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Harros said: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.
RadioRon said: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.
Harros said: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...
RadioRon said:Yes, it is a magnetic field antenna well suited to H field probing.
Harros said: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?
RadioRon said: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:
https://www.electronics-tutorials.com/basics/wide-band-rf-transformers.htm
Harros said: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...
RadioRon said: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.
RadioRon said: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:
**broken link removed**
Yes, standard FR4 fibreglass is fine. In fact, just about any kind of pcb is ok for this.Harros said: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?
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.Harros said: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?
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.Harros said: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…
That is alright.Harros said: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?
Harros said: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 ), I have looked through the op-amp in Ti website, most of the op-amps provide very low unity gain with 3MHz bandwidth...
Harros said: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?
Harros said:but i cant find the oscillator for 1.07 MHz... Do I need to change my operating frequency for the system?