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)?

 

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.

__________________
RadioRon

 

Can I measure the inductance of the ferrite antenna using this method?
http://engr.nmsu.edu/~etti/fall96/el...ct/induct.html

 

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

__________________
The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

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.

 

Yes, this method is good.

__________________
RadioRon

 

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?

Attached Files

Loop Antenna.pdf (288.9 KB, 6 views)

 

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?

__________________
RadioRon

 

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?

 

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.

__________________
RadioRon

 

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?

 

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.

__________________
RadioRon

 

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?

 

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

 

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.

__________________
RadioRon