Hello Mike,
Thank you for your answer. I am sure there are plenty of 'lurkers' on
THIS reflector :-))
Am 24.07.2012 20:37, schrieb Mike Underhill:
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.
Well, i still don't know the actual location where i want to test the
antenna. I have some ideas though. I want to try to get a antenna
orientation beaming to G and PA, where the most RX stations are present.
Anyway, the tests will be temporary, i.e. i will remove the antenna
after a few hours of testing. So if the project looks promising after
the first tests i will certainly repeat it at different locations. I
also want to try different locations because i want to see the effect of
different ground conductivities, so one test will be done close to a
lake (Neckar, Heidelberg, maybe this location:
http://no.nonsense.ee/qthmap/?qth=JN49HK81MN&from=JN49HK80RG) and
another on a stony hill. There will be many tests and hopefully the
radiated signal is acceptable so it is an alternative for many OMs with
limited estate...
I will also try different heights above ground, starting from 0 to maybe
2m or 5m AGL.
Ocasionally broadcast signals in the 31m and 25m bands were found to
be be stronger on the underground antenna than on the reference antenna.
A stronger S/N or a stronger signal level?
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 may be an idea to test on MF too. Elevated ends as a compromise of
effort compared to a completely lifted antenna. But first i want to see
if the efficiency is at least in thge range of 0.5%, otherwise it is not
worth the effort i find. 100W RF power may be the absolute limit for a
/p setup...
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
Oooh i think there will be an excessive discussion on this description
by other amateur here in thge group. Or not?
I can imagine a positive effect from lifted ands since the voltages and
the impedance is maximal at the ends. So the negative effect of the
conductiong soil, imagined as a parallel loss resistor will be reduced
by that. But things may be much more complex on LF and MF compared to
HF, due to the skin effect.
I must admit that i am rather focused on practical tests than by reading
in the literature. Also there are some very good teachers in THIS group.
I wonder if this interesting discussion will be copied by someone to
yahoo to suggest that the real LF/MF discussions are done there, whereas
the "getting rid of..." messages from the "live and let die"-minded
fraction are permanently copied here... (Sorry).
change-over height to be about 10 to 15 meters at 470kHz or 500kHz.
That's not so easy but possible anyway.
BTW it is not at all easy to evaluate the differences of such different
antenna arrangements. I have never seen such a QSB on any band. Probably
it is the best to have a local RX station and do the tests on groundwave
distance. Probably the antenna radiates in a very high angle But the
difference (of the simulation) between 90 deg and 20 deg is just 5 dB
while the antenna gain is stated to be -5 dBi...
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.
OK Mike, go back lurking, HI. And thank you for the nice words. I'm glad
to be able to pull some lurkers out of the background from time to time :-)
As i said the results may be very different to your observations on the
31m and 25m band. But there will be results :-)
Best 73 and i hope we can discuss the results in a QSO on the band in 2013!
Stefan/DK7FC
Mike - G3LHZ
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