Antenna matching 1

[Leiser]

Hi,

I have to match a DAB antenna to my LNA. The antenna S11 is very bad as it´s very narrow band. Attached you will find the S11.

I was wondering if anybody has experience matching antennas and can help me with that, as I´m trying to design a filter that has minimum transmission losses within the band and I am strugling a little bit.

DAB band is from 174 to 240MHz.

Thanks!

 

[zappedagain]

It is pretty ugly, but at least you are 'only' 3dB down across your band. There is a small chance you could flatten the response over your required band, but you are more likely to open up some nasty interference paths. Think about your likely interference sources; will they be lower frequency so you want to match with a high pass, or higher frequencies so you want to match with a lowpass, or both so you need to match with even more elements? Minimum insertion losses typically means fewer matching components.

I still like "RF Circuit Design" by Chris Bowick (published by SAMS) for the basics on impedance matching.

John D

 

[Higgler]

Usually with that shape return loss, you can't just add some magic matching and cover your entire band. Are you sure this antenna is supposed to be 31% bandwidth?

What type antenna is this? Maybe it's inherently narrow band because your antenna volume is too small (law of physics issue) to achieve good return loss over your bandwidth.


example;
To get good VSWR over that band, if I made a notch antenna, it would be 22" x 22" x thin (0.060") in size and would work from 174 to 500 Mhz.

kch

 

[Leiser]

Hi,

this is a fractal antenna. I have checked the VWSR and is 42% below 3 in the frequency range o 174 to 240MHz.

what do you mean by "Are you sure this antenna is supposed to be 31% bandwidth?" where did you get the 31% figure?

FM and TV bands are very close so I need the higher attenuation the better.

 

[VEBill]

"...where did you get the 31% figure?"

It comes from "174 to 240MHz".


Some might say that so-called fractal antennas have a promises/reality ratio that exceeds unity.

 

[Leiser]

Thanks for you replies, they were very useful and quit!

 

[Leiser]

sorry, I meant quick!.

Here we are working with fractal antennas, and what I have found is that they are very narrow band and from my side, as a LNA designer, it´s very difficult to match and achieve some kind of gain flatness over the bandwindth.

 

[Higgler]

Fractals are typically used to slightly shrink the physical size of a narrow band antenna by about 10-20%. My company has never uses a fractal antenna.

Your plot shows S7,7, is this an array of 7 elements and you're measureing one of them?

Is your data from your own measurements or a manufacturer?

kch

 

[Leiser]

Hi Higgler,

I work in a team that integrate antennas in cars. I design the LNA´s and the other guys design the fractal antennas, so they give the the S-parameters. I don´t have much experience in antenna design so I don´t really know how realistic is to match these kind of antennas to the LNA.

VSWR is what it is, I have attached the VSWR plot for the DAB band III. The problem I found is that to match the antenna to the LNA and at the same time have minimun insertion losses across the band was nearly imposible. Typical maximum insertion losses were around 7dBs which I believe impact in noise directly.

I´m not simulating an array, I used S(7,7) because I am comparing different circuits on the same sheet.

All I want to know is if working with fractal antennas and tring to match them is well-known problem, as I am new on that and I would like to know what I´m dealing with.

Thanks!

 

[Leiser]

Sorry, I forgot to add the attachment.

 

[Higgler]

fractals are uncommon antennas. One company "fractalantenna.com" pushes their greatness, but when I compared his limited data to antennas I've built, they were the same size and performance. Seems likes it's all a marketing job.

Your antenna engineers stuck you with a bad antenna design based on your VSWR plots. You have no chance on matching that puppy at 2:1 VSWR.

Normal antennas make spec. If not, the antenna engineer adds their own matching to the circuit card the antenna is printed on. Or they find the matching circuits and give them to the LNA designer.

ask the antenna engineers if they've matched this antenna in the past and have them show you the matching circuits with proof they work.

Otherwise, change antennas. Sorry if that's blunt.

Can you give the dimensions (ballpark) of this antenna and how it's mounted on the vehicle. I'm sure I can give you a better antenna design in a minute if your location is friendly.

kch