|Information about the KD4RLD/W4DEX
VLF Transmitting Exmeriments
KD4RLD 8800 Hz signal as seen by W4DEX over a distance
of 10 km
I am pleased to correspond directly with you. I have read about
many experiments and accomplishments.
I will try to answer some of your questions. I feel sure you will
some humor in our lack of technical expertise.
First I will describe the receiving system as I was on the receiving
of this test. The antenna I used was a 160 meter dipole fed with
wire with the feed line tied together. Effectively a T 33 meters
with a 80+ meter flat top. I added inductance at the feed point
resonate the T at 8 kHz. The receiver was a HP 3586B selective
meter operating with the 20 Hz filter selected. Output of receiver
sound card of course. I have experimented with VLF directly into
sound card but I have not been able to achieve the sensitivity of the
for this frequency.
On the transmit end Brian was using a digital audio oscillator driving
100 watt solid state audio amp. Now here's the questionable part.
the antenna he used two commercial AC power distribution poles that
separated by about 145 meters. Each pole has a ground wire running
the AC ground wire on the pole to a copper plate on the bottom of the
pole. Of course the ground wire on each pole is connected by
ground wire between the poles. From one pole at ground level
connected a wire to the pole's ground wire and ran the wire laying
the ground to the base of the other pole. At that point he connected
the audio amp output to the wire laying on the ground with the amp's
ground to the ground wire of the pole. So the DC loop the amp
driving was effectively a loop 10+ meters high by 145 meters long.
used no loading and had no way to measure antenna current. I
said the amp was drawing about 10 amps from the 12V vehicle battery.
We received several messages saying we were just exciting the AC
distribution grid and that his signal was arriving at my location via
the power lines and not being radiated. We don't argue that.
did was to state the facts of the experiment making no claims.
some point there must have been some signal radiated or I don't think
receive antenna would have picked it up.
Brian will be transmitting again, soon we hope, with a loop antenna
completely isolated from the power lines. I'll let you know the
We would certainly appreciate your thoughts and ideas about our test.
Also any help you can offer as to how we can match the amplifier output
to a loop or any other antenna and how to measure the antenna current.
Hi Dexter and Brian,
tnx a lot for your immediate response. Very interesting indeed, I really
your idea of the powerline antenna.
> I feel sure you will find some humor in our lack of technical expertise.
... just the slightest note of understatement ;-)
> ... So the DC loop the amp was
> driving was effectively a loop 10+ meters high by 145 meters
> used no loading and had no way to measure antenna current.
I believe he
> said the amp was drawing about 10 amps from the 12V vehicle
> We received several messages saying we were just exciting the
> distribution grid and that his signal was arriving at my location
> the power lines and not being radiated..
A q&d calculation (literally on the back of an envelope) for the
Rrad = 31.2kOhm * area^2 / wavelength^4 = 5*10^-8 ohms,
assuming that most of the current actually returns through the overhead
and not through the ground. For a transformer-less bridge-type class
we could perhaps assume
I_RF ~ I_DC = 10 A, and
Prad = 5 microwatts = -23 dBm,
which at 10km would produce a fieldstrength of
E = -23 + 49.6 - 20 = 7 dBuV/m.
From measurements last year, I estimated the background noise level
En = 5 dBuV/m in 1 Hz.
If that were also applicable to your location, using 0.08 Hz BW you'd
SNR = +7 - 5 + 11 = 13 dB.
So in theory, it is quite possible to achieve the contact without the
powerline conduction. However at 6 kHz the budget becomes much worse,
radiated power scaling down with the 4th power of frequency and noise
higher. It may have worked by a combination of both mechanisms: RF
the powerline system, and this then acts as a much larger radiating
> Also any help you can offer as to how we can match the amplifier
> to a loop or any other antenna and how to measure the antenna
Well, you probably have not been far off a good low-impedance match:
145 m of
wire will have a couple of ohms, and a rough estimate of inductance
something on the order of 200 microhenry or j 12 Ohms, maybe less.
possibly get more current by series-tuning with a capacitor, perhaps
1 or 2
microfarad. This would also reduce the risk of feeding induced 60Hz-currents
back into the amp.
In this country, the small power poles for household distribution have
completely been replaced by underground cables, but of course the medium
high voltage lines are still there. And then there exist the overhead
of our 16.7Hz railway system. These are lacking the grounded top wire,
despite their moderate height they might be an effective radiator,
devised an effective (and safe) means of coupling.
Happy and successful experimenting, and please keep us informed.
73 de Markus, DF6NM