Re: Shifted Resonant Frequency between HFSS simulation and PCB antenna
I am looking at your file. Few things still unclear. First, antenna measurement:
1. Your NA cable is calibrated, correct?
2. When you measure antenna antenna is laying on table? The material of table loads antenna, shifts it's resonant frequency down a little bit. If you plan to use your antenna on the table it is ok. Otherwise you want your antenna to be mostly in the air. Place couple of styrofoam blocks underneath. Styrofoam is mostly air, it is like air.
3. To measure antenna only your energy harvesting circuit and traces leading to it must be disconnected. Cut traces with knife, only few mm away from connector. Later you connect traces with little pieces of wire.
Do all that make another measurement sent it to the board.
You do not need new antenna we can always tune existing one
1. I will check the NA cable. I do not know how to correct the calibration I just connect to the SMA and measure, I will look at the catalog and check it.
2. That is right Sir. Every time I measure the frequency change a little bit.
3. I will follow your suggestion Sir.
I learned a lot of things. Have a good day Sir.
1.There is nothing wrong with cable. You need to calibrate NA at the end of the cable before you do measurements. You do it by connecting calibration standards. Do you?
2. Frequency changes a bit because all things around the antenna affect resonant frequency. This is normal.
1. Yes, I did it by connecting calibration standard Sir, it looks like a adapting connection for the cable to SMA. Now I came back home, tomorrow I will keep doing that, it's so interesting.
Ok, we assume that 1, 2, 3 are all ok. (post pictures of your antenna measurement setup and NA measurement). Assume you still getting dip resonance below the required 915 MHz. (You changed the measurement scale of Network Analyzer. NA probably asked you to interpolate calibration results, you answered 'yes'.) Add two more Markers. Place marker 2 at 902MHz, marker 3 at 928MHz. This will show your 900MHz ISM band.
Now it is time to tune your antenna.
- Record the frequency of the lowest point of your resonant 'dip'.
- reduce length of antenna elements by 1 mm from each side - cut it off with a knife
You will see that the 'dip' will come up in frequency.
- record new frequency and calculate difference
- calculate difference between new frequency and 915 MHz. That's how much you still have to come up.
- make ratio and calculate how much more you need to cut off.
- cut less then that and repeat the process.
- continue until your 'dip' right on 915 MHz.
You should be tuned now.
These are the new results (I put antenna on the styrofoam), now I will follow your instructions to tune the frequency. Your suggestions are awesome.
Thank you. Please send pictures showing how antenna is set - show antenna styrofoam feed cable and everything. Send before you start shortening antenna.
Are you really in VietNam? Where abouts in VietNam?
Thank you Sir, these are all of steps I measured antenna.
I am staying in Vietnam, HoChiMinh City Sir.
1. Remove black computer display. Remove mouse, remove disconnected cables in front of network analyzer any other conductive bodies. You want to create like a bubble around the antenna with no conductive bodies it.
2. Did you cut traces to multiplier? If not, cut them close to connector just leave few mm's so you can solder then back later.
3. Set up network analyser: F = 500MHz to 1 GHz; Markers: M1 to min, M2=902MHz, M3=928MHz.
show me plots: Return loss, Smith chart.
1. I removed all of conductive bodies.
2. I cut traces to multiplier Sir.
3. I set as you mentioned.
Very good. Adjust the foam and antenna placement so the cable is generally perpendicular to the antenna.
After that start tuning:
Now it is time to tune your antenna.
- Record the frequency of the lowest point of your resonant 'dip'.
- reduce length of antenna elements by 1 mm from each side - cut it off with a knife
You will see that the 'dip' will come up in frequency.
- record new frequency and calculate difference
- calculate difference between new frequency and 915 MHz. That's how much you still have to come up.
- make ratio and calculate how much more you need to cut off.
- cut less then that and repeat the process.
- continue until your 'dip' right on 915 MHz.
Send me pictures again after one or two iterations
Note: I see a bit of a problem: the two traces connecting antenna to multiplier circuit. They make up transmission line. The way it is now the impedance of this transmission line is around 300 Ohms. This will result in some mismatch. We will use them as they are for now but later if you will be re-doing the board you want to make them short and make gap between them narrow, ~0.5 mm. Connector should be right at the antenna edges. Later. Right now we use what we got.
You have good equipment in HoChiMinh City. Your Network Analyzer is better then the one I have. Who are you - student or work for a company?
The antenna connection shown in post #29 will never give correct impedance measurements. The antenna is detuned by the cable which becomes part of the antenna. Please compare with the setup shown by AndreyG in post #6.
There are various previous threads about the necessity of using a balun or cable isolation means for antenna measurements.
FvM, if cable is perpendicular and feed point is in the center currents on cable will be low, it will be close enough. It would also help having thin cable but ... we have what we have.
I shortened the length of antenna. These are the experiment results.
I routed the cable perpendicularlt to the antenna.
Very good! Show us the antenna - how you cut it. Play with it a little - see how much impedance changes when you move cable around, when you move hand along the cable, holding cable in your hand. To make it convenient to see the changes you store this trace in memory and display 'data and memory'. Keep frequency range 500 MHz to 1000 MHz. Show us more results.
After that we switch to multiplier.
Who are you - student or work for a company?
It will help me to understand what the end result should be.
Yes, and unbalanced although connected to a dipole. That's generating a completely different antenna type with different impedance. I thought you know why placed the ferrite tubes with your dipole antenna?
There's however a simple test. Touch the cable near the antenna. If the VNA display doesn't change, everything is O.K.
FvM, I know exactly why I placed choke on my antennas and I know when I can do without. FYI I spent 25 years designing antennas, earned 5 patents. Let's see the results BiNa send us.
I am a student Sir. I will test it and post the results soon.
These are my results AndreyG and FvM, thank you for your assistances.
I have 4 markers, the fourth is the resonant frequency.
Very Good! As you can see resonant frequency wanders somewhat showing there is some current running on the cable but it is not much. Bandwidth of the antenna is wide enough to accommodate the changes. Return loss is always -10 db or better across the band. The antenna is done. Now we need to figure out how to match multiplier. The problem there: Impedance of multiplier schematic which you mentioned earlier is low is transferred to antenna through impedance of transmission line which is high. Transmission line is short ~1/10th of wavelength. Right now I do not have feeling what to expect. Let's measure what we see on the connector. Before we do that, solder all the components, solder transmission lines you earlier cut, cut away antenna transmission lines. then measure and send us plots again. And we need to figure out where we install the components.
WHat is the load to the multiplier? Not sure how critical but it would be safer to connect the load and power it up. Also please send multiplier circuit