Dear Rik,
thank You for the
internet
address concerning the details of DI2AM. I have tried to find them again
but
without any success.
But after sleeping on our discussion I
came to
the conclusion that we should regard the problem I have raised in a
different
way.
When transmitting I concede that any losses in small
antennas
can be compensated by higher transmitter power.
But when
receiving such a
compensation is not possible, and if a small antenna has
losses weak
signals may get lost.
When recalculating the figuress I have
mentioned of 7 meters and 10 meters antenna height respectively, related
to 500
kHz this increase in height means that the radiation resistance (which
is
independent of the antenna being tuned or not) of the higher
antenna has
doubled, and as the antenna signal current may have remained equal the
receiving
power will have quadrupled!
This would also mean than an OM using
a small
antenna for transmitting and receiving, as is common among radio
amateurs,
will have a reduced receiving range compared to a station using a higher
antenna.
Therefore I would like to repeat my statement that we
should not
disregard antenna height, unless we do not want to achieve anything
else than just beaconing.
HW?
73 Ha-Jo,
DJ1ZB
"Rik Strobbe" <[email protected]> schrieb:
Dear HaJo,
I can not find any reason to believe that the same ERP radiated from 2
antennas with the same radiation pattern could produce different field
strengths.
But I am far from an antenna specialist or propagation specialist, so
maybe others can come up with these reasons.
Another thing:
At the moment we talk about skywave (ionospheric) propagation there
might
be no such thing as "the best antenna". An antenna that has a
high takeoff angle may be superior to an antenna with a low takeoff
angle
at shorter distances (let's say less than 1000...1500km) while the low
angle antenna will do better at long distances.
Regarding DI2AM:
http://www.seefunk-fx-intern.de/radiobeacon/radiobeacon_en.htm
provides some figures:
- The TX has 18W input, at 75% efficiency this means about 13 W output.
- The antenna current is +/- 1.7 A, assuming 13W output the total
resistance (Rrad + Rloss) is only 5 Ohm.
- The antenna top is 35m asl. Based on the picture I estimate that the
antenna bottom is +/- 10m asl, so antenna hight is +/- 25m. The
topload seems about twice as long as the antenna height so let's
estimate
50m. According to "mmana-ga"l Rrad is 2.3 Ohm, the antenna
capacitance is 840pF (the website notes +/- 800pF, so thta's pretty
close) and the gain is 4.64dBi (2.5dBd).
- Putting 1.7A into 2.3 Ohm means 6.65 Watt radiated or +/- 12 Watt
ERP.
No wonder they are putting out such strong signal.
73, Rik ON7YD - OR7T
At 00:51 18/06/2008, you wrote:
Dear Rik,
thank You for your contribution. Especially your last sentence is
interesting for me because this may explain the success of MF-stations
operating close to the coast.
Another example for exceptionally high antennas on MF for me is DI2AM,
located on a museum ship in the harbour of Rostock. I have been
informed
that the antenna has been put up between two masts of 30 meter in
height.
Other favourable conditions are the low ground loss onboard of a ship
and
also the vicinity of water, of course.
In general I do not have any objections to Finbar's setup; it shows
that
effective MF antennas can be built up on a rather small area.
But I am not yet convinced that height of MF antennas should not
matter,
and I will continue to observe the scenery.
OK?
73 Ha-Jo, DJ1ZB
"Rik Strobbe" <[email protected]>
schrieb:
- Dear HaJo,
- But in general I feel that antenna height should not be
disregarded.
Even with equal ERP sky waves may develop better from higher
aerials.
- HW?
- I guess that the only cause that 2 antennas fed with the same ERP
produce different signals is a difference in the (vertical) radiation
pattern, ie. the takeoff angle.
- I simulated a 30m, 10m and 3m vertical (at 502kHz) with mmana_gal
and
found only very small differences: from 18.2 degrees for the 30m
vertical
to 18.6 degrees for the 3m vertical. This for a uniform ground with a
conductivity of 10mS/m.
- Making the ground poor (1mS/m) results in a takeoff angle of about
26
degrees (+/- a few tenths depending on the height).
- Increasing the conductivity to 100mS/m reduces the takeoff angle to
12-13 degrees.
- 73, Rik ON7YD - OR7T
- Disclaimer:
http://www.kuleuven.be/cwis/email_disclaimer.htm for more
information.
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for more information.
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