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Message-ID: <3.0.6.32.20000202093713.0084b7c0@laatikko.saunalahti.fi>
Date: Wed, 02 Feb 2000 09:37:13 +0200
To: rsgb_lf_group@blacksheep.org
From: =?iso-8859-1?Q?V=E4in=F6_Lehtoranta?= <vaiski@dlc.fi>
Subject: Re: LF: Change of ground resistance due to frozen ground
In-reply-to: <3897CCC5.533A@xtra.co.nz>
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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: vaiski@dlc.fi - alias: oh2lx@dlc.fi & oh2lx@sral.fi