Tom Rauch wrote:
>> 43.5 nH to 49.3 nH measured; 48.6 nH calculated from the standard
>> formula for solenoid coils.
>>
>> Maybe that was lucky, but it seems like the formula is a good place to
>> start, for those of us without network analyser.
>
>Gosh, what luck! ;-)
Yes, probably some luck is involved, but this is actually twice in a row
that the formula has been useful for design purposes.
The other one involved a bunch of very small 1k 2W metal film resistors
(Philips PR02 from Farnell/Newark) used for grid swamping in a 144MHz
PA. Once again the resistors are of spiral construction, but they're
physically small enough that they look much more like resistors than
inductors, even at 144MHz. For a scoping calculation I used the inductor
formula, and then added some inductance to account for the short leads.
Note added after reading about John's measurements:
It's interesting that the inductance seems to be pretty much independent
of the resistance (except for the 5.1 ohms). That implies that the
spiral construciton is the same, and they vary the resistance by
changing the material/thickness/width of the MO film.
>
>All I usually do is scrape the outside off and look at the guts. If
>it's a coil with multiple turns, I don't use it as a suppressor
>resistor without measurements.
>
Fair enough, since you have the means to measure it at the relevant
frequencies. If I had, I wouldn't be fooling with no stinkin' formula.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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