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VNA and antenna matching : calculating reactance needed to match

 

Hello,

Having read through the Group, I've not been able to find an answer to my question, but I have found a lot of information that suggests several members of the Group will know ... here's the question:

Let's say one measures the impedance of an antenna, reading the S11 parameters from the NanoVNA via USB, converting the reflection coefficients to real & imaginary impedances (thank you Rich NE1EE for your spreadsheet). Does anyone have a spreadsheet to calculate the necessary reactance (L C) that needs to be added in a L match tuner (with series L)? Effectively, a spreadsheet, or formula, that replicates what one would normally use a Smith chart to achieve.

Thanks in advance.
 

Hi Andrew
For me, SimSmith/SimNEC is the right software to do those calculations.
Very helpfull.
73 - Antonio EA7HJ
 

SimSmith includes various matching topologies as part of the Smith Chart
presentation:



Dave - W?LEV

On Thu, Aug 24, 2023 at 7:46?PM Andrew (G1RVD G0Z) <andrew@...>
wrote:

Hello,

Having read through the Group, I've not been able to find an answer to my
question, but I have found a lot of information that suggests several
members of the Group will know ... here's the question:

Let's say one measures the impedance of an antenna, reading the S11
parameters from the NanoVNA via USB, converting the reflection coefficients
to real & imaginary impedances (thank you Rich NE1EE for your
spreadsheet). Does anyone have a spreadsheet to calculate the necessary
reactance (L C) that needs to be added in a L match tuner (with series L)?
Effectively, a spreadsheet, or formula, that replicates what one would
normally use a Smith chart to achieve.

Thanks in advance.






--

*Dave - W?LEV*


--
Dave - W?LEV
 

Hi Andrew.

You can also apply a formula, use a pencil and paper and a calculator and obtain a simple
match that will accomplish most Z matches to an antenna.

If you have a representative frequency, S11 and conversion to it's Rs and Xs values, lets take a look and run an example of your case.

Also, consider looking at this site, very easy to apply:
 

Also, know that with L-Networks, the shunt element goes on the high-Z side.

Dave - W?LEV

On Thu, Aug 24, 2023 at 9:49?PM alan victor <avictor73@...> wrote:

Hi Andrew.

You can also apply a formula, use a pencil and paper and a calculator and
obtain a simple
match that will accomplish most Z matches to an antenna.

If you have a representative frequency, S11 and conversion to it's Rs and
Xs values, lets take a look and run an example of your case.

Also, consider looking at this site, very easy to apply:







--

*Dave - W?LEV*
--
Dave - W?LEV
 

On Thu, Aug 24, 2023 at 12:46 PM, Andrew (G1RVD G0Z) wrote:


Let's say one measures the impedance of an antenna, reading the S11 parameters
from the NanoVNA via USB, converting the reflection coefficients to real &
imaginary impedances (thank you Rich NE1EE for your spreadsheet). Does
anyone have a spreadsheet to calculate the necessary reactance (L C) that
needs to be added in a L match tuner (with series L)? Effectively, a
spreadsheet, or formula, that replicates what one would normally use a Smith
chart to achieve.
A couple of things to consider:
- When you say "measure the impedance of the antenna" in this case you are really measuring the impedance at the end of the coax feedline. It will be much different at the antenna feedpoint.
- Make sure that you don't have any current flowing on the outer surface of the shield (the "3rd wire"). This usually requires a balun or RF choke ("isolator"). If you don't do this the measured impedance will change when you connect the USB cable which has a ground shield on it. You can check by measuring the impedance on the NanoVNA with and without the USB cable attached.
- There is a LC match feature in recent DiSlord firmware for the NanoVNA
- If you have a copy of the ARRL Antenna Book the CD comes with a program called TLW that will design various type of tuners and gives their matching efficiency.
- Here is an online LC match calculator:


Roger
 

Although the ARRL Handbook states, without qualification, that the shunt element always goes on the high-Z side (2021 Handbook, the latest I have), that is strictly true only when the impedances to be matched are real or for a specific range of complex impedances. For a considerable range of complex impedances, there will be multiple solutions to the matching equations and the network may be reversed if necessary or desirable.

This is discussed in detail with GNU Octave code to provide solutions in "More Octave for L-Networks," QEX, May/June, 2012. A graph is provided to determine whether an L-network may be reversed to match a particular pair of complex networks.

73,

Maynard
W6PAP

On 8/24/23 15:35, W0LEV wrote:
Also, know that with L-Networks, the shunt element goes on the high-Z side.
Dave - W?LEV
On Thu, Aug 24, 2023 at 9:49?PM alan victor <avictor73@...> wrote:

Hi Andrew.

You can also apply a formula, use a pencil and paper and a calculator and
obtain a simple
match that will accomplish most Z matches to an antenna.

If you have a representative frequency, S11 and conversion to it's Rs and
Xs values, lets take a look and run an example of your case.

Also, consider looking at this site, very easy to apply:
 

Thank you for all the superb replies.

I'm looking for a source that either has the relevant formulas or, where I can figure out the formulas. Ultimately to implement them in some form of auto tuner. (An intelligent version of an autotuner matching algorithm - if that makes any sense).

I'm investigating: GNU Octave code and TLW (i have a version of the ARRL Antenna Book).
I've used SimSmith, it's a great programme. (Effectively, I'm looking for the underlying math to automate the match.)


Kind regards,
Andrew
 

B. Whitfield Griffith, Jr., "Radio-Electronic Transmission Fundamentals," McGraw-Hill, 1962 discusses the design of L-networks and provides graphical methods for such designs. ARRL used to sell this book, but I don't see it at the ARRL website now. Some used book dealers on the Web have it for under $10, although some of them want fantastic prices for it.

And there are many other text and reference books that provide design information for various networks.

"Octave for L-Networks," QEX, Mar/Apr, 2011 provides GNU Octave code for implementing Griffith's methods and points out that networks can often be oriented either way if the impedances to be matched are complex.

"More Octave for L-Networks," which I referenced earlier, provides Octave code that determines whether a network is reversible and, if so, provides element values for both orientations.

73,

Maynard
W6PAP

On 8/25/23 00:14, Andrew (G1RVD G0Z) wrote:
Thank you for all the superb replies.
I'm looking for a source that either has the relevant formulas or, where I can figure out the formulas. Ultimately to implement them in some form of auto tuner. (An intelligent version of an autotuner matching algorithm - if that makes any sense).
I'm investigating: GNU Octave code and TLW (i have a version of the ARRL Antenna Book).
I've used SimSmith, it's a great programme. (Effectively, I'm looking for the underlying math to automate the match.)
Kind regards,
Andrew
 

Based on my limited knowledge I think with regard to L networks there are choices in topology, depending on antenna impedance and desired phase shift in the L network. Also practical concerns such as desire to isolate DC from the transmitter, or to put an otherwise floating antenna at DC ground potential without having to use (and analyze) a separate "static choke".
 

I have traps doublets (3 or 4 bands) connected by about 10 m of coax to the station. I have always measured the impedance of my antennas at the level of the center by calibrating (open short load) at the end of the caoxial removed from the antenna, from my PC and nanovna-saver. I then calculated and made my 'L' adapters to place them in the center of the doublet. It worked without a problem, without a chock balun. The ROS becomes correct and it is correct at the station, at the calculation frequency only, obviously

When I go to the nanoVNA of the chock balun, I doubt that we can measure anything correct beyond the chock balun. But hey, hope gives life, as well as those who sell chock-balun.
73
--
F1AMM
Fran?ois
 

If you calibrate with the measurement plane at the antenna end of the choke
balun, you can measure the antenna impedance correctly. You need to
calibrate out the choke balun.

Dave - W?LEV

On Fri, Aug 25, 2023 at 3:32?PM Fran?ois <18471@...> wrote:

I have traps doublets (3 or 4 bands) connected by about 10 m of coax to
the station. I have always measured the impedance of my antennas at the
level of the center by calibrating (open short load) at the end of the
caoxial removed from the antenna, from my PC and nanovna-saver. I then
calculated and made my 'L' adapters to place them in the center of the
doublet. It worked without a problem, without a chock balun. The ROS
becomes correct and it is correct at the station, at the calculation
frequency only, obviously

When I go to the nanoVNA of the chock balun, I doubt that we can measure
anything correct beyond the chock balun. But hey, hope gives life, as well
as those who sell chock-balun.
73
--
F1AMM
Fran?ois






--

*Dave - W?LEV*
--
Dave - W?LEV
 

Thank you for all the replies.

The first six pages of seem to address my question. is very similar content.

"Notes on Antenna Tuners: The L-Network and Impedance Matching" by K6JCA is interesting

Kind regards,
Andrew

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