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Stefan
If your 600m wire is average 6 metres above the
ground then you have an elevated grounded loop whose natural resonance is the
length of wire plus the same length earth return plus 6 m vertical at each
end ie 1200m plus 12 metres == 1212 metres.
This is an old trick used on 80m and 160m to
make a vertical loop antenna where space is restricted. ie one half of
the antenna is elevated above ground and the other half is made up by the
ground, and can be in the form of a rectangle or square or triangle. Half
the rectangle or square or triangle is elevated normally as high as possible
above ground and the ground is a MIRROR image to complete the loop.
An earth mode antenna is the type of antenna used
by the CAVE fraternity to communicate through ground ie below ground in a CAVE
to the ground above.
ie short distance communications.
In my opinion an earth mode antenna is intended to
communicate short distances EARTH to EARTH, TX into the earth using a probe
rod and RX this signal
using another probe rod . Your grounded loop is
propagating the signal into the ether with the intention of it being received by
an elevated RX antenna some hundreds of metres or Kilometres away at a
distant location.
de mal/g3kev
----- Original Message -----
Sent: Tuesday, December 14, 2010 9:55
PM
Subject: Re: LF: Earth Electrodes
Mal,
No.
It
depends on the frequency. On 8970 Hz and lower it is an earth antenna.
Mal, it was so often explained how an earth antenna works. Do we
really have to repeat that each few weeks? Really? OK, once again:
And
earth electrode antenna is a loop antenna where the current return path is in
the earth. So there are two electrodes and the TX can be applied anywhere
between these two electrodes. Furthermore i have never said to have 2 wires
(600m each) but just one wire. The TX ground is connected to the first (near)
ground rod(s) and the center of the coax is applied to the wire which is in 6m
height (average). The total wire length is about 700m and the electrode
spacing is about 600m. So the far away electrode is in 600m distance.
Rogers and my earth antenna is not a grounded loop antenna since the
wire is not closed! The ground is a part of the loop, thats the difference
between a usual loop and an earth antenna.
I can run a current at any
frequency. I have tried 10 Hz (just for testing) and DC and 100 Hz and so on.
I hope you can agree that the current MUST come back trough the ground if the
frequency is 0 (DC)!! So, the lower the conductivity, the higher the effective
area of the loop (skin effect!!!) and thus the efficiency. Since i am on a
hill there (500m ASL) and there are almost only stones, the conductivity is
very low. This causes more effort in arranging a suitable earth rod system ( i
am using 30 rods, 0.4m long each, spaced about 1m) but is is interesting
:-)
There is no matching from 8 Ohm to 50 Ohm since my PA has not an
output impedance of 8 Ohm and my antenna has not an input impedance of 50 Ohm
;-)
The goal is to get as much antenna current as possible, so there is no
need to match something to 50 Ohm. Since the cable length is always
<<< Lambda, there is no need to match something! I can measure the
antenna current directly by using a digital multimeter(attached picture). The
power can be measured on the DC side of the PA, since the efficiency is quasi
100%. So you can calculate the losses.
If the earth antenna has a
overall resistance of 500 Ohm and you can and want to generate 500 W (assuming
the reactive Z component is negligible) you just have to apply 500 V (rms) on
the wire, thats all!
Earth antenna does not mean that the wire is on
the ground but that the current comes back in the earth!
So, hope to
have answered the questions, in december.
73, Stefan
Am
14.12.2010 20:52, schrieb mal hamilton:
Stefan
You now admit that your antenna is in FACT a
LOSSY DIPOLE and not an earth antenna
g3kev
-----
Original Message -----
Sent:
Tuesday, December 14, 2010 6:12 PM
Subject:
Re: LF: Earth Electrodes
Chris, LF,
Am 14.12.2010 17:27, schrieb Chris:
HiStefan,
Yes, but 600m is VERY long! The wire even
laying on the ground I would expect to radiate quite well at 137 with
that length! And how many watts?? ERP??
I am sure 8970 would do well through
the ground at the sort of powers being spoken about. Probably ideal
for submarines, as Roger says!
Vy 73, Chris,
G4AYT. Well, my earth antenna is grounded at the
ends and TX power was about 250 W. The mode was DFCW-600. The receiver was
at Michael Oexner using a PA0RDT design. ERP? No idea, probably some few
uW. Its not so easy to calculate the losses.
Yes, on 137 kHz the
wire length comes close to lambda/4 and above so it acts not just like a
pure earth antenna but rather as a combination of this and a very loss
dipole. On 137, my power was 100 W at this test.
The antenna was
built to do local tests but seems to work suitable on 137. So it is a
fixed antenna, my only fixed one. And it is a very quiet location. This
makes it interesting to use it as a RX antenna too!...
73,
Stefan
-----
Original Message -----
Sent:
Tuesday, December 14, 2010 2:49 PM
Subject:
Re: LF: Earth Electrodes
Hi Chris,
But how do you explain that i have
crossed 49.6 km with a 600m spaced earth electrode antenna on 8970 Hz?
It was rather summer time than winther (regarding QRN)! With the same
antenna, my 137 kHz signal was seen 20 dB above noise near Paris in
DFCW-3!?????
73, Stefan
Am 14.12.2010 15:15, schrieb
Chris:
Hi Mal,
Yes, that is what I had concluded years
ago. No harm in trying though!
Further to your previous e-mail to LF,
looks to me like beacons are now becoming the norm on 136/7kHz band
and below. I have no problem with that personally, indeed, I think
it preferable to a QSO taking forever! I woud like to see full
idents though, no matter how slow.
Vy 73,
Chris,
G4AYT.
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