Hi Peter, VLF,
Did you see a trace on your grabber? Can you show me a link or a screenshot?
The signal is well visible on Paul Nicholson's excellent VLF grabber,
http://abelian.org/vlf/fbins.shtml#p=1387116000&b=110&s=sp but far from
stable. Is instability is due to my signal of course. Distance to Paul
is 880 km, http://no.nonsense.ee/qth/map.html?qth=IO83XQ&from=jn49ik00wd
You are right, there are many things to optimise. One thing is the
housing of the PA. Now it is just mounted on a wooden plate. It is a H
bridge using 4x IRFP460, switching directly at 230V*1.41, so the output
of the PA is on mains potential! There is no galvanic decoupling
transformer on the output (which would allow to use a grounded coax
then) because such a transformer would have to be a VERY big ferrite
core with many many turns (> 100). So the PA output is directly
connected to the primary winding of the loading coil, which couples its
field into the resonance winding (1200 turns). So the feed line is
floating on an unknown voltage which depends on the capacitive coupling
to sourrounding things. In the field experiments 2010/2011 this was no
problem. But now it is. I guess this is causing EMC problems to the /2
frequency divider, sometimes. Thus, i will put it into a metallic box
which is grounded to earth potential.
Another problem is the parallel capacitor which i have to add to come on
resonance. It's capacity is not high enough and i have to add about 10m
or wire hanging arround. This additional wire is moving in the wind
blasts and causes further instability of the signal (kind of AM due to
modulated resonance peak, i.e. antenna current). Another way would be to
add more turns to the coil but this antenna system is voltage limited
and it doesn't matter at all if i have to put 400W or 500W into the system.
BTW this night/day it was 850 mA antenna current and about 400W RF
power. I will plan to do more QRO, when the new capacitor is ready to use.
Here is an image of the arrangement, taken 30 minutes ago:
https://dl.dropboxusercontent.com/u/19882028/VLF/20131215_165406.jpg
As soon as the C of "DK7FC" is finished, i will run some OPDS32
sequences for DF6NM and Paul Nicholson and others :-)
BTW i have disconnected my VLF grabber antenna since the RX is
overloaded anyway. Now it is easier to identify eventual phase glitches
(could be on the RX side as well, though).
73 and thanks for watching.
Stefan/DK7FC
Am 15.12.2013 14:49, schrieb PA1SDB, Peter:
Stefan,
Signal is O.K., but stabillity is'nt :-(
To much spread around center frequency.
(I know.... I'm complaining about 0.01 Hz drift..........)
73's Peter
----- Original Message ----- From: "Stefan Schäfer"
<[email protected]>
To: <[email protected]>; "Paul" <[email protected]>
Sent: Sunday, December 15, 2013 4:14 AM
Subject: Re: LF: A short VLF experiment at DK7FC
Since 4:10 UTC, the signal is available without interruptions now on
8.97000500 kHz.
73, Stefan/DK7FC
Am 14.12.2013 20:50, schrieb Stefan Schäfer:
Hi all,
Tomorrow the rain probability is just 10% and so i think i can risk
to run a short VLF experiment on 8970.00500 Hz. I'm using the fixed
antenna at JN49IK00WD. These days, the QRN is very low for quite a
long time! The plan is to run about 500W RF power, or more, depending
on the voltage limit of the antenna. And the plan is to start
transmitting tonite, after the experiment with VO1NA, i.e. i'll come
on air arround 2 UTC, hopefully. I will run the test until the late
afternoon. First i'm TXing a stable frequency until 9 UTC, later some
DFCW-600, and then maybe some tests in OPDS32.
Suggestions welcome.
BUT it all depends on the WX. If it starts to rain then i have to stop.
Will someone give it a try and watch for me?
73, Stefan/DK7FC
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