How to measure S11 on active antenna using Network Analyzer

[ccwjames]

Hi, I have a antenna with LNA connected. Is there any way that I can measure S11 of the antenna without disconnect the LNA?

 

[young.microwave.eng]

dear roommate
I think at least you should have the parameters of LNA.like S21,S12 and S22
then measuring S11, you can find ΓL and the value of mismatch consequently

 

[ccwjames]

hi there, maybe let me explaine a bit more. Normally when we want to measure S11 for a passive antenna, we can directly connect to the Network Analyzer. Right now this device that I have which the antenna is connected with a LNA. I am not interested on the LNA part, I just want to measure the antenna to check the performance without having disconnect the LNA from the antenna. So how should I go around about it?

 

[biff44]

About all you can do is sweep the antenna with another broadband antenna from a few feet away and figure out if its resonant freuquency is correct.

 

[ccwjames]

Thanks for the info. Possible to elaborate more on how to do the sweeping? Which equipment to be use?

 

[flanello]

As biff44 mentioned, you can place a broadband antenna in front of your system and then measure the S11 of the broadband antenna.
The resonant frequency should be depicted with a lower S11.
However, with this method you characterize the whole system: antenna + LNA (and the broadband antenna of course) and not the antenna itself .
As far as i know you have to brake the connections in order to only characterize the antenna.

 

[FvM]

I think, you would pereferably measure |S21| (from auxilary antenna to LNA output) with a VNA or any kind of scalar instrument (e.g. generator + spectrum analyzer or power meter). It's a reasonable way to check antenna tuning and gain. The LNA gain and input impedance must be known, however. If you can't rely on the LNA or want to measure strictly antenna S11, you have to break the connection.

 

[biff44]

I should have been clearer. You need to sweep the frequency while measuring S21 on the network analyzer. If one of the antennas is a known good broadband one, then the transmission hump should equal the center frequency of the antenna under test.

In practice, sometimes this works, sometimes not. If your antenna under test is far off...there may not be a clear resonance hump, or worse there could be many humps and you would not know which one is the right one.

Bridging over the LNA to measure S11 might be useful. Although as a minimum you will want to put a couple ferrite clamps on the coaxial cable where it attaches to the PCB.

 

[ccwjames]

I am testing on the GPS active antenna,may I what kind of broadband antenna is needed? And where to get it?

 

[FvM]

If an active anatenna has been designed with a dedicated amplifier, S11 would be meaningless because the antenna can be matched to an arbitrary amplifier impedance (or vice versa). If the amplifier is a standard LNA with 50 ohm matching, S11 can be a reasonable criterion.

Im my opinion, a gain measurement with a reference dipole is sufficient to check a GPS antenna quality (despite of amplifier noise level). Directional characteristics of the antenna (I guess, a usual patch antenna?) should be considered, of course.

 

[goal]

A broadband antenna can be a horn antenna. you can get is for rent or sale. ask any RF instruments/chambers supplier.

 

[matt11]

I think, you would pereferably measure |S21| (from auxilary antenna to LNA output) with a VNA or any kind of scalar instrument (e.g. generator + spectrum analyzer or power meter). It's a reasonable way to check antenna tuning and gain. The LNA gain and input impedance must be known, however. If you can't rely on the LNA or want to measure strictly antenna S11, you have to break the connection.

FvM,

Am I understanding correctly, to measure the GPS performance doing S21: connect horn antenna to port 2 or network analyzer, connect GPS antenna with LNA (biased to proper LNA voltage) to port 1 of network analyzer, then take S21 measurement?

Thanks,

- - - Updated - - -

I think, you would pereferably measure |S21| (from auxilary antenna to LNA output) with a VNA or any kind of scalar instrument (e.g. generator + spectrum analyzer or power meter). It's a reasonable way to check antenna tuning and gain. The LNA gain and input impedance must be known, however. If you can't rely on the LNA or want to measure strictly antenna S11, you have to break the connection.

FvM,

Am I understanding correctly, to measure the GPS performance doing S21: connect horn antenna to port 2 or network analyzer, connect GPS antenna with LNA (biased to proper LNA voltage) to port 1 of network analyzer, then take S21 measurement?

Thanks,

 

[SoCal-RF-Apps]

Take a look at
https://www.johansontechnology.com/...p-antenna-mounting-and-tuning-techniques.html

https://www.johansontechnology.com/en/technical-notes.html

 

[FvM]

On most VNAs, port 1 is the in/out port and port 2 the input port, so you connect it the other way around.