Stefan
As a long time 'lurker' on this LF group I am particularly interested in
what orientation you plan for your low height 630m band dipole antenna and
what azimuthal radiation pattern you find. A few years ago I did some work
(on a contract) on low height, on-the-ground and underground antennas in the
1 to 10MHz region. The underground antenna was an end fed wire about 45m
long in a 10cm 'land drain' in damp clay soil at a depth of about 45cm.
(The wire was put in the drain using five sets of 10 drain rods.) These
antennas radiated quite successfully with received and transmitted signals
generally about 10 to 30dB down on a reference horizontal 83m loop at 15m
height. Some of the tests were performed and winessed on a regular Sunday
morning net on 3722kHz.
Ocasionally broadcast signals in the 31m and 25m bands were found to be be
stronger on the underground antenna than on the reference antenna. So local
field strength measurements were then made just above ground on the
under-ground and on-ground (dipole) antennas. A 35cm tuned loop antenna on
an AOR hand-held scanning receiver was use as the field sensor. This could
sense the field polarisation as well as the field strength. The conclusions
were that such antennas "radiate and receive best from the ends with
vertical polarisation". This was confirmed earler this year at the IET
Ionnospheric Radio conference in York in a conversation with the conference
chairman Les Barclay, G3HTF. He had found similar results some years
earlier.
The interesting question is why this might be so. The following is now
offered as a possible explanation:
Electric field lines above the antenna extend to a height that is a
substantial fraction of (the square root of) a wavelength and terminating
on the two ends of the antenna. This electric field creates displacement
currents as half loops above the antenna from one end of the antenna to the
other. The half loops of displacement current radiate with vertical
polarisation with maxima in the direction of the ends of the dipole antenna.
As the antenna height is raised the E-field lines and displacement currents
become established below the antenna and above the ground. The currents are
in the same direction as the currents above the antenna and so they start
to cancel the loop radiation from the upper half loop(s) as the antenna is
raised. At some critical height the conventional dipole mode takes over and
the loop mode becomes suppressed. The critical height has been found to
proportional to the square root of wavelength in accordance with the
properties discovered for 'electromagnetic coupling'. (Measurements have
been made for antennas at different heights and form the minimum horn size
variation with frequency for the Goubau single wire transmission line.)
From measurements at higher HF frequencies I estimate the critical
change-over height to be about 10 to 15 meters at 470kHz or 500kHz. At
136kHz these figures are increased by the square root of the freqency ratio,
which is about 1.9. Real height of the antenna is important!
Whether this hypothesis or suggested theory is correct depends on real and
not simulated measurements. So you can guess the reason for my interest.
Looking forward to your results with interest, so I can go back to lurking
either with a warm glow of satisfaction or with a red face of embarrassment
and disappointment.
Mike - G3LHZ
----- Original Message -----
From: "Stefan Schäfer" <[email protected]>
To: <[email protected]>
Sent: Tuesday, July 24, 2012 4:04 PM
Subject: LF: Pretests for the 630m band dipole
MF,
This week i intend to start my tests with a full size dipole for the 630m
band. All the equipment for this /p is available now.
Since the dipole is VERY close to ground (relative to Lambda) there will
be a capacitive component and the Z=R at resonance will be lower than 36
Ohm of course.
I have just done some simulations in EZNEC and found that it is useful to
feed the dipole assymetrical, i.e. not in the center. The simulation
tells that a wire length of 315m will be necessary. The resonance
frequency does not change by different feed points. I hope this applies in
practice as well :-)
The electrical parameters will depend on the soil conductivity and epsilon
r, which i don't know exactly of course. But if the settings in the
simulation are reasonable, then there will be a real component of Z=R
(=resonance)of 46 Ohm. So i would not even need a transformer to match the
antenna to the PA.
85% means that one wire is "just" 47m. So if someone lives close to a
forest, there would be a chance to build such a TX antenna and run the
major part of the antenna outside the garden. Just a short rest has to be
inside the own property. For a 90% feed, R would be 96 Ohm. This could be
matched to 50 Ohm very easily and the shorter part of the antenna would
only be 31.5m...
I have a bundle of old telephone wire (once found a 1800*0.4mm x 10m
length cable) and so i can connect several pieces in series for the test.
So i can make a table of measurements. The modified MFJ-259B antenna
analyzer will help, hopefully.
Looking forward to that test.
Due to the permanent number of OP4 listening stations, this mode may
actually help to get an impression about the efficiency and directivity.
73, Stefan/DK7FC
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