what is and isn't in the nec2 output file, in the picture above you clearly see a value for SWR, yet there is no SWR listed in the nec2 output file, it isn't there. The modeling software actually calculates this from the complex impedance. From what I can tell, there is nothing in the nec2 output file that has anything to do with the impedance of the feed line, it simply doesn't care, that information is all handled by the modeling software that you choose to use.


The DB