Antenna analyser

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greggdaug
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Re: Antenna analyser

Post by greggdaug » Sat May 14, 2016 12:07 am

You need a coupler or bridge to measure reflections from a 1-port device. The setup as shown appears to be measuring gain through a 2-port DUT. When using probes you would need to insure they are 1:1 to maintain 50-ohms to/from the DUT.

Example 1-port VNA configurations:
vna_1port_configs.png
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pavel
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Re: Antenna analyser

Post by pavel » Sat May 14, 2016 9:37 am

You need a coupler or bridge to measure reflections from a 1-port device. The setup as shown appears to be measuring gain through a 2-port DUT.
I'm not sure if I understand this.

In my understanding, we try to find the impedance of the DUT by measuring voltages.

If the probes are shorter than 0.5 m, then the wave effects could be neglected for the frequencies lower than 60 MHz.

The length of my probes is about 1.5 m. But I expect that calibration compensates for the transmission line effects. Unfortunately, I don't have a coupler or a bridge to confirm this.

Could you please try to compare the measurement results obtained with the ZFBDC20 coupler and with the terminated probes?

A simplified schematics of a series fixture and of a bridge:
Image

Expressions for the impedance calculations:
Image

greggdaug
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Re: Antenna analyser

Post by greggdaug » Sat May 14, 2016 3:40 pm

These fixtures (using the equations you show) work ok over a limited range of impedances. Due to the low directivity, measurement error is higher than when using a coupler. Also, the series fixture does not provide all four of the 2-port S-parameters.

Microwave VNAs use calibration techniques outlined in:
Network_Analyzer_Error_Models_and_Calibration_Methods.pdf
Plots below show percentage measurement error using a bridge at 60 MHz for 1) Rdut = 0.1 - 2000 ohms, 2) Ldut = 1 - 1000 nH and 3) Cdut = 1 - 1000 pF.

Rdut sweep (Freq = 60 MHz):
Rdut_err.png
Ldut sweep (Freq = 60 MHz):
Ldut_err.png
Cdut sweep (Freq = 60 MHz):
Cdut_err.png
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Drachenfrucht
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Re: Antenna analyser

Post by Drachenfrucht » Mon May 30, 2016 10:22 pm

pavel wrote:I could suggest to try the Ham VNA program with the SDR/HPSDR application.
Hi Pavel,

I did read your post here.
out of curiosity. What would be the difference (pros/cons) between:

a) using the official RP OS image 0.95-1 and there install your app called "SDR transceiver compatible with HPSDR". Then run this app (=server) and on a windows computer install and run "HamVNA" (=client).

b) using the official RP OS image 0.95-1 and on that install your app called "Network Vector Analyzer". Then run this app (=server) and on a windows computer install and run your "VNA controll software" (=client).

If I understood correctly one should prefer using the app "Network Vector Analyzer" from RedPitaya's Application Marketplace because it's faster and better?? Am I right or completely wrong? What combination do you suggest to use?

a) OpenHPSDR running on the official RedPitaya image (=server) and HamVNA running on a WindowsPC (=client) ?

-or-

b) Vector Network Anlyzer running on the official RedPitaya image (=server) and your VNA controller software running on a Windows PC (=client) ?

And please tell me, which button/mode do I need to choose in "HamVNA" and "VNAclient" when I want to measure a Balun ? In HamVNA should it be "reflection" or "transmission" and in VNAclient ??

pavel
Posts: 531
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Re: Antenna analyser

Post by pavel » Mon May 30, 2016 10:44 pm

Drachenfrucht wrote: a) using the official RP OS image 0.95-1 and there install your app called "SDR transceiver compatible with HPSDR". Then run this app (=server) and on a windows computer install and run "HamVNA" (=client).
It won't work. Since I didn't succeed in making my SDR application to reliably work with HamVNA, I developed a new VNA application for Red Pitaya and removed all the VNA related code from the SDR transceiver compatible with HPSDR.
Drachenfrucht wrote: b) using the official RP OS image 0.95-1 and on that install your app called "Network Vector Analyzer". Then run this app (=server) and on a windows computer install and run your "VNA controll software" (=client).
As far as I know, it's the only VNA application that currently works with Red Pitaya.
Drachenfrucht wrote: And please tell me, which button/mode do I need to choose in "HamVNA" and "VNAclient" when I want to measure a Balun ? In HamVNA should it be "reflection" or "transmission" and in VNAclient ??
My knowledge of the VNA measurements is still very basic. So, I'm not sure how to measure a balun with my application.

In the current version of my VNA application there is no equivalent of the transmission mode. If I'm not mistaken, the only measurement available in the transmission mode is gain.

What parameters of your balun you want to measure?

Drachenfrucht
Posts: 34
Joined: Fri Mar 18, 2016 12:18 am

Re: Antenna analyser

Post by Drachenfrucht » Mon May 30, 2016 11:05 pm

Hi Pavel,

thanks for your kind reply. So I definitely can uninstall HamVNA from my Windows PC and focus into your new released "VNA" server application on RP's appMarketplace and your "VNA" client software for Windows, right?

Well, related to my question measuring a balun. I want to measure the attenuation/loss in relation to the frequency. Such results looks like that:
VNA_current_balun_1_to_1.jpg
The results on this graph show that this current balun attenuates quite well beginning from 3,5MHz where the attenuation is -20dB which is a quite satisfying value. At 29MHz the attenuation is 34dB which is really good for a current balun.

I'd like to measure that. Should that be the point "Return loss" on your "VNA" client program ?
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Drachenfrucht
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Re: Antenna analyzer

Post by Drachenfrucht » Mon May 30, 2016 11:49 pm

pavel wrote:
Drachenfrucht wrote: A german documentation is available as PDF.
Could I use the mentioned reflexion bridge for my goal?
I could not find any schematics of the bridge in the PDF document. So, I don't know how to compare it with the bridge suggested by the author of the HamVNA program.
Is there any particular reason that you need the reflection bridge?

The series fixture is easier to build (just a few resistors) and even the author of the HamVNA program writes that he uses the series fixture more often because it works both in the transmission and reflection modes.

I've just tried two scope probes as a series fixture. Using my RX antenna as a DUT I could see some resonances. Unfortunately, I don't have any reference antenna analyzer or VNA to compare with.
Hi again,

as we already said, I won't focus on HamVNA any more but instead I will focus on your application called "VNA". I want to ask again you or Gregg related to the reflexion bridge I am using:
fa_reflexionsmesskopf.jpg
THIS IS THE PDF for that reflection bridge in german. I am not good in translating but I will give a try to explain what this manual says about this reflection bridge:

For reflection measurements with the Network Analyzer a measuring head is needed that provides a sharpness of directivity more than 35dB at the range from 0,1 to 160MHz and at 160MHz this sharpness of directivity should be at least 30dB. Such a unit as described here is needed for measuring passive elements like antennas, filters at the load of 50 Ohm (usual values are SWR, return loss, etc.). Criteria of quality for the measurement accuracy is the return loss, also known as sharpness of directivity. This value shows how much power is reflected unwanted even on a matched impedance -here 50 Ohm-.

This reflection bridge measuring head consists of a special coupler called TDC-10-1 manufactured by Mini Circuits. As already mentioned it provides a very good sharpness of directivity better than 30dB which is quite enough for the ham radio world. The following table shows the directivity attenuation, reflection factor and minimum displayable SWR:
table.jpg
This is how the reflection bridge looks like inside:
reflection_bridge_1.jpg
reflection_bridge_2.jpg
The following illustration shows how the reflection bridge is connected to the VNA board:
reflection_bridge_connection.jpg
I connected it exactly like that. The text with red colour "Zx (Messobjekt)" is the DUT.

In the following the PDF shows some results of measurements and discuss several parts. The network analyzer used there is not the RedPitaya.

So I run your VNA application on my RP and start the VNA client on my windows PC. On the right side of that reflection bridge (=where you plug the DUT) I disconnect everything and in VNA client I click on "OPEN". Then I short circuit this ende with a piece of wire and then in VNA client I click on "Short". Then I remove this short circuit wire and I insert a 50Ohm resistor into that jack and in VNA client I click on "Load". That looks like that:

On that link you see the three screens in that order OPEN - SHORT - LOAD

But when I plug in my current balun into the DUT jack and in VNA client clicking on DUT I get this picture here:
Image

I cannot understand that, why do I get such a result? I want to add that on the end of my current balun I have attached a 50 Ohm resistor so the impedance is matched. This 50 Ohm simulates an antenna.

Any help appreciated.
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pavel
Posts: 531
Joined: Sat May 23, 2015 5:22 pm

Re: Antenna analyser

Post by pavel » Tue May 31, 2016 7:26 am

I looked for some information on the web and found the following interesting documents:

http://www.testequity.com/documents/pdf ... N9340B.pdf
http://cp.literature.agilent.com/litweb ... -7917E.pdf

The first document contains some information about the transmission and reflection measurements.

If I'm not mistaken, the first screenshot that you posted yesterday, showed the gain measurements in the transmission mode:
Image

According to the HamVNA page, only the series fixture could be used for the transmission mode.

In the current version of my VNA client program there is no gain calculation but it can be easily added.

So, if I add the missing calculations to my VNA client program, then you'll be able to measure the transmission parameters of your balun using a series fixture.

Drachenfrucht
Posts: 34
Joined: Fri Mar 18, 2016 12:18 am

Re: Antenna analyser

Post by Drachenfrucht » Tue May 31, 2016 8:19 am

Hi Pavel,

thanks for that information. I also own a series fixture. It consists of two 100Ohm resistors that are connected parallel each other so in result the path has 50 Ohm:
shunt.jpg
I just need to know the connection schematic how I need to connect and use it to measure transmission which is needed for measuring a balun.

EDIT: Oh, sorry. I think now I understood what "series fixture" mean :-) I don't need the shown unit, I just need to connect the DUT between OUT1 and IN1 of my Red Pitaya as shown here??
series_fixture_dut.jpg
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pavel
Posts: 531
Joined: Sat May 23, 2015 5:22 pm

Re: Antenna analyser

Post by pavel » Tue May 31, 2016 9:10 pm

I did some reading about the balun measurements. Looks like the problem is far from trivial. Here are the links to the documents that I could find so far:
http://www.arrl.org/files/file/QEX_Next ... ov-Dec.pdf
https://www.anaren.com/sites/default/fi ... ting_0.pdf
http://cp.literature.agilent.com/litweb ... 5635EN.pdf
http://cdn.rohde-schwarz.com/pws/dl_dow ... Z53_0E.pdf

If you have two identical baluns, then you could try the back-to-back measurement technique. described on page 3 in BalunTesting_0.pdf.

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