Hello Paul,
Yes, 1500 turns and 1,2m high is good for handling voltages well above 50 kV
rms and you may get a proper Q. If you can get isolated wire instead of
enameled wire, you will get some distance between the turns and thus even more
Q and a higher withstand voltage but the coil will be bigger with the same
inductance.
Yes, top wires are the usual way to increase the C.
Ruducing the size of the coil, without choosing a smaller wire, is best reached
by increasing the antenna C, as mentioned. Furthermore it is important to
decrease the voltages! It may sound challenging and imposing to handle with
100kV rms at 8,97kHz but the effort to handle these voltages will take much
time and complicates the arrangement without getting the ERP as high as with a
high antenna...
So, just start building your coil and show us some impressing photos :-)
73 es gl, Stefan/DK7FC
________________________________
Von: [email protected] im Auftrag von Paul A. Cianciolo
Gesendet: So 28.03.2010 20:27
An: [email protected]
Betreff: RE: LF: Question about ground impedance at 8.97 KHZ **UPDATE
Hello Stefan,
Thank you for the response. I hope to increase the "C" with top wires?
The coil I have calculated is very large as you say.
Using one of Reg Edwards programs, it appears that a coil 1.2 meters tall, .6
meters in diameter, using #21 wire, .76 MM dia would result in a 545 mHy
inductor.
It would take 1500 turns, have a dc res. of 110 ohms and consist of 9300' of
wire for a total of 23 lbs of copper. I chose #21 because of the its larger
than the .46 MM you uses and hopefully would yield a higher "Q"
and because I found a 50 LBs spool in possession already helps also. :>) The
predicted "Q" of the inductor is 300.
I have to read more about tophat/wires at 9 KHz, but anything that would reduce
the size of the coil would be helpful.
PauLC
W1VLF
-----Original Message-----
From: Stefan Schäfer [mailto:[email protected]]On Behalf Of
Stefan Schäfer
Sent: Saturday, March 27, 2010 7:23 PM
To: [email protected]
Subject: AW: LF: Question about ground impedance at 8.97 KHZ **UPDATE
Paul, the values you measured seem to be realistic for me. Especially the
measured antenna C does not really differ very much between 137kHz and 9 kHz.
It is a good value that allows you to dimension your needed L. The needed L is
nearly independent from the earth losses. I generally recommend a variometer
part of 1...5% of the fixed L to compensate all effects that influence the
antenna C. But think about if it is possible to increase the antenna C instead
of building a very huge coil that causes further losses.
Good luck and have fun with building up! :-)
73, Stefan/DK7FC
________________________________
Von: [email protected] im Auftrag von Paul A. Cianciolo
Gesendet: Sa 27.03.2010 22:01
An: [email protected]
Betreff: RE: LF: Question about ground impedance at 8.97 KHZ **UPDATE
Hello Folks,
Thank you for all the help with measuring my antenna impedance.
Let me explain further what I was using to measure with and then add some
updates.
The homebrew bridge I was using was built as the one in the following link.
The diagram at the very bottom of the page is the schematic I used.
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=2285
This really my first venture into things LF and I had really no idea of
what to expect for numbers
After posting my results here, and talking to W1VD, he forwarded me a link
to Alan and Finbar's bridge experiments here
http://www.alan.melia.btinternet.co.uk/aelossbr.htm
Basically I duplicated that design, making sure that the transformer had
100 mHy of inductance to work at 10 KHz.
Sure enough the bridge worked, and balance with my antenna, but would not
balance with my antenna simulator load.
The simulated load is 2 X 390 ohm .1% resistors in series with a 500 pf
silver mica.
Reading on Alan talks about stray capacitance.
"The problem with the usual trifilar wound transformer is that if driven
from an unbalanced source the secondary suffers unbalanced strays to ground.
This would mean that if you were to swap the components on the arms of the
bridge the balance would be different"
With that in mind I built another transformer to balance the drive to the
bridge. After doing this the bridge will now balance with the simulated load
and the antenna.
See the following numbers for results:
Measurments were taken 10Khz
The "R" and "C" "known" side of the bridge were then measure measured
Sim load = 780 Ohms and 507 PF on LCR meter
Sim load = 940 Ohms and 460 pf on the bridge.
Antenna = 1111 Ohms and 330 PF on bridge
Antenna also measured at 137 KHz
Antenna = 85 ohms and 340 pf @137 Khz.
I think these are more in line with what I should be reading????
What do you folks think?
Thank you to everyone who responded to this post and helping me get started.
PauL C
W1VLF
-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Johan H. Bodin
Sent: Friday, March 26, 2010 4:32 PM
To: [email protected]
Subject: Re: LF: Question about ground impedance at 8.97 KHZ Stefan.
Hi Paul,
it sounds like your impedance bridge is measuring the absolute impedance
(that is Z = (R^2 + X^2)^0.5 where R is the real part, resistance, and X
is the imaginary part, reactance). R is probably a 10 to a few hundred
ohms, most of it is ground resistance - it can be neglected - the major
part of your impedance is the capacitive reactance of the antenna (Xc).
At 9 kHz, where C = 1/(2*Pi*9000*Xc), 70 kOhms means 253pF which is in
the ballpark for your 170' wire (about 5pF/m).
73
Johan SM6LKM
----
Paul A. Cianciolo wrote:
>
> Hello All,
>
> Below is a quote from the Stefan's web page concerning his latest
"Dreamers Band" DX contact Congratulations again Stefan.
>>From this information alone is it possible to calculate or know the
antenna system impedance.
>
>
>
>
> "100m Vertical wire antenna, coil/antenna voltage was abt 15kV rms,
average antenna current about 480mA, giving up to 1,7mW ERP. Applied power was
up to 250W."
>
> Pertaining to the coil "Technical and mechanical data: L=553mH, Q=82,
R(DC)=283 Ohm; 0,4mm diameter enameled copper wire, about 1200 turns! Average
diameter 0,55m, height abt 0,5m. So, about 2000m wire! :-)"
>
>
>
> I have built a small impedance bridge that operates at 10 KHZ and shows
my sloping 170' wire as 70K Ohms. I can also see approx 500 PF that can be
nulled out using the bridge?
>
> Can the 70K value possibly be correct? (3) 6' ground rods about 6'
apart are the ground... plus the electrical ground of the house.
>
> I am hoping to attempt a local transmission on 9KHZ
>
> Any information would be helpful.
>
> Thank you
>
> Paul
>
> W1VLF
>
>
>
>
>
<<winmail.dat>>
|
