In Finland we have some experience with frost.
Information about its effects on ground wave
propagation is not very well established.
Same with TV & FM space wave (I am not talking
about frost on antennas). What effect frost can
have on higher angle sky wave component on
LF, MF or even HF frequencies, I just wonder...
I do have some frost depth statistics from
Southern Finland and have tried to find some
correlation between measured LF/MF ground wave
values and frost. The stations both flight and
and ground (fixed qth) measured consist of:
- One LF BC station, Lahti 254/252 kHz, 200 kW,
- Several MF BC stations, two still active,
- More than 100 MF aviation beacons (NDB & L),
- A total of some 10 maritime MF DGPS beacons.
YLE measurements began in the 1950's but only
VHF/UHF will continue after 1st Jan 2000. Private
LF measurements continue from my poor qth...
Finnish NDB's and locators (LO/LI) presently use
either 11 m or 14 m high top loaded monopoles.
Finnish DGPS beacons use a 36 m high top loaded.
Locator with a 14 m high monopole and 500 W out
radites between 5 and 10 W EMRP (1% to 2%).
DGPS with a 36 m monopole and 100 W out radiates
approximately 5 W EMRP (5%), or even more.
Raimo, OH2MRX is real aviation beacon expert
and he reads these messages. He and other CAA
fellows may confirm whether frost plays some
significant role on NDB/L behaviour.
According to our experiences, as propagation
researches something happens in beacons close
environment when the water content changes
from liquid to solid or vice versa.
From long time ground wave measurements we can
easily see that the most significant variable
to effect indicated F-s values is air temperature.
This was documented already in the 1940-1950's.
Earth net efficiency varies from case to case.
73 de Vaino, OH2LX
At 19:20 2.2.2000 +1300, you wrote:
Hans-Joachim Brandt wrote:
Besides just noting the change It would have been interesting, of
course, to
find out into what direction the ground resistance has changed. On one
of the
last cold days and before the snow had disappeared I checked it with a
resistive
bridge to find that the total feed point resistance of my antenna (when
tuned to
resonance) had decreased from 100 ohms down to 60 ohms!!!
I do not have a reference at hand to quote a definitive response, but my
recollection of ground parameter data is that frozen ground is "bad
news" for ground wave loss, as is fresh water. Your measured result is
the other way round. I will try to find a text book reference on frozen
ground losses versus unfrozen.
By the way, the temperate climate here in Wellington, New Zealand
involves a few frosts in the winter but no frozen ground as such, so I
have not experienced the type of conditions that Hans-Joachim is more
familiar with.
73, Bob ZL2CA
----------------------------------------------------------
V.K.Lehtoranta, OH2LX, POBox 50, FIN-05401 Jokela, Finland
------ Tel: +358-9-4173965 ---- Fax: +358-9-4173961 ------
E-mail: [email protected] - alias: [email protected] & [email protected]
|
