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newbie: Measureing Er constant without good equipment, FET vs Bipolar, why not LDPE?

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Terminator3

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newbie: Measureing Er constant without good equipment

1. My first question is about measuring relative dielectric constant Er of copper clad laminate.
I read about ring resonators and other microstrip methods. But what if we do not have Network Analyser or any Spectrometer able to deal with high frequencies? Is using a RCL-meter is the only alternative? I just made RCL-measurement of raw 85x45mm PCB material:
calculating area
S=0.085*0.045
constant in formula
E0=8.85*10^-12
E0 = 8.85e-12
thickness 0.5mm
D=0.5*10^-3
D = 500e-6
RCL-meter gives measurement of 0.22nF
C=0.22*10^-9
C = 220e-12
Final computation:
Er=(C*D)/(E0*S)
Er = 3.249510727
is it right way of calculation? Er is 3.24 Farad/Meter?

And this method can be inaccurate for high frequencies. So what we can do if there is no handy Network analyser/spectrometer?

For example, we have frequency source of 5GHz. I come up with some stupod method: etch 20 bandpass filters for this frequency, then connect each filter and measure amplitude (simple RLC-voltage with some detector element), make chart, choose filter with maximum voltage amplitude. Or maybe we can come up with some clever microstrip power dividers and mixers, then etch them and analyse IF output?

Method#2: Etch copper on both sides of CCL, so no any copper exists. Take 5GHz antenna, measure signal power using RLC-meter with hf "detector". Take etched dielectric board and put in in front of 5GHz antenna, measure signal power again. do some calculations. I think it can be possible, we know air constant, then we know how it changed after inserting plate, right?

2. My second question is about HJ-FET and Bipolar transistors.
How engineer decide wich type of transistor can be used? I mean he knows the price, noise and power characteristics already. So here we have two transistors: cheap HJ-FET and cheap Bipolar with almost the same characteristics. What will he choose and why?
As an example: **broken link removed**
we can put BFP405 bipolar with common emmitter, +v to collector pin and we get RF from collector microstrip. (emitter pin to Gnd)
ALSO we can put some HJ-FET, +v to Drain pin and again we get RF from Drain microstrip. (source pin to Gnd)
If price, power and noise is the same, maybe there are some other pros and cons?

3. And the third question is. Why LDPE is not used as dielectric material for microwave CCL?
I just read http://members.tm.net/lapointe/Plastics.htm table and see that:
LDPE is 2.2 at 1GHz
PTFE is 2.0 at 1GHz
So i see that LDPE can be put as dielectric. Why not? Is it aging effects, problem with water absorption(?), can't resist high temperatures?
Good points: easy adhesion, cheap(?).
Thanks!
 
Last edited:

is it right way of calculation? Er is 3.24 Farad/Meter.
Yes.

You're apparently measuring a RF substrate like Rogers. It has tightly specified dielectric parameters, thus I wonder what's the intention of your measurement? It will be rather difficult to reproduce the manufacturers specification without suitable RF equipment.
 

i bought some cheap plates, because it was thinner than usual, then guess it is good to try them. But I do not know Er, so can't choose microstrip lengths. Seller does not know about Er =)
 

Update! I edited a post a little. Please check my "RF laminate dielectric constant measurement device" design idea.

Cheap and effective Er-meter design:
1. First we buy good laminate with known Er and etch the following microstrip PCB
- 5 000 000 Hz oscillator using DRO
- Mixer for generating 5GHz+1000Hz = 5001000 Hz frequency
- Two Wilkinson power dividers for 5GHz signal and for 5.001GHz signal.
- Signals divided as follows: LO1=5GHz, LO2=5GHz, RF1=5.001GHz, RF2=5.001GHz
- Two mixers for generating IF1 and IF2 from LOn and RFn.
- It looks like balanced mixer, BUT there is no 90 degree phase shift (read: microstrip lines are equal in length for LOn and RFn)

How it works:
- because microstrip lengths are equal then IF1 and IF2 signals are equal (phase and frequency of 1000Hz).
- we use knife and extract small rectangular part about 1x1cm of LO2 microstrip area
- then we take UNKNOWN laminate of 1x1 cm with etched microstrip and put it in 1x1cm hole. Maybe a little welding or connectors.

If we put original 1x1cm peace of laminate, then IF1 and IF2 phases are equal
If we put UNKNOWN 1x1cm peace of laminate, IF2 phase will change according to Er costant of dielectric: for example, phase can be "faster" or "slower" in this dielectric.

So we can use coplex FFT transform on IF1 and IF2 signals, calculate phase difference between this two signals, and convert it to Er constant. Because we know size of the 1x1cm peace, microstrip length is 1cm. All we need to examine UNKNOWN rf laminate samples is 1x1 or bigger peace of laminate with etched microstrip. The higher frequency.

It is cheap, easy to produce, we only need:
- 1 cheap RF transisto
- some DRO
- 3 or 4 Schottky diodes for mixers
- computer sound card to record signal
- the only expensive thing: piece of good laminate for device.

And also it is good: we can make this device for some frequency we use, and get real results for laminate.

Is it good idea or i reinvented some old trick?

ALSO i still have a question about Er measurement without spectrum analyzer and/or network analyzer: Can i do it with two antennas and fully etched copper (PCB without copper between antennas, measure power with and without dielectric plate)?

Thanks!
 

i want critic of my method (the post above).
We can use double-balanced mixer, with replacable microstrip area. Put the sample with unknown Er to this place and get Er from phase using complex FFT. With pretty good resolution i think, if dimension of patch is precise..
 

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