Need Help in Loop Antenna Building

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

1. HarrosNew Member

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Well, any idea on the way to determine the inductor Q?

How to determine the capacitance range of the tuning capacitor for the ferrite core loop antenna (the rx loop attenna)?

Last edited: Feb 25, 2008

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For the inductor Q, one way is to simply guess. Of course, it would be better to first look at the published Q of similar sized coils made by coil manufacturers, but these may be hard to find.

Or, you could measure using this technique:
http://users.tpg.com.au/ldbutler/QMeter.htm

For the ferrite antenna, you could measure the inductance and then use the formula for the L and C of a resonant circuit to determine C. Then, for tuning range, use 10% of the overall C for a variable capacitor value.

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5. k7elp60Active Member

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There is a modification of this formula,Fr=1/[2 x pi x sqrt(LC)], that I have used often. It is LC=(25330/f*f) with the following conditions. L= inductance in uh, C= capacitance in pf and f= frequency in Mhz.
The last frequency mentioned was 1Mhz so LC=25330.
I have done some work with circuits on the US AM broadcast band,approximately(550Khz to 1650Khz) and have used approximately 250 uh off the shelf inductors and then calculated a appropiate capacitor using the above formula to create a resonate circuit.
Just thought this info might be helpful.
Ned

6. HarrosNew Member

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It seems both of the equations give the same value. Anyway, this formula really opens my eye (I only know Fr=1/[2 x pi x sqrt(LC)] so far ). Thank you.

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8. HarrosNew Member

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Hi, I found a book titled "Small Antenna Design". A formula is introduced in this book to approximate the radiation resistance of the loop. Is it suitable to estimate the resistance in the loop for our case? By the way, it is quite fussy for me (I am running out of time to get this project done ) to measure the Q of the loop...

Can I put the secondary loop next to the primary loop? Or is there any better orientation that would maximize the antenna gain?

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Last edited: Feb 27, 2008

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The text reference seems OK to me so you can use that to estimate your radiation resistance.

I'm not sure I understand what you mean by secondary loop. Can you explain?

10. HarrosNew Member

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Sorry for my unclear question. Well, for the tx antenna, how should i put the secondary loop? Should I put it next to the primary loop (as shown in the picture)? Or is there any other way to put it?

Do you mean that i may use the equation in the text to estimate the Q of the loop (the loop in tx antenna)?

Another question: I have found that in many RF pcb designs, the designer intentionally leave most of the copper on the pcb (They only remove the copper nearby the circuitry, by the way, I usually remove all the useless copper but the copper for circuitry on the pcb). Why they do so?

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The secondary loop is usually put right beside the primary for best chances of the two loops sharing the same magnetic field. Adjust the number of turns of the secondary to get the best impedance match.

The equation allows you to estimate radiation resistance. This is the value that accounts for the loss of power to radiation, it is not a resistance that turns power into heat. There is some of that too, but that is conventional wire resistance and it is added to the radiation resistance to find the total resistance. So, the overall Q of the antenna will be the ratio of inductive reactance to the sum of the radiation resistance plus wire resistance. I think the text formula only estimates the radiation resistance so it is up to you to guess or measure or estimate the wire resistance.

RF designers usually leave a lot of copper on the board because they want to have a "ground plane". This is usually a sheet of copper on one side of the board or on an inner layer of a multilayer board that acts as reference for all your circuit's return currents. I think it is too difficult to explain the reasons we use a ground plane in detail in a post, so try looking up some references that discuss the purpose of a ground plane and see how far you get with understanding it.

12. HarrosNew Member

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So, can we estimate the inductive reactance using this formula: X=2*pi*f*L? Do you mean that the equivalent series resistance of the coil = radiation resistance + conventional wire resistance?

Well, in my design, should i provide ground plane to all the circuits in this project? (From the text that I read from internet, a ground plane will make easier the design of the pcb circuit)

Another question: Which type of capacitors are suitable for filters building in this project? Can I mix the surface mount components (inductors) with non surface mount components in circuit building?

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Yes, that formula is correct for inductive reactance. Yes, the total equivalent series R of the coil is the sum of those two.

Yes, you should use a ground plane.
You should use ceramic capacitors for all the RF circuits. Surface mount multilayer ceramic or leaded ceramic are both ok. Plastic dielectric capacitors, tantalum, aluminum electrolytic and similar types are not suitable above a few hundred kHz. Yes, you can mix surface mount parts and leaded parts as you like, but it is best if you keep the leads short on those leaded components to minimize unnecssary lead inductance. At 1 MHz this is not really critical, but above 50 Mhz it can be.

14. HarrosNew Member

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For the secondary loop of the tx antenna:
As mentioned in your previous post, the number of turns of the secondary loop should be adjusted to get the best impedance match. How should i infer the impedance of the secondary loop (the number of turns) for the impedance matching? Does the impedance of the secondary loop equal to the conventional wire resistance of the second loop?

15. HarrosNew Member

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Again, what is the suitable gain of the discrete amplifier (at the receiver) that i should build?

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Well, in theory you would estimate the total resistance in the primary loop at resonance (where total resistance is the sum of radiation resistance plus heating resistance), then you would use basic transformer theory which teaches that the impedance ratio is the square of the turns ratio to estimate the effect of placing a resistive load across the secondary. In practice, and if I'm in a hurry, I think I would simply use a turns ratio of 1:10 and then measure the resulting input impedance on a vector network analyzer. Then I would tune it by varying the turns ratio, the number of primary turns, the spacing and diameter of the turns, and so on, to achieve a pleasing result, consisting of a good impedance match and a narrow bandwidth.

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If you mean the first amplifier after the antenna, then a reasonable gain would be 20 dB.

18. HarrosNew Member

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Hi, is the BJT amplifier suitable for a sin wave with a swing around 4vpp amplification? Or any suggestion on the amplifier that should i build for the above-mentioned purpose?

By the way, there is only one amplifier for each channel in my design (as shown in the diagram). Will the amplifier provide enough amplification (20dB) for the received signal? Or should i add another amplifier in the design (each channel in receiver part)?

I have designed an amplifier using 2N2222, however the gain is just around 10... Is there anything that i have done wrongly in designing this amplifier?

Are 2N2222 and 2N2222A the same BJT transistor?

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I do not understand. You have not defined the purpose of this amplifier enough yet. Is this amplifier shown in your diagram? If you are referring to the RF amplifier between the filters on your diagram, then I do not expect the signal to get as high as 4Vpp. This is meant to be, as we call it, a small signal amplifier, that is, one that does not have to deal with large voltage swings.

For each channel, we have the choice of having only 20 dB to start with, using one transistor, or we can decide to put two transistor amplifiers in cascade to get 40 dB. More than this would not be a good idea. The sensitivity of the phase detector is about -60 dBm (if I recall correctly), so we can expect that one amplifier stage in front of that chip will improve the sensitivity to about -75 dBm. This might be enough for your purpose. I suggest that we settle for using one amplifier for each of the two channels in hopes of getting sensitivity of -75 dBm. It is important that the amplifier provide not only power gain, but also a reasonable noise figure. For now, we will focus on gain and not worry too much about noise figure since this is difficult to measure anyway. We might assume that if we build a good amplifier, we might hope for a noise figure of about 4 dB or less.

Your design is not good for high frequency use. I suggest that you change it to become a tuned RF amplifier. Instead of using a resistor load on the collector, you should use a tuned circuit (inductor and capacitor in parallel) resonating at 1 MHz, and then tap the inductor about 20% down from the DC power supply to feed the next stage. Here are some examples of tuned rf amplifiers:

http://www.tpub.com/neets/book8/31g.htm

Usually, the collector uses a tank circuit instead of a resistor, but a tuned circuit is often also used at the base. You don't need the tuned circuit on the base, but the one on the collector is essential to getting good performance.

these links may give more useful example info:

http://www.isp.ca/ve3nh/w7iuv.htm
http://www.isp.ca/ve3nh/w1fb.htm
http://freespace.virgin.net/geoff.cottrell/Images/G3PQA-amp-1.jpg
http://www.n3ox.net/projects/rxloop/

Last edited: Mar 5, 2008
20. HarrosNew Member

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Sorry, I have misconceived the usage of small signal amplifier. By the way, i am planning to add a power amplifier to the output of the crystal oscillator to boost the transmitting power of the transmitter to 20dBm (100mW) if possible. Do you have any suggestion on the type of amplifier that i can build?

I have rebuild the small signal amplifier, and it works quite well where it produces 20*log (37/2) = approximately 25dB of gain. I can only simulate the circuit using the signal ranging from 1micro to 1nano Vpp. The simulator went wrong when I was trying to simulate the circuit using 1 pico Vpp signal. On your opinion, does this amplifier work well? (The diagram and waveform included).

Did i connect the tank filter to the amplifier correctly? By the way, I cant get the output signal from the amplifier... (Please refer to tank amp.pdf)

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21. HarrosNew Member

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Well, I have finished the RF amplifier design, it seems working well (with a gain of approximately 30dB). It seems the tank filter greatly improve the gain of the amplifier... What do you think about this amplifier?

However, there is a problem where the output of the amplifier for the signal with nano volt range is not desirable as the output waveform (the waveform is perfect) is shifted up and down in simulation. It seems its due to input coupling capacitor as I try varying the value of that capacitor and investigate the result waveform for the 1uV-range input, the waveform shifted up and down (similar to the above-mentioned situation) when i adjust the capacitor with the value smaller than it supposed to be... What do you think?

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