Message-ID: <4EAAEAEA.7030800@iup.uni-heidelberg.de>
Date: Fri, 28 Oct 2011 19:48:26 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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To: rsgb_lf_group@blacksheep.org
Subject: LF: Q measurements at DK7FC
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LF,

For those who are interested, i have a small report of my todays Decca 
RF litz coil measurements. Maybe this will give newcomers some ideas how 
to proceed...

Since i do not want to cut that wire into to many pieces, i first took 
the fulll 170m piece to see where i come out. The calculated value is 
160m for a 3.00 mH coil. The program says that i will achieve 3,08 mH 
with the actual dimensions i achieved last night after winding the wire.

I did some pre-tests a few weeks ago and measured the antenna C as 
precise as possible. I put one single turn arround my (old) coil and 
measured the resonance frequency by turning the wheel as long as the 
highest amplitude was shown on a nearby probe, connected to a portable 
oscilloscope. Then i replaced the antenna wire by a variable cap and 
turned it to the highest amplitude peak again, without changing the 
frequency and the hardware arrangement, as good as possible. I got a C 
of 447 pF. Resonance was found at 137.3 kHz, so i have 3.0 mH on that coil.

Today i placed the coil on the position where it will be placed later, 
to include the sourrounding losses in the measurement. These cannot be 
avoided completely.

Now its time to show some photos of another excellent autumn day in 
finest WX conds and a good view on the top of that building where one 
can do many individual experiments without beeing inderrupted or disturbed:
http://dl.dropbox.com/u/19882028/LF/LF%20stuff%20on%20the%20top%20of%20building%2C%20euphorism%20WX.jpg
http://dl.dropbox.com/u/19882028/LF/Litz%20coil%20in%20final%20position.jpg
http://dl.dropbox.com/u/19882028/LF/Alternative%20position%20for%20Q%20measurement.jpg 
(Heidelberg castle in Background)
Sorry for the bad quality, it's a 4 year old mobile phone camera with 
scratched lens...

I found it was quite difficult to accurately measure such a high Q coil. 
I saw that the peak amplitude significantly decreased when i come a bit 
closer to the coil. Furthermore that cheap oscilloscope allows not very 
accurate reading of the values. So the measurements are rather showing 
the region where i will come out later. But very promising!

When connecting the antenna or the variable cap to the coil, the 
resonance was found near 130.2 kHz. So the wire is to long and L to 
high, better than to low! :-) If C is 447 pF, then L is about 3.35 mH, 
10% to much. Probably the 6mm thick glass adds some C between the turns 
due to its relatively high relative permittivity of 7. But the body has 
to be non-burning, this was the idea.
I did 3 Q measurements and achieved 3 Q values ;-) These are 566, 627 
and 734. During the 734 measurement i took care to be as far away from 
the coil as possible. So i rather expect values arround 700+ are 
realistic, later.

Expecting a L of 3 mH and a Q of 750 would imply losses of  3.4 Ohm 
which is a loss reduction (compared to the old/current coil) of 60% ! 
BTW the DC resistance of that coil is 1.0 Ohm.

The ratio of length to diameter is about 3, which is not ideal but more 
secure regarding voltage per turn, which will be an issue.

I expect higher Q values for the second version of that coil since:
-The distance of the lowest turn to ground will increase
-The length/diameter ration will decrease
-The wire length will decrease

Enough data to fill some formulas of Rik/ON7YD's fine LF antenna web 
page at http://www.strobbe.eu/on7yd/136ant/

Now I'll remove some turns to come closer to 137.0 kHz...


73 to all the homebrewers

Stefan/DK7FC