Re: Coil efficiency

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: Re: Coil efficiency
From: Tesla List <tesla-at-stic-dot-net>
Date: Mon, 17 Nov 1997 23:25:40 -0600
Approved: tesla-at-stic-dot-net

From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: 	bert.hickman-at-aquila-dot-com
Sent: 	Monday, November 17, 1997 8:38 AM
To: 	Tesla List
Subject: 	Re: Coil efficiency

Tesla List wrote:
> 
> From:   Ted[SMTP:tedric-at-generation-dot-net]
> Sent:   Sunday, November 16, 1997 2:44 PM
> To:     tesla-at-pupman-dot-com
> Subject:        Coil efficiency
> 
> Hi all,
> 
> I have this question in my mind for weeks: According to the 1996 (or any)
> edition of "The ARRL Handbook For Radio Amateurs", "the (coupling)
> coefficient with air-core coils may run as high as 0.6 or 0.7 if one coil is
> would over the other." This is logical to say that the maximum efficiency of
> a regular (3 air-core coils) magnifier system can never beyond 70%, yet I
> remember that in one of Richard Hull's tapes, Richard said that it can
> approaches 100%. How come?
> 
> Also, what is the maximum efficiency of a soild state magnifier?
> 
> Ted


Ted,

The coupling coefficient ("k") only refers to the portion of the
electromagnetic field lines that are common to both the primary and
secondary, and is a function of coil geometries and spacing. It has no
direct bearing on the efficiency with which energy can be transfered
from the primary to the secondary. It turns out that coupled, dual tuned
circuits transfer energy between the primary and secondary tuned LC
circuits over a number of cycles - the lower the coupling coefficient
the longer this process takes. For example:

                     # of
          k       half-cycles
         ===      ===========
         0.60         2
         0.385        3
         0.28         4
         0.22         5
         0.18         6
         0.153        7

Now if there were no losses (from the gap, resistance, radiation, etc),
fully 100% of the energy initially stored in the primary circuit could
be transferred from the primary to the secondary, back to the primary
again, and so on forever! However, in the real world, losses are
continually removing energy from the system. Greater efficieny occurs
when we transfer the energy as quickly as possible, using higher "k"
value. 

With 2-coil systems, the practical limit for "k" is between 0.22 and
0.28 due to insulation limitations. For magnifier drivers, "k" is
ideally in the 0.385 - 0.60 range, but the insulation and gap quenching
problems can be horrendous. Hope this helps!

-- Bert --