Inductor for RF transmission and a few questions

[JimB]

Because the coils are so large, could it be that the transmitter is actually on about 45 to 50 Mhz and when Banana can hear the tx on an FM radio he is actually hearing the second harmonic?

JimB

 

[bananasiong]

Because the coils are so large, could it be that the transmitter is actually on about 45 to 50 Mhz and when Banana can hear the tx on an FM radio he is actually hearing the second harmonic?

JimB

Is it true? This transmitter works at around 96 MHz to 100 MHz, not higher or lower. But I can receive the signal by using a super regen receiver which can also receive a few radio station. The digital radio from my mp3 player can receive the signal too. I didn't try the max distance, but it works trough wall and around 10 meters tested.

Thanks

 

[RadioRon]

Because the coils are so large, could it be that the transmitter is actually on about 45 to 50 Mhz and when Banana can hear the tx on an FM radio he is actually hearing the second harmonic?

JimB

That's quite possible. Good thinking.

 

[audioguru]

Don't larger coils make the inductance lower because the coupling between the turns is lower? The larger coil is approaching being just a long piece of wire.

 

[JimB]

Don't larger coils make the inductance lower because the coupling between the turns is lower? The larger coil is approaching being just a long piece of wire.

No,

more turns = more inductance
turns closer together = more inductance

At some frequency a coil will become self resonant with its own capacitance. Above the self resonant frequency, the coil will become a capacitor.
Similarly, capacitors have a self resonant frequency, above which they become inductive.

RF is fun isn't it!

JimB

 

[audioguru]

At least this is RF. Microwaves use plumbing parts for tuned circuits.

 

[bananasiong]

Then it is possible also that my superregen receive at around 40 to 50 MHz, or maybe from 40 to 90 MHz because I can receive from other station too, just two stations.
Isn't the inductance of the coil increased when the area is increased?

Thanks

 

[Hero999]

Yes, the inductance increases with area, look at the coild winding formula.

 

[bananasiong]

Ya, I'll reduce the area of the coil when I got a chance. Maybe it performs better after the modification

I've read some article saying that the quality, Q is determined by L/R. So by using larger inductance and lowering the capacitance of the tank circuit, can the quality be improved? What about the R from that equation? Resistance of what?

Thanks

 

[audioguru]

The resistance of the coil reduces the Q. They are talking about a low frequency coil with many turns of thin wire so the resistance of the winding is high.

In an FM transmitter, the low impedance of the antenna is in parallel with the tuned circuit of the RF amplifier which also lowers the Q.

 

[Hero999]

Isn't the the Q factor the ratio of resistance to inductance?

So a 100nH inductor with a resistance of 1mhm: has the same Q as a 1:mu:H inductor with a resistance of 10mhm:.

 

[audioguru]

Coil resistance reduces the Q of a parallel resonant LC.
A capacitor with leakage resistance which is the same as a load resistance which also reduces the Q.

 

[RadioRon]

Isn't the the Q factor the ratio of resistance to inductance?

So a 100nH inductor with a resistance of 1mhm: has the same Q as a 1:mu:H inductor with a resistance of 10mhm:.

That's not correct. The Q of an inductor is the ratio of inductive reactance to its resistance at a particular frequency. The inductive reactance is equal to frequency x inductance x 2 x pi.
The resistance at high frequency is not the same as the DC resistance. It tends to be a lot higher due to skin effect and to material losses that increase with frequency.

A 100nH air-core coil made of reasonable sized wire might have a Q between 50 and 150 or thereabouts at 100MHz. The larger amount of wire needed to make a 1 uH inductor might increase the R as well and we often find that larger value inductors have worse Q than smaller ones. It is quite unusual to see Q values for a wire inductor higher than a couple of hundred and a value of 50 is pretty normal for a lot of air-core inductors at VHF frequencies, I think.

 

[bananasiong]

Hi,
Another question. For LC tank circuit, usually I fix the L and use a trimmer cap to tune. Is it good to use larger L so that it won't be so sensitive when I tune it? For example 30 MHz:
A 12 pF trimmer with a 2.3 uH
A 33 pF trimmer with a 850 nH
which one is preffered? or both are the same?

EDIT: I have a 300 MHz crappy rf module that uses cmos oscillator as the modulating signal, doesn't the output of the cmos oscillator produce square wave? Can the square wave being modulated?

Thanks

 

[RadioRon]

Hi,
Another question. For LC tank circuit, usually I fix the L and use a trimmer cap to tune. Is it good to use larger L so that it won't be so sensitive when I tune it? For example 30 MHz:
A 12 pF trimmer with a 2.3 uH
A 33 pF trimmer with a 850 nH
which one is preffered? or both are the same?

EDIT: I have a 300 MHz crappy rf module that uses cmos oscillator as the modulating signal, doesn't the output of the cmos oscillator produce square wave? Can the square wave being modulated?

Thanks

While it is true that both combinations will provide the same resonant frequency, there will be a difference. The Q of each combination will be different, partly because the reactance is different at resonance and partly because the resistance will be different in each of the two coils. The Q determines both the tank efficiency (which you want to be high) and the bandwidth (which you want to be low for an oscillator, but not so low for the output amplifier).

When using a 12pF trimmer, the highest capacitance will be 12pF and the lowest might be about 2 pF (guessing). In this case the tuning range would be from 30MHz to 74.2MHz. The other combination, using a 33 pF trimmer that has a minimum C of about, say, 4 pF, will tune from 30 MHz to 86.3 MHz. Similar result, but not the same. The lowest value that a variable cap can go to depends on the design of the capacitor and can vary widely.

To finally answer the question of which L value and which C value are the best, this is often determined by building the circuit and measuring performance and then altering the L and C values then remeasuing performance. This can be done over and over to find the best values. After many years of building similar circuits within one frequency range, engineers and amateurs have settled on a practical range of values vs frequency. You can see what they favor by studying examples of successful circuits in your frequency range.

Another way to start is by checking out what an inductor manufacturer recommends for, say, a 100MHz application. For example, Murata makes RF inductors. Their catalog has a selection tree that first asks you the question "above or below 100MHz". If you answer "above", then they recommend values only as high as 270nH. This implies that they don't think you should go above 270nH at 100MHz. That's a pretty good clue. Then, when I look at the characteristics for a 270nH SMT inductor (eg. LQW18A series), I can see that it has a minimum Q of 30 at 100MHz (which is not bad) and a self resonant frequency of 960 MHz, far enough away to not be a worry. So 270nH would work, but I would use a lower value, like 100nH, just based on my experience.