Hello, an interesting discussion.
How about high angle radiation from the toploading wire at DI2AM? As I
understand it the shape is more or less an inverted-L.
Quoting Rik Strobbe <[email protected]>:
> 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:
> 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.
> >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.
> >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
> >for more information.
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