Hi Gerhard,
I think the acoustic phenomen is coming from partial discharges anyway.
These can be present without beeing visible. The PD inception voltage
depends on the humidity as well. So the problem can become extreme in
hot and humid summer days. You can hear it if you are standing below a
400 kV HV power line. I have the same problems on LF...
It may be worth to add a spheric conductor on the ends like i did on my
sparcing spark gap on the LF antenna (lightning protection for the coil
and isolator), http://dl.dropbox.com/u/19882028/LF/Electrode.JPG
But maybe the thin wire is the problem too. So you could try a kind of
ladder line to reduce the maximum field strength of the wires surface,
like here http://www.pfalzwerke.de/dokumente/h_220kv-portal(1).jpg or
here http://de.wikipedia.org/wiki/B%C3%BCndelleiter Or you could
increase the capacitive load of ypour system even more. This lowers the
voltages as well, you know.
73, Stefan/DK7FC
Am 28.03.2012 19:37, schrieb Gerhard Hickl:
Hello Markus, VLF!
This might be true at least partially. In my setup, I'm observing
acoustic noise without the presence of corona discharges. I observed my
antenna in the dark, running about 600mA of antenna current. There was
no "corona" visible but the noise was clearly heard. Definitely it was
not emitted by the loading coil behind the house but I have got the
impression, that it was emitted by the ends of the top-load wires. One
of those ends is even 50m away from the coil, in my neighbours garden.
When walking from my property (where I also have an end of a top-load
wire) up to his plot of land, the noise is decreasing and then getting
louder again under the end of the wire on his property.
In this respect I agree with you. The sound is emitted by the ends of
the top-load wires but it doesn't need a corona-discharge to make the
antenna "noisy" during transmit.
In fact, this behaviour of my antenna is the biggest problem in
transmitting during warm summer (spring) days. My neighbour allowed me
to use one of his trees as a hook of the top-load so I can't bother him
with noise when he's out in the garden.....that's way more than good
neighbourhood could bear.
The more I would be interested in suppressing this noise by suitable
measures.
73
OE3GHB
Gerhard
Am Dienstag, den 27.03.2012, 17:22 -0400 schrieb Markus Vester:
It's a beautiful night! At 20:25, I have started up a straight carrier
on 8970.002 Hz (DHO locked). If all stays well I will leave it on
until after sunrise.
An interesting observation on the side: In the first few minutes, I
drove the amp up to 400 mA antenna current. A little corona became
visible near the ends of two of the three topload wires - it wasn't
very bright, not unlike 3rd magnitude stars when viewed from the
ground 10 m below. But the 9 kHz sound from antenna itself could be
heard quite loudly, which is a potential cause for neighbourhood
trouble.
Then I reduced the drive to 350 mA where the corona disappeared
completely. So did the noise, the antenna became absolutely quiet! All
that was left was a little noise from the coil in the dustbin on the
balcony, and the transformer indoors. Thus the predominant cause for a
squeaking antenna must be corona. The fact that it can be heard at 9
rather than 18 kHz demonstrates that the discharge must have an
asymmetric dependence on the voltage polarity.
At the same time, wideband electrical noise on the nearby LF grabber
antenna went down by 20 dB.
BTW If you like you can actually read my VLF frequency from an alias
line on the LF TA grabber: It is created by the 24th VLF harmonic,
intermodulating with DCF 77, thus appearing on 24 x 8970.002 - 77500 =
137780.048 Hz.
Best regards,
Markus (DF6NM)
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