How to Match Antenna - Page 1
Forum section: electronics / beginners

author: randyyork
author_rank: Newbie
timestamp: May 04, 2014, 04:52:18 am
content: Hi there!I'm trying to build a sort of wide-band grid dip meter to operate between 5 MHz and 10 MHz. This will be based around a microcontroller. I've hit a few snags with coil design and would be grateful for any suggestions. I realize there are a many simple designs out there for this, but I'd like to use this project to learn a bit more about RF design.Some background:The idea is to use a single-loop, un-tuned PCB "antenna" which will give a relatively flat response throughout the wide frequency sweep range. This will allow measurement of changes in response in presence of external tank circuit.What I did:I etched a test PCB comprising of square loop 60 mm x 60 mm and a track thickness of 2 mm.To experiment, I hooked up a function generator across the PCB antenna terminals and injected a 5 Vpp swept sine signal. Using an oscilloscope, I expected to measure a stable 5 Vpp signal at the coil, across the frequency range. The problem is that I see significant roll-off and an increasing amplitude as the frequency is increased. This is without any external LC circuits nearby or interference.I think this is due to an improper impedance matching and reflections. My function generator has an output impedance of 50 ohms and the oscilloscope has a very high input impedance. I am using a 50 ohm through-terminator at the oscilloscope, with 10X probe, and 50 ohm coax cable to drive the antenna.Firstly, I'm not entirely sure how I can match the antenna impedance without using expensive impedance analyzer. Is there a rule of thumb I can follow?Secondly, does the antenna still have to be matched to 50 ohms if it is, instead, driven by a power amplifier and a VCO present on the same circuit board? I am confused because the "transmission line" would not really be a 50 ohm coaxial cable anymore, but instead a very short PCB trace.Lastly, in some designs, directional couplers are used to prevent loading of coil by the detection circuitry. Instead of expensive directional couplers, can multiple coils be used (one as a transmitter coil with injected sine wave, one as receiver coil where the amplitude is measured) to circumvent this issue?I'm grateful for any suggestions you may have for me.Thank you very much,Randy

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2014, 07:04:59 am
content: 0. How...exactly, are you intending on doing this with a microcontroller? Do you know what a GDO is, to begin with?1. It's approximately ahttp://www.technick.net/public/code/cp_dpage.php?aiocp_dp=util_inductance_square0.126 uH inductance. It is not an antenna by any means, not until a GHz or so. At 5-10MHz it will behave quite well as an inductance with magnetism active in the near field.2. If you're testing with a function generator, and you want to deliver all the FG's power to the loop, yes, it needs to be matched. Note that you're matching to the resistance (which being an inductor rather than an antenna, will be the loss component of that inductor -- its Q factor), so you must do two things: cancel the reactance (so it looks resistive again), and get the resistance up (or down) to 50 ohms. Both of these are accomplished with various LC networks. The basic idea is you're resonating the inductor with a capacitor, and coupling to that with an L network (series X, parallel Y -- X and Y are a capacitor and inductor respectively, or vice versa), which is also resonant. Obviously, this works best at exactly one frequency; the tuning components have to be adjusted in parallel for it to operate at another frequency.3. No, or not entirely. The sense coil should also be resonant, which means it will present some load to the remote coil. The fact is, you're sensing power flow; if everything is tuned the same, then power flow is limited by proximity alone. Too close and the coils become detuned, too far and not enough is coupled in. In the middle, coupling is ideal, and (I'm guessing here, I haven't actually calculated this) as much power will go into the target coil as your source coil. Which can be sensed at the source coil as a change in voltage, current, impedance or power flow. A third coil should follow the same pattern, though its size and power coupling can be much smaller, thus having less effect on the two main resonators. But at a relatively large distance, you can't really tell if you're sensing the source or load coil the most: at enough of a distance, they both look the same. On the upside, power flow is power flow, so it doesn't matter much.Tim

author: Fank1
author_rank: Regular Contributor
timestamp: May 04, 2014, 08:05:44 pm
content: What you are really after is the primary of a transformer.At RF the general rule is that Xl should be at least 5 times the output impedance at the lowest frequency.

author: randyyork
author_rank: Newbie
timestamp: May 05, 2014, 03:58:34 am
content: Tim,Thanks very much for the response:Quote from: T3sl4co1l on May 04, 2014, 07:04:59 amThe GDO basically senses a dip in voltage across the transmitter coil when a resonant circuit is brought near (when the sweep frequency matches the tuning frequency of the circuit). I'd like to use a microcontroller to control the sweep and to detect this dip automatically. The plan is to utilize simple diode detector to detect peak amplitude (with some filtering) and a buffer to feed the uC ADC.I should have made my intentions clear, my apologies.Quote from: T3sl4co1l on May 04, 2014, 07:04:59 amVery informative! If I understand this correctly, the need for impedance matching is to deliver maximum power to the load, and in special cases, to prevent reflections from appearing back at the source which can create standing waves and therefore reduce overall integrity of the signal. Because the overall length of the transmission line is magnitudes below the minimum wavelength along the sweep (30 m at 10 MHz (and length of line << wavelength of signal)), reflections aren't really a concern: The impedance matching is to allow maximum power delivery from function generator to the load to maximize the detection distance. This issue will appear again when trying to drive the coil from a RF power amplifier as the output will have to be matched (albeit differently) for maximum power delivery in that case.As matching will require the use of another tuned circuit, there will be frequency dependency and thus maintaining a flat response will be fairly complicated for this broadband range.I hope I've understood it all right so far.Perhaps the easiest thing to do is to leave the load unmatched with the compromise of a shorter read distance. Or to match it to form a "resistive looking" load for a particularly high frequency and hope the response is not largely affected (remains relatively flat) in the expected range.Quote from: T3sl4co1l on May 04, 2014, 07:04:59 amSo it seems the coupling factor is important to get just right to distinguish which coil is being sensed. But if we're looking at relative responses, assuming the transmit and sense coils are fairly closely coupled, relative changes in response should all be caused by an external circuit, yes?From what I understand, a similar thing is used for the EAS 1-bit transponder tag detectors used in stores. I realize those don't sweep large frequency bands, only a couple hundred kHz above and below ~8 MHz, but in this case, the sense coil is used to pick up the dip in the voltage. (Please see attached image.)Could the dip meter utilize this setup (obviously not in the form of a gate but more two closely positioned coils) as long as they're positioned in the ideal zone which you mentioned?Image source:http://www.rfid-handbook.de/downloads/E2E_chapter03-rfid-handbook.pdfThank you for all your help,-Randy

author: randyyork
author_rank: Newbie
timestamp: May 05, 2014, 04:27:10 am
content: Quote from: Fank1 on May 04, 2014, 08:05:44 pmFank1,Thanks very much.Based on this, I was able to find some information:http://users.tpg.com.au/users/ldbutler/OutputCoupling.pdfQuoteIt seems by using a broadband transformer, a 50 ohm transmitter coil circuit can be driven by a higher output impedance RF power amplifier section. As the secondary has typically 50 ohm source impedance, driving of the transmitter coil is simple (as long as the transmitter coil is also matched to 50 ohms).My concern is, with the simple PCB loop coil I etched, I must figure out a way to broadband-match the impedance of the transmitter coil first. This must be done so it looks like a 50 ohm resistive load across the whole frequency sweep in order for this broadband transformer to be useful. This doesn't seem trivial... Is that correct?Thank you very much,-Randy

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 05, 2014, 11:28:22 am
content: The easiest way to get a match is to use a tracking source. In the olden days, you'd use a variable capacitor to tune the coil directly, and a self excited oscillator for signal. Additional matching is eliminated by judicious choice of coil and capacitor values.Doing this with a planar coil isn't impossible (you'll probably need more turns of finer traces), but electronically tuned, you'll have a hard time with. Varactors are out, since we're talking large AC voltages. Bandswitching diodes and a binary series of tuning capacitors may suffice. You'll want a self-excited oscillator again (perhaps with AGC, so that the output voltage remains fairly constant, and you can effectively measure output power by the AGC control voltage), and a frequency counter because the oscillator frequency will never be exactly the same at a given tuning setting.Tim

author: Fank1
author_rank: Regular Contributor
timestamp: May 05, 2014, 09:36:52 pm
content: From your original post, I assumed that you were just attempting to find the resonant frequency of a circuit.This is really what a GDO does.It does not have anything to do with the impedance.Simply put the GDO coil emits RF when it is within the field of a resonant circuit, that circuit absorbs some of the energy.I assumed that your microprocessor is actually providing the RF and that the coil is just a means of coupling.A simple solution is to put some resistance in series with the coil and measure the drop across it.When the coil is coupled to the resonant circuit, the circuit will absorb some of the RF and the voltage drop across the resistor will increase.I don't think the 50 ohms has much bearing on the situation in this case.If however you are using the coil as the oscillator tank circuit we need to look again.Please clarify.

author: Teledog
author_rank: Regular Contributor
timestamp: May 05, 2014, 11:31:25 pm
content: For higher frequencies, I use a directional coupler with a tracking generator ..on the spectrum analyzer..Pretty cool seeing it "live".. ie: hand/objects near the antenna(s){Look for the "dips"}

author: randyyork
author_rank: Newbie
timestamp: May 28, 2014, 06:40:51 pm
content: Thanks very much for the replies.Fank1, that sounds exactly right. I am indeed trying to create a digital GDO. It seemed this simply in theory, I’ve been trying a variety of experiments but I’m noticing bizarre behavior.To verify the idea, first I applied the function generator output directly to the PCB coil.The oscilloscope measures across this same coil (no series resistance). As the frequency is swept, I notice the amplitude (without external resonant circuit) increase significantly, as if there is some roll off. I thought this might be capacitance from the probe tip (10X) causing the coil to behave like a resonant circuit. I’m applying a 6 Vpp signal. I do, however, notice a measurable dip in voltage when the tank circuit is brought near.I’m at a loss at what might be causing this roll-off. I’m using 1-foot twisted-pair BNC cable connected to function generator but this shouldn’t cause too much of an issue at this frequency unless using very very long wires.As suggested, I’ve also tried a couple of experiments by placing a 50 ohm-10 kohm resistor in series with the coil. In this case, the voltage remains relatively flat across the sweep (no roll-off) but there’s no measurable dip across the resistor. Does this mean the dip in current is so minute that the voltage drop isn’t measurable needs to be amplified (differentially with baseline)?I also tried using two coils, one as a pickup and the other as a generator but the roll-off issue persists.Thanks again,-Randy