Inductors for Receiver

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jollyrgr
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Inductors for Receiver

Post by jollyrgr » Fri Apr 01, 2011 6:52 pm

It has been a while since I've had to wind a coil for a receiver project. I want to build a low frequency receiver and found a schematic I want to use. It requires a 10mH and 1mH coil so I figured I'd wind my own. The calculator web pages keep coming back with coils needing to be up to one pound in some cases. The cost of the two coils to order them cost more than the rest of the circuit (I have the caps, resistors, transistors, case, etc.) combined. Am I missing something? I'm sure there has to be a simple core wound coil or "resistor like" inductor in these values. The circuit is not critical; this is just a 400kHz receiver.
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Re: Inductors for Receiver

Post by dyarker » Fri Apr 01, 2011 8:28 pm

What gauge wire did the web calculators say to use? At a pound, sounds like they're giving you instructions for a switching power supply inductor.

Anyway, try Mouser p/n 542-5800-103-RC; 10mH, 50mA, $1.33 each.
At Mouser home page left side pick Passive Components > Inductors > RF Inductors then pick inductance and max DC current.

Still want to wind it your self, find a web calculator that lets you set max current less than an amp, or one that specializes in RF coils vs power supplies.

Haven't seen you here for a while. Cheers,
Dale Y

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Re: Inductors for Receiver

Post by Robert Reed » Fri Apr 01, 2011 9:19 pm

Hello Jollyrgr
Thats the problem with some calculators, miss one parameter and it makes assumptions for you. First of all you did not mention required 'Q' of coil and whether its air core or slug tuned. But I will tell you, you do not want to wind coils in that inductance range. Even with a ferrite core its a LOT of turns. A coil winding machine would be the only way you could make a decent coil. Doing it by hand would end up so sloppy and probably with a lot of excess distributed capacitance. Commercial coils in that range are usually compound wound - a method of cross winding that minimizes distributed capacitance. As Dale pointed out, these coils are commonly available and quite cheap. Depending on the range of 'Q', your looking at 1/2" to 1" in length and weighing in at a gram? Costs - $0.5 to $3 in a fixed value.

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Re: Inductors for Receiver

Post by jollyrgr » Sat Apr 02, 2011 4:55 am

I am missing something. Both of you note that these can be found for pennies (which I thought should be the case). When I searched different sites (Parts Express for example) I came up with devices meant for a speaker or amplifier (cost $13 to $40). Digikey had devices in the 10uH but I didn't find one in the 10mH. Google searches brought up ones meant for speakers and were similar to what the web calculators were saying (up to a pound of copper).

I am trying to build a VLF receiver (basically a radio for below the AM broadcast band). I figured there must be a coil similar in appearance to a resistor or at least one that is a slug (fixed or tuned) similar to an electrolytic cap. The inductors are in series which is then in parallel to a capacitor forming a tank circuit. Not a super critical tuning either; as long as resonance winds up between 300kHz and 500kHz it should work.
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Re: Inductors for Receiver

Post by dyarker » Sat Apr 02, 2011 5:18 am

????
I got 16 hits for "In Stock" 10mH inductors at Digikey, $1.13 to $10 depending on current. (And that was only through hole type.
So again ??????? What did you search on?

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Re: Inductors for Receiver

Post by Robert Reed » Sat Apr 02, 2011 10:00 am

For starters look at mouser Pgs. 1047 & 48. 18 cents to $2. Conformal coated will be the cheapest and varnished compound wound the highest. These are Miller brand , but Xicon and others have a wide range of inductors. And this only scratches the surface - they are everywhere!
Parts Express is almost strictly an Audio Outlet and probably doesn't know what an RF coil is. Also they have a negative attitude and a poor tech group when it comes to customer service.
One thing I might mention is that some 10 MH coils will have a self resonant frequency near to your upper limit of 400 KHz, so keep this in mind when designing the tank circuit.Spec sheets will bear this out so that you can adjust tuning cap value accordingly.

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Re: Inductors for Receiver

Post by k7elp60 » Sun Apr 03, 2011 6:21 am

I think 10mH is way to high in inductance. I have built a number of AM broadcast band receivers and about 250uH is good for about 550Khz. I use a formula that is from the formula for calculating resonate frequency of a LC tank. The formula is LC= 25330/(f x f) where L is in uH, C is in pf and f is in Mhz. For 400Khz LC=158.31E03. If you divide 158.31E03/1000pf the inductance in uh = 158.31.

In the old days a 365pf variable capacitor had a range of abot 40pf to 365 pf and with a 230uh slug tuned coil the circuit tuned 550Khz to 1650Khz.

I think you could get a lower value inductor around 300uh and with a fixed ceramic capacitor and a large value trimmer capacitor in parallel you could tune it right to the desired frequency.
Ned

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Re: Inductors for Receiver

Post by jollyrgr » Sun Apr 03, 2011 1:29 pm

I did a search on Alltronics, All Electroncis, Parts Express, and GOOGLE. The first two I came up with nothing that met my needs. Parts Express came up with the audio amp device. Google even came back with the audio type. I must have done something wrong at Mouser the first time around as they only had one listed and it was an 8000 piece minimum order. Digikey as a $25 order. That is when I figured I needed to roll my own as it were. I used online calculators and came up with something similar to what was on the Parts Express site which I knew could not be right for what I wanted to build. I knew something was wrong and posted asking for advice.

As I stated in my second posting "I missed something". Thanks for the assistance. I went to where I should have started in the first place; eBay. The inductors are available from several vendors in China for $3 for ten devices SHIPPED.

Here is what I want to build:

Image


Full details can be found here:
http://www.techlib.com/electronics/lightning.html

This is basically a 300kHz receiver. I suppose I could redesign the tank as Ned suggested. The "easy" thing would be to just get an AM radio and tune it to the bottom of the band; or anywhere in the band for that matter.
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Re: Inductors for Receiver

Post by rshayes » Sat Apr 09, 2011 4:09 am

I once measured the inductance of a trigger transformer used to fire flashtubes. As I remember it, the inductance was in the 10 mH range and the stray capacitance was surprisingly low, on the order of a few pF. These are not specified for inductance and may be hard to get, and might require modification (find one over 10 mH and remove turns).

J W Miller used to make a line of RF chokes that probably went this high with low shunt capacitance. These appear to be discontinued. If you were very lucky, you might find one at an amateur radio swap meet. (Not very likely)

The resonant frequency calculated for an 11 mH inductance and 10 pF is actually about 450 KHz and not the 300 KHz claimed. This indicates that the inductor used has about 10 pF shunt capacity. Further, the Q of a small part such as an axial lead choke will probably be very poor.

The parts sold for crossover networks are usually wound with multiple layers and will probably have excessive shunt capacitance.

A single layer air wound coil can be wound by hand but it won't be small. Starting with Wheeler's Formula:

L(microhenries) = (n^2)*(r^2)/(9*r+10*l)
r is the coil radius (inches) and l is the coil length (inches)

choose the length equal to the diameter, l = 2*r

then L = (n^2)*r/(29)

let d equal the wire diameter, then the number of turns is n = 2*r/(d)

and L = 4*(r^3)/(29*d^2)

solving for r, r^3 = (l*29*(d^2))/(4)

for #28 wire, d = .0126 inches

A 10,000 uh coil would have a radius of about 2.26 inches

The coil would be about 4.5 inches in diameter, 4.5 inches long, and have about 360 turns of #28 wire close wound. The form could be made of cardboard using an empty jar as a form or a plastic jar of about this size.

Smaller wire would allow a smaller coil diameter.

You see coils like this in pre WWI radio equipment, when the frequencies used were in the 50 KHz to 500 KHz range.

The 1 mH coil can be eliminated by tapping the 10 mH coil at about one third of the turns away from the ground end.

This coil might also be wound on a ferrite or powdered iron toroid. With a low frequency, high permeability type of core of reasonable size this inductance can probably be obtained with one to two hundred turns. Shunt capacity might be high, since this winding would probably be multilayer.

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Re: Inductors for Receiver

Post by Bob Scott » Fri Apr 15, 2011 3:08 pm

Visit:
http://www.amidoncorp.com
Check the book section and order a FREE copy of their tech flyer. No amateur making his own cored coils should be without one.

"Free Tech Data Brochure contains information on materials and specifications on parts. If not placing an order at this time, you may send a message for a free Tech Data Brochure through our feedback form by clicking "contact us". Be sure to include a mailing address. "

EDIT: Amidon has a history of selling to hams. They have toroids, E cores, pot cores, bobbin cores, balun cores, and shielded can slug tuned transformer/coil forms in various ferrites and iron powder mixes. Make your own IF's...
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Re: Inductors for Receiver

Post by rshayes » Sat Apr 16, 2011 4:53 am

Amidon is basically a distributor. You can get more detailed information (including application notes) from manufacturers' web sites (Examples: Micrometals for powdered iron cores, Ferroxcube and Fair-Rite for ferrite cores).

Easiest cores to get are probably toroids and possibly the EC series of pot cores.

Pot cores have tighter tolerances and better temperature stability. These are probably not necessary for this application, since frequency is not critical.

Moderately large ferrite toroids (about 1 inch diameter or so) might give you 10 mHy with a few hundred turns. This is tedious to wind by hand, but not impossible. Stray capacitance may be high, since this will probably need a multilayer winding.

The cores from surplus 88 mHy loading coils (originally used by the telephone companies) are powdered permalloy and may still have reasonable "Q" in the 100 KHz range.

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Re: Inductors for Receiver

Post by Dean Huster » Tue Apr 19, 2011 5:12 pm

My gosh, no wonder the inductor has to be so big, with only 10pF in parallel! The "old" BCB receivers of yore used a 365pF variable.
Dean Huster, Electronics Curmudgeon
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Bob Scott
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Re: Inductors for Receiver

Post by Bob Scott » Wed Apr 20, 2011 6:01 am

Dean Huster wrote:My gosh, no wonder the inductor has to be so big, with only 10pF in parallel! The "old" BCB receivers of yore used a 365pF variable.
??? I wonder why the designer chose such a high impedance for the tank circuit? High Q/ high selectivity for a lightning receiver? ? At 500KHz, 10pF is about 30K ohms capacitive reactance. The inductor's would be the same.
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Re: Inductors for Receiver

Post by rshayes » Thu Apr 21, 2011 7:38 pm

The antenna specified is very short compared to a wavelength (less than 1 meter compared to 1000 meters wavelength at 300 KHz) and probably has a very high impedance. The low capacitance/high inductance tuned circuit may be an attempt to load the antenna as little as possible and thus maximize the signal voltage to the receiver.

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Re: Inductors for Receiver

Post by Dimbulb » Sun Apr 24, 2011 7:29 am

for a lowfer receiver you don't need heavy coil.
there are alot of misguided radio designs on the internet.

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