Message-ID: <FBF0B7FE4F7A469FA77C02CBB09C7F06@konda03bf4520e>
From: "Joze Konda - S52AB" <s52ab@siol.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4EAAEAEA.7030800@iup.uni-heidelberg.de>
Date: Sat, 29 Oct 2011 07:55:09 +0200
MIME-Version: 1.0
Subject: LF: Re: Q measurements at DK7FC
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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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