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Need Help in Loop Antenna Building

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Harros, Feb 11, 2008.

  1. Harros

    Harros New Member

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    Urgent!!! Need Help in Winding the Air-Core Loop Antenna

    Hi, guys.

    Does anybody know the way and the formulas (if any) to wind a portable air-core loop antenna (using PVC pipe if possible...)? Thank you.
     
  2. mvs sarma

    mvs sarma Well-Known Member

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    Please try this

    http://www.elexs.de/drm6.htm
     
  3. Harros

    Harros New Member

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    Hi guys,

    Is it possible to use the existing external AM loop antenna (the one that bundled with the receiver or Hi-Fi) for signal receiving in other receiver circuit? Do I need to put extra circuitry to optimize the receiving? Thank you.
     
  4. dave

    Dave New Member

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  5. RadioRon

    RadioRon Well-Known Member

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    Antennas are typically tailored to specific frequency bands. The loop antenna for AM is not useful for other frequency bands beyond about 2.5 MHz, so if your other receiver circuit is also 500Khz to 2 MHz, then yes, this loop would be useful.

    You don't need much extra circuitry, but you do have to pay attention to impedance matching in a way suited to such an antenna.
     
  6. Harros

    Harros New Member

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    Thank you for your information. Does the AM loop antenna serve the same function of a H-field probe antenna? I am thinking of using this antenna to measure the H-field signal...
     
  7. RadioRon

    RadioRon Well-Known Member

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    Yes, it is a magnetic field antenna well suited to H field probing.
     
  8. Harros

    Harros New Member

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    Thank you. By the way, the H-field probe antenna is too expensive for me. :( Should I add a tuning capacitor to the antenna? Or all I need to do is just to connect the antenna to the receiver circuit (If the impedance is matched)? What should I do if the impedance do not match? I learned from the internet that most of people do the autotransformer style coupling to match the impedance... Does it work? How to calculate the coupling turns?
     
  9. RadioRon

    RadioRon Well-Known Member

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    One of the main reasons an H-field probe may be espensive is that they are calibrated to a known standard. So when you make a measurement and get a value of, say, 32, you know what that means. If you make your own, you will need to find a way to calibrate it too.

    Without seeing the antenna, I can't recommend one connection circuit vs another. A tuning capacitor is commonly added but not necessarily to the same winding that is used to couple to the receiver. This capacitor tunes the system to resonance which adds selectivity and sensitivity. This is useful for a radio receiver, but sometimes we want an H field probe to be broadband, and a tuning capacitor would then interfere.

    It is common to use an RF transformer or autotransformer for matching, but without knowing much more about the antenna and your reciever input, I can't recommend a turns ratio.

    This site may help:
    http://www.electronics-tutorials.com/basics/wide-band-rf-transformers.htm
     
  10. Harros

    Harros New Member

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    By the way, i am going to use a monopole rod antenna to measure the e-field signal and the loop antenna to measure the h-field signal because i want to observe the phase difference between both of the signals (the devices are working in near field). Is it possible for me to achieve the above-mentioned purpose using the commercially available antennas (the AM loop antenna and rod antenna)? I dont know the way to calibrate the antenna as i have only a little bit of knowledge in antennas field...
     
  11. RadioRon

    RadioRon Well-Known Member

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    Ah, so your goal is Near Field Electromagnatic Ranging. Yes, you can do this with these antennas, if your operating frequency is low enough. And calibration won't be necessary after all since you are mainly interested in relative phase angle, not intensity. I'm assuming that you will have an appropriate receiver for each antenna. I think that the rod antenna will have a very high capacitive impedance so it will need an impedance buffer amplifier.
     
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  12. Harros

    Harros New Member

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    Yes, I do this ranging system for my final year project. There are only a few people know how the system is working, so do with my supervisor lecturer, he know nothing with it, and i can seek nobody for help... :( The operating frequency is around 1.07MHz. Is the impedance buffer amplifier designed using the op-amp? In my design idea, each receiver shall consist: (in order) Antenna> preselect filter> RF amplifier> Phase Detector> Micro Controller (For data manipulation and communication with computer through USB, I am considering using PIC18F4550). Can I seek advisory from you on this matter?
     
  13. RadioRon

    RadioRon Well-Known Member

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    Your design idea seems reasonable. You should draw it out as a block diagram though to make it clearer for others. If the operating frequency is fixed at about 1 Mhz, then the loop will work well. As a matter of fact, you may go ahead and add a parallel capacitor to tune this loop antenna which will help its selectivity and improve its sensitivity.

    One significant problem that you may face is that there are many other signals around 1.05 MHz which will also be received by your antenna and which will interfere with your desired signals. How will you deal with those? Typically it is wise to make the receiver bandwidth quite narrow and also you should decide on your final frequency by listening to the airwaves with an AM radio both night and day and finding a quiet frequency.

    For the impedance buffer amplifier I recommend using a discrete transistor amplifier made out of a FET. Here is an example:
    http://www.uoguelph.ca/~antoon/circ/activant.html
     
  14. quixotron

    quixotron New Member

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    Hello. I have also been working with inductively coupled loop antennas. Although I work in the 900 Mhz band.

    Just a few questions:

    1. Loop antennas have poor radiation resistance thus are poor radiators. They also have high input impedance. Tapped Capacitor loop antennas have a pair of capacitors that allows you to tune your antenna to resonance and tune its input impedance. In your case, what are the dimensions of your antenna? Its diameter, cable/trace thickness? This is important, since you need to tune your antenna to resonance in order pick up the desired signals. The loop size also affects its inductance, wire capacitance and loss resistance.

    2. If you have access to an impedance analyzer, you can find the equivalent circuit of the loop antenna, which is usually a parallel combination of a cap and a inductor in series with a resistor for a tapped cap loop. This will give you the vital parameters to tweak your antenna.

    3. What range are you trying to measure? Be warned in the near field the polzarization of the EM fields are not stable. The waves behave as surface waves and "couple" to devices. In the far field, waves travel far enough to disperse and be treated as plane waves. 1 Mhz is LONG! near field would be lambda/2PI=50 meters.

    4. I have some calibrated loop probes on a portable spectrum analyzer. I'll ask the EMC guys to what are they calibtrated to? (probably 50 ohm cables) and if they can detect h-field parameters. although, we use antennas to pick up E-field. I wonder if we can extrapolate since in the far field, the field are plane waves and are in phase. we know E/c =B.

    So yeah get back on those.

    BTW Ron, I had some problems with a RG cable picking up radiation from an antenna, behind the groundplane. groundplane wasnt sufficient enough to cover the whole field pattern. I suggested to a colleague that the radiation would incude currents on the outer jacket of the conductor, the currents had to go somewhere, probably back the connector and into the antenna port. He placed three clip on toroids. This restored the receiver erformance. However it was also determined that the cable was halfwave. another fellow suggested to detune the cable by shortening or lengthening it. This also restored the system performance. So thanks.
     
  15. Harros

    Harros New Member

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    I think this frequency should be suitable since there is no AM radio channel in my country... Here is my block diagram of my system... Any comments on this system?
     

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  16. RadioRon

    RadioRon Well-Known Member

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    Your diagram is very good. We have several steps to do now. We must record the gain or loss of signal (preferably in dB) that we expect through each block. We must also determine whether each block is capable of handling the frequency. (judging by the chosen parts, it seems you've already done this, but I would like to know exactly what gain you expect in each stage).

    I have some observations about your choices so far. First of all, I wonder if you are choosing the easiest kind of filter topology for input. You indicate the filter to be a Chebyshev LP + HP, which will become fairly complicated with many components. A simpler and more selective filter can be realized using multiple resonant tank circuits which are coupled together. In this configuration, a single resonant tank is simply one capacitor and one inductor connected in parallel with one side to ground and the other side being called the "hot" side. One of these tank circuits can have high Q, so if you couple several of these tank circuits together, you can get extremely good selectivity which is desireable in this project I think. The methods for coupling energy into the first tank, and then coupling from one tank to the next are simple, but you will have to look them up in a book. The ARRL Handbook covers this pretty well, but it is a common practice in radio design so most radio circuit books would teach this.

    Next, I see that the op amp you have chosen has plenty of performance and will provide you with at least 20 dB of gain, and has the potential for quite a bit more, although exactly how much isn't clear. Perhaps 70 dB. I think that this amplifier is overkill for your application. It offers much more bandwidth than you need or want which may get you into trouble. The problem with too much bandwidth capability in an amplifier is simply that you invite instability at frequencies well away from the frequency you are operating on. Circuit board layout, decoupling and shielding may become quite critical, especially if you try to get a lot of gain out of one opamp. For example, you may lay out your circuit board with practices that are suitable for 1 MHz, but the amplifier can amplify up to 100MHz and more, so you would have to use pcb layout practices that are suitable for 100MHz to avoid having the amplifier become an oscillator due to unintentional feedback.

    If you are interested, we may research some simpler choices, but you would have to do the work, I can only guide.

    The phase detector AD8302 looks like a very good choice. It includes considerable additional gain, resulting in an input sensitivity of about -60 dBm which is very good. So, although it has very high performance bandwidth, much more than you need, it looks fairly easy to use so I approve.

    The output of the phase detector will require a low pass filter, whose cutoff frequency you will have to choose. You should show this in your diagram. I presume that you will use an A/D converter in the PIC 18F4550, and this converter will have a limited bandwidth, so your low pass filter frequency should be chosen to avoid aliasing. Since you expect a fairly slow rate of change of phase, I think this filter cutoff can be quite a low frequency to be safe.

    Judging the sensitivity of the system will require a knowledge of the gain of each stage, and then we will have to estimate or find out the expected noise figure of each stage. But worrying about the sensitivity is probably not very important just yet. Getting the thing basically operational is most important first.

    What you are building here is essentially two Tuned Radio Frequency receivers (an old term). This simply means that each receiver amplifies the incoming signal to a level that is useful for demodulation (or in our case, for phase detection) without any frequency translation. This sort of receiver is not commonly used in simple analog broadcast receivers because it can be difficult to maintain stability of the amplifiers when they are working at 1 MHz and with gain as high as 80 dB or so (plus it is very tough to get good selectivity when you are tuning the amp from 550 KHz to 1700 MHz but we don't face that problem here). But this configuration is the best choice for your system. For additional selectivity and performance, you should strongly consider adding another bandpass filter between the RF amplifier and the Phase detector IC. I also want to add that you must assume that a PCB layout is needed as there is no way this circuit will work on plugboard, perfboard, or with point to point wiring. You must use a pcb properly layed out for high frequencies.
     
    Last edited: Feb 13, 2008
  17. Harros

    Harros New Member

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    Yes, I am going to work on the PCB layout design right after all the circuits have been confirmed since there are only two months left before my hardware presentation. I will use the fiber glass PCB for PCB etching, is it alright to use this type of PCB in etching?

    By the way, I am planning of doing ranging measurement for ~100 meter (About 0.35 lamda. Note: the measurement can reach 0.45 lamda using this system theoretically). Is the transmitter’s power high enough for the purpose? How we can determine the antenna gain if there is no datasheet for the antennas? I cant determine the gain for each stage as I don’t know the antenna gain… Any suggestions on the way to determine the gain for each stage?

    Regarding the bandpass filter between the amplifier and the phase detector, is it alright to implement a crystal filter there? I have no idea of any filter with better cutoff/ rolloff…

    I am thinking of putting a 2nd Order Butterworth LP Filter before the ADC input of the MicroController, is it alright for me to do so?

    Do you have any clues on the op-amp that I should use (I mean the spec of the op-amp)? Preferably the op-amp from TI (Since i can get the sample from there :p ), I have looked through the op-amp in Ti website, most of the op-amps provide very low unity gain with 3MHz bandwidth...

    I have enclosed a new diagram of the system, as well as the schematic of the Colpitts Oscillator used in the transmitter... Any comments on the oscillator?
     

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    Last edited: Feb 14, 2008
  18. RadioRon

    RadioRon Well-Known Member

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    Yes, standard FR4 fibreglass is fine. In fact, just about any kind of pcb is ok for this.

    So many difficult questions! I'm not sure if the transmit power is high enough without doing some analysis. What is the legal maximum for such radiation in your country? Do you care about complying with rules, since this is only for demonstration? You diagram shows 75 mW which is a good level, perhaps enough to do the job. Again, some analyis is needed and I will show you how.

    The rod antenna is very easy to simulate and I can do that quickly and tell you the gain. But the loop antenna is difficult and I cannot use my simulator for that one. We will have to find out the antenna gain by finding similar antennas that others have built and who have measured their gain figure and reported this in a report or website. This will take some time. Another way is to measure the gain of the antenna. This is possible if we are not too fussy about our accuracy. Can you provide any details about the tx antenna including a picture?

    You could use a crystal filter and in fact the input impedance of the phase detector is suited to that. But you will have to find a crystal at the 1.05 MHz frequency. Since it is ok for the bandwidth to be quite narrow, you can get away with a crystal oscillator resonator instead of a filter. But we might have to cascade two of these and warp them a bit to get a few hz of bandwidth. If I were you, I would not use crystal to start with as it is an extra complication. I would build a cascaded tank circuit filter, same as recommended for the input filter.
    That is alright.

    Yes I have many clues. If I were designing this, I would not use an op amp. Instead, I would use an RF amplifier IC (not sure which one, this would require some research), or I would build a discrete tuned amplifier using a FET. At 1.05 MHz it is not hard to get a FET amplifier to work well for small signals.
    Yes, it looks very strange. This circuit is not appropriate. You must use a crystal in the resonant circuit to stabilize the signal. Also, I would never use an opamp as a 1 MHz oscillator when it is so much simpler with a FET or bipolar transistor as an oscillator followed by a simple tuned buffer amplifier again made out of a FET or bipolar transistor. It seems that you are afraid of, or not knowedgeable of, design of amplifiers with discrete transistors. Such amplifiers are much more commonly used than opamps at frequencies like 1 MHz, especially in a narrowband application like this one where the tuned amplifier works well. But this is up to you. In any case, you must use a crystal.

    Do you know how to make a simple crystal oscillator for use as a logic clock? This is usually done with a simple CMOS logic inverter. This would be the ideal oscillator and transmitter for you. Use a HEX Buffer IC (eg 74HCU04) and configure one buffer as a crystal oscillator, then feed the other buffers all in parallel and combine their outputs and then pass the signal through a 1 MHz chebyshev lowpass filter to remove harmonics. This is far simpler and is easy to do. I have seen links in another thread and will look for the example.
     
  19. Harros

    Harros New Member

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    Thank you for your advices. I am now working on redesigning the oscillator with crystal oscillator using 2N2222, but i cant find the oscillator for 1.07 MHz... Do I need to change my operating frequency for the system?
     
  20. RadioRon

    RadioRon Well-Known Member

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    I presume that you mean that you cannot find the crystal for 1.07 MHz. If you can't find 1.07 MHz, then you have the choice of changing your frequency to a common crystal frequency, like 1.0 MHz (which might not be a good choice as there are many devices running at this frequency and they may interfere), or you can order a custom crystal for 1.07. Such custom crystals are not expensive, but they take quite a few weeks to manufacture and may not fit your schedule. Consider 1.0 MHz as there are many suppliers at this frequency and you will need several crystals, for each tx and perhaps four for the rx.
     
  21. Harros

    Harros New Member

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    Do you mean Pierce Oscillator for the oscillator built using HEX Buffer IC and crystal? I have built the circuit of it, but the output suffers from severe distortion... I have built the oscillator using crystal as well... But it is not working...

    Again, for the filters, is the enclosed filter is the one in your mind?
     

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