Content-Transfer-Encoding: 8bit
Message-ID: <3617AC3245C2D1118A840000F805359C01752857@PDW-MERCURY-1>
From: "Talbot Andrew" <ACTALBOT@dera.gov.uk>
To: "'LF Group'" <rsgb_lf_group@blacksheep.org>
Subject: LF: Coil winding conundrum
Date: Wed, 12 Apr 2000 13:48:41 +0100
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

Moving from LF to MF for a bit, here's a problem for the topband
operators and loading coil winders.

At the weekend I made a loading coil to resonate the LF Tee antenna on
topband.  34uH was needed and the only suitable former to hand was 40mm
plastic drain pipe - the white type with no significant RF loss.

First off I used 27 turns close wound of 1mm enamelled wire (which was
all that I had of that wire), and completed the coil with 20 turns of
0.7mm diameter tinned copper with turns wide spaced at 3mm to allow
tapping points.

This worked fine but I noticed the coil got a bit warm with 100 Watts
and decided I needed a higher Q.... 

So, I made some Litz wire by twisting 20 strands of 0.25 mm diameter
enamelled, which gives a copper X-sectional area a bit more than 1mm
diameter equivalent.  Overall Litz wire diameter about 1.8mm. 
40 turns of this, close wound with a few taps gave near enough the same
inductance value for a winding length of 72mm

Q was measured by connecting a 680pF cap in parallel, exciting the
circuit with a one turn coupling coil driven from a synth, very loosely
coupled and monitoring voltage across the coil with a x10 scope probe.
Peak response frequency was noted and then the points either side where
the response fell to -3dB (0.707) to get the bandwidth.  Then  Qu = CF /
BW 

Now for the interesting bit.   When it came to measure the unloaded Q of
each coil, the original one was a fair bit higher  at Qu = 140, compared
with the Litz wound coil with Qu = 95.   Both coils were the same
diameter, same inductance, and roughly the same length in total.   So
why was the one made of plain wire better ?   Self capacitance ?    Q
was only measured at 1 MHz, perhaps I should try measuring gain at a
lower frequency.  

Puzzled and curious..................

One interesting aside to come out though.  Rayners formula for
inductance predicted the values of each coil to better than 10% and
bears out observations I've made over the years  -  such a simple
formula for wound inductors is too good to be true, working over a range
in excess of  1000000 : 1

	L(uh)  =    (D.N) ^ 2 / (460.D  +  1020 * length)     D = mean
diameter, mm     N = turns

Andy  G4JNT


-- 
The Information contained in this E-Mail and any subsequent correspondence
is private and is intended solely for the intended recipient(s).
For those other than the recipient any disclosure, copying, distribution, 
or any action taken or omitted to be taken in reliance on such information is
prohibited and may be unlawful.