coils in a fm receiver

[preeste]

hi there

i would like to know how to or what the exact properties are of a wire in the radio receiver in order to have an inductance value of 78nH and 70nH.

I mean what type of wire, the dimensions , how many turns etc would i need to get an inductance from a coil of the above values......help please!

thanks in advance

 

[Nigel Goodwin]

There's no real answer to that - because of the high frequency and low inductance, there are far too many other factors affecting it to wind the coil from theoretical calculations.

 

[Hero999]

There are formulae around for calculating the inductance of and air coil and it's true that at higher frequencies they aren't that acurate. These can be found from Google but here is the page I find the most useful.

 

[stevez]

The website that Hero999 mentions appears to be excellent. An alternative - if you can get a copy of the Radio Amateurs Handbook published by the American Radio Relay League you will find some theoretical and practical information as well.

 

[Nigel Goodwin]

Or six turns of thick copper wire around a pencil?.

 

[Papabravo]

The usual way to solve your problem requires an instrument that may be a bit out of your price range. It is called a network analyzer. It will measure the s-parameters of a network as a function of frequency. If you need this for a short time only you might be able to rent one.

 

[JimB]

Dont forget the old fashioned instrument, the Q -Meter.
Just as effective at measuring inductance and capacitance, and somewhat cheaper than a network analyser!

JimB

 

[audioguru]

Whatever capacitor you tune the coil with will also have an unknown value due to stray capacitance in parallel with it.
That is why radios have trimmer capacitors and trimmer inductors to adjust as part of their alignment.

 

[stevez]

A dip meter is another way to measure the approximate inductance - maybe that's what JimB is calling a Q-meter though I am not sure.

The inductor is combined with a known value capacitor - the dip meter coil is placed near the inductor. The dip meter is an oscillator with a current meter - and when the frequency of the dip meter oscillator matches the resonant frequency of the ind/cap pair it dips. By knowing the frequency and capacitance one can calculate the inductance.

 

[JimB]

maybe that's what JimB is calling a Q-meter

No, a dip meter or GDO is a GDO and a Q-meter is a Q-meter!

A Q-meter is a combination of an oscillator, a voltmeter and a set of calibrated variable capacitors. Usually there is a set of calibrated plug-in inductors, or you use your own inductor ie the device under test.

It is set up so that the capacitors and coil are connected in parallel.
There is usually a low value (1 ohm) resistor in series with the capacitors, the oscillator is used to inject the excitation current into the LC circuit across this low value resistor.
The voltmeter measures the voltage across the capacitor.
From memory, the Q of the circuit is then the ratio of the voltage across the capacitor to the voltage across the 1 ohm resistor.
All sounds a bit weird if you have not seen one before.

Have a look at the circuit attached here which I have cut from an old textbook.

JimB

 

[stevez]

JimB - thanks! I recall reading a procedure for estimating the Q of a tuned circuit with a GDO. One would tune to resonance then some point either side to establish some half-power or other benchmark. I can't comment on the validity of the procedure.

A little extra for preeste - once you've got the coil you'll soon discover that if it is part of an oscillator that it will be extremely sensitive to nearby objects as well as vibration or deformation. Quite often some dope or other glue like substance is applied to provide some firm support for the turns.

 

[Hero999]

There are many way of measuring the Q, you could also connect the capacitor in parallel with the inductor and a current sense resistor in series with them both; if you know the voltage across the voltage across the resistor and inductor then you can work out the Q.

 

[Papabravo]

The Q-meter and the Grid Dip Oscillator are interesting in a historical context. There use was primarily for the HF bands in the 3-30 MHz. range. Would they continue to work at VHF? Do you suppose either one would be helpful to the original poster in his quest to get design and build coils for his FM Receiver?

 

[Russlk]

I have made filters in the several hundred megacycle range by winding the coil from formula then resonating it with a known capacitor and compressing or streaching the coil for a fine tune. The size of the wire is not important in a receiver, but I like to make it stiff enuf that it is self supporting. I surface mount the coil by bending the start and finish of the coil at 90 degrees so it doesn't roll over. The equation is from the "Engineer's Handbook" : L=n^2(r^2/(9*r+10*l)) where L=inductance in microhenries, n=number of turns, r=radius of coil to the center of the wire, l=the length of the winding. This is very accurate if you remember that the length of the leads is part of the inductance not included in the formula.

 

[JimB]

The Q-meter and the Grid Dip Oscillator are interesting in a historical context. There use was primarily for the HF bands in the 3-30 MHz. range. Would they continue to work at VHF? Do you suppose either one would be helpful to the original poster in his quest to get design and build coils for his FM Receiver?

In the context of the original question, about as useful as a network analyser I would say.

JimB

 

[Papabravo]

I'm asking if they are useful in the VHF range. The ones I am familiar with top out at 30 MHz.

 

[JimB]

The now ancient Q-meter which I used many years ago was the Marconi TF1245 which would work up to 300 Mhz.
I have a dip meter which works up to 75 Mhz OK, and I have seen designs which work up to 450Mhz using "hairpin" type coils.

JimB

 

[Papabravo]

Then maybe the original poster should try to find one on ebay since it sounds like it might be useful for him. What do you think they should go for?