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Here is my next iteration (PDF) of a summary of "Antenna Fundamentals and Measurements with a Nano VNA".
I am still learning a lot about this gadget and how to interpret the various graphs.
I've heard you need to do a careful "calibration" to have meaningful results.
I mostly work at HF and mostly on 80m or 20m.
Once you have the fundamentals (new vocabulary) it is rather amazing that a $50 device can tell you so much about
resistance, reactance, impedance, resonance, SWR and phase.
Comments are appreciated as I am going to be giving a talk with SOME of these slides at local radio clubs.
TU
Barry k3eui
Philly region
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I bought one some time ago but never used it much.
Had issues yesterday trying to upgrade the firmware so I ordered a newer one from Amazon.
Should be here today.
I'll take it into the lab tomorrow and see how accurate it is.
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On Mon, 9 Nov 2020 at 18:03, Barry Feierman <k3euibarry@...> wrote: Here is my next iteration (PDF) of a summary of "Antenna Fundamentals and
Measurements with a Nano VNA".
Comments are appreciated as I am going to be giving a talk with SOME of
these slides at local radio clubs.
TU
Barry k3eui
Philly region
A couple of comments, after a very quick skim read. 1) You say the loss in a coax is due to I^2 R and V^2 / R. However, it's not the same value of R. You might consider using Rc for the conductor losses, and Rd for the dielectric losses, where Rd >> Rc. However, these are not resistors you would measure on a multimeter. Copper losses will certainly increase with frequency, which I think you cover, and dielectric losses may too - it depends on the dielectric. 2) You talk about efficiency of an antenna, but don't define it. Most people don't have a clue what efficiency is. The generally accepted definition amoung professionals comes from IEEE standard 145. Efficiency is the power radiated divided by the power absorbed by the antenna. Note the word absorbed - it has nothing to do with the incident power. So if you have a crap SWR, you transmit 100 W, 99 W gets reflected, and 0.95 W gets radiated, then the antenna is 95% efficient, despite you would probably not consider it a very good antenna.
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On 11/9/20 12:51 PM, Dr. David Kirkby, Kirkby Microwave Ltd wrote: On Mon, 9 Nov 2020 at 18:03, Barry Feierman <k3euibarry@...> wrote:
Here is my next iteration (PDF) of a summary of "Antenna Fundamentals and
Measurements with a Nano VNA".
Comments are appreciated as I am going to be giving a talk with SOME of
these slides at local radio clubs.
TU
Barry k3eui
Philly region
A couple of comments, after a very quick skim read.
1) You say the loss in a coax is due to I^2 R and V^2 / R. However, it's
not the same value of R. You might consider using Rc for the conductor
losses, and Rd for the dielectric losses, where Rd >> Rc. However, these
are not resistors you would measure on a multimeter. Copper losses will
certainly increase with frequency, which I think you cover, and dielectric
losses may too - it depends on the dielectric. It's actually I^2*R and V^2*G in the traditional formulation, where G is the dielectric "conductivity" (= 1/R in some sense). And yes, they are different. G tends to be increasing with frequency linearly. R increases as 1/sqrt(f) due to skin effect. For most coax, up to 100 MHz, the IR losses are much bigger than the dielectric losses. That's why the usual transmission loss formula has two coefficients - loss = k1*sqrt(f) + k2*f 2) You talk about efficiency of an antenna, but don't define it. Most
people don't have a clue what efficiency is. The generally accepted
definition amoung professionals comes from IEEE standard 145. Efficiency is
the power radiated divided by the power absorbed by the antenna. Note the
word absorbed - it has nothing to do with the incident power. So if you
have a crap SWR, you transmit 100 W, 99 W gets reflected, and 0.95 W gets
radiated, then the antenna is 95% efficient, despite you would probably not
consider it a very good antenna. I agree - Efficiency is a tricky word when associated with antennas. I try to stay away from it, and find some other way to describe what it is - there's "mismatch effects", and "loss effects" - you could have an efficient antenna (low antenna loss) but with system loss because of mismatch and a lossy transmission line.
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On 09/11/2020 20:51, Dr. David Kirkby, Kirkby Microwave Ltd wrote: 2) You talk about efficiency of an antenna, but don't define it. Most
people don't have a clue what efficiency is. The generally accepted
definition amoung professionals comes from IEEE standard 145. Efficiency is
the power radiated divided by the power absorbed by the antenna. Note the
word absorbed - it has nothing to do with the incident power. So if you
have a crap SWR, you transmit 100 W, 99 W gets reflected, and 0.95 W gets
radiated, then the antenna is 95% efficient, despite you would probably not
consider it a very good antenna.
David, surely the antenna in your example is fine - it radiates the power it actually receives.?? What's crap is the matching ! Hardly the fault of the antenna..... Richard G7RVI
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On Tue, 10 Nov 2020 at 12:07, Richard Hankins <g7rvi@...> wrote:
On 09/11/2020 20:51, Dr. David Kirkby, Kirkby Microwave Ltd wrote:
2) You talk about efficiency of an antenna, but don't define it. Most
people don't have a clue what efficiency is. The generally accepted
definition amoung professionals comes from IEEE standard 145. Efficiency is
the power radiated divided by the power absorbed by the antenna. Note the
word absorbed - it has nothing to do with the incident power. So if you
have a crap SWR, you transmit 100 W, 99 W gets reflected, and 0.95 W gets
radiated, then the antenna is 95% efficient, despite you would probably not
consider it a very good antenna.
David,
surely the antenna in your example is fine - it radiates the power it
actually receives. What's crap is the matching ! Hardly the fault of
the antenna.....
Richard
G7RVI
Richar, my point is, that on a document aimed at hams, on fundamentals, to use efficiency, without defining it, is not a good idea. If this was an IEEE Antennas and Propogation journal, it would be different. As Jim Lux said, in response to my post *"I agree - Efficiency is a tricky word when associated with antennas. I try to stay away from it, ..."* If the antenna impedance is 0.5 ohm, then it would have the 100:1 VSWR I mention when measured in a 50 ohm system. That would be tricky to match to. Efficient yes, but not easy to use. I personally feel, that on something aimed at amateurs, to use the word "efficiency" is not a great idea, unless one is going to describe in detail about what efficiency is. It's one of those words, that 99% of hams will not know the true meaning, but will all think they have a fairly good idea of what it meant by an efficient antenna. The original poster asked for feedback, so that is my thought on the matter. Dave
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Thank you for the suggestions.
My casual use of "efficiency" of an antenna did not add anything to the slide show.
I mainly wanted to show the relationships of
Reflection coefficient, return loss, SWR, impedance, R and X, and phase
But without getting too deep into the weeds and the math of complex numbers.
When a dipole is operating at its resonant frequency, the current and the voltage at the feed poin are in phase.
WHY?
Most hams I have talked with cannot tell me WHY that is so.
And why does operating below resonance get you into capacitive reactance?
Why does operating above resonance get you into inductive reactance?
Why at a resonant frequency do these two cancel?
And, what can you tell by just looking at a Smith Chart graph of an antenna?
That was what I wanted to concentrate on.
And I admit, I am a rookie at this stuff, but learning fast.
Agn, TU for suggestions
Barry k3eui
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On Tue, 10 Nov 2020 at 14:57, Barry Feierman <k3euibarry@...> wrote: Thank you for the suggestions.
My casual use of "efficiency" of an antenna did not add anything to the
slide show.
That's personally where I see a problem - the *casual* use of a technical term. In my opinion, it would be more accurate to remove the word "efficiency" and replace it with something that gets your point over, without using that word. Another word that is abused is directivity. People have some vague idea what they mean by that, but use it incorrectly. I recall an article in RadCom where the author suggested the reader check the directivity of the antenna by rotating it 180 degrees, and looking at the S-meter. Apart from the fact that S-meters vary wildly in their readings, this is a poor way to measure the front to back ratio, not the directivity. Barry k3eui Dave G8WRB
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Hello Nano VNA folks
Again, thank you for the feedback. Yes, this is what I asked for.
I try to be careful with language, but the slide on antenna "efficiency" was just sloppy.
My main point of putting this together was to look at how to interpret the various graphs that a VNA shows.
I am not an Electrical Engineer, and had no prior experience with the VNA vocabulary.
Six months ago I had no idea what a '"return loss" was, let alone explain why it is positive or negative.
Hams talk about SWR all the time. On nets, I often hear folks say they are worried since their SWR is 1.8:1 at some frequency, and they WANT 1:1.
The book by Walt Maxwell W2DU was fantastic reading for me and taught me a lot about SWR and feedlines and impedance.
I wanted to start my presentation with some basic antenna characteristics, like resistance, reactance, impedance and phase. Then branch into the VNA world to discuss how these parameters can be measured at least with reasonable accurately at HF with a $50 gadget and free software.
So that was my goal. I am adding some slides on "reflection coefficient" (rho) in tems of what various values of rho look like on a Smith Chart. Rho of zero is just one point on that graph: R = 50 ohms and zero reactance (with 50 ohm coax). So if you visualize the Smith Chart as a Dart Board, and the center is where you are aiming, then a SWR of 1:1 is only at the exact center of the dart board (Smith Chart).
So if ou miss the Bull's Eye, your reflection coefficient is bigger than 0. And how much you miss will indirectly tell you LOTS of information. The perimeter of the Smith Chart is a reflection coefficient of 1. Of course, that is just awful. Reflection Coefficient (0-1) leads to RETURN LOSS (dB) and that leads to SWR.
I think I now have an 8th grader understanding of all of these terms now.
What puzzled me was how to interpret the PHASE curves on simple one-band dipoles.
At resonance, reactance is zero (I get that) and PHASE crosses over from neg to pos.
WHY? If a ham (non engineer) can explain that in a few sentences, then that is a pretty good understanding of the concept of resonance. Of course, these are measurements made in my shack, about 100-150 ft of RG213 from the antenna being measured.
I find most hams (I talk with) think the SWR changes with feed line length (other than attenuation).
Many hams (I talk with) do not see why impedance changes with line length if Z is other than 50 ohms.
These can be easily confused by most of us in our (casual) discussions.
Running around 180 degrees in a Smith Chart was a real "eye-opener" for me, personally.
I noticed that QST article last May had a quick "review" of the Nano VNA but did not go into any detail explaining the relationships of the various graphs. Perhaps a second article for QST would help us non-engineer types see the value of a $50 VNA so we can adjust our antennas to work better.
Again, from a rookie at VNA language (but a ham for over 60 years)
Thank you. This IO group obviously has some very talented hams.
Barry k3eui
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Barry,
I¡¯ve reviewed your slide show and must say I am impressed with your ambitiousness. That¡¯s a LOT of material to cover in a single presentation. I would think most ham clubs members would be hard pressed to absorb that much information in a single presentation. Perhaps breaking it up into a few parts would be more effective, but you know your audience. I have taught such material to amateur level students and found it a little daunting.
As far as content is concerned I don¡¯t have much to add to what Dr. Kirkby and others mentioned, but here¡¯s a thing or two.
Antenna resistance: You present antenna ¡°resistance¡± (along with reactance and phase) but don¡¯t specify ¡°Radiation Resistance¡±. Most people will think of wire resistivity, or resistance associated with loss when presented with this term. I think the natural ¡°radiation resistance¡± of an antenna is an important concept to distinguish. This being the resistive load presented to the RF source by the radiating field of energy the antenna creates. Generally speaking, the longer the antenna the higher the Radiation resistance. I describe it (mechanically) like casting a fishing pole. A very long pole exhibits quite a bit more resistance than a very short pole.
Another factor that is important to consider in this regard is that as reactance varies from resonance the net Z of the antenna rises, as does SWR. Hence, even a mismatched antenna will exhibit its lowest SWR at resonance. (There are minor deviations to this rule for reasons beyond the scope of amateur discourse. So I pray the experts don¡¯t beat me up on that :). ) But for things like multi-band verticals and short mobile antennas this becomes critical as Reactance to (low) Radiation Resistance ratios rapidly swing the antenna Z, resulting in high Q, low bandwidth antennas. You see this in some of your latter VNA Smith curves as tight snail shell curves. Whereas antenna with high resistive losses, like short mobile antenna with thin wire, swamp this out and present wider bandwidth. A seemingly "better" antenna... NOT even a little.
SWR "Losses", etc: The misapplication of this term make me nuts sometimes. If you¡¯ve read Walt¡¯s book you know his position on the Amateur obsession with SWR! With that in mind, I like to point out to my audience that the increase in loss due to SWR is simply because the reflected RF passes through the feedline multiple times from it's mismatched load multiple times with the SAME loss, which simply accumulates. Using a matching network at the system input can be misleading as it only accounts for a SINGLE point of mismatch in the system. For example: Matching a 50 ohm exciter to a 300 ohm line, feeding a 300 ohm folded dipole will match with great success. Matching that same exciter to a 300 ohm line feeding a 75 ohm dipole, not so much. The 300 to 75 mismatch WILL result in additional feed-line reflections and commensurate loss regardless of the tuner. Hence, it is important to stress the need to match the line to the load as best as possible. Many installations using mismatched antennas and feed line exist. The most popular being the OWL fed dipole. The belief that the OWL is so much less lossy than coax being a benefit is (IMHO) not warranted given the trade-offs. Not the least of which is (depending on line length) the extreme swings in voltage and current that can exist in such a system.
I¡¯ll forego my mantra on preferring resonant antenna and broadband transformer matching for radically long or short antennas. Best of luck with your presentation and kudos for your efforts. I hope I¡¯ve given you some useful things to consider conveying.
Mark
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Greetings:
I always love to use a 50 Ohm load in a presentation about antennas,
impedance and resonance.
The 50 Ohm load is a 'perfect' load across its frequency range, but is a
lousy antenna!
All that energy is wasted as heat, and poorly radiates any signal.
Get over that, and begin making your antennas radiate, you are making
progress.
SWR has always been far too over rated. Personally, return loss means far
more to me, as does phase angles and characteristic impedance across a
specific range of frequencies.
The greater the return loss (signal being reflected back), the better the
overall match. High return loss, equals less signal loss due to heat and
mismatch.
toggle quoted message
Show quoted text
On Fri, Nov 13, 2020 at 4:28 PM Mark Sedutto <ka2qfx@...> wrote: Barry,
I¡¯ve reviewed your slide show and must say I am impressed with your
ambitiousness. That¡¯s a LOT of material to cover in a single presentation.
I would think most ham clubs members would be hard pressed to absorb that
much information in a single presentation. Perhaps breaking it up into a
few parts would be more effective, but you know your audience. I have
taught such material to amateur level students and found it a little
daunting.
As far as content is concerned I don¡¯t have much to add to what Dr. Kirkby
and others mentioned, but here¡¯s a thing or two.
Antenna resistance: You present antenna ¡°resistance¡± (along with reactance
and phase) but don¡¯t specify ¡°Radiation Resistance¡±. Most people will think
of wire resistivity, or resistance associated with loss when presented with
this term. I think the natural ¡°radiation resistance¡± of an antenna is an
important concept to distinguish. This being the resistive load presented
to the RF source by the radiating field of energy the antenna creates.
Generally speaking, the longer the antenna the higher the Radiation
resistance. I describe it (mechanically) like casting a fishing pole. A
very long pole exhibits quite a bit more resistance than a very short
pole.
Another factor that is important to consider in this regard is that as
reactance varies from resonance the net Z of the antenna rises, as does
SWR. Hence, even a mismatched antenna will exhibit its lowest SWR at
resonance. (There are minor deviations to this rule for reasons beyond the
scope of amateur discourse. So I pray the experts don¡¯t beat me up on that
:). ) But for things like multi-band verticals and short mobile
antennas this becomes critical as Reactance to (low) Radiation Resistance
ratios rapidly swing the antenna Z, resulting in high Q, low bandwidth
antennas. You see this in some of your latter VNA Smith curves as tight
snail shell curves. Whereas antenna with high resistive losses, like
short mobile antenna with thin wire, swamp this out and present wider
bandwidth. A seemingly "better" antenna... NOT even a little.
SWR "Losses", etc: The misapplication of this term make me nuts
sometimes. If you¡¯ve read Walt¡¯s book you know his position on the Amateur
obsession with SWR! With that in mind, I like to point out to my audience
that the increase in loss due to SWR is simply because the reflected RF
passes through the feedline multiple times from it's mismatched load
multiple times with the SAME loss, which simply accumulates. Using a
matching network at the system input can be misleading as it only accounts
for a SINGLE point of mismatch in the system. For example: Matching a 50
ohm exciter to a 300 ohm line, feeding a 300 ohm folded dipole will match
with great success. Matching that same exciter to a 300 ohm line feeding a
75 ohm dipole, not so much. The 300 to 75 mismatch WILL result in
additional feed-line reflections and commensurate loss regardless of the
tuner. Hence, it is important to stress the need to match the line to the
load as best as possible. Many installations using mismatched antennas and
feed line exist. The most popular being the OWL fed dipole. The belief that
the OWL is so much less lossy than coax being a benefit is (IMHO) not
warranted given the trade-offs. Not the least of which is (depending on
line length) the extreme swings in voltage and current that can exist in
such a system.
I¡¯ll forego my mantra on preferring resonant antenna and broadband
transformer matching for radically long or short antennas. Best of luck
with your presentation and kudos for your efforts. I hope I¡¯ve given you
some useful things to consider conveying.
Mark
|
And for those that may need a reference. A 14dB return loss equates to a VSWR of 1.5:1. *Clyde K. Spencer* On Fri, Nov 13, 2020 at 9:09 PM ERNEST AEC-RADIO <aecradio1@...> wrote: Greetings:
I always love to use a 50 Ohm load in a presentation about antennas,
impedance and resonance.
The 50 Ohm load is a 'perfect' load across its frequency range, but is a
lousy antenna!
All that energy is wasted as heat, and poorly radiates any signal.
Get over that, and begin making your antennas radiate, you are making
progress.
SWR has always been far too over rated. Personally, return loss means far
more to me, as does phase angles and characteristic impedance across a
specific range of frequencies.
The greater the return loss (signal being reflected back), the better the
overall match. High return loss, equals less signal loss due to heat and
mismatch.
On Fri, Nov 13, 2020 at 4:28 PM Mark Sedutto <ka2qfx@...> wrote:
Barry,
I¡¯ve reviewed your slide show and must say I am impressed with your
ambitiousness. That¡¯s a LOT of material to cover in a single presentation.
I would think most ham clubs members would be hard pressed to absorb that
much information in a single presentation. Perhaps breaking it up into a
few parts would be more effective, but you know your audience. I have
taught such material to amateur level students and found it a little
daunting.
As far as content is concerned I don¡¯t have much to add to what Dr. Kirkby
and others mentioned, but here¡¯s a thing or two.
Antenna resistance: You present antenna ¡°resistance¡± (along with reactance
and phase) but don¡¯t specify ¡°Radiation Resistance¡±. Most people will think
of wire resistivity, or resistance associated with loss when presented with
this term. I think the natural ¡°radiation resistance¡± of an antenna is an
important concept to distinguish. This being the resistive load presented
to the RF source by the radiating field of energy the antenna creates.
Generally speaking, the longer the antenna the higher the Radiation
resistance. I describe it (mechanically) like casting a fishing pole. A
very long pole exhibits quite a bit more resistance than a very short
pole.
Another factor that is important to consider in this regard is that as
reactance varies from resonance the net Z of the antenna rises, as does
SWR. Hence, even a mismatched antenna will exhibit its lowest SWR at
resonance. (There are minor deviations to this rule for reasons beyond the
scope of amateur discourse. So I pray the experts don¡¯t beat me up on that
:). ) But for things like multi-band verticals and short mobile
antennas this becomes critical as Reactance to (low) Radiation Resistance
ratios rapidly swing the antenna Z, resulting in high Q, low bandwidth
antennas. You see this in some of your latter VNA Smith curves as tight
snail shell curves. Whereas antenna with high resistive losses, like
short mobile antenna with thin wire, swamp this out and present wider
bandwidth. A seemingly "better" antenna... NOT even a little.
SWR "Losses", etc: The misapplication of this term make me nuts
sometimes. If you¡¯ve read Walt¡¯s book you know his position on the Amateur
obsession with SWR! With that in mind, I like to point out to my audience
that the increase in loss due to SWR is simply because the reflected RF
passes through the feedline multiple times from it's mismatched load
multiple times with the SAME loss, which simply accumulates. Using a
matching network at the system input can be misleading as it only accounts
for a SINGLE point of mismatch in the system. For example: Matching a 50
ohm exciter to a 300 ohm line, feeding a 300 ohm folded dipole will match
with great success. Matching that same exciter to a 300 ohm line feeding a
75 ohm dipole, not so much. The 300 to 75 mismatch WILL result in
additional feed-line reflections and commensurate loss regardless of the
tuner. Hence, it is important to stress the need to match the line to the
load as best as possible. Many installations using mismatched antennas and
feed line exist. The most popular being the OWL fed dipole. The belief that
the OWL is so much less lossy than coax being a benefit is (IMHO) not
warranted given the trade-offs. Not the least of which is (depending on
line length) the extreme swings in voltage and current that can exist in
such a system.
I¡¯ll forego my mantra on preferring resonant antenna and broadband
transformer matching for radically long or short antennas. Best of luck
with your presentation and kudos for your efforts. I hope I¡¯ve given you
some useful things to consider conveying.
Mark
|
Is this presentation available to view?
Darrell
N5FTW
Sent from my over-rated IPhone 7 Plus. Any Mis-spellings or grammar errors are due to my IPhone auto correct feature.
toggle quoted message
Show quoted text
On Nov 13, 2020, at 17:28, Mark Sedutto <ka2qfx@...> wrote:
?Barry,
I¡¯ve reviewed your slide show and must say I am impressed with your ambitiousness. That¡¯s a LOT of material to cover in a single presentation. I would think most ham clubs members would be hard pressed to absorb that much information in a single presentation. Perhaps breaking it up into a few parts would be more effective, but you know your audience. I have taught such material to amateur level students and found it a little daunting.
As far as content is concerned I don¡¯t have much to add to what Dr. Kirkby and others mentioned, but here¡¯s a thing or two.
Antenna resistance: You present antenna ¡°resistance¡± (along with reactance and phase) but don¡¯t specify ¡°Radiation Resistance¡±. Most people will think of wire resistivity, or resistance associated with loss when presented with this term. I think the natural ¡°radiation resistance¡± of an antenna is an important concept to distinguish. This being the resistive load presented to the RF source by the radiating field of energy the antenna creates. Generally speaking, the longer the antenna the higher the Radiation resistance. I describe it (mechanically) like casting a fishing pole. A very long pole exhibits quite a bit more resistance than a very short pole.
Another factor that is important to consider in this regard is that as reactance varies from resonance the net Z of the antenna rises, as does SWR. Hence, even a mismatched antenna will exhibit its lowest SWR at resonance. (There are minor deviations to this rule for reasons beyond the scope of amateur discourse. So I pray the experts don¡¯t beat me up on that :). ) But for things like multi-band verticals and short mobile antennas this becomes critical as Reactance to (low) Radiation Resistance ratios rapidly swing the antenna Z, resulting in high Q, low bandwidth antennas. You see this in some of your latter VNA Smith curves as tight snail shell curves. Whereas antenna with high resistive losses, like short mobile antenna with thin wire, swamp this out and present wider bandwidth. A seemingly "better" antenna... NOT even a little.
SWR "Losses", etc: The misapplication of this term make me nuts sometimes. If you¡¯ve read Walt¡¯s book you know his position on the Amateur obsession with SWR! With that in mind, I like to point out to my audience that the increase in loss due to SWR is simply because the reflected RF passes through the feedline multiple times from it's mismatched load multiple times with the SAME loss, which simply accumulates. Using a matching network at the system input can be misleading as it only accounts for a SINGLE point of mismatch in the system. For example: Matching a 50 ohm exciter to a 300 ohm line, feeding a 300 ohm folded dipole will match with great success. Matching that same exciter to a 300 ohm line feeding a 75 ohm dipole, not so much. The 300 to 75 mismatch WILL result in additional feed-line reflections and commensurate loss regardless of the tuner. Hence, it is important to stress the need to match the line to the load as best as possible. Many installations using mismatched antennas and feed line exist. The most popular being the OWL fed dipole. The belief that the OWL is so much less lossy than coax being a benefit is (IMHO) not warranted given the trade-offs. Not the least of which is (depending on line length) the extreme swings in voltage and current that can exist in such a system.
I¡¯ll forego my mantra on preferring resonant antenna and broadband transformer matching for radically long or short antennas. Best of luck with your presentation and kudos for your efforts. I hope I¡¯ve given you some useful things to consider conveying.
Mark
|
Yes... and No. Check your email OM.
|
Barry K3EUI
Dr. David Kirkby, Kirkby Microwave Ltd
Darrell Carothers