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Antenna Queries de k3eui Barry prior to Nano VNA talk

 

Questions to think about concerning antennas and feed lines:
K3EUI's Zoom talk on Nano VNA Part II (Under the Hood)

1) Why do antennas have resistance, reactance, and impedance?
What are typical values for a one-half wavelength dipole mounted
clear of the ground and surrounding conductors?

2) Why does every feed line have a "characteristic impedance"?
What factors affect its value?

3) Why does every feed line have an SWR of some value at some frequency?
Is my measured SWR in the shack the same value as (or pretty close to) the SWR at the antenna terminals?
If so, why? If not, why not?
What exactly is the SWR telling us about the "quality" of the match of feed line and antenna?

Let's take a specific example now:
4) Say I have 100 ft of 50 ohm coax connected to a one-half wave 80m center-fed dipole about 120 ft long and 50 ft above ground. It is tuned to the center of the band.

Why is the SWR high (above 3:1) at the 80m band edges?
What kind of loss (in dB) is acceptable on this antenna on 80m?
Can I use this antenna on 40m? Why or why not?
Can I do anything (in my shack) to improve its performance on 80m?


5) What do the following impedance values all share in common:
25 + j0 40 + j30 40 - j30 100 + j0

How can my Nano VNA tool help me to get the answers to ALL of these questions?

Send your responses to
Barry K3EUI K3euibarry@...
 

On 7/6/21 8:55 AM, Barry K3EUI wrote:
Questions to think about concerning antennas and feed lines:
K3EUI's Zoom talk on Nano VNA Part II (Under the Hood)

1) Why do antennas have resistance, reactance, and impedance?
What are typical values for a one-half wavelength dipole mounted
clear of the ground and surrounding conductors?

This could get into an interesting discussion of electromagnetics and physics.

Ultimately, antennas have a combination of radiated and non radiated fields with energy moving back and forth between the stored and radiated fields. The ratio is the Q of the antenna (where Q is defined as "energy stored"/"energy lost per cycle" - it happens to be *similar* to a RLC resonant circuit, but the mechanism is different.
 

Your NANOVNA is a wonderful educational tool. However, it doesn't do
calculus. To answer all your questions, you really need a good graduate
level reference addressing antennas and transmission lines. WARNING: Some
pretty deep calculus.

Dave - W?LEV

On Tue, Jul 6, 2021 at 9:55 AM Barry K3EUI <k3euibarry@...> wrote:

Questions to think about concerning antennas and feed lines:
K3EUI's Zoom talk on Nano VNA Part II (Under the Hood)

1) Why do antennas have resistance, reactance, and impedance?
What are typical values for a one-half wavelength dipole mounted
clear of the ground and surrounding conductors?

2) Why does every feed line have a "characteristic impedance"?
What factors affect its value?

3) Why does every feed line have an SWR of some value at some frequency?
Is my measured SWR in the shack the same value as (or pretty close to)
the SWR at the antenna terminals?
If so, why? If not, why not?
What exactly is the SWR telling us about the "quality" of the match of
feed line and antenna?

Let's take a specific example now:
4) Say I have 100 ft of 50 ohm coax connected to a one-half wave 80m
center-fed dipole about 120 ft long and 50 ft above ground. It is tuned to
the center of the band.

Why is the SWR high (above 3:1) at the 80m band edges?
What kind of loss (in dB) is acceptable on this antenna on 80m?
Can I use this antenna on 40m? Why or why not?
Can I do anything (in my shack) to improve its performance on 80m?


5) What do the following impedance values all share in common:
25 + j0 40 + j30 40 - j30 100 + j0

How can my Nano VNA tool help me to get the answers to ALL of these
questions?

Send your responses to
Barry K3EUI K3euibarry@...





--
*Dave - W?LEV*
*Just Let Darwin Work*
Anne Ranch
 

My favorite question , not directly related to nanoVNA

Why is having minimal SWR important ?

Optional answer:

SWR is an indicator of IMPEDANCE match between x and y devices -
directly related to "power transfer" between. source - TX - and load - antenna.

Demonstrate by applying Ohm's law.
 

On 7/6/21 8:46 PM, Anne Ranch wrote:
My favorite question , not directly related to nanoVNA

Why is having minimal SWR important ?

Optional answer:

SWR is an indicator of IMPEDANCE match between x and y devices -
directly related to "power transfer" between. source - TX - and load - antenna.

Demonstrate by applying Ohm's law.

Which breaks horribly if your source has very high or very low impedance - Consider a DC power supply.
 

A DC source has no complex portion and is not characterized by impedance,
only the real term in the R ¡À jX. Further, if no current is drawn from the
source, the presence of the R term isn''t there. The ¡À jX term requires AC
and is absent for DC. However, once you draw a varying current from that
DC source as in SSB or keyed RF (CW), then the complex portion appears.

Example: Take AWG #000 (or whatever) large solid copper conductor. At DC,
the cross sectional current density is homogenous. However, even at 60 Hz,
skin effect due to resistance and inductance (+ jX) is evident and
measurable. The cross sectional current density is no longer homogenous
even at 60 Hz.

Dave - W?LEV

On Tue, Jul 6, 2021 at 9:53 PM Jim Lux <jim@...> wrote:

On 7/6/21 8:46 PM, Anne Ranch wrote:
My favorite question , not directly related to nanoVNA

Why is having minimal SWR important ?

Optional answer:

SWR is an indicator of IMPEDANCE match between x and y devices -
directly related to "power transfer" between. source - TX - and load -
antenna.

Demonstrate by applying Ohm's law.

Which breaks horribly if your source has very high or very low impedance
- Consider a DC power supply.







--
*Dave - W?LEV*
*Just Let Darwin Work*
Charlie N2MHS
 

I remember wbw a 100 foot piece of -48V 500 MCM being modeled as R+jinductance and C to trough. We were wondering what impulse it would take to raise -48V to 0. Turned out a disgruntled employee was pulling the fuse. KISS

On Wednesday, July 7, 2021, 12:56:52 PM EDT, David Eckhardt <davearea51a@...> wrote:

A DC source has no complex portion and is not characterized by impedance,
only the real term in the R ¡À jX.? Further, if no current is drawn from the
source, the presence of the R term isn''t there.? The ¡À jX term requires AC
and is absent for DC.? However, once you draw a varying current from that
DC source as in SSB or keyed RF (CW), then the complex portion appears.

Example:? Take AWG #000 (or whatever) large solid copper conductor.? At DC,
the cross sectional current density is homogenous.? However, even at 60 Hz,
skin effect due to resistance and inductance (+ jX) is evident and
measurable.? The cross sectional current density is no longer homogenous
even at 60 Hz.

Dave - W?LEV

On Tue, Jul 6, 2021 at 9:53 PM Jim Lux <jim@...> wrote:

On 7/6/21 8:46 PM, Anne Ranch wrote:
My favorite question , not directly related to nanoVNA

Why is having minimal SWR important ?

Optional answer:

SWR is an indicator of IMPEDANCE match between x and y devices -
directly related to "power transfer" between. source - TX - and load -
antenna.

Demonstrate by applying Ohm's law.

Which breaks horribly if your source has very high or very low impedance
- Consider a DC power supply.







--
*Dave - W?LEV*
*Just Let Darwin Work*
 

Nice topics Barry

just one small (nitpicking) remark on the wording in
3) Why does every feed line have an SWR of some value at some frequency?

Actually it's not the feedline that has an SWR 'of some value'.
The line has a characteristic impedance (see item 2) )
It's the (mismatched) load that is transformed into something else along the line, causing the SWR.

Also a fun topic : how can a capacitive load suddenly be measured as some inductive impedance elsewhere along the line ? ... call our friend Smith to help.

73,
Luc ON7DQ
 

SimSmith:

Excellent Tutorial:
<>

Dave - W?LEV

On Thu, Jul 8, 2021 at 3:13 AM Luc ON7DQ <on7dq@...> wrote:

Nice topics Barry

just one small (nitpicking) remark on the wording in
3) Why does every feed line have an SWR of some value at some frequency?

Actually it's not the feedline that has an SWR 'of some value'.
The line has a characteristic impedance (see item 2) )
It's the (mismatched) load that is transformed into something else along
the line, causing the SWR.

Also a fun topic : how can a capacitive load suddenly be measured as some
inductive impedance elsewhere along the line ? ... call our friend Smith to
help.

73,
Luc ON7DQ





--
*Dave - W?LEV*
*Just Let Darwin Work*
 

I'm not explaining this well.

Goal: understand more about how an antenna works using a Nano VNA tool in S11 mode
Translation: understand the graphs of R, X and Z vs. frequency that I get with Nano VNA SAVER
Also: understand what the S11 PHASE graph is revealing about resonance and reactance.
Finally: see that the Smith Chart in S11 mode tells me just about everything I need to know - if I can interpret the meaning of the graphs. I am always using 50 ohm coax feed line and dipole-like antennas.

So to help me understand these graphs on a real antenna, I put together a simple RLC circuit board.
I used parts from an old MFJ tuner: 200 pf air variable cap, 10 microHenry tapped coil, and a few different resistors (10 ohm to 100 ohm).

What I forgot: the S21 port is already terminated with a 50 ohm resistor.
I realized that later when my Smith Chart missed the "bull's eye" ..... rookie error.

So I can manipulate the R and L and C of my breadboard circuit and play with how changing these components (in series for now) changes the Smith Chart, the R and X graphs and the Z graph and more importantly, the PHASE graph.

Once again, a Nano VNA rookie playing and learning as I go.

The Nano VNA is just a gadget that probes my understanding of all of these cocepts.
Yes, it does not do calculus.
Perhaps WHY an antenna radiates is a bit too broad a question - for now.

But it is like trying to read my EKG to tell me how my heart is behaving.
Actuallly I think I can read an EKG with better accuracy than read my antenna's performance.

TU
De k3eui barry
 

To get a good idea how antennas behave check out 4NEC2 an antenna modeling program. It can be downloaded from the net free. There are other programs as well like EZNEC that might be easier to start with. Start with a dipole that has been already designed and change the parameters to see how the radiation pattern, the feed point impedance and other characteristics change.
It is well worth the time to learn how to use a basic antenna modeling program and use it conjunction with the Nanovna. Antennas can be scaled down to make them easy to support and modify.
I have made antennas for 200 MHz or higher to make them compact. In one case I just taped it to a large piece of cardboard. With the Nanovna you can use it to drive the antenna with ch0 and monitor the radiation from it with ch1 using a small sniffing antenna. The sniffer can give you an idea of the pattern and polarization. Years ago I wanted to prove that a half square antenna actually was a vertical polarization radiator with its horizontal half wave of wire connecting the two verticals. In the far field it was perfectly vertically polarized. I also did the same with a discone and learned that it does have a polarization tilt at some frequencies (old radio shack version tested at 2 meters).
 

On 7/9/21 6:22 AM, WB2UAQ wrote:
To get a good idea how antennas behave check out 4NEC2 an antenna modeling program. It can be downloaded from the net free. There are other programs as well like EZNEC that might be easier to start with. Start with a dipole that has been already designed and change the parameters to see how the radiation pattern, the feed point impedance and other characteristics change.
It is well worth the time to learn how to use a basic antenna modeling program and use it conjunction with the Nanovna. Antennas can be scaled down to make them easy to support and modify.
I have made antennas for 200 MHz or higher to make them compact. In one case I just taped it to a large piece of cardboard. With the Nanovna you can use it to drive the antenna with ch0 and monitor the radiation from it with ch1 using a small sniffing antenna. The sniffer can give you an idea of the pattern and polarization. Years ago I wanted to prove that a half square antenna actually was a vertical polarization radiator with its horizontal half wave of wire connecting the two verticals. In the far field it was perfectly vertically polarized. I also did the same with a discone and learned that it does have a polarization tilt at some frequencies (old radio shack version tested at 2 meters).
Copper foil tape on paper !

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