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For the sake of making an instruction for an antenna tuner I placed a dipole of length about 2 times 8 meters in my garden about 1 meter up from the ground. The idea was to get some not ideal values that could be handled by an antenna tuner. I used my NanoVNA to get a Smith-diagram between 3 and 8 MHz according to the enclosed picture. Point 1 is 3 MHz and point 8 is 8 MHz.
It is usually stated that resonance occurs when +/- jX is zero in Z=R+jX. I began to think about where I had resonance for my dipole. There are four transitions of the curve over the pure resistance line so is it correct to say that all those are points of resonance? Point 7 is obviously close to 50 ohm, SWR is actually 1.2, but nevertheless I have pure resistance at three other points, all of which by the way could be handled by the antenna tuner to get 50 ohm for the transmitter.
I am used to say that an antenna only has one resonance frequency apart from multiples of the basic resonance. My Smith-diagram clearly shows that is not correct. Point 3 is 4.6 MHz, Point 6 is 6.5 MHz and point 7 is 7.2 MHz. (I should have noted the frequency for the transition between points 6 and 7, but it is a bit less than 7 MHz.)
Can someone please shed some light on the definition of resonance in conjunction with an antenna.
73/Torbjorn
PS. I manually added the SWR 2 and SWR 3 circles afterwards, but it could be handy if the FW had the option to show them.
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A couple of points.
- You did not say how you connected your antenna to the NanoVNA. If you connected with a piece of coaxial cable and did NOT calibrate at the end of the cable you are NOT measuring the feedpoint impedance of your antenna. If you just calibrated the VNA at the SMA terminals and then connected the coax you are measuring the impedance at the input of the coaxial cable. The coaxial cable will be an "impedance transformer" and the impedance the NanoVNA measures will not represent the impedance at the antenna so reactance = 0 does not mean the antenna is resonant. However you will know the "approximate" resonant frequency of the antenna when the SWR is at a minimum.
- The dimensions of the antenna you gave will result in a resonance that is higher than 8 MHz. The resonant frequency will vary depending on the height off the ground and the ground conditions at your location. The feedpoint impedance will also vary considerably especially the closer you get to ground. I ran two simulations for you in EZNEC using 8M elements. One was 8m off the ground and the other 1M. Average ground conditions for both. Note that when reactance = 0 the antenna is resonant by definition.
Roger
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On 9/7/20 11:34 AM, Torbj?rn Toreson wrote: For the sake of making an instruction for an antenna tuner I placed a dipole of length about 2 times 8 meters in my garden about 1 meter up from the ground. The idea was to get some not ideal values that could be handled by an antenna tuner. I used my NanoVNA to get a Smith-diagram between 3 and 8 MHz according to the enclosed picture. Point 1 is 3 MHz and point 8 is 8 MHz.
It is usually stated that resonance occurs when +/- jX is zero in Z=R+jX. I began to think about where I had resonance for my dipole. There are four transitions of the curve over the pure resistance line so is it correct to say that all those are points of resonance? Point 7 is obviously close to 50 ohm, SWR is actually 1.2, but nevertheless I have pure resistance at three other points, all of which by the way could be handled by the antenna tuner to get 50 ohm for the transmitter. yes, there are resonances at approximately any multiple of a half wavelength. The even multiples have fairly high resistance, the odd multiples tend to be lower resistance. When you get to a very large number of multiples, the impedance converges to a value similar to twice that of a balanced transmission line, where the "other half" of the transmission line is the image in the earth under the antenna. This is like any resonant system which is based on the propagation of something (sound or EM wave) - Organ pipes, trumpets, wind instruments in general all can resonate at multiples (that squeak on a clarinet or recorder when you overblow). In acoustics or string vibration, the wave equation that controls things is slightly different than the wave equation for EM resonance, but ultimately, they're all solutions of a 2nd order differential equation. Just like with stringed instruments (where particular harmonics/overtones can be suppressed by striking the string at a particular point), you can do the same with an antenna by driving it off center. However, the value of an adjustable matching network for most applications is in being able to adjust out the reactive component. And then on top of that, transform the R to something more suited for your source. You can actually model this as two separate networks. One is purely a L or C (either shunt or series) to "cancel" the reactive part. And the other is a LC network to transform the resistive impedance. Then, you can collapse the two networks into one. if the two port impedances are R1 and R2 T network Z1 Z2 are series, Z3 is shunt Z1 = -j * (R1 *cos(beta) - sqrt( R1 * R2))/ sin (beta) Z2 = -j * (R2 *cos(beta) - sqrt(R1 * R2))/sin(beta) Z3 = -j * sqrt(R1*R2) / sin(beta) Pi network Za and Zb are shunt, Zc is series Za = j *R1*R2 * sin(beta) / (R2 * cos(beta) - sqrt(R1 * R2)) Zb = j * R1*R2 * sin(beta) / (R1 * cos(beta) - sqrt(R1 * R2)) Zc = j * sqrt( R1 * R2) * sin (beta) You can transform impedance AND get a phase shift (beta) except beta can't be zero or pi. Typically, what's done is some sort of iterative approach to get the Zs "doable", because in the general matching problem, you don't care about phase shift. If you're building a phase array, you do. In many practical applications, the load or generator impedances may be reactive (i.e. Z (port 1) and Z (port 2) are some general R+jX). This can be accomodated by absorbing the external reactive impedance into the network, reducing or increasing the series or shunt impedance as requred. For instance, if a T network is required to connect between two impedances: 50+j0 and 100-j20 with 45 degrees of phase shift: First, calculate the Z's assuming resistive impedances: R1=50, R2= 100 Z1 = -j * (50 * .707 - sqrt(50*100))/.707 = +j 50 ohms Z2 = -j * (100 * .707 - sqrt(50*100))/.707 = 0 ohms Z3 = -j * sqrt( 50 * 100) / .707 = -j 100 ohms (the example is somewhat contrived, and it winds up creating an L network for the resistive case). Now, a reactive component is added to Z2 to exactly cancel the external reactive component. This changes Z2 from 0 ohms to +j20 ohms. The final network is then: Z1 = +j50, Z2= +j20, Z3 = -j100 ohms If you are working with a pi network, you would want to transform the external impedances into their corresponding shunt forms first, so that the reactive component is a shunt value, which would be absorbed (or combined) with the corresponding shunt component of the pi network. This produces a "point solution" - in a lot of systems, you might want to evaluate the design over a band of frequencies. And, if you've got non-ideal components with loss, some solutions will have more or less loss. I am used to say that an antenna only has one resonance frequency apart from multiples of the basic resonance. My Smith-diagram clearly shows that is not correct. Point 3 is 4.6 MHz, Point 6 is 6.5 MHz and point 7 is 7.2 MHz. (I should have noted the frequency for the transition between points 6 and 7, but it is a bit less than 7 MHz.)
Can someone please shed some light on the definition of resonance in conjunction with an antenna.
73/Torbjorn
PS. I manually added the SWR 2 and SWR 3 circles afterwards, but it could be handy if the FW had the option to show them.
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With all this chatter about resonance (some of which I know won't hold water) I will have to go and look again. I have been working with resonance as being when inductive reactance and capacitive reactance are equal. That may or may not occur at 50 ohms resistive. The "tuner" really is a "matcher" and if it is in your shack near your transmitter it is used to show your transmitter a match to the feedline - usually 50 ohms. The other end of the feedline at the antenna is randomly affected by the matcher at the opposite end (in your shack). It would be rare for the feedline and antenna to be well matched with parts thrown up out in the wild. It may work just fine anyway.
To get a good match at the antenna to the feedline out there the "matcher" must be located at the antenna feedpoint. Now I will go look up resonance once again and see what it is. When I was a child and wanted to build a radio I was completely frustrated. I completely ignored resonance. Who needs that foolishness anyway. Finally I got a grasp on resonance and next I built a working radio:) I don't believe the definition has changed in 65 years.
73,
Bill KU8H
bark less - wag more
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On 9/7/20 2:34 PM, Torbj?rn Toreson wrote: For the sake of making an instruction for an antenna tuner I placed a dipole of length about 2 times 8 meters in my garden about 1 meter up from the ground. The idea was to get some not ideal values that could be handled by an antenna tuner. I used my NanoVNA to get a Smith-diagram between 3 and 8 MHz according to the enclosed picture. Point 1 is 3 MHz and point 8 is 8 MHz.
It is usually stated that resonance occurs when +/- jX is zero in Z=R+jX. I began to think about where I had resonance for my dipole. There are four transitions of the curve over the pure resistance line so is it correct to say that all those are points of resonance? Point 7 is obviously close to 50 ohm, SWR is actually 1.2, but nevertheless I have pure resistance at three other points, all of which by the way could be handled by the antenna tuner to get 50 ohm for the transmitter.
I am used to say that an antenna only has one resonance frequency apart from multiples of the basic resonance. My Smith-diagram clearly shows that is not correct. Point 3 is 4.6 MHz, Point 6 is 6.5 MHz and point 7 is 7.2 MHz. (I should have noted the frequency for the transition between points 6 and 7, but it is a bit less than 7 MHz.)
Can someone please shed some light on the definition of resonance in conjunction with an antenna.
73/Torbjorn
PS. I manually added the SWR 2 and SWR 3 circles afterwards, but it could be handy if the FW had the option to show them.
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Hi,
I did go back and look at what resonance *is*. The source this time was published in 1976 but has not changed from the more ancient texts I first read (early 50s). Jim has explained things in much more detail but essentially he says inductive reactance is equal to capacitive reactance and may or may not occur when R is 50 ohms.
Our matchers are used to make capacitive and inductuve reactance equal.
73,
Bill KU8H
bark less - wag more
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On 9/7/20 3:33 PM, Jim Lux wrote: On 9/7/20 11:34 AM, Torbj?rn Toreson wrote:
For the sake of making an instruction for an antenna tuner I placed a dipole of length about 2 times 8 meters in my garden about 1 meter up from the ground. The idea was to get some not ideal values that could be handled by an antenna tuner. I used my NanoVNA to get a Smith-diagram between 3 and 8 MHz according to the enclosed picture. Point 1 is 3 MHz and point 8 is 8 MHz.
It is usually stated that resonance occurs when +/- jX is zero in Z=R+jX. I began to think about where I had resonance for my dipole. There are four transitions of the curve over the pure resistance line so is it correct to say that all those are points of resonance? Point 7 is obviously close to 50 ohm, SWR is actually 1.2, but nevertheless I have pure resistance at three other points, all of which by the way could be handled by the antenna tuner to get 50 ohm for the transmitter. yes, there are resonances at approximately any multiple of a half wavelength. The even multiples have fairly high resistance, the odd multiples tend to be lower resistance.? When you get to a very large number of multiples, the impedance converges to a value similar to twice that of a balanced transmission line, where the "other half" of the transmission line is the image in the earth under the antenna.
This is like any resonant system which is based on the propagation of something (sound or EM wave) - Organ pipes, trumpets, wind instruments in general all can resonate at multiples (that squeak on a clarinet or recorder when you overblow).
In acoustics or string vibration, the wave equation that controls things is slightly different than the wave equation for EM resonance, but ultimately, they're all solutions of a 2nd order differential equation.
Just like with stringed instruments (where particular harmonics/overtones can be suppressed by striking the string at a particular point), you can do the same with an antenna by driving it off center.
However, the value of an adjustable matching network for most applications is in being able to adjust out the reactive component.? And then on top of that, transform the R to something more suited for your source.
You can actually model this as two separate networks. One is purely a L or C (either shunt or series) to "cancel" the reactive part. And the other is a LC network to transform the resistive impedance.
Then, you can collapse the two networks into one.
if the two port impedances are R1 and R2
T network Z1 Z2 are series, Z3 is shunt
Z1 = -j * (R1 *cos(beta) - sqrt( R1 * R2))/ sin (beta)
Z2 = -j * (R2 *cos(beta) - sqrt(R1 * R2))/sin(beta)
Z3 = -j * sqrt(R1*R2) / sin(beta)
Pi network
Za and Zb are shunt, Zc is series
Za = j *R1*R2 * sin(beta) / (R2 * cos(beta) - sqrt(R1 * R2))
Zb = j * R1*R2 * sin(beta) / (R1 * cos(beta) - sqrt(R1 * R2))
Zc = j * sqrt( R1 * R2) * sin (beta)
You can transform impedance AND get a phase shift (beta) except beta can't be zero or pi.? Typically, what's done is some sort of iterative approach to get the Zs "doable", because in the general matching problem, you don't care about phase shift.
If you're building a phase array, you do.
In many practical applications, the load or generator impedances may be reactive (i.e. Z (port 1) and Z (port 2) are some general R+jX). This can be accomodated by absorbing the external reactive impedance into the network, reducing or increasing the series or shunt impedance as requred. For instance, if a T network is required to connect between two impedances: 50+j0 and 100-j20 with 45 degrees of phase shift:
First, calculate the Z's assuming resistive impedances: R1=50, R2= 100
Z1 = -j * (50 * .707 - sqrt(50*100))/.707 = +j 50 ohms
Z2 = -j * (100 * .707 - sqrt(50*100))/.707 = 0 ohms
Z3 = -j * sqrt( 50 * 100) / .707 = -j 100 ohms
(the example is somewhat contrived, and it winds up creating an L network for the resistive case).
Now, a reactive component is added to Z2 to exactly cancel the external reactive component. This changes Z2 from 0 ohms to +j20 ohms. The final network is then:
Z1 = +j50, Z2= +j20, Z3 = -j100 ohms
If you are working with a pi network, you would want to transform the external impedances into their corresponding shunt forms first, so that the reactive component is a shunt value, which would be absorbed (or combined) with the corresponding shunt component of the pi network.
This produces a "point solution"? - in a lot of systems, you might want to evaluate the design over a band of frequencies. And, if you've got non-ideal components with loss, some solutions will have more or less loss.
I am used to say that an antenna only has one resonance frequency apart from multiples of the basic resonance. My Smith-diagram clearly shows that is not correct. Point 3 is 4.6 MHz, Point 6 is 6.5 MHz and point 7 is 7.2 MHz. (I should have noted the frequency for the transition between points 6 and 7, but it is a bit less than 7 MHz.)
Can someone please shed some light on the definition of resonance in conjunction with an antenna.
73/Torbjorn
PS. I manually added the SWR 2 and SWR 3 circles afterwards, but it could be handy if the FW had the option to show them.
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Thanks for the input so far. I should have mentioned that the antenna (total length 16 meter) was connected in the middle with a 20 meter long RG58 cable. The VNA was calibrated "standalone". This was intentional because I wanted to see what values of impedance I would get at the feedpoint, i.e. the point to connect to a tuner. I am quite aware that the measured values as shown in the Smith-diagram are not the same as those at the antenna connection. But I am still curious how to interpret the four points where reactance is zero, possibly this could be an effect of the impedance transformation in the cable at various frequencies? But nevertheless I have four points with zero reactance in my system consisting of antenna plus cable, all at lower frequency than could be anticipated with regard to the length of the antenna, so can those four points be called resonance points of the system? I am not interested of getting 50 ohm, only to interpret/understand the Smith-diagram zero-reactance points.
73/Torbjorn
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Jim Lux, as usual, did a very credible explanation of what is going on - far better than I could! Second order differential equations are the key to understanding. I'd like to add a practical point: Again, understanding resonance is, indeed, defined as the frequency at which the complex portion of the impedance goes to zero yielding only resistance. That is gospel! However, let's completely *disconnect* the concept of SWR from resonance. Point 7 exhibits a low SWR and is only real and does, indeed represent resonance as it's purely resistive. However, the other resistance-only crossings of the horizontal line on the Smith Chart are also true resonances. Point 7 exhibits a low SWR due to the fact that our system impedance for determining 'resonance' (low SWR - improper confusion of the two measurements) is defined to be in a 50-ohm system. If we had assigned the center of the chart to any of the other resistive-only crossings to the resistive-only value of those crossings and measured in that system impedance (not our 50-ohm system SWR meter), we would also measure a low SWR for each point. However, point 7 would then have a high SWR in those other systems of measurement. Resonance does not depend on system impedance as does measurement of SWR. SWR and the system impedance standard (usually 50-ohms) in which we measure SWR is dependent on our defined system impedance. All the resistive-only crossings represent resonance, but not to be measured for SWR in a 50-ohm system impedance. For example, if our system impedance were 100-ohms and our measurement instrument was designed to measure in a 100-ohm system, a 100-ohm resistor would measure 1:1 SWR. In that same system a 50-ohm resistor would measure 2:1 SWR. The 100 ohm resistive load could be said to be at 'resonance' (if we were dealing with a tuned circuit) in spite of the 100-ohm standard system impedance. As a further example going to acoustics, download an audio spectrum analyzer onto your PC and enable the microphone. Now whistle at a constant tone and amplitude. Note the spectrum. Your ear hears just one tone, usually the strongest as indicated on the spectrum. However, the spectrum indicates a very complex display of many tones. All these tones represent resonant conditions of the Helmholtz resonator formed by your mouth cavity and the opening of your lips. Pipes on a pipe organ work in a similar manner. There are many resonant modes besides the basic fundamental. You can repeat the experiment with stretched rubber bands, guitar strings, or anything that makes a tone your ears hear. A pan flute is also a good experimental subject. A violin, string base, bassoon, trumpet.......they are all interesting in their unique harmonic content. And, NONE of this has anything to do with SWR!! Different resonant structures yield the different sounds from differing instruments, even though they may all be playing concert A. Take a concert guitar and a banjo. They are both very similar stringed instruments. The guitar sound is much more mellow than the banjo, both playing the same note. Why? The resonant cavity behind the strings of both instruments are very different in volume and construction. That of the guitar is large and quite deep. It enforces the lower frequency overtones and harmonics of the plucked string. The banjo is just the opposite with a small and shallow cavity behind the strings. Therefore, that cavity enforces the higher overtones and harmonics. Something very similar applies to EM waves and resonant circuits. Dave - W?LEV On Mon, Sep 7, 2020 at 6:34 PM Torbj?rn Toreson <torbjorn.toreson@...> wrote: For the sake of making an instruction for an antenna tuner I placed a
dipole of length about 2 times 8 meters in my garden about 1 meter up from
the ground. The idea was to get some not ideal values that could be handled
by an antenna tuner. I used my NanoVNA to get a Smith-diagram between 3 and
8 MHz according to the enclosed picture. Point 1 is 3 MHz and point 8 is 8
MHz.
It is usually stated that resonance occurs when +/- jX is zero in Z=R+jX.
I began to think about where I had resonance for my dipole. There are four
transitions of the curve over the pure resistance line so is it correct to
say that all those are points of resonance? Point 7 is obviously close to
50 ohm, SWR is actually 1.2, but nevertheless I have pure resistance at
three other points, all of which by the way could be handled by the antenna
tuner to get 50 ohm for the transmitter.
I am used to say that an antenna only has one resonance frequency apart
from multiples of the basic resonance. My Smith-diagram clearly shows that
is not correct. Point 3 is 4.6 MHz, Point 6 is 6.5 MHz and point 7 is 7.2
MHz. (I should have noted the frequency for the transition between points 6
and 7, but it is a bit less than 7 MHz.)
Can someone please shed some light on the definition of resonance in
conjunction with an antenna.
73/Torbjorn
PS. I manually added the SWR 2 and SWR 3 circles afterwards, but it could
be handy if the FW had the option to show them.
-- *Dave - W?LEV* *Just Let Darwin Work*
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You have the impedance looking into the feedline from the transmitter end, but now you are asking what the antenna is doing.
An arbitrary amount of feedline will rotate a point on the smith chart trace around the center by an arbitrary angle, easily turning most anything into a reactance of zero.
And that one point is only for one frequency, at other frequencies it gets rotated by a different amount, because the feedline is a different number of wavelengths long.
Somebody might be able to figure out what's going on, given an exact measurement of the length of your feedline and the velocity factor of the feedline.
Not me, at least not with an awful lot of head scratching.
A couple weeks ago we had a discussion about doing an O-S-L calibration through the feedline to calibrate that out.
This looks like an excellent example of why one would want to do that.
The smith chart will look far different when you do, and tell you much more about exactly what is going on at the antenna.
Jerry, KE7ER.
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On Mon, Sep 7, 2020 at 01:38 PM, Torbj?rn Toreson wrote:
Thanks for the input so far. I should have mentioned that the antenna (total
length 16 meter) was connected in the middle with a 20 meter long RG58 cable.
The VNA was calibrated "standalone". This was intentional because I wanted to
see what values of impedance I would get at the feedpoint, i.e. the point to
connect to a tuner. I am quite aware that the measured values as shown in the
Smith-diagram are not the same as those at the antenna connection. But I am
still curious how to interpret the four points where reactance is zero,
possibly this could be an effect of the impedance transformation in the cable
at various frequencies? But nevertheless I have four points with zero
reactance in my system consisting of antenna plus cable, all at lower
frequency than could be anticipated with regard to the length of the antenna,
so can those four points be called resonance points of the system? I am not
interested of getting 50 ohm, only to interpret/understand the Smith-diagram
zero-reactance points.
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Thank You very much. I am sorry that I did not mention the feeder cable originally, and I did not clarify that the antenna was not the main object but the combination of the antenna and the feeder and also not the SWR (as that as mentioned is computed to the center 50 ohm point). The various inputs have given a clear understanding of my measurement and I am happy to find that all my zero reactance points really can be called resonances, as well as all such with just a cable attached to the NanoVNA resulting in a long spiral. Thank You all who gave input!
73/Torbjorn
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On 9/7/20 1:58 PM, David Eckhardt wrote: Jim Lux, as usual, did a very credible explanation of what is going on -
far better than I could! Second order differential equations are the key
to understanding.
I'd like to add a practical point: Again, understanding resonance is,
indeed, defined as the frequency at which the complex portion of the
impedance goes to zero yielding only resistance. That is gospel! However,
let's completely *disconnect* the concept of SWR from resonance. and one might want to disconnect "resonance" from "radiation efficiency" - a 10 meter long dipole (resonant at around 15 MHz) will efficiently *radiate* over a huge range of frequencies, probably down to 4-5 MHz and up to 100 MHz, in the sense of efficiency as "power pushed into the antenna"/"power radiated into the far field".
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Ok.
We all agree free space theoretical resonance would be something like 9.35 MHz.
We also agree closer to ground would lower fundamental resonance to something lower, like 9.125 or so.
But I have not seen an answer as to why he sees three resonances, two of which are below Fo. In my experience this may be due to common mode current, which when cleared with a suitable choke, yields a single resonance.
Of course I can understand more resonances (x= 0 zero crossings) at half-waves at higher frequencies.
But could someone take another shot at why he is seeing resonances at 4.6, 6.5 and 7.2 MHz on an an antenna cut for 9 MHz?
And, true to the advice of calibration at the far end of the feedline, can¡¯t he ALSO calibrate at the VNA connector, and walk-back along the Smith Chart, or otherwise subtract the effect of the coax feed with a reasonable chance of accuracy?
Ed McCann
AG6CX
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This is a correction to my first post. Unfortunately that simulation was not done in high accuracy mode so it did not correctly model the effect of ground so close to the antenna. The following plots were done in high accuracy mode and show a reasonable SWR at the antenna feedpoint .
The first drawing is an excerpt from the ARRL Antenna Handbook showing the effect of ground on a dipole's impedance. The other two are for a dipole with 8M elements at a height of 1M and 8M.
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Ed, Looks to me like he really does have all those resonances at the transmitter end of his feed line. An appropriate length of feedline will turn any arbitrary impedance into a pure resistance. See post /g/nanovna-users/message/17233Jerry, KE7ER
On Mon, Sep 7, 2020 at 03:20 PM, AG6CX wrote:
But I have not seen an answer as to why he sees three resonances, two of which
are below Fo
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Ed,
He is not seeing more "antenna resonances". The antenna is only resonant at 9 Mhz., 27 MHz. 45 Mhz. etc...
What he is seeing is the reactance at the input of the coaxial cable become 0 at some frequencies. This is because the transmission line is acting like an impedance transformer. Reactance going to zero at the input of the cable is not the same thing as resonance. The plot below is the complex impedance of the antenna under discussion. On the right hand side I have calculated one frequency where the reactance equals zero at the input of the transmission line. There are others. If the transmission line length is changed or a different cable with different transmission characteristic is used the transformation will result in a different cable input impedance at that frequency.
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Whether the system of transmission line plus antenna is resonant is a matter of how you define terms.
Me, I'd say a specific length of transmission line is as acceptable as a lumped LC impedance matching
device at the antenna feed point, if they both give the desired input impedance at the transmitter
at the frequency of interest.
The issue with using a length of transmission line is that you don't get many choices
as to the pure resistance the transmitter will see at system resonance.
Also, it's a pain to adjust when you move to a different frequency.
Jerry, KE7ER
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On Mon, Sep 7, 2020 at 05:34 PM, Roger Need wrote:
Reactance going to zero at the input of the cable is not the same thing as
resonance.
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Apologies if I'm wrong but nobody seems to have asked if there is a balun at the dipole feed point. If there is no balun the coax will be part of the antenna, which would explain lower frequency resonances.
There's also a question about height, at least for modelling. The antenna would have to be very, very tight or be supported along its length to remain 1m above ground. Is that the case or is it a V shape with higher ends?
73
Mike
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On 2020-09-08 02:08:-0700, Mike Brown wrote: There's also a question about height, at least for modelling. The antenna would have to be very, very tight or be supported along its length to remain 1m above ground. Is that the case or is it a V shape with higher ends? I have a spreadsheet that I use...developed after much research into the subject of sag and tension... I set it up for a nominal Synthetic Textiles support rope - 5mm and antenna for a support span of 20 m (the antenna is 16 m) and a sag of 0.2 m the tension is 1.2 kg. I used this when I installed my 40 m EF supported on 60 m of the same 5 mm. It correctly calculates that I needed ~2 kg for a 1 m sag. -- 72/73 de Rich NE1EE On the banks of the Piscataqua
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Can someone please shed some light on the definition of resonance in conjunction with an antenna.
Let's agree that "resonance occurs when +/- jX is zero".
It does not take knowledge of x-degree differential equation , modelling , size of wire , moon phase etc.
It is applicable to ANY AC circuit, antenna included.
Let's also agree that dipole , by definition resonant radiator, exhibits APPROXIMATELY 50 Ohms impedance
at the center feed point.
For a sake of simplification let's also agree that variation of the center point impedance with - insert your favorite parameter here - is not fundamental to the discussion .
Now for the punch line
By definition , ANY length of transmission line of characteristic impedance will transfer impedance at the load - AKA terminal impedance EQUAL of transmission line characvteristric impedance TO the input of such transmission line.
Place note - I am still using general terms, adding SPECIFICS - such as velocity factor of RG58 coax is immaterial - for the sake of this discussion.
Described ideal system ,for purpose of staying with basic, not woo-doo electronics , with "real data of 50 Ohms substituted " parameters of
source impedance of 50 Ohms ,
transmission line of characteristic impedance of 50 Ohms
and load / antenna impedance of 50 Ohms
will PRIMARILY exhibit FUNDAMENTAL resonance @ ONE frequency.
IF these parameters are SAME and EQUAL on harmonics , resonance @ harmonics will be observed.
So if the load (antenna) impedance at random frequency NO LONGER matches the transmission line characteristic impedance - everything else staying same , balum or no balum, you see the results.
.
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Anne,
I think your central point is that with a 50+j0 ohm load and a 50 ohm transmission line,
the transmitter sees a 50 ohm impedance looking into the coax regardless
of the length of the coax. That is correct.
The question is, what happens when the antenna itself is not being used at its
resonant frequency, so it has significant inductive or capacitive reactance?
In this case, a carefully chosen length of coax can rotate the antenna impedance
around the smith chart center point until the impedance seen by the transmitter
is purely resisitve. And thus the system of coax plus antenna is resonant.
However, the pure resistance seen by the transmitter is not likely to be 50 ohms.
Alternately, you can have whatever length of feedline is convenient,
and use an antenna tuner at the transmitter end to make the entire system
of antenna tuner plus feedline plus antenna be resonant, presenting the
transmitter with a pure 50 ohm resistance.
None of the above methods has an inherent inefficiency, it all comes down
to the quality of components.
A matching network placed at the antenna feedpoint to make the coax
see 50+0j ohms will have some losses, often mostly due to resistance in the inductor.
An antenna tuner at the transmitter end transforms the impedance in exactly the same way,
the only extra loss is that reflections may make several round trips through the coax before
going out the antenna, being attenuated with each trip.
Just a piece of coax also transforms the impedance, but in this case can only rotate it
around the center of the smith chart, so not as good a solution in the general case.
I'd say the system is resonant when the transmitter sees a pure resistance,
assuming resistive losses are not swamping out the complex impedance of the antenna.
And that the system is fully matched when the transmitter sees 50 ohms of pure resistance.
Jerry, KE7ER
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On Tue, Sep 8, 2020 at 10:17 AM, Anne Ranch wrote:
Can someone please shed some light on the definition of resonance in
conjunction with an antenna.
Let's agree that "resonance occurs when +/- jX is zero".
It does not take knowledge of x-degree differential equation , modelling ,
size of wire , moon phase etc.
It is applicable to ANY AC circuit, antenna included.
Let's also agree that dipole , by definition resonant radiator, exhibits
APPROXIMATELY 50 Ohms impedance
at the center feed point.
For a sake of simplification let's also agree that variation of the center
point impedance with - insert your favorite parameter here - is not
fundamental to the discussion .
Now for the punch line
By definition , ANY length of transmission line of characteristic impedance
will transfer impedance at the load - AKA terminal impedance EQUAL of
transmission line characvteristric impedance TO the input of such transmission
line.
Place note - I am still using general terms, adding SPECIFICS - such as
velocity factor of RG58 coax is immaterial - for the sake of this discussion.
Described ideal system ,for purpose of staying with basic, not woo-doo
electronics , with "real data of 50 Ohms substituted " parameters of
source impedance of 50 Ohms ,
transmission line of characteristic impedance of 50 Ohms
and load / antenna impedance of 50 Ohms
will PRIMARILY exhibit FUNDAMENTAL resonance @ ONE frequency.
IF these parameters are SAME and EQUAL on harmonics , resonance @ harmonics
will be observed.
So if the load (antenna) impedance at random frequency NO LONGER matches the
transmission line characteristic impedance - everything else staying same ,
balum or no balum, you see the results.
|
Can someone please shed some light on the definition of resonance in
conjunction with an antenna.
I'll make an attempt.
Let's agree that "resonance occurs when +/- jX is zero".
It does not take knowledge of x-degree differential equation , modelling ,
size of wire , moon phase etc.
It is applicable to ANY AC circuit, antenna included.
True. The definition of resonance requires the capacitive reactance equals
the inductive reactance or: -jX = +jX. Since the two cancel eachother,
only real resistance remains at resonance. For an antenna in free space,
that resistance amounts to the sum of the radiation resistance plus loss
resistance in the conducting structure. In practice, losses in the field
return structure ("ground" or earth losses required in some installations
as in a 1/4-wavelength vertical) also contribute to losses.
Let's also agree that dipole , by definition resonant radiator, exhibits
APPROXIMATELY 50 Ohms impedance
at the center feed point. For a sake of simplification let's also agree
that variation of the center point impedance with - insert your favorite
parameter here - is not fundamental to the discussion .
A dipole is understood to be a resonant radiator. A doublet is a dipole
configuration with resonance structure outside the frequencies of interest,
but still exhibits resonance(s).
At typical amateur heights above soil surface, not DX antennas and
installations which are far above the typical amateur budgets, yes, 50-ohms
feed or radiation resistance of a dipole is in order.
Now for the punch line
By definition , ANY length of transmission line of characteristic impedance
will transfer impedance at the load - AKA terminal impedance EQUAL of
transmission line characvteristric impedance TO the input of such
transmission line.
Absolutely true. That's what the Smith Chart is all about. A coaxial
transmission line of known length and characteristic Zo acts as an
impedance transformer. In reality, why is the *antenna* terminal
impedance so important when it must be connected to a transceiver through
that coaxial transmission line (or any other transmission line)? What
really counts is the terminal impedance at the *station end* of the
transmission line! It's academically interesting for the design engineer
(like myself) to know the antenna terminal impedance just to verify the
results of putting the whole system through the Smith Chart exercise, but
from a strictly practical viewpoint what's really important is what the end
of the feedline presents to our station inside where its climatically
controlled. So, measure at the station end of the transmission line and
forget about the transformative properties of the transmission line.
Others: please don't flame me for that statement, but from a practical
standpoint, that's all that is important.
Place note - I am still using general terms, adding SPECIFICS - such as
velocity factor of RG58 coax is immaterial - for the sake of this
discussion.
Described ideal system ,for purpose of staying with basic, not woo-doo
electronics , with "real data of 50 Ohms substituted " parameters of
source impedance of 50 Ohms ,
transmission line of characteristic impedance of 50 Ohms
and load / antenna impedance of 50 Ohms
will PRIMARILY exhibit FUNDAMENTAL resonance @ ONE frequency.
IF these parameters are SAME and EQUAL on harmonics , resonance @
harmonics will be observed.
Yes, resonances at harmonically related frequencies to the fundamental
1/2-wavelengths will be present. However, they likely will not exhibit
the same resistance as the intended resonant frequency. Again, decouple
the concept of SWR from resonance!
Hope this helps just a little.....
Dave - W?LEV
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On Tue, Sep 8, 2020 at 5:17 PM Anne Ranch <anneranch2442@...> wrote: Can someone please shed some light on the definition of resonance in
conjunction with an antenna.
Let's agree that "resonance occurs when +/- jX is zero".
It does not take knowledge of x-degree differential equation , modelling ,
size of wire , moon phase etc.
It is applicable to ANY AC circuit, antenna included.
Let's also agree that dipole , by definition resonant radiator, exhibits
APPROXIMATELY 50 Ohms impedance
at the center feed point.
For a sake of simplification let's also agree that variation of the center
point impedance with - insert your favorite parameter here - is not
fundamental to the discussion .
Now for the punch line
By definition , ANY length of transmission line of characteristic
impedance will transfer impedance at the load - AKA terminal impedance
EQUAL of transmission line characvteristric impedance TO the input of such
transmission line.
Place note - I am still using general terms, adding SPECIFICS - such as
velocity factor of RG58 coax is immaterial - for the sake of this
discussion.
Described ideal system ,for purpose of staying with basic, not woo-doo
electronics , with "real data of 50 Ohms substituted " parameters of
source impedance of 50 Ohms ,
transmission line of characteristic impedance of 50 Ohms
and load / antenna impedance of 50 Ohms
will PRIMARILY exhibit FUNDAMENTAL resonance @ ONE frequency.
IF these parameters are SAME and EQUAL on harmonics , resonance @
harmonics will be observed.
So if the load (antenna) impedance at random frequency NO LONGER matches
the transmission line characteristic impedance - everything else staying
same , balum or no balum, you see the results.
.
--
*Dave - W?LEV*
*Just Let Darwin Work*
|
Roger Need
Rich NE1EE