Is resonance really where you get best antenna performance?

[TheBlaster]

Given a 5/8 is electrically a 3/4 wave with the coil match does that also make a 5/8 wave resonant after all ? I am a bit confused by that.

Which factor defines a resonant antenna, physical length or electrical length ? As I gather the 5/8 wave length element physically produces the flattening low angle gain effect and the 3/4 electrical length produces a closer impedance match.

 

[freecell]

a 5/8 antenna is incapable of self resonance (X ≠ 0) because of the excessive level of capacitive reactance present at the feedpoint and requires an L network at the input to dump the reactance and provide an input impedance compatible with 50 ohm feedline. 3/8 wl., 1/2 wl,, 5/8 wl., 7/8 wl, & 1 wl. radiators are all examples of antennas that are not capable of self-resonance.

the radiation pattern of a full size 3/4 wl. vertical gm monopole differs quite a bit from the pattern of a 5/8 wl. vertical.

"which factor defines a resonant antenna, physical length or electrical length ?"

the difference between the two when speaking of a self resonant 1/4 wl. vertical that is electrically 90 degrees is very small, any variations are caused by differences in the length to diameter ratio (physical size) of the conductor, other variables having only minor effects.

here's a real life scenario covering some other variables.


given:

100W into a 3/4 λ gm vertical monopole with a total length of 8.281419 meters and a diameter of 38.10 millimeters designed to operate @ resonance @ a frequency of 27.205 mhz. over 16 radials with a length of 0.7 meters and a diameter of 3.175 millimeters over average ground.

0.752 λ @ 27.205
feedpoint input impedance: 70.7 ohms
feedpoint reactance: jX = 0.0 ohms,
load is purely resistive.


loss in T match coil: 0.6% (not used)
loss in antenna conductor: 0.1%
loss in soil vicinity: 0.0%
loss in radial system: 3.4%

ground electrode system:
dc resistance: 42.1 ohms
rf resistance: 2.4 ohms
rf reactance: 0.0 ohms

power radiation efficiency: 95.96%
loss relative to ideal system:
0.2 dB. or 0.0 S-Units

confirm radiation efficiency:
total resistance - loss resistance = radiation resistance
(impedance at resonance is purely resistive)

70.7 ohms - 2.4 ohms = 68.3 ohms radiation resistance
radiation efficiency = radiation resistance / radiation
resistance + loss resistance (total loss)

2.4 ohms of rf resistance is part of the 70.7 ohms of
total antenna resistance.

68.3 / 70.7 = 0.9660 = 96.60%
96.60 - 95.96 = .64. since the
T match used by default is not
needed the discrepancy between
the two figures is resolved to .04%,
i.e., 96.56 - 96.60%

since the T match is not used the swr
on the feedline @ both the transmitter
and the antenna is 1.414:1 @ 27.205 &
equal to or less than 1.5:1 from 26.915
to 27.465 mhz..

the importance of resonance in an antenna system:

1.optimizes current in the antenna. (see #3)
2.makes the antenna easier to feed.
3.indicates frequenc/y/ies where the load represents a pure resistance.
4.establishes the center of the band in reference to operational bandwidth
5.tracking changes in antenna systems due to changes, weather, etc..
6.provides a point from which antenna "Q" can be calculated


references:

James F. Murray introduction to the concepts of energy resonance

https://duckduckgo.com/?q=antenna+resonance+in+high+performance+receivers+&t=h_&ia=web

 

[TheBlaster]

And there was me thinking 1/2 waves were resonant. 1 wavelength also would shout that it is resonant to me. I always considered something that was 1 wavelength long to be resonant as a wavelength of RF is an identical length to the radiating element.

Now I question if I even have the correct meaning of resonance when it comes to antennas. (Though thinking about it it must be the same as any tuned circuit, when Xc and Xl are equal)

From the above it seems if reactance can be 0 then an antenna is resonant. I have never used analyzers as I never seen them as necessary (owning an SWR capable radio) making known designs I have not gone into the definition more than was required to pass an exam.

I agree a 3/4 wave is a poor antenna for DX compared to a 5/8 wave in the models I have produced.

Electrically 90 degrees is very small is an interesting statement. It's big enough to cause increased calculation complexities in capacitors and inductors... small compared to what ?

A 1/4 wave is current fed but why is a 1/4 wave resonant ? Surely it has the same voltage and current phase difference as any of the other antennas (90 degrees) I can only imagine that by virtue of it being current fed something is happening to allow capacitance and inductance to be equal in that design.

Edited to add : Ok the penny has dropped I think... no matching required = resonant antenna.. if you need matching circuit it is not resonant.

 

[freecell]

Blaster posted:

"and there was me thinking 1/2 waves were resonant."

an end fed 1/2 λ is not self-resonant however a center fed 1/2 λ is self-resonant.

a wire 90, 270, 450 (any odd number of 1/4 wl, in length) degrees long @ any given frequency is i.e., SELF RESONANT, not "resonant." theres a difference.
1 λ = 360 degrees. a full size, self resonant 1/4 λ can be fed directly with 50 ohm feedline without matching as in the example posted earlier.

any antenna not capable of SELF RESONANCE can be made RESONANT if the reactance can be dumped by an opposite reactance of equal amplitude, where as in the case of a SELF RESONANT antenna there is no reactance present when the antenna is adjusted for X = 0 simply by adjusting the length, the same thing that is really taking place when you ADJUST the length of a loaded 1/4λ mobile antenna while mistakenly believing that you are changing the swr. all you're changing is the resonant frequency of the antenna up and down across the antenna bandwidth to find a lower swr at some other frequency. that's not impedance matching. just look at any swr bandwidth chart to see what is actually happening. if you're going to affect the swr on the feedline then something needs to change @ the antenna-feedline or transmitter-feedline terminal connection in the case of either reflectionless load matching or feedline input matching.

 

[The DB]

Given a 5/8 is electrically a 3/4 wave with the coil match does that also make a 5/8 wave resonant after all ? I am a bit confused by that.

Which factor defines a resonant antenna, physical length or electrical length ? As I gather the 5/8 wave length element physically produces the flattening low angle gain effect and the 3/4 electrical length produces a closer impedance match.

To answer the first question, it depends on your point of view. I have seen both methods used by hobbyists. For example, shortened mobile antennas that are 5 foot long are often referred to as shortened 1/4 wavelength antennas, and some people don't bother to put the shortened. The Imax 2000 has been shown to be electrically 1/2 wavelength, yet it is still considered a 5/8 or, for those that worship a certain length, the so called "almighty .64". If you have a 5/8 that has a coil as you said that makes it an electrically shortened 3/4 wavelength, people still call it a 5/8 wavelength antenna, although in reality these are actually very rare. Most 5/8 wavelength antennas are actually impedance corrected electrical 5/8 wavelength antennas, there typically is not electrical shortening involved.

Further still, and this is really nothing more than snake oil, some fiberglass mobile antenna manufacturers will call their shortened 1/4 wavelength antennas 5/8 because they were wound with a 5/8 wavelength physical length of wire, but this is nothing more than marketing.

I could actually go on here, I guess the short answer is it depends on the person and what they are trying to sell you.

And there was me thinking 1/2 waves were resonant. 1 wavelength also would shout that it is resonant to me. I always considered something that was 1 wavelength long to be resonant as a wavelength of RF is an identical length to the radiating element.

Anything that measures as X = 0 on an Antenna Analyzer/VNA/whatever is self resonant by the definition of self resonance. Some people put an additional requirement of it also needs to be near the impedance of the coax it is attached to, but there is nothing on the engineering side that supports this idea. This is part of something I call the "coaxification" of radio users, essentially people who are dependent on the limits of coax and can't see anything beyond.

Now I question if I even have the correct meaning of resonance when it comes to antennas. (Though thinking about it it must be the same as any tuned circuit, when Xc and Xl are equal)

You will note that bot freecell and I have both used the phrase "self resonant". This is key as what most hobbyists call resonance is more accurately defined as self resonance, and is in reality a special case. I don't recall freecell using this phrase before this thread so I have to give him props for this change. Any impedance can be made resonant with a matching circuit of some sort, although this isn't always efficient. In general, if the length of the element/antenna is less than 20% of a wavelength (say a 5 foot CB antenna), it is generally more efficient to use some form of loading up on the antenna over a matching circuit.

Just a note here, it is also technically accurate to call "self resonant" just "resonant" as self resonance is a part of resonance. The thing is, when a hobbyist talks about "resonance" they are almost always referring to "self resonance".

From the above it seems if reactance can be 0 then an antenna is resonant. I have never used analyzers as I never seen them as necessary (owning an SWR capable radio) making known designs I have not gone into the definition more than was required to pass an exam.

Don't fret over this, as I stated above, you will never notice the difference between the low SWR and the self resonant points performance wise. Also, if you have an antenna that gets a low SWR without some form of matching, the low SWR point will always be close to the self resonant point. As self resonance is required for a perfect SWR match, if you have such a match one of two things happened, either the self resonant point lined up with the SWR match point, or you have something else affecting the reading, including but not limited to to many losses or common mode currents on the feed line. This is, of course, assuming there is no other form of matching in the antenna system.

Electrically 90 degrees is very small is an interesting statement. It's big enough to cause increased calculation complexities in capacitors and inductors... small compared to what ?

I agree with this as in the hobby world this is a common length that is used from MF all the way through VHF, and even beyond those limits. I would be curious as to why he said such a thing, but he does say some strange things sometimes so I just put it in that category.

A 1/4 wave is current fed but why is a 1/4 wave resonant ? Surely it has the same voltage and current phase difference as any of the other antennas (90 degrees) I can only imagine that by virtue of it being current fed something is happening to allow capacitance and inductance to be equal in that design.

That is a good question. I'm not so sure that a 1/4 wavelength antenna, by itself, is self resonant. Every time we set up a 1/4 wavelength antenna, electrically shortened or otherwise, there is always something attached to the shield side of the coax, and if there isn't the antenna simply doesn't tune without drastic help. That being said, I haven't really researched this directly... yet... so I will hold off on any opinion at this time. I know a lot of people jump to the 1/4 wavelength sections being self resonant, but may simply be them ignoring the part of the antenna attached to the shield side of the coax...

Edited to add : Ok the penny has dropped I think... no matching required = resonant antenna.. if you need matching circuit it is not resonant.

A lot of people think this way, but I do not.

On the topic of resonance, I have two additional comments. The first is a quote from the 23'rd edition of the ARRL Antenna Book, page 1-6 and I'm pretty sure it is in other recent versions as well.

Is Resonance Required?
Please recognize that an antenna need not be resonant in order to be an effective radiator. There is in fact nothing magic about having a resonant antenna, provided of course you can devise some efficient means to feed the antenna. Many amateurs use non-resonant (even random length) antennas fed with open wire transmission lines and antenna tuners. They radiate just as well as using coaxial cable and resonant antennas and as a bonus can usually be used on multiple frequency bands. It is important to consider an antenna and its feed line as system in which all losses should be kept to a minimum.

In the charts I showed above, the addition of the feed line and its losses had noticeably more of an effect on overall gain than weather or not you were near the "resonant" part of the antenna. Further, those losses were not based on reactance/resonance, but SWR.

Another thing I've noticed recently, if you look up resonant vs non-resonant, if its written by an engineer for other engineers, they will almost always say that resonance doesn't matter, while the hobbyist writing for hobbyists will almost always say it does, so take that for what it is worth.


The DB

 

[TheBlaster]

Great replies I will read more deeply soon.. and I was thinking a 1/2 wave is exact multiples so it must be resonant (but was wondering why a end fed has to be matched with a 49:1 approx) and a dipole does not.

I will read with interest when I have some time, every now and then great threads appear here.... well done for starting The DB.

 

[Handy Andy]

IF no one minds I'll inject this...

• if a 50 ohm load shows low SWR on a meter - I hope we're not considering the Dummy load as Self-resonate.

That is a good question. I'm not so sure that a 1/4 wavelength antenna, by itself, is self resonant. Every time we set up a 1/4 wavelength antenna, electrically shortened or otherwise, there is always something attached to the shield side of the coax, and if there isn't the antenna simply doesn't tune without drastic help.

Bless you...

Because of this - many consider the viewpoint of 1/4 LENGTH as being resonate - even though there's 75% of the other stuff is still missing...

So yes, along comes the coax and the counterpoise to the rescue ...

​

What I find interesting; as this evolves, as we "build this antenna" the SWR then tends to be a correlation to the Feed point impedance - as you adjust the opposing elements' ANGLE the shield is connected to - you can obtain the "low SWR" oriented like I had shown earlier - apply the notion that both elements are 1/4 wave in LENGTH. You and your antenna, in free air, you can acheive LOW SWR not by changing length - but by changing ASPECT (orientation) of the elements to each other.

Oops, better add this...

Center-Fed - of course...

 

[TheBlaster]

I have often thought of a dipole as a 1/4 wave with 1 radial... they really are rather similar. I have regularly loaded a 1/4 wave for 11m/10m and 20m and 40m against a vehicle. I have a metal to metal SO239 mount and vehicle panels fairly well bonded. In addition I run a bunch radials off at roughly 45 degrees and peg them down just for good measure.

I get something like 1.8:1 with no radials and once radials on it produces a better match to 1.2 /1.3:1 - I run the 10m wire for 40m up a 10m fishing rod. Technically I could drive so that is some mobile antenna, though obviously I would not want to do that.

It is a rather odd antenna including the vehicle and radials but it works pretty well.

It just made me question why a GPA is called a 1/4 wave and a dipole a 1/2 wave. I suppose you don't 100pct need the radial for the 1/4 wave and could use a ground pin if the ground was salt marsh or sea water.

The simplest of antennas can keep your mind going for months and years... still learning more and more about them and this thread is helping. It never ceases to amaze when they are well set up what is achievable with quite literally a piece of wire and favourable conditions... it seems nothing less than miraculous. For most antennas I have an inline coaxial ferrite choke patch lead then my feeder just to make the pattern what it can be without the influence of feeder.

 

[Wire Weasel]

All this is good but how about a Simplified Roundup
.
Maximum Antenna Performance (best TX & RX) Signal to the other station you're communicating with is an overall combination of these factors - audio and watts aside. Also to assume it's best signal to a LOCAL Ground Wave contact. DX not included as once you throw in variable propagation most of everything else isn't thrown out the window exactly but propagation can't ever be nailed down so as to measure.
.
Best resonance and Lowest SWR
Gain
Height Above Ground

Perfect resonance and flat SWR will provide greatest Effective Radiated Power

A Higher Gain design will beat a Lower Gain Design - A perfectly resonant 5/8 wave antenna will beat a perfectly resonant 1/4 antenna at same height and power output.

An antenna 100' above the ground will beat an antenna 30' above the ground.

 

[338_MtRushmore]

Perfect resonance and flat SWR will provide greatest Effective Radiated Power

It seems to me that a 1:1 feedline swr is nearly always a bad thing unless matched. Can't you can match anything to 50 ohms? I'm assuming "flat swr" is referring to 1:1, but I'm really not certain what it means.

I guess I don't know exactly what "perfect resonance" means either.

 

[unit_399]

All this is good but how about a Simplified Roundup
.Perfect resonance and flat SWR will provide greatest Effective Radiated Power.

This is not necessarily true. Read this article by BOB, KONR.

"Let’s start with a simple example: a dummy load. A dummy load is basically a resistor that replaces the antenna so we can test transmitters without actually radiating a signal. For ham radio use, a dummy load will be a 50 Ω resistor, consistent with the fact that most ham radio systems operate at 50 ohms. A good dummy load provides an excellent impedance match at all frequencies of interest and it is not resonant. I say its not resonant because the load looks like 50 Ω for all frequencies (within a specified range). Its antenna efficiency is zero, because it does not radiate any of the power. So here we have a great example of a low SWR but no resonance and no radiated power.

Now let’s look at the classic center-fed half-wave dipole (1/4 wave per side) in free space. At the resonant frequency, the antenna has a characteristic impedance of 73 Ω, purely resistive. The SWR can be calculated by taking the ratio of the impedance to 50 ohms, giving SWR = 73/50 = 1.5. (By the way, for impedances less than 50 ohms, the SWR is calculated using SWR = 50/R.) Dipole antennas generally work well, so the antenna efficiency will be high and depends on the actual construction of the antenna. Note that the SWR is not equal to 1 at resonance, it is a bit higher. However, an SWR of 1.5 does represent a good match and is normally considered just fine.

Now let’s take a look at an antenna that is no where near 50 Ω at resonance, the half-wave folded dipole antenna. This antenna has an impedance of about 280 Ω at its resonant frequency. If we connect this antenna to a 50 Ω transmitter, the SWR is 280/50 = 5.6. So here is an example of a resonant antenna that has a high SWR. At the resonant frequency, this antenna will radiate efficiently but will present a difficult impedance match to a 50 Ω transmitter, which will struggle to deliver power from the transmitter into the antenna. While we might choose to accept this high SWR, a more practical approach is to add a matching network to produce a 50 Ω impedance match.

Many of the antennas we use are designed to be close to 50 ohm (SWR = 1) when they are resonant. For these cases, SWR is a good indicator that the antenna is resonant, which is why most hams associate low SWR with resonance. Low SWR does not tell us anything about how well the antenna is working (antenna efficiency). A dummy load has excellent SWR but fails to radiate. Some antennas are like that, too."

73, Bob K0NR

 

[freecell]

"Perfect resonance and flat SWR will provide greatest Effective Radiated Power".

not so. an antenna exhibiting a radiation resistance of 14 ohms and total loss resistance of 36 ohms will perform poorly when compared to another antenna exibiting a radiation resistance of 36 ohms and total loss resistance of 14 ohms.

it's no mystery here that they both add up to 50 ohms and swr is 1.0:1 for both antennas. even though the total resistance of both antennas is 50 ohms, the radiation efficiency of the first antenna will be only 28% while the radiation efficiency of the second antenna will be 72%. assuming no line loss to keep it simple, the first antenna will radiate 28W. and the second antenna will radiate 72W with 100W input in both cases. the difference in directivity between them is 4.1 dB..

"Low SWR does not tell us anything about how well the antenna is working. (antenna efficiency) Bob got that right.

 

[TheBlaster]

ARRL Antenna book : "Where the current and voltage are exactly in phase, the impedance is purely resistive with zero reactance and the antenna is resonant"

By what means is V and I phase changing in a 1/4 wave by 90 degrees to make it purely resistive ? (compared to any other antenna)

I know quarter wavelengths can be significant in creating sleeve balun's and impedance matching but cannot think why I and V change phase just because wire happens to be a 1/4 wave of the freqeuncy you are sticking up it.

Something is not adding up in this ARRL statement as we know for a 1/4 and a 1/2 wave "I max" is in centre and volts are higher at the end of the antenna. That does not appear to be the definition of "in phase" as far as I understand it.

 

[freecell]

your "education" provided by commercial antenna manufacturers and "hobbyists" is in error, the opening statement is irrefutably correct.

maximum feedpoint current (low impedance) only occurs when the 1/2 antenna is center fed, when end fed feedpoint voltage is maximum. (high impedance)

from the center of a center fed dipole the impedance increases in direct proportion to the shifting phase relationship between current and voltage as they move towards the ends of the dipole elements. this is perfectly understandable once you realize the impedance is simply a mathematical ratio between the two.

here is the relationship expression based on 100W input to a reflectionless load in a 50 ohm system.

E / I = Z
E = 70.7V
I = 1.414A
Z = 50 ohms

both current and voltage are in phase with one another in this 50 ohm system.
there is also a mathematical ratio expressed when voltage is divided by current.

70.7 / 1.414 = 50 ohms, the ratio between voltage and current is 50:1.

this same E/I relationship also exists in any feedline that is terminated with a perfectly matching load. when this is not the case, the relationship between voltage and current all along the feedline changes in direct proportion to the mismatch (phase shift) resulting in reflection from the load and the ensuing standing waves on the feedline.

the impedance of the feedline is no longer 50 ohms at every point along its length.

 

[TheBlaster]

I'll repeat the question...as I have no idea who you are replying to Freecell, but make a guess it is me.

"By what means is V and I phase changing in a 1/4 wave by 90 degrees to make it purely resistive ? (compared to any other antenna)"

Your statement makes some sense as I recall what you say as Delta match based on this concept I recall from the exams. However why is current vs volts phase shifting along length of the elements ? I would say VF but this is unlikely as that is a constant along the element assuming it is the same material and width.


As we know V and I are NOT in phase, they are 90 degrees out of phase with respect to each other.

How come this is not so in a 1/4 wave or a 1/2 wave centre fed dipole ?

These 2 waves are not in phase they are 90 degrees out of phase and at any point remain 90 degrees out of phase with respect to each other.

https://www.electronics-notes.com/a...na/current-voltage-waveforms-distribution.php

Cannot post the image alone, sorry.