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G4WYW > COILS 05.09.05 18:58l 57 Lines 2223 Bytes #999 (0) @ WW
BID : ED9627G4WYW
Read: GUEST DL1LCA OE7FMI
Subj: Re: Pile wound coil problem.
Path: DB0FHN<DB0MRW<OK0PKL<OK0PPL<DB0RES<ON0AR<GB7FCR
Sent: 050905/1655Z @:GB7FCR.#16.GBR.EU #:172 [Blackpool] FBB-7.03a $:ED9627G4WY
From: G4WYW@GB7FCR.#16.GBR.EU
To : COILS@WW
G0FTD wrote:-
> I previously spewed:-
>> Can anyone shed any light on the following problem ?
>>
>> I have been constructing pile wound solenoids approx 8 x 3cm with 3 layers
>> in them but I'm not convinced that the measurement I'm getting is correct
>> on my equipment.
>>
>> Depending upon the frequency that I measure the inductance at (10Khz - > 400Khz)
>> I see an inductance value between 150uH - 250uH which has several peaks and
>> troughs when plotted against measurement frequency.
>>
>> I'm confused !
>>
>> How do I determine what the correct value is ?
>
> Some bloke missing his ribs replied:-
>
>>but I'm wondering if the ambiguity might be caused by such a simple thing as
>>the differences between using solid state metering and analogue (coil) type
>>metering.
>
> Dunno - but I've come to the tentative conclusion that being pile wound
> means theres going to be lots of stray capacitance caused.
>
> As such I guess varying the test frequency when measuring the inductance
> causes the odd variation.
>
> I then hit upon the idea of finding a frequency where the coil produces
> a purely reactive impedance on a test bridge, noting the freq and then
> working out the corresponding reactance to impedance value.
>
> This seems to be giving a more reliable value which I'm happy with.
>
> Unfortunatly I'm out of reach of a decent network analyser at the moment
> so I was trying to make do with simpler equipment to hand :(
Further to my helpful response above, I have now had a chance to discuss
Andy's problem with a Professional Radio Designer friend of mine who is a
Professor of Electronics. He tells me that this is very simple to resolve!
Here's what he said:
Load the coil at one end with a known value of resistance, NONE INDUCTIVE.
He suggests 100k ohms but says you could come down as far as 5,000 ohms. He
reckons this will yield a more stable measurement and that if you still get
two or more values of measurement, to interpolate between them all and my
friend claims the result is accurate enough for the even the most stringent
applications.
He added, "Don't forget to remove the loading resistor before using."
MeL G4WyW
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