LF,
I have experienced that any lossy elements around the antenna affect feedpoint resistance,
and hence antenna current.
With my antenna on the roof, the roofing material was lossy enough,
the antenna mounting "leg"s were lossy,
the feeding wire (PVC) was lossy also,
and covering them with wire mesh, replacing PVC-wire to PE-wire, etc.,etc,
reduced my feedpoint resistance, from about 60ohm to 30ohm, at 136kHz,
when I was using a coil of 15ohm(136kHz) and ground resistance of 10ohm (AF, measured).
A few ohms was left as unsolved unknown loss.
I felt,
ground is an imperfect conductor, is lossy,
many surrounding materials are imperfect insulator, are lossy,
(the roof, the house, trees, ....)
unless they are chosen to be low loss.
Hideho YAMAMURA / JF1DMQ
Markus,
Well said.
73, Jim AA5BW
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
into a parallel shunt cunductance, whose real part will be
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.
----- 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
