> From: km1h@juno.com (km1h @ juno.com)
> Subject: Re: [AMPS] Q Swing
To: <amps@contesting.com>
> Date: Wed, 20 Aug 97 23:09:45 +0000
Karl wrote:
> >> diameter and TPI and look it up in a B&W sheet. Since the inductance
> >is
> >> linear all you need to do is look up the total L for a full 10"
> >length
> >> and then measure your lengths.
I said:
> >Not a good method in the real world.
KM1H:
> Maybe in your version of reality but not in the world the rest of us
> inhabit.
I had no idea most people didn't know such a simple fact!!
The reality of the world is this. Air coils are greatly affected
by the surroundings, since the field... unlike a capacitor... extends
outside the coil.
You can find this published in virtually any text dealing with RF
components, or observe the results if you own an amplifier (an
excellent example would be a Dentron) and watch the tuning change as
the cover is removed and installed.
In any book, from the ARRL Handbook to top notch
engineering books (like Termin or Giacoletto's books), they
universally indicate:
1.) Inductance does not change linearly with turns
2.) Air coils are greatly affected both in Q and inductance by
surroundings.
The proper way to get an inductance correct is to mount the
inductor in its end location with all nearby components and
sheet metal in place. Install a small high quality capacitor that
resonates near the working frequency with the desired value of
inductance. You can then measure the resonant frequency and adjust
the inductor accordingly.
By the way, even the shorted turns (most tanks short unused turns)
will change the inductance of the section you are using, rendering
"chart estimates" one step above a wild guess.
73 Tom
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|