Message-ID: <0F769D54D6F042F0AD2CAD7C1AC6C84A@WarrenPC>
From: "K2ORS" <k2ors@comcast.net>
To: <rsgb_lf_group@blacksheep.org>
References: <000901c83a65$6404c040$7f157ad5@w4o8m9> <000701c83a7a$ed72bd40$0900a8c0@Lark> <002401c83ac6$f343c6a0$792a8351@w4o8m9>
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Date: Tue, 11 Dec 2007 15:19:32 -0500
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Subject: LF: Re: Re: Re: Antenna tests on 136k and 503k - Results
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Jim,

   Interesting treatise on short antennas and the non-propagating 
"evanescent" modes that couple in to surrounding objects on AB2EZ's web 
page:

"Typically, the evanescent modes interact with nearby objects, such as the 
ground, buildings, trees, etc. The evanescent mode fields produce currents 
in these objects. which will be converted to heat unless the objects are 
perfect conductors. Evanescent modes can also produce magnetic fields in 
objects which also produce heat through hysteresis effects.
The shorter the antenna is compared to a wavelength, the larger the strength 
of the non-propagating, evanescent field near the antenna (the sum of the 
propagating and the non-propagating fields is often referred to as the "near 
field" of the antenna), compared to the strength of the propagating field 
near the antenna. and thus the larger the percentage of power that is 
converted to heat by losses associated with the evanescent modes."

http://mysite.verizon.net/sdp2/id21.html


73 Warren K2ORS


----- Original Message ----- 
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Sunday, December 09, 2007 7:52 PM
Subject: LF: Re: Re: Antenna tests on 136k and 503k - Results


>
> Dear LF Group,
>
> Thanks for the comments...
>
> K2ORS wrote:
>
>> Bob has an open field with no vegatation. Recently Bob has made field
>> strength measurements at 500 and confirmed that the calculations are
> indeed
>> very close.
>
> I have just re-calculated the ERP based on values of Rrad obtained using a
> NEC-based antenna simulator (MMANA-GAL). This should give a more accurate
> value of Rrad than the simple formulas based on capacitance, because a 
> more
> complicated geometry can be modelled, and the interaction of the fields of
> the horizontal and vertical parts of the antenna will be accounted for. 
> This
> resulted in slightly higher values of Rrad, and slightly higher expected
> ERP. This gave even better agreement between calculated and measured ERP 
> at
> the open-field site - at 136k the measured ERP is now +0.3dB on the
> calculate value, and at 503k it is -0.1dB. This is probably well within 
> the
> likely measurement uncertainties.
>
> G3NYK wrote:
>
>> I suppose you could see the Rrad reduction due to nearby structures and
> the
>> environment as being due to some of the feed current being diverted by
>> capacitive coupling into the lossy surroundings and so not traversing all
> of
>> the distributed Rrad.
>
> I think this ties up with the "tree current" experiment I did some months
> back - it was clear then that, especially on 136k, a substantial 
> proportion
> of the antenna current was returning to ground through the trees near the
> antenna. In the far field, the total radiation would be a function of the
> superposition of the currents flowing "up" the antenna and "down" the
> trees - the bigger the trees, and the more current flowing in them, the 
> more
> they would tend to cancel the radiation from the antenna. The measurements
> of the "tree current" were very rough, but at 500kHz it appeared that the
> currents were a smaller fraction of the antenna current, which would tie 
> up
> with the smaller observed reduction in Rrad at 500kHz.
>
> G3YMC wrote:
>
>> What would be interesting is if you could quantify any differences in
>> the type of ground at the two QTHs. The much lower earth loss at the
>> open site may well be due to totally different soil characteristics
>> rather than solely due to the surrounding trees.
>
> The ground was probably as similar as it is possible to be at two separate
> locations - the open field site was only about 4km from my home QTH, and
> geographically part of the same landscape. The soil in both places is a
> heavy clay that is waterlogged at this time of year. If anything, the 
> ground
> was wetter at the /P location, because my QTH is on a slope and relatively
> well drained, but I generally find that wetter conditions lead to higher
> loss resistance. It would certainly be interesting to see how the type of
> ground afffects loss resistance, but it would probably be quite hard to 
> find
> a suitable selection of sites.
>
> G3LDO wrote:
>
>> At one time there were a row of conifer trees along the bottom of my
>> garden, which caused a reduction of antenna current when they were wet.
>
> At my QTH, over the period I have been making antenna measurements, the
> antenna has stayed basically the same, but the surrounding trees have 
> grown
> quite a bit larger. During this time, the apparent Rrad has decreased to
> about half of its initial value, and Rloss has increased by around 50%.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>