Maybe in help abouth Q calculation:
http://hamwaves.com/antennas/inductance.html
73,Joze S52AB
----- Original Message -----
From: "Stefan Schäfer" <Stefan.Schaefer@iup.uni-heidelberg.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Friday, October 28, 2011 7:48 PM
Subject: LF: Q measurements at DK7FC
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
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