[Lowfer] LF: DK7FC's 2nd VLF TX test...

[Mike.WE0H]

Just so you guys are somewhat up to speed, the German's are working on 
8.97kc now and getting some serious distance with low power and 
reasonable vertical antenna size. The old 10km and 16km distance records 
got totally blown away today.

Mike
WE0H



-------- Original Message --------
Subject: AW: LF: DK7FC's 2nd VLF TX test...
Date: Tue, 16 Mar 2010 01:15:17 +0100
From: Stefan Schäfer


Dear LF group, RXers and 9Dreamers and doubters,

Thank you for your assistance on that very successful experiment! It is 
a good day for the VLF ham operations and it won't be the last. YOU are 
a part of this success since i wouldn't have done all the effort to 
become QRV there if you all wouldn't have been so interested in the 
discussion about <9.

So, perhaps there will come up further TX stns, i hope so. With the 
results we can see what can be reached with a given ERP in a given 
distance. So, all the theoretical calculations become less speculative...

First, i confirm that i was QRV to the described time at 8,9700kHz.

I will detailed describe now what happend on this experiment.

Starting with buying a plastic spray paint and fixing the windings on 
the water barrel was the first work. Than applying the variometer and 
the primary coil.

The primary coil is inductive coupled to the secondary and isolate 
mounted on the bottom side of the water barrel. It has just 9 turns out 
of 4*0,4mm diameter isolated copper wire. The 9 turns was an estimation 
but later it is shown that it is optimal for getting the maximum output 
power at moderate voltages and currents.

On http://www.qrz.com/db/DK7FC i will show many photos i have taken on 
the hill. So there may come up some questions and some may be answered 
by the pictures ;-)

After building up the kite (the 8,1m^2 type due to a little unstable 
wind) and applying the whole equipment i started some resonating tests. 
The variometer part has a litte too much L so it is sensitive to adjust 
but never mind. It was easy to resonate and after transmitting a long 
carrier i noted that the heat sink of the PA remained absoultely cold. 
Then i increased the output voltage and got an average DC current (PA 
input at 13,8VDC by a SMPS driven by my fuel generator) of about 13A 
during the whole transmission time. The PA still remained cold but it 
was fused with 10A and i had a amperemeter of 20A full range, so i 
didn't further increase the power. The current came up to 18A and go 
down to 9A during the kite movement.

During the whole transmission, there were no HV problems! Just one time 
the wind turned and suddenly the antenna wire got disconnected with a 
8,97kHz noisy 2cm long spark ;-) The DC current immediately decreased to 
< 1A so that case is no problem for the PA.

I was in telephone contact with Bernd/DF8ZR and started to transmit some 
undefined sequences. Later i built up the notebook and installed the LF 
sequencer and keyed the TX in QRSS30. Bernd copied it difficultly in a 
distance of 16km. He clearly catched a 2 minutes transmission to that 
time i transmitted it.

I forgot to take the 1 Ohm current shunt resistor and the probe to 
watch/measure the current with the oscilloscope but later i rembered 
that i have a "true rms" digital multimeter with a range up to 10 kHz 
and so i could easily measure the accurent antenna current. The average 
antenna current was about 480mA!

So, the voltage was abt 15kV rms. Since there were no HV problems there 
is the possibility to increase the ERP at least by 3dB.

Andrey: No, the coil does almost not sing. The sound of the PA 
transformer is louder (only a little, not too nerving) but all that is 
blown in the wind ;-)

Paul: My exact QTH is 
http://maps.google.de/maps?hl=de&ie=UTF8&ll=49.77647,8.699525&spn=0.001607,0.003428&t=h&z=18 
(see the coordinates in the link). So: >Bearing of this signal was 
117/297 deg.  Frankfurt is on 114.  This is pretty exact! Congrats to 
your RX antenna system, Paul, very fine!! So, what is the new WR? I do 
not have a program that calculates the distance accuratly out of the 
coordinates...

Later i got a mobile call from DF6NM who told me that the signal is 
visible on his grabber. To that time i tried an unsuccessful QRSS10 test 
with Bernd. Later i switched to QRSS120 to write "DK7FC/P" there but it 
took 2,5 hours just for these few letters. Finally i just transmitted 
"DKM" since during the "7" the antenna wire (copper+steel-litz) broke. 
You can clearly see the "M" at the DF6NM grabber, the last sent letter 
for this day. The broken wire was due to strong wind: The kite needs a 
braid that is elastic (110% length at maximum force) to withstand the 
wind blasts, so the ant wire needs to have some sag and this sag was not 
enough in that moment, so the whole force turned to the wire. No problem 
about the wire, i have enough for the next test...

To the earth resistance: R(coil)=300Ohm and P(PA (with abt 100% 
efficiency))=180W, so the earth resistance seems to be about 480 Ohm. 
Hmm, seems to be a little much but it includes all the other additional 
losses such as coil losses due to the H stray field near the earth... I 
will measure this more accurately the next time. BTW i measured the 
voltage waveform with a loosely capacitive coupled HV probe. The sinus 
form was OK but could be better. But the C coupling is a high pass and 
so the harmonics may be not so strong as displayed... Listening to the 
harmonics with my IC706 gave nothing, nice.

My first question (others surely will come up later): What is the 
achievable groundwave distance on the 33422m band? ;-) And what do you 
expect is possible at night? Maybe, if we now have 830km (?, not 840?) 
we could even get much more...

Maybe i have forgotten to answer some questions but this mail is already 
long enough. So, will go on tomorrow. Now uploading the pics...

73 es GL, GN, 9 dreaming...

Stefan/DK7FC