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The apparent contradiction (why is higher impedance
better?) is easy to understand if you look at surrounding trees as capacitively
coupled shunt resistors. The loss
contribution At the antenna can be deduced by transforming the series
impedance
-j X + R
into a parallel shunt cunductance, whose real part
will be
G = R / (R^2 + X^2)
Obviously G disappears when R is infinite (tree is
transparent) or zero (tree is a metal pole), and reaches a broad maximum
when R ~ X.
While the coupling capacitance X is determined
by shape and size, the resistance of the tree trunk and twigs
varies with humidity and temperature. During frost conditions, the sap is
retracted and R becomes much larger than X, thus G becomes very small.
On the other hand, when trees are wet, R is approaching X, and antenna
losses G go up. For a backyard amateur antenna, capacitive coupling to
vegetation can easily be the dominant loss mechanism, exceeding the series
resistances of the ground connection and the coil.
Another effect is that partial shunting of
displacement currents to ground reduces effective height - with a given input
current less is getting out to the far field. A few years ago, I found by
measurements that my antenna efficiency almost doubled in a cold
winter night: The loss resistance went down by 30%, and the effective hight grew
by 20%, so each effect was contributing roughly 1.5
dB.
Best 73,
Markus (DF6NM)
----- Original Message -----
Sent: Saturday, February 01, 2014 9:40 PM
Subject: Re: LF: Re: Ant current
Christian, LF
"Ice is an insulator"
If this is true
it also means that all lossy trees in the surrounding
may have turned
into nearly insulators.
Unless there is ice or frost on my antenna
and insulators I usually
have a lower Rtot (more antenna current) in
the winter. This is in
line with your findings. It may well be lower
environmental losses and
not the ground loss that are resonsible for
this phenomenon here as my
antenna has lots of trees in the
vincinity.
BR
Paul-Henrik, OH1LSQ
Quoting Alan
Melia <[email protected]>:
>
Wet ground is lossy, ice is an insulator, antenna current is mainly
> determined by Rloss ?? If you measure the unloaded antenna with
a
> bridge, you will measure values for C and Rtotal. Rtotal
include
> Rrad and Rloss. Rrad is very smal for amateur size
amtennas and
> doesnt change much. So the Ground loss must be
reduced.
>
> Alan
> G3NYK
> ----- Original Message -----
From: "C. Groeger" <[email protected]>
> To: <[email protected]>
>
Sent: Saturday, February 01, 2014 2:51 PM
> Subject: LF: Ant
current
>
>
>> Hi all
>> How can one explain that
ant current rises when soil is frozen?
>>
>> Conductivity of
water should be much higher than that of ice.
>>
>> So earth
resistance would be higher and the current should drop
>> when
soil is frozen...
>>
>> 73,
df5qg
>>
>>
>> Christian
Groeger
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