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73 de Jean-Louis F6AGR
Hello Stefan,
ground waves (surfaces waves) are a tricky
thing.
The assumption you made (+6dB TX power = double distance) takes only
the 2D spreading loss into account. This would be correct if the ground would
be a perfect conductor and if the earth would be flat.
In reality you have
2 additional losses: diffraction loss (due to the earth curve) and ground
loss.
The bad message is that these losses both have a more or less 1D
behavior, and thus their attenuation is more or less linear to the
distance.
The late G4FGQ wrote a very good DOS application (named
GRNDWAVE3) where you can put in a lot of parameters (distance, antenna
efficiency, frequency, ground type, TX power) and it gives you the path
attenuation, field strength at RX end and RX antenna voltage.
It must be
on the web on several places (google it), but in case you cannot find it I can
send it to you.
Just as an example the path loss this programme gives
for 137kHz and an average ground:
250km = 55.1dB
500km = 65.3dB
750km
= 74.7dB
1000km = 83.5dB
2000km = 115.4dB
3000km = 144.5dB
4000km
= 172.2dB
5000km = 199.0dB
As you can see doubling the distance "costs"
far more that 6dB (by surface wave, sky wave is a different story).
I
haven't kept any records by I think that the surface wave limit for most
amateur stations is 1000-1200km (on 137kHz), maybe a bit more in QRSS. Beyond
that you are far better of with sky waves.
73, Rik ON7YD -
OR7T
At 02:51 29/01/2010, you wrote:
Hi Alan and LF,
I know there are some of you who can
easily answer my question that follows :-)
The maximum distance of the groundwave at a specific fieldstrength E
is (about) linear increasing with the antenna current of the TX antenna,
right?
So, if i have an antenna current of 0,5A and get a maximum
distance of 1000km, i would reach 2000km with 1A (same RX, same surrounding
noise level, same average ground properties, same OP ;-)
)?.
I expect, that the groundwave does not
immediately stop beyond this 2000km border but rather decreases with 1/r,
just as before.
So, if we assume one is increasing the
antenna current in the above example to 7A, is then a distance of 14000km
possible? Sure, thats a very theoretical question since there will not be
the same ground conductivity on the whole distance but
anyway.
And it is said that the groundwave is (nearly) not
affected by the daytime, by the season and so on. There must be
interferences with the sky wave, so QSB, but this does not affect the
groundwave at an other RX QTH, where no sky wave is
present!?
If there is so much sea water between a transatlantic
distance, why is it so difficult to do it with the groundwave? On HF or MF
it is clear but on LF?
Tnx for enlightning
answers...
Stefan/DK7FC
Von: [email protected] im
Auftrag von ALAN MELIA
Gesendet: Fr 29.01.2010 01:51
An:
[email protected]
Betreff: Re: LF: Ok its a sea path ..
but this is getting silly
Ah this 500k stuff is
too easy Graham :-)) oh for 73kHz again !
Alan
G3NYK
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