Re: [Amps] Measuring tank coil inductance

["Carl" <km1h@jeremy.mv.com>]

----- Original Message ----- 
From: "Bill Turner" <dezrat1242@yahoo.com>
To: "Amps" <amps@contesting.com>
Sent: Tuesday, April 23, 2013 3:56 PM
Subject: Re: [Amps] Measuring tank coil inductance


> ORIGINAL MESSAGE (may be snipped):
> On Tue, 23 Apr 2013 20:17:41 +0100, Chris wrote:
>
> > When you see specs in ARRL build plans for say a
> > .8 uH indctor, or see a 5 uH inductor advertised, there must be a
> > definitive measuring method to arrive at the value, surely?
> >
> > Glad you posted, I feel I ask too many questions ;)
> REPLY:
> No such thing as too many questions, within reason of course.  :-)
>
> According to theory, pure inductance does not vary with frequency, however
> the effective inductance DOES vary due to the effect of distributed
> capacitance within the coil.  To get the "true" inductance of a coil it
> should be measured at a very low frequency. However the effective 
> inductance
> should be measured at the frequency of operation. If a coil is used at
> multiple frequencies, you will have multiple readings. Can't be helped.
>
> Probably the most useful way is with an accurate grid dip meter and a
> precision capacitor so the combination is resonant near the operating
> frequency. Done carefully, you should get within about 1% of the effective
> measurement. Any closer than that will probably be thrown off by 
> unavoidable
> stray capacitance.
>
> Bill, W6WRT

I wonder how anything got built before all these wunnerful, wunnerful toys 
came along to supposedly help us.
The GDO from a 1949 CQ I built in 1955 still works as does the Measurements 
59 that National Radio used in R&D that I bought at the IRS auction in 92. 
National (and everyone else at the time) built many commercial and military 
transmitters and amps up to the 10KW range with it and a slide rule.
Since stray and tube C will always be there just ignore it for now and use a 
known cap value that conforms to whatever you use for calculating LC values. 
I use G3SEK's program since it works well and allows you to make changes 
with a keystroke. Ian is also a regular on here and can answer some 
reasonable questions but dont overload him (-;
http://www.ifwtech.co.uk/g3sek/

Using the GDO I built in 1955 from a 1949 CQ article and the Measurements 59 
that National used in R&D Ive yet to have a problem with any amp LC 
circuits. National (as well as all the other companies) used it for ham, 
commercial, and military products to 10KW. Using known value caps select the 
coil taps. I use a small  variable and fixed caps and a hand held LCR meter 
to take measurements as I could care less at what frequency the caps were 
measured with. Ive compared to both a military and HP Q Meter and the 
differences at HF arent even worth talking about.
If you cant eyeball a rough inductance then use the solenoid coil graphs in 
an old Handbook or one of the several online programs. Strip the insulation 
off a long hunk of #12 solid house wire and practice with diameters and 
spacings to get a feel for the relationships. Read carefully about diameter 
to length ratios and how they affect performance.
It will be little different with refrigerator copper tubing at the power 
levels most of use care about and the #12 can easily be used at 160/80M at 
well over 1500W; use one of the many ribbed ceramic forms available at 
hamfests, Fleabay, etc, for those bands or go with toroids.
The strays and tube C is usually only important at 10/12M and determines the 
type of  Tune cap minimum C needed and available....or you can fudge on the 
Q and likely never tell the difference. All the common commercial ham amps 
make do with air variables as a cost effective compromise.
Figure on 10-15pf of strays in a well laid out amp plus the tubes output C. 
A well laid out amp does not crowd inductors right next to metal (which 
includes those type of tubes) or parallel to each other.
If you run out of C on 10M tap its coil a turn at a time in from the 
blocking cap until it tunes; spreading the coil turns also helps a bit at 
times. Do not go past half way. Some add a small coil before the cap with 
varying results; I dont care for adding any more L on a band where 
efficiency is already hard to obtain at times. There is no perfect Q altho 
many formulas use 12; Ive used 8 to 16 with very little efficiency change as 
long as the tube isnt some antique that can barely make 10M in a perfect 
world. Watch those harmonics with a low Q if not using a Pi-L.
Carl
KM1H



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